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Detecting Regenerated Voltages
11

Detecting Regenerated Voltages

Detecting Regenerated Voltages

(OP)
I would like to discuss the details of how a motor acts when a phase is lost and if votlage imbalance and single phase protection devices actually work. Here is what I know about the behavior of the motor:
 If you lose a phase on a 3 phase motor, you are losing a pole. The speed of the motor drops, meanwhile the other pole is acting like a generator. What is the amplitude of the voltage generated? I think the voltage of the generated leg will be in phase as the 'lost'.
 The voltage protection devices I am familiar with protect against single phase, low voltage, and voltage unbalance conditions. They assume that the generated voltage will not be the same amplitude as the line voltage. Is this a correct statement? What affect, if any, does multiple motors on the same line have on the ability of voltage protection device?
Thank you.

RE: Detecting Regenerated Voltages

(OP)
I should add another question: Does the location of the single phase condition affect the ability of the voltage protection device, I mentioned, to detect the loss of the a phase?

RE: Detecting Regenerated Voltages

2
You are right that speed is decreasing, but the reason is not that you are "losing a pole". (actually every pole has windings from all 3 phases.... deenergizing one phase will distort and weaken the field but not change the number of poles).

So the syncronous (no-load speed) of the motor will not decrease with loss of one phase. But the flux is lower so the torque developed at any speed is lower. With a lower torque-speed curve, the intersection with load torque-speed curve will be lower... and speed drops further below syncrounous.

Someone has recently posted here an analysis of the increase in current on remaining phases due to open circuit. Based on the assumption that the change in speed is relatively small, the power will be constant.  Neglecting change in power factor and efficiency, we can calculate the required increase in current on the two remaining phases to deliver the same power as the case with no open circuit.  I don't remember the exact answer.

Generated voltage on the open phase.  That's a good question too.  And will it depend on motor load?  Hmm.



RE: Detecting Regenerated Voltages

After thinking about it for five minutes, my GUESS is that any voltage generated on the open phase will be far less than the line voltage. Here is my reasoning.

Look at 2-pole motor with pole-phase groups a, b', c, a', b, c'.

Assume c phase is open... what will be the voltage induced in c?

The c group overlaps 120 mechanical degrees with a' and b'.  It also overlaps 60 degrees with a and b.

Since this is now a single phase system  the flux from a is equal and opposite magnitude to flux from b.  Contributions from 120 degrees of a' and b' cancel each other.  Contributions from 60 degrees of a and b cancel each other.  Voltage induced in C should be zero.  Similar logic would show voltage induced in c' should be zero.

There may be some assymeteries and in particular the slight tangential component of the flux in the airgap which will cause unequal contributions from a and b. But this will be a small effect resulting in an induced voltage much lower than line.

That's just a guess.



RE: Detecting Regenerated Voltages

(OP)
Let me make a summary of your first post and tell me if I am right: The speed of the motor will drop, even at no-load, with the loss of a phase voltage. This speed will be reduced even further of a fully loaded motor.
Second post: I agree that the loss of any phase results in a single phase power supply. I think the current increases by a factor of 1.73 (if I remember the previous post you referred too).
I agree with all of your statements I just could explain why. When I was a newbie to motor protection design, the company I worked for touted protection against regenerated voltages. I was told this is because newer devices (not new now) monitored for voltage unbalance and this is why regenerated voltages were not a problem. I never pursued the real answer much, until now. If what I was told was true, then if the device was monitoring for low voltage, for example, then why wouldn't the average voltage protect against regenerated voltages? Maybe they only monitored one phase? Thank you.
 

RE: Detecting Regenerated Voltages

I believe the no-load speed of motor will NOT drop upon loss of one phase.  The number of poles remains the same, even with loss of one phase.

The slip will increase for a given load.

I can't provide any info on the subtleties of motor protection for loss of phase.  Also bear in mind my response that there is no substantial generated voltage is a guess. I haven't read anything on the subject or done any measurements... I could be way off base.


RE: Detecting Regenerated Voltages

The motor will keep the voltage on the open phase at a fairly high level.  It is usually high enough that normal undervoltage relays cannot be used, because the required setting would be too high for normal operation with occasional voltage sags.  Some 3-phase relays operated on an average of the three phase voltages, and this won't work either usually.  If protection is to be provided by voltage detection, the best bet is a true negative sequence voltage relay, but even these may not detect the open phase if there is mostly motor load.  The more static load on the bus, the more unbalanced the voltages will be for an open phase, and the more likely that a voltage relay will detect the condition.  The best protection is to monitor current unbalance or negative sequence current.  An open phase will increase the current unbalance significantly (zero current in the open phase and 1.73 x normal current in the remaining two phases) and can be quite reliably detected.

RE: Detecting Regenerated Voltages

Thanks jwerthman. Your statement about regenerated voltage is similar to what busbar said on the other thread and I'm sure it's correct.

Can anyone figure out where I went wrong in my reasoning in analysizng the flux linked to a given pole-phase group (above). It seems pretty straightforward to me.

RE: Detecting Regenerated Voltages

3
When you single phase a motor the big effect is a loss of torque. In general, I believe that the motor will only produce about 57% of the 3-phase torque.

Anyway, for no load operation the motor will run at speed. Performance loaded depends on the load torque requirements (duh!). This means that the motor will run at whatever speed the (available 57%) torque curve intersects the load torque. In some cases the motor will not turn at all and the starter will trip on the LRA of the active phases (overload trip). In some cases it will run at reduced speed due to load (not loss of poles) but during this situation the active phases will have a great slip and the starter will trip on overload. Finally, in some cases the motor will actually run at speed, but this is not common since most motors are not that lightly loaded.

With respect to the generated voltage in the open phase, I need to think about that one....I am not certain right now what the magnitude would be and it is not clear to me where you would connect the relay to detect this voltage since the circuit could be open anywhere between the incoming power supply and the motor or the open could be in the motor winding itself.

I believe that the most reliable way to detect a single phase condition would be a current loss relay. This would actuate whether the problem was an open circuit anywhere downstream of the starter or a voltage loss upstream.  


RE: Detecting Regenerated Voltages

(OP)
I think we are all in agreement that current monitoring will detect a single phase condition.
I have heard a lot of stories about voltage monitors being effective and not effective at detecting regenerated voltages. No one has yet been able to explain one way or the other. Again, does anyone know where experimental data can be obtained? I have been looking for this for quite sometime. Given the arguments both ways, I am not content to believe either way just because someone says it is true. I do know, that even large motor protection companies make this claim and I tend to agree with them at this point. Maybe one of you guys can do an experiment and see what happens, just kidding. Single phase conditions are generally caused by a blown fuse or utility issues. If a phase in the motor opens then there should be no voltage on the phase to ground (or phase to phase for that matter) outside of the motor, depending on where the open winding is located.
My concern is not with a no-load motor operation because this is not a real world scenario. Lightly loaded maybe. Any real data would be greatly appreciated. Thanks to all for your opinions.  

RE: Detecting Regenerated Voltages

  
buzzp, apologies for the slow followup.  See:

  www.sea.siemens.com/motorsbu/product/White%20Papers/Troubleshooting.pdf

Based in info in the Siemens paper, a {probably longer-term} 5% voltage unbalance calls for a ~¼ reduction in load to prevent premature failure.  Estimate a level of protection—is 5% voltage unbalance {highest level in the Siemens graph} an OK starting point?
 
Some manufacturer’s literature contains statements like:
•    “The relay releases when one phase-neutral voltage drops below 70% of the other phase-neutral voltages or when the phase sequence is wrong.”
•    “Phase Loss: 18% Low Voltage in one phase”
•    “Phase Loss: <75% of set point”
Maybe I’m misinterpreting their statements, but it seems like those numbers translate to several multiples of the chosen 5% level [and also reflected in IEEE guides/standards.]

But, some literature contains unabashed statements like, “Phase Monitor Relays protect against unbalanced voltages or single phasing regardless of any regenerative voltages” in claims for some socketed or din-rail modules.  That is simply not always the case.  Significant temperature drift and very low transient immunity are problems with a fair number of socketed/din-rail so-called protective devices.  A cheesy ¾-turn ‘full-range’ adjustment does not give me any additional confidence.  I am not saying that this type of relay is completely useless, but the “specs” {and lack of same} should be carefully read and understood.  Be careful in looking for a device that promises to end to all your motor problems.  The specifier needs to review claims and compare them to published numbers, to see that they mesh.  I would consider reliance on voltage-based protection during motor running conditions as giving a false sense of security.

I believe that voltage-based protection can be effectively used, within defined limitations, to inhibit starting on one or a group of motors, but that a current-based device is best applied to individual motors to terminate running.  The need should be based on the historic, statistical likelihood of phase loss/imbalance, the cost of new or repaired motors and probably most important, the cost of production outages.  A $120 device should not be tasked as expected to furnish faithful burnout protection.

It the end, the answer must be, “what’s going to cost the least amount of money,” and be based on the long run; not just initial cost.  Electrical protection is often described as a philosophy, so economic operation has many opinions.  The objectionable degree of unbalance and the time it is allowed to persist should be carefully reviewed.


Minor related discussion in Thread237-20355 Thread238-20643
  

RE: Detecting Regenerated Voltages

Electripete is correct on the speed and Rhatcher does a nice job explaining the reasons. Rhatcher gets a star!

RE: Detecting Regenerated Voltages

Suggestions to the original posting "buzzp (Electrical)May 8, 2002" marked ///\\\
I would like to discuss the details of how a motor acts when a phase is lost and if votlage imbalance and single phase protection devices actually work. Here is what I know about the behavior of the motor:
 If you lose a phase on a 3 phase motor, you are losing a pole.
///Or more poles or more exactly pole-pairs, depending how many pole-pairs the motor has.\\\
 The speed of the motor drops,
///The slip increases at the induction motor, the synchronous motor keeps its synchronous speed.\\\
 meanwhile the other pole is acting like a generator.
///Not necessarily, the delta connection will have the winding somewhat energized. The Wye winding will be disconnected at one star corner, therefore, there will be no noticeable current flowing outside of the motor terminal with loss of phase.\\\
 What is the amplitude of the voltage generated?
///At the wye connection, it will the wye leg Erms x sqrt2.\\\
 I think the voltage of the generated leg will be in phase as the 'lost'.
///On the Wye connected leg, the rms generated voltage will be about the same as the lost rms voltage across that leg.\\\
 The voltage protection devices I am familiar with protect against single phase, low voltage, and voltage unbalance conditions. They assume that the generated voltage will not be the same amplitude as the line voltage.
///There are various principles on which the voltage protective devices operation is based on. Some of them use sophisticated computations of the voltage negative sequence.\\\
 Is this a correct statement? What affect, if any, does multiple motors on the same line have on the ability of voltage protection device?
///It depends on a protection device type, a principle of operation, and what kind of motors are on the same line.\\\
  

RE: Detecting Regenerated Voltages

3
When you operate a motor off a three pahse supply and then remove one phase, the torque potential is dramatically reduced, but provided that the motor does not stall, the non driven phase will act as a generator generating a voltage that is correctly phased. If the shaft load is very low, the gnerated voltage will be comparative in magnitude and phase to the missing phase. This renders voltage monitoring devices next to useless unless the sensitivity is high and then there are nuisance trips due to supply variations. As the shaft load increases, the generated voltage reduces and voltage monitoring devices can work.

If the loss of supply is well upstream from the motor / starter installation, the generated voltage can supply other electrical equipment on that supply. This can render current monitoring protection less effective also. If the electrical load is high, then the generated voltage can be expected to drop making the voltage sensor effective.
If you monitor the current vectors, you can discriminate between current drawn from the supply and current generated by the motor.

The two phase operation of a three phase motor is a known means of regenerating the absent third phase.

Mark Empson
http://www.lmphotonics.com

RE: Detecting Regenerated Voltages

(OP)
I am not concerned with line-ground readings because the motor could care less about this. Any device being used to protect a motor that monitors line-ground is a waste of money, unless it is a single phase motor. Everyone keeps talking about lightly loaded motors in this thread, my concern is with almost fully loaded or fully loaded motors. Also, I wanted to talk about a real single phase condition (total loss of phase) not a brown out condition, sag, dip, or anything along those lines as the voltage unbalance detection is suitable to handle this phenomenon with no problems.

Well thanks for all the posts, unfortunately I have not learned what I wanted to from this post. I will keep searching for some real data and will post it here when and if I find it. Thanks to all.

RE: Detecting Regenerated Voltages

Hi Buzzp,

From my experience when a three phase motor running on full load looses a phase it will run with greatly increased current and slight loss of speed. The voltage generated in the third phase will be very close to the voltage in the live phases and will be in the correct phase, i.e 120 out of phase with the live phases. In this condition a great deal of heat is generated and the motor is running for a rewind. I appreciate this is an re-iteration of most of what has been said previously. I just thought I might simplify it it a bit.

Regards,
gjones33

 

RE: Detecting Regenerated Voltages

If your concern is for motors that have a high loading, then a) the motor will begin to stall and draw a high current on the two remaining phases and b) the regenerated voltage on the open phase will be very low and a voltage imbalance relay will trip.

Mark Empson
http://www.lmphotonics.com

RE: Detecting Regenerated Voltages

ok guys, I have to admit that I don't get it.....would marke and/or gjones33 please explain the how an induction motor that is operating in a single phase condition can generate voltage on the missing phase that is similar in magnitude to the missing phase and is 120 degrees out of phase with each of the active phases.

Also, for marke...please further the explanation by detailing the 'known means' that an induction motor operating in a single phase condition can be used to generate the third phase and drive three phase loads. If I am reading you correctly you could, for example, boost the 220V supply from your house to 460V, attach a 3-phase motor (single phased of course) to the transformer output, and then use that motor as a power supply for 460V 3-phase equipment.

PS: the only analytical approach to answering this question has come from electricpete (stars to you buddy on all posts in this thread). Unfortunately, his analysis is that it won't happen. I am hoping that you guys can clarify what he and I don't see.

RE: Detecting Regenerated Voltages

Hello rhatcher and others.
First, let us consider the induction motor and how it works. With a three phase induction motor, there are effectively three sets of windings that are displaced by 120 degrees that are energised by the applied three phase. These windings are split into subwindings to create the extra poles for multipole machines. When the stator winding is energised with three phases, there is a rotating magnetic field generated spinning in one direction and at a speed determined by the supply frequency and number of poles. This rotating magnetic field is coupled to the short circuited winding in the rotor and induces a current to flow. The frequency of the current flowing in the rotor is equal to the difference between the stator magnetic field frequency and the rotor speed. The rotor current in turn creates a magnetic field that is equal in speed to the stator field speed. This is because the rotor field is spinning relative to the rotor and the rotor current frequency. Relative to the stator, it is the rotor current frequency plus the rotor speed.

If we consider a situation of a single phase supply (or two phase two wire) then we have two counter rotating magnetic fields in the stator. Both have equal magnitude and frequency but spin in opposite directions. A stationary rotor will develop equal torque in both directions so will not rotate. A spinning rotor will however synchronise onto the torque field spinning in the same direction. This is why we need to have a start winding on a single phase motor. The start winding creates a second magnetic component that is displaced and effectively enhances the rotating field in one direction.

If the motor is spinning at full speed, there is a rotating magnetic field in the rotor and that induces a voltage in the open circuit winding. The physical positioning of the winding establishes the 120 degree offset. If the slip is low, the voltage will be of the same order of magnitude as the missing phase.

In terms of the rotating two phase to three phase converter, yes, once ou have th machine up to full speed, you can actually draw three phase from a three phase motor driven by two phases, but you do need to get to full speed first. This can be done by using a single phase motor in conjunction wih the three phase motor. Obviously, the impedance is relatively high and the motor must be unloaded.

Mark Empson
http://www.lmphotonics.com

RE: Detecting Regenerated Voltages

Actually, an unloaded or lightly loaded three phase motor can be used as a crude 1-phase to 3-phase converter.  This has been done on farms for decades.  The voltages become more unbalanced as more 3-phase load is placed on the system, but it will work.  If the motor is loaded very heavily, it will overheat from the unbalanced current, hence the need for unloaded or lightly loaded.

RE: Detecting Regenerated Voltages

(OP)
Thanks for the last post Marke, you get a star. This has been my thinking it is just nice to have someone explain why.
So for normally loaded motors, the regenerated voltage will be picked up as a voltage imbalance. This is the case for installations I am worried about.
How could you detect regenerated voltages with a voltage monitoring device on a lightly loaded motor? Is it even possible?

RE: Detecting Regenerated Voltages

thanks for the quick answer marke, i havent had time to really absorb what you have said but am working on it.

RE: Detecting Regenerated Voltages


Hi Guys,
Yes I agree with Marke. If the motor is running the rotor must be producing an induced magnetic field, just like a transformer. (All electric motors operate on the reactions between two magnetic fields). As Marke stated, the physical position of the phase windinngs determines the phase difference. The voltage induced in the open circuit stator  winding is proportional to the magnitude of the rotor magnetic field which is the same for all the windings and to the speed which is again equal to all the windings.

Best wishes,
G

RE: Detecting Regenerated Voltages

Hi, Ive been reading this post with interest for a while now and it seems to me that people are posting opinion and not much in the way of facts.Most of what has been written here is just plain wrong if the contributors took the trouble to try and prove there assersions they would see that they are wrong.The important facts are 1 the regenerated voltage is dependent on the rotor speed. 2 a fully loaded motor stops if you lose a phase.

RE: Detecting Regenerated Voltages

Not all motors will stall when fully loaded with a loss of a leg. Different applications have different effects on a single phased motor. Centrifugal pumps, blowers and fans can keep running until they burn up.

RE: Detecting Regenerated Voltages

Buzzp, from my experience, voltage monitoring protection is adequate for loaded motors but ineffective for unloaded motors. Current monitoring is adequate for unloaded and loaded motors provided the phase loss isolates the phase to the motor. Current imbalance protecion also should have a sliding scale of acceptable imbalance. Generally, motors operating at very light load will experience a moderately high current imbalance and this is acceptable, but under high load conditions, the imbalance most be small. The light load imbalance is very much influenced by the supply imbalance and is much higher in magnitude.
Sorry, don't have a magic answer, as usual there are compromises and you need to ascertain the level of prtection you wnat and susceptability to supply variations that you can tolerate.

Mark Empson
http://www.lmphotonics.com

RE: Detecting Regenerated Voltages

(OP)
Summary of all of the above:
- The amplitude of regenerated voltages vary depending on   
load
- The 120 deg difference stays the same (obviously)
- Voltage monitoring will protect against single phase conditions if the motor is loaded as it was designed for or is heavily loaded
Questions:
- What we don't know is where to distinguish between lightly loaded and loaded
- We also don't know the exact affect of the momentum or inertia of the load when a single phase condition exists except that it could potentially add to the amplitude of the regenerated voltage (this can not go on forever and will decay with time, we don't know how much time, assuming the motor OL doesn't trip first or the motor stalls/dies)
- The data in the above two statements will, presumably, vary from mfg to mfg under the same conditions
Thanks to all and a star to cbarn for his the last sentence of his last post.

RE: Detecting Regenerated Voltages

It's a great discussion touching on a lot of areas.

It appears that several people have real-world experience that suggests that the missing 3rd phase will have a generated voltage almost identical to what it would be if you had 3-phase voltage applied (120 degrees apart and same magnitude as other 3 phases).  I cannot dispute that, but I still do not understand the explanation proposed above.

The explanation proposed is that the induced voltage on the 3rd phase arises from the rotor field.  I agree that the rotor field is rotating at sync speed but I don't see that it should be important in this problem.

Let's go back to the basic model of a [balanced] induction motor, neglecting leakage reactance.  The air-gap flux is established by the stator exciting current (which lags stator voltage by 90 degrees in time).  That air-gap flux is NOT affected by the rotor field.  How can that be?  Very simply that any current flowing in the rotor is balanced by an equal-opposite (on an amp-turn basis) load current in the stator.

If you don't believe me look at the model. Istator=Im+Irotor.  The stator current contains two components: one Im which established the magnetizing field and one Irotor (referred to the stator) which is the load current which exactly cancels the effect of the rotor field.

If that much doesn't make sense then look at the load dependence. As load increases Irotor increases and Brotor increases, but airgap flux does not change (again neglecting leakage reactance).  The reason airgap flux does not change is because the increase in Irotor and Brotor is exactly cancelled by a load-component of the stator current and associated field.

I'll admit that the above is based on balanced conditions. Which parts of it do not apply during the serverly-unbalanced single-phase conditions requires much careful thought (more than I am capable of at the moment).  

But the bottom line is that simple analysis suggests that rotor field has nothing to do with the airgap flux which links the stator.  What establishes the airgap flux is the exciting component of the stator current.

Looking at stator exciting flux alone I see no reason why any significant voltage would be created as discussed above.  Above I analysed 2-pole scenario with pole-phase groups a, b', c, a', b, c', with C phase missing. To add clarity to that analysis, consider that b must be equal an opposite to a (any current flowing in b must flow in opposite direciton in a... assuming wye connection). On that basis substitute a' for ever b and a for every b'. We will see the sequence of pole phase groups is a, a, c, a', a', c'.  and if we add additional poles beyond two the sequence will repeat.

These letters represent the physical location of the coils as we move around the stator. It should be obvious from the sequence a, a, c, a', a', c', a, a, c, a', a', c' etc that there is a symmetry which would tend to prevent inducing any voltage in c since it is in equal proximity to a and a'.  The same applies to c'.

I don't dispute the contention that the missing 3rd phase voltage is "regenerated" but I certainly don't understand why it should be so.

RE: Detecting Regenerated Voltages

Hey Pete...I have been trying to resolve how this could be using the same reasoning as yourself but I don't see it either. By the way, a star for your good explanation.

While researching this I have seen some references to 'home made' rotary phase converters which are essentially 3-phase motors operated from a single phase supply with no load for the purpose of generating the third leg, but these all have capacitors connected between one (or both) of the hot legs and the third generated leg. Presumably this is to provide excitation for the generated leg. The articles did not get into details about theory of operation, fluxes, etc. but did suggest that the voltages were not balanced and that the magnitude and phase of the voltages generated varied greatly with load. Some of the commercial units I checked into did claim true balanced 3-phase output at any load, but no info was provided to suggest how this was accomplished. I suspect a LC/RLC network and/or electronics in place of the capacitors.

I have only found one reference to support that a motor running single phase would generate voltage in the third leg without capacitors or other external devices, but that was not from a technical source and it did not provide any more explanation of the phenomenon than any of the posts in this thread.

Anyway, I am still trying to think this through but in the meantime I am going to take the old fashioned approach to finding the answer. In a little while one of our techs is going to test a small 3 phase motor that was just rewound (I didn't want to do this on a 500HP unit...). When he is done I will run the motor single phased and measure the voltages...I am limited right now to mulitmeter measurements, but believe that phase-phase and phase-neutral checks of the voltage at the two hot legs and the one generated leg will give me a good idea of whats going on.

So, now it is time for "Famous Redneck Last Words" from your friend in South Carolina...
 
- "Hey guys, watch this."
+ "What's gonna happen?"
- "Ummm, I'm not sure but we'll know in a minute."

RE: Detecting Regenerated Voltages

Hi Guys,

Perhaps the bottom line on this post is that three-phase induction motors are used as three-phase generators in several well known wind generation schemes. In this case the rotor flux is produced from the load current in the stator coils.

Best wishes
G

RE: Detecting Regenerated Voltages

In my plant, we use a blown fuse indicator intended to trip when we lose a fuse on a 575V motor. This device fits in the starter and has three wires terminating on the load side of the motor fuses. The company that sells this claims regenerated voltages can reach 90% of supplied voltages. The relay trips when it senses a loss of about 8% voltage.

The motor model of a stator branch, magnetizing branch, and rotor branch is a simplification. A more accurate model has a stator branch feeding a magnetizing branch and an ideal transformer primary. The ideal transformer secondary feeds a voltage source and rotor branch. (The transformer represents the air gap). The magnitude of the rotor voltage source is related to the slip.The rotor current generates a magnetic field that cuts through the stationary stator winding.Since the rotor current is AC, the rotor flux is AC. This results in a changing magnetic flux being seen within the coils of the open winding. This generates a voltage proportional to the rate at which flux is changing (E= - d(flux)/dt).

The reason the voltage on the "good" phases is slightly higher is to supply the energy to supply power to the magnetizing branch.

RE: Detecting Regenerated Voltages

Thanks gord and gary for responding to my question. I know that it appears an academic discussion.  But I am interested in understanding where my model fails.

gjones - I am not terribly familiar with induction generators. To the best of my knowledge they require to be connected to the grid. Ifit is true that an induction generator must be connected to an external voltage source, then the excitation which establishes the air-gap flux would comes from the grid voltage applied at the stator terminals (the same as a motor).  The air gap flux would have nothing to do with the rotor field.  If an induction motor can operate without being connected to a grid or external voltage source, then I would be interested to hear more.

gords - it sounds like your blown-fuse-indicator manufacturer has presented some relevant and unambiguous info.  I'm still at a loss to understand the theory.  Your explanation does not ring a chord with me. You focus on rotor flux without regard to the other fluxes. I focus on the fact that the "airgap flux" = "magnetizing flux" = "resultant flux" is the difference between the stator field and the rotor field. This is a mirror of the fact that
Imagnetizing=(Istator_total) - (Irotor)
which can be readily observed from applying KCL to the equivalent circuit.
ie the stator current is comprised of a magnetizing component and a torque-producing load component which cancels the rotor current. Likewise in a transformer a primary load current I1 will flow which is approx equal to N2I2/N1 and therefore "cancels" the flux contribution of the secondary current.  The flux in the core will remain on it's excitation level which is directly dependent on the primary voltage, but is INDEPEDENT of the load currents (neglecting leakage reactances).  

So once again I present to an apparent contradiction:  IF the rotor flux is responsible for the induced voltage in the open winding, then why doesn't the induced voltage increase in direct proportion to motor load?    After all:
s~ P (load).  Vrotor~s. Irotor~Vrotor. Brotor~Irotor.

And if the induced voltage varies with load, what load corresponds to full induced voltage? Is there any reason to suspect that full nameplate load would just happen to be the load at which the induced voltage matches the missing voltage (I can see no reason whatsoever).  I don't believe that rotor field is the whole story.

One possible flaw in my original argument is that I have assumed that stator current can flow to completely cancel the rotor field (as is the normal model).  It may be a little more complicated when stator current can only flow in 2 phases.

RE: Detecting Regenerated Voltages

If I can oversimplify it I will. The three phase motor, single phased, now has a set of windings with NO power flowing into it. The other two phases are driving the motor and inducing a rotating magnetic field (North and South poles) into the rotor and dragging the rotor along with it. The rotor is now a rotating magnetic field spinning past the OPEN phase. It just a simple as moving a magnetic field past a wire to generate a current in the wire. It all falls back to Motion, Flux and Current. Its just a generator.

 As for a three phase motor generating power when on a grid ... the motor is DRIVEN faster than is synchronous speed. Just as any generator underpowered on a grid will not produce an output...A three phase motor DRIVEN faster will pick up the load and have an output of electricity! You induce a field in a rotor and spin it...it is now a generator. Not being an engineer my technical explanation would be the CEMF of the rotor is now GREATER than the applied power.

RE: Detecting Regenerated Voltages

Thanks steve for your discussion. Thank rhatcher for your kind words.

It is certainly possible that you all are right that the rotor field is responsible.

But let me clearly explain that during NORMAL operation the rotor field has no effect on the air-gap flux (flux linking the stator coils) whatsoever. I believe the reason that rotor field may come into more importance in this single-phase condition is that the assumption of amp-turn balance which holds during normal breaks down during single-phase operation. More details follow:

In general magnetic theory there is a premise that an amp-turn balance will always hold.  That is that for a transformer N1I1=N2I2+N1Im.  Im is a magnetizing component of I1 that is small and can sometimes be neglected for simplicity.  The magnitude of Im is dependent on V1 and geometry factors, but not upon the load.

For motors the equivalent amp-turn balance is I1=I2+Im where all quantities are referred to the stator.  For motors Im is not as small but is in quadrature to the load currents I1 and I2 so for high loads its effect becomes small.

Inherenent in all of the above formulations is that the core has such high permeability and low reluctance that any small mismatch between N1I1 and N2I2 creates a huge voltage which will drive that mismatch down.  As a result, the mismatch between N1I1 and N2I2 (N1Imag=N1I1-N2I2) always remains relatively small.

If the above model is true, then Irotor does create a field but that field is irrelevant to determining air gap flux because the load component of Istator (I1-Im) will exactly cancel I2=Irotor.  This gives rise to the well-known fact that transformer core flux and motor air gap flux do not vary with load, except for the effect of series leakage reactances.

The above model is good for normal situations but I think that perhaps the breakdown does occur when I assume that the primary currents will continue to cancel the rotor mmf even with one phase open. The amp-turn balance holds in most situations but I'm not sure if it holds here.  

If we artificially considered the stator windings to be non-overlapping, then it would be clear that when the max rate-of-change of rotor field is adjacent to the open-circuited phase C, then that entire voltage must appear in the open-circuited phase.

But in the actual case, there is substantial overlap between phases.  There is no portion of c phase that is not overlapped by a "b" phase or an "a" phase coil in some manner.  Whether or not those other stator phases are capable conducting current to cancel the rotor field, given the phase relationship of the voltage is tricky.  My initial gut feel was that amp-turn balance could not be substantially violated under any circumstances.

It is certainly plausible that my assumption of amp-turn balance is incorrect in this scenario. If that is the case, then rotor field does provide the explanation.

RE: Detecting Regenerated Voltages

electricpete, I think that perhaps this is one of those cases that we come accross so often that models we apply to things to describe them are based on certain conditions and when the conditions change, then the model does not hold up. I suspect that you are correct, applying the model that you wold normally apply to a motor where all three windings are driven by the supply does not cover this particular situation. Unfortuately, a fully comprehensive model to describe behaviour under all conditions gets too complicated. Under normal circumstances, the flux in the gap is related to the magnetising current in the stator and that is constant and independant of the load. In this case, there is no magnetising current in the stator and so there is no flux in the gap, well thats not actually true, the model falls over because there is flux coming from the rotor and I am not sure how much of the gap flux is directly dependant on the rotor current and how much is effectively from the residual magnetism of the rotor poles. If the flux was due to the rotor current, then I would expect the output voltage to increase with shaft load whereas the regenerated voltage actually reduces with shaft load. I suspect that the flux at the open circuited winding is related more to either the residual magnetism, or the current through the inductive component of the rotor winding or a combination of both.
If we consider the rotor, effectively we will have a voltage induced in this winding byt the stator flux field. The rotor winding has both an inductive component and a resistive component. As the shaft load is increased, the rotor current increases. I would expect that the effective rotor voltage will then reduce due to the effective series impedances and therefore the rotor magnetisng current will also reduce. This would explain the reducing voltage with load or speed but I am not sure that this is a correct model. Someone may be able to shed more light on this interesting subject.

Mark Empson
http://www.lmphotonics.com

RE: Detecting Regenerated Voltages

For gjones, induction or asynchronous generators are common for wind generation and small hydro applications. This is not the same case as 'regenerative voltages' since the asynchronous generator must be excited from a 3 phase line and is driven at greater than synchronous speeds by a prime mover.

------
About my 'experiment', the test conditions were not ideal. The motor was a 7.5 hp, 2 pole, 1 delta, concentric winding rated for 460V. No problem there. The power supply was a hipotronics motor test set, 500kVA, 0-4160V...essentially a 'variable autotransformer'. The part that is not ideal is the base condition of the voltage output for the test set with no load:

Vac= 246   Vab= 246  Vbc=245
Va= 151  Vb= 139  Vc= 75   (phase-ground)

Similarly

Vac= 444  Vab= 443  Vbc=442
Va= 253  Vb= 247  Vc= 158   (phase-ground)

The first difficulty was that once the motor was at speed on the 3 phase source that I could not kill the power, disconnect a phase, and then reset the test set before the motor stopped. It was a small motor with little inertia. In the end I found the minimum voltage at which I could 'pull the plug' on one phase and the motor would remain rotating (200V). Do not try this at home.

Three test were done. First, single phase applied to motor at stop. Obviously no rotation, but I wanted to establish the case where there was no potential for rotor flux contribution. Next, the motor was started on 3-phase power, 'switched' to single phase once at speed, and then operated at voltages of approximately 240V and 440V. In all tests phase B was the dead phase, Vac represents the single phase input voltage, and the autotranformer tap was set for 480V. The phase to ground volts are included but are of little value due to the fact that my system 'neutral' is obviously different from ground. Of course, if any of you can draw conclusions from the phase-ground data please do.

No rotation: Vac=63  Vab=31  Vbc=31 Va=56 Vb=31 Vc=10

240V:  Vac=246  Vab=226  Vbc=209 Va=145 Vb=124 Vc=101

440V: Vac=440  Vab=399  Vbc=401 Va=248 Vb=224 Vc=206

I haven't really formed any conclusions yet. Clearly the 'no rotation' case shows straight voltage division (Vac parallel with Vab and Vbc). Equally as clear is the fact that for the 240V and 440V cases where the rotor was at speed something else is going on. Although the voltages are not balanced and therefore not 120' out of phase, they also do have some phase difference from the single phase supply

I have to admit that I went into this as a 'non-believer' and that I was surprised by the results. However, I will state now so there is no confusion that until I can quantify this based on motor theory that I will not draw any conclusions one way or the other.

I have some ideas on the theory of all of this but it is late. I will throw this idea out to pete (and marke)though. I have not thought this through yet but here it is...you are right on with your analysis but it would seem that in the case of the 3 phase motor operating single phase that the only winding contributing to the flux is the single phase that is fully energized and the transformer action takes place between the rotor and that phase. The rotor mmf wave will be ideally 90' (electrical) behind the voltage of the active phase at low slip. Although this does not correspond with the physical position of any of the adjacent windings or the 'electrical' position in terms of the 120' model, that is not to say that a voltage may not be induced in the dead stator windings. Of course, the frequency would be less than synchronous based on slip and the apparent difference in phase at any moment different than 120'. With greater load, although the rotor mmf would increase, the slip would increase leading to a greater difference in frequency and greater difference in apparent phase. Of course, this does neglect the fact that in the case of a delta motor the dead legs do assume some contribution as shown in your previous post so I am not sure that it is a useful line of reasoning.

RE: Detecting Regenerated Voltages

Pete...looking at the last part of your argument... I suspect you are saying (at least I hope I understand where your coming from) that since the windings overlap each other they cancel each other out ... as in the paragraph just before where you say if they where NON OVERLAPPING then you could believe what we say.

If that were true, how can the motor run at all with everything overlapping in the stator!

Even if you were to separate each winding on its own pole piece (like the poles on a DC motor) the poles field flux would cross over each winding in the same manner as a lap wound stator. The flux traveling from north to south to make a pole will have to have the same path no matter how you mechanically set the windings or the motor would not work.

Let me try a different tack...just as you take the vector sums to plot a voltage you also take the vector sums of the flux. At a point of the flux a vector is SINGLING out the opened winding (C) and imparting a voltage on it! At that particular vector it is different on phase C than phases A & B!

RE: Detecting Regenerated Voltages

Steve - yes, the span of windings in phase A will DEFINITELY overlap the span of windings in phases B and C.

Anyone who has traced the wiring of a standard induction motor can confirm this for you.

Although I am usually a man of many words (to the dismay of my peers on this forum!), at this late hour I can come up with no simple way to explain it in a reasonable manner without diagrams.

Do you have access to a textbook which shows a lap-wound winding?  Maybe rhatcher can pitch in some explanation.

RE: Detecting Regenerated Voltages

A follow up discussion for Steve.  I think that parts of the reason for using overlapping winding have to do with minimizing harmonics (the same reason we use distributed windings).  Maybe someone else can provide more insight.

It's also worth pointing out that the overlapping does not decrease the peak flux, but actually increases it above that which would come from a single pole phase group alone.  At the moment when pole phase group A is at it's peak flux, adjacent pole phase groups B' and C' (60 electrical degrees from A) will have magnitude of 1/2 in the same polarity as A.  If we also consider the 60 degree spatial (vector) separation and compute vector sum of  the contributions from B', A and C', we get 3/2 of the peak.  See Fitzgerald page 149 if you have it.

RE: Detecting Regenerated Voltages

rhatcher - Thanks for some good data regarding actual performance of a delta-connected motor.   I vote you a star.

Clearly the induced voltage in the open phase is much closer to full line voltage (as others have said) than it is to zero (as I had predicted).

RE: Detecting Regenerated Voltages

To rhatcher,

The point I was making about generators is that a rotating squirrel cage rotor will induce a voltage in a stationary stator winding. This is a fact of life whether the rotor is driven by a prime mover, or, in this case the two live phases. The only reason you need to drive at a higher than synchronous speed in the generator mode is to ensure power flows to the electrical load. If the speed is less than synchronous then power flows to the mechanical load. In a generator this would drive the prime mover.

How does the power know which way to flow? The only link between the mechanical and electrical system is the magnetic field in the airgap, which, in turn, is dependant on currents in the stator and rotor windings.

Cheers
G


RE: Detecting Regenerated Voltages

Pete - being in the motor repair business I have rewound hundreds of three phase motors and am quite intimate with how the phases are placed in a stator. The overlapping is very pronounced in a two pole machine. I was just trying to elaborate on your earlier post of ... "It is certainly plausible that my assumption of amp-turn balance is incorrect in this scenario. If that is the case, then rotor field does provide the explanation".
    
    

RE: Detecting Regenerated Voltages

Hi, have you ever seen a little single phase indution motor? Ever woundered what that shorted turn(BIG CLUE HERE) on the pole does?

RE: Detecting Regenerated Voltages

Steve - my deepest apologies on your meaning. I thought you were disagreeing that there is any overlap at all.  I remember being quite surprised that you would say that.  Sorry to misinterpret you.

The reason I discussed overlap is that it would be the determining factor of induced voltage in the open coil IF we make the [questionable] assumption that rotor load current is still cancelled by the two remaining stator phases (wye winding). For example portions of C (open phase) which overlap with adjacent B' will cancel those portions of C which overlap with adjacent A' (because A and B currents are equal and opposite during single-phase condition).  More details on my earlier messages this thread…although I am sympathetic to the fact that they are not very understandable.

gjones - If an induction generator is operating in parallel with an external voltage source (grid), then I believe the excitation to establish the airgap flux comes from the external voltage source (not necessarily the rotor).  I believe the induction generator will not produce vars but absorb vars as a consequence of this excitation current drawn from the source. That is quite a different scenario from a motor with an open phase where the open phase cannot recieve any excitation from the external voltage source.

I'd be happy to reconsider those statement if you tell me that induction generators are capable of operating without being connected to any external grid or other source of excitation (can they?) or that an induction generator can be made to produce more vars than it consumes (can they?).

cbarns - The shorted turn does not provide any insight to me. In fact if you analyse a shorted turn which does not significantly perturb the overall flux, you would find that the flux which causes the circulating current in the shorted turn arises from the excitation component of the stator current.  This flux is directly dependent upon the stator applied voltage (NOT the load-dependent rotor current).

RE: Detecting Regenerated Voltages

To electricpete.
Where I believe we differ in concepts is that a voltage or potential cannot produce a magnetic field. Current has to flow and current needs a circuit and only then will a magnetic field be produced. In the case of an induction generator the load provides a convenient circuit.

However, if a supply is not available it is well known that self-excitation of an induction generator can be obtained by connecting in parallel with a suitable capacitor and running up on residual.

Cheers
G

RE: Detecting Regenerated Voltages

Hi All,

On reading the post again, I think the concept of overlapping poles can be very misleading. When you think about it there is no such practical entity as overlapping poles. When a phase is energised it will produce however many pairs of poles as the design dictates. Each phase of a two pole motor will produce just two poles; each phase of a four-pole motor will produce just four poles and so on. If two phases are energised at the same time they will only produce the same number of poles as one phase. Two phases energised in a two-pole motor will only produce two poles but this time the poles will be phase shifted from the positions of the poles due to one phase. (The poles will be midway between the two if the voltages are in phase).

This phenomenon (energising two-phases) is used in stepper motors to produce half steps etc.

From the physics viewpoint magnetic fields comprise of lines of force, i.e a line joining points having equal field strengths. It follows lines of force cannot cross and therefore poles cannot overlap.

Cheers
G

RE: Detecting Regenerated Voltages

Good discussion!


gjones - I never said that voltage in the absence of current created a field. What I said was that the external voltage supplies the excitation current (vars into the generator) which creates the airgap flux/field.  The magnitude of that flux is proportional to the magnitude of the terminal voltage.  I felt it necessary to distinguish between the three fluxes: #1 - voltage-dependent excitation flux which creates the air-gap flux under normal conditions, #2 - load-depedent rotor flux, #3 - load dependent stator flux (equal and opposite to load-dependent rotor flux). The relationship between these three: total stator flux=#1+#2.  Total flux=total stator flux +rotor flux =(#1+#2)-#3=#1.  #1=(total stator flux)- (loaddepednet stator flux)=(#1+#2)-#2=(#1+#2)-#3.   #1=(#1+#2)-#3 is the direct analogy to N1IM=N1I1-N2I2.  

External capacitors certainly can fulfill the similar role as external voltage source since capacitors are sources of  vars.

I remember hearing as you say that induction machines can generate without interconnection to a grid PROVIDED that caps are attached. But that is hard to understand.  Slip no longer has any meaning since there is no syncronous frequency in the picture... only rotor frequency.  So if there is no slip, how do we interpret the torque vs slip curve? What defines and controls the torque and power in such a machine?  (just curious).

I never said that poles overlapped… only that pole-phase groups overlapped.  (Please let me know if you want me to elaborate on the difference between a pole and a pole-phase group.)   We need to consider the physical overlap of the stator windings in order to determine what stator flux from one phase will link an adjacent phase.

RE: Detecting Regenerated Voltages

gj - I will elaborate on the term pole-phase group. It is a group of physically adjacent coils connected in series which are associated with the same phase and the same pole.

Pole phase groups DO overlap in induction motors. That is a fact.  I have already made the mistake of trying to explain that fact to someone who already understood it, so I won't try to explain it again.  Let me know if you want to discuss more.

RE: Detecting Regenerated Voltages

To electricpete

Consider your #1 and #3. How can they be different? We agree it is current that produces the magnetic field so how can you have two values of current in the same coil at the same instant? You can see how it is very misleading to state and I quote "The magnitude of that flux is proportional to the magnitude of the terminal voltage." It is not correct the flux is proportional to the current which can only flow in one direction at a time, the error is then self evident. Similarly the load cannot produce a current or magnetic field.

If you adopt the concept that a current is necessary to produce a magnetic field then you will understand why it is not necessary for the induction generator to be connected to a source. It only needs a suitable electrical load and by load I mean a current sink which can be the grid or a capacitor.

There is no argument that the coils forming poles overlap. For example in a three-phase machine if there are two coils forming a pole then there is be a span of 4 slots between the inner coil, the next two slots taken up by the two coils of the previous phase and the next two taken up by the coils of the third phase, thus the coils overlap but the magnetic poles which was the subject of the original post do not.

Do we agree now?

Cheers
G

RE: Detecting Regenerated Voltages

gjones - I can think of no way to respond other than responding to each of your statements one by one.

Consider your #1 and #3. How can they be different?
If you accuse me of being wordy and hard to understand, then I plead guilty.  It is unfortunate that I chose to introduce the numbered fluxes #1,#2#3 which are very non-descriptive. It adds precision but does not add clarity.

Let me try to clarify. Draw the equivalent circuit all referenced to the stator and you will see a node where the exciting branch is hooked. KCL at that node gives I1=Im+I2 where I1 is total stator current and I2 is rotor current, as referenced to the stator.  This means that there are two components of stator current I1…. a voltage-dependent component Im and a load-depdent component I2=I1-Im. With simplifying assumptions (neglect leakage reactances) I2 will be in phase with applied voltage and Im will be 90 degrees out…. making it easier to distinguish these two components.  

We can associated each of the fluxes with the currents discussed above: Flux #1 is associated with Im. Flux #2 is associated with I2,  Flux #3 is associated with the load component of I1… specifically I1-Im (which happens to be equal to I2… flux #2).  

We agree it is current that produces the magnetic field so how can you have two values of current in the same coil at the same instant?
We have two components of stator current (in the same stator coil at the same instant). One is magnetizing Im, one is load-related (I1-Im).  Add them together and you get I1.  

You can see how it is very misleading to state and I quote "The magnitude of that flux is proportional to the magnitude of the terminal voltage."
"that flux" was referring to magnetizing flux=airgap flux = flux#1.  It is proportional to the terminal voltage under normal three-phase conditions. Doesn't seeem misleading to me. Do you disagree?

It is not correct the flux is proportional to the current which can only flow in one direction at a time, the error is then self evident.
No error is evident to me.  Please be more specific.

Similarly the load cannot produce a current or magnetic field.
I have used the term load-related to refer to the current components and flux components which vary with load. (as distinct from the voltage-dependent flux exciting flux #1).   The flux associated with the load current varies with changes in load.


If you adopt the concept that a current is necessary to produce a magnetic field…..

I wholeheartedly agree that a current is necessary to produce a magnetic field. I have never suggested otherwise.

then you will understand why it is not necessary for the induction generator to be connected to a source. It only needs a suitable electrical load and by load I mean a current sink which can be the grid or a capacitor.
The fact that an induction generator needs to be connected to either a grid or a capacitor suggests that vars need to be supplied to the induction generator to establish its field. The reactive vars flow into the generator in while real power watts flow out.  Why should we suspect otherwise?

There is no argument that the coils forming poles overlap. For example in a three-phase machine if there are two coils forming a pole then there is be a span of 4 slots between the inner coil, the next two slots taken up by the two coils of the previous phase and the next two taken up by the coils of the third phase, thus the coils overlap but the magnetic poles which was the subject of the original post do not.
Magnetic poles may have been the subject of your "original post", but not mine.  I first discussed overlapping of pole-phase groups.  You second objected to my posting with a discussion of magnetic poles.

Do we agree now?
Getting closer I think.
Cheers!
electricpete

RE: Detecting Regenerated Voltages

Wow guys, I am glad tomorrow is Saturday. I need the time off to catch up with the posts from the last two days. I am about 1/2 through now and will finish in the morning. This is certainly an interesting thread and if nothing else it demonstrates what I like about our field:

'When you go deeper into it, some questions get answered but then other questions pop up. Your mind plays tricks on you, then you play tricks back...'

RE: Detecting Regenerated Voltages

Suggestion to Marke (Electrical) May 13, 2002 marked ///\\\
When you operate a motor off a three pahse supply and then remove one phase, the torque potential is dramatically reduced, but provided that the motor does not stall, the non driven phase will act as a generator generating a voltage that is correctly phased.
///Could you clarify the generated voltage in terms of wye and delta connections. The wye connection lost branch will have no current flowing in it. The delta connection branch will have current flowing across the missing voltage supply terminal. Therefore, the motor terminal voltage at the missing phase motor terminal will be different between wye connection and delta connection.\\\
 If the shaft load is very low, the gnerated voltage will be comparative in magnitude and phase to the missing phase.
///This statement needs a clarification in terms of the motor wye and delta winding connections.\\\
 This renders voltage monitoring devices next to useless unless the sensitivity is high and then there are nuisance trips due to supply variations. As the shaft load increases, the generated voltage reduces
///This needs a clarification in terms of the motor connection; especially, the wye connection.\\\
 and voltage monitoring devices can work.

RE: Detecting Regenerated Voltages

Suggestion to electricpete (Electrical) May 9, 2002 marked ///\\\
 Generated voltage on the open phase.  That's a good question too.  And will it depend on motor load?  Hmm.
///There will be a different generated voltage on the missing phase terminal of wye connection and missing phase terminal of delta connection since there is no current flowing in the missing phase winding of wye connection and there is current flowing across the missing phase terminal of delta winding. This current will approximately be one half of the abnormally powered delta leg.\\\

RE: Detecting Regenerated Voltages

To jbartos,
Have you missed our post on induction generators.

When you loose a phase in a three phase induction motor you do not loose torque unless the motor is loaded to almost pull out torque. What happens is the motor slip increases, the BEMF falls and current is increased in the other two phases which may be sufficient to burn the motor out.

There will be a voltage close to the line voltage generated in the third phase whether it is wye or delta connected and this is a function of the rotor magnetic field and the speed of the rotor not the current in the phases as you suggest.
Cheers,
G

RE: Detecting Regenerated Voltages

gjones
you say "the slip increases" while jbartos says that "we lose torque". I say you're both right. If the torque speed curve decreases for all speeds (as is the case for loss of phase), the new operating point is at a higher slip. You're right that the change will not significant impact operation unless we reach breakdown torque.

I don't believe the discussion on induction generators sheds any light on generated voltages in the case of a lost phase. An induction generator recieves it's excitation (which establishes the airgap flux) from vars which are supplied by either an external power source or connected caps or both. This is the same as an induction motor. From this perspective, we might say that an induction generator really doesn't "generate voltage" (except for small amount due to residual rotor magnetism), it generates real power.  The external var source supports the voltage.  

If you wish to draw a direct analogy between an induction generator and a motor with open-circuited phase, you must ask the question what will happen to the induction generator terminal voltage when we remove the external var source (caps or grid). I believe the voltage would decrease to a small fraction of normal.

RE: Detecting Regenerated Voltages

The purpose of my last statement of last post was to illustrate that we can't draw any useful analogy between induction generator and induction motor with open phase which would explain the observation that voltage remains on the open phase. (I was not trying to argue that induction motor open phase does not generate voltage.)

RE: Detecting Regenerated Voltages

To electricpete,

Reading your last post,I now believe we have been on diferent tracks for a while. In one of my earlier posts I mentioned residual magnetisim of the rotor to explain how an induction generator can build up it's terminal voltage from stationary when driven by a prime mover without any electrical input power.

The rotor residual magnetism is the magnetisim left over from previous events and when the machine is rotated it builds up a voltage in the stator windings which is fed into a capacitor (or other suitable load) to phase shift the stator current and thereby the stator magnetic field so as to induce a voltage in the rotor bars, which, in turn produce the rotor magnetic field which, in turn interact with the stator magnetic field to produce a torque. Whew! I hope you followed that.

The capacitor does exactly the same job as a shorting turn on a shaded pole motor as charn24050 suggested in the post above. Just like rotor bars, the shorting turn produces a magnetic field which is out of phase with the main field.

Now when the motor is running on two phases, the rotor magnetic field is still induced from current in the other two phases and we therfore have an open circuit winding surrounding a rotating magnetic field, the stuff generators are made of.

The alternative is that if the voltage in the open winding is zero or low, then there is no rotor magnetic field and no bemf in the other phases either so the current will be almost v/r. Whoops!. The point I'm trying to make is that and induction machine operates as both motor and to a lesser extent a generator in the motor mode and as a gnerator and to a lesser extent a motor in the generator mode. It is all down to the direction and phasing of the currents in the stator windings.

Cheers,
G

 

RE: Detecting Regenerated Voltages

gjones - lots of info in the last post.  I'm not sure if I disagree with any of it. I do object to repeated previous inferences that a motor with one open phase will generate a voltage by the same mechanism as an induction generator.  The induction generator cannot generate any significant  voltage without an external supply of vars... it is no different than a motor in that respect. Both have external supply of vars to create the magnetizing field.... but the torques and real power flows are reversed.  The rotor residual magnetism plays only a minor role in the steady-state excitation of the machine.

I stand by all my previous statements in this post.

RE: Detecting Regenerated Voltages

I'll be signing off for a few days. Look forward to continuing te discussion after next Thursday.

RE: Detecting Regenerated Voltages

to electricpete,
Thank you for your post, I hope you enjoy your break.
Cheers,
G

RE: Detecting Regenerated Voltages

I lied. I'm not gone yet. Still a few hours before my flight.

I should back off on my statement that "The rotor residual magnetism plays only a minor role in the steady-state excitation of the machine."  

What I should say instead is: I don't think the induction generator can generate significant voltage without the caps.  (analogous situation to open-circuited phase...no caps).

Here's what I think is going on in the standalone induction generator with caps attached.

Caps are selected to be in resonance with the motor magnetizing reactance.  Rotor residual magnetism induces a small voltage in the magnetizing reactance.  This small voltage induces a large circulating current within the loop formed by the magnetizing reactance and the cap (assuming single phase motor where both are connected line-to-ground...other connections must be considered separately). The voltage drop of that current flowing through either the magnetizing reactance or the cap (equal and opposite impedance) creates the high terminal voltage.

One thing that bothered me was the the busbar's link mentioned the induction generator maintained very close to 120v. That seems like quite a coincidence.

I'm curious what fraction the voltage would drop to if caps removed. I think it would be very low.

RE: Detecting Regenerated Voltages

To electricpete,

Pete, I just happened to have one of those single phase shaded pole motors they used to drive record players with handy and out of curiosity I hooked it up to my 'scope and spun the shaft by hand, I could easily obtain 1.6v p-p with it.

Now if a small motor spun by hand with no other input or connection to an external load other than an oscilloscope probe can produce an appreciable voltage then I'm sure you will be fully convinced that a larger motor hooked up to a supply can do it.

I dont think there is anymore I can say on this topic and I'll leave the last word with you.
Bon voyage.
Cheers,
G

RE: Detecting Regenerated Voltages

(OP)
You guys have been busy! I feel lost now. But in reading through the posts I propose one thing: not knowing the internal connections of a motor that well let me propose a scenario and you guys tell me if I am missing something- say we have a single phase condition on a 30HP motor. The voltage protection is at the contactor (where monitored) and the single phase condition exists on the line side of the contactor. Wouldn't the internal connections of the motor produce a reading on the so called missing leg by means of the internal connections, ignoring regenerated voltages? Forgive me if this is too elementary, it is Monday morning though. Granted it wouldn't be the same magnitude as line voltage. Thank you.

RE: Detecting Regenerated Voltages

Hi buzzp,

In a word yes

Cheers,
G

RE: Detecting Regenerated Voltages

(OP)
So what error does this produce when trying to measure regenerated voltages??

RE: Detecting Regenerated Voltages

Hi buzzp,

Can I ask a silly question but like you said it is Monday. Where are you going to measure your voltage between? The missing phase motor terminal and one of the other phases or between the missing phase terminal and neutral assuming a wye connected motor or, across the break in supply, assuming it's available?

In any case the voltage will depend on the load on the other two phases, the slip, etc. In other words it is not a reliable indication. Was this your point?

Cheers
G   

RE: Detecting Regenerated Voltages

(OP)
Yes, this was my point! I am glad someone caught it.

RE: Detecting Regenerated Voltages

Suggestion to gjones33 (Electrical) May 19, 2002 marked ///\\\
There will be a voltage close to the line voltage generated in the third phase whether it is wye or delta connected
///Please, could you prove this statement or refer to any literature?\\\
 and this is a function of the rotor magnetic field and the speed of the rotor not the current in the phases as you suggest.
///It can be proved that the degenerated delta to the two branches of windings have the current flowing from one of the remaining phase terminal to the other remaining phase terminal through the two stator motor windings; one winding is normal, e.g. having impedance Zab and the second winding is created by Zbc in series with Zca. Certainly, there will be some voltage generation due to the rotor BEMF; however, there will also be the voltage drop I x Zbc and I x Zca. Additionally, one can simplify the motor's loss of one phase by considering a delta connected transformer on its primary. The delta connected transformer on its primary will deliver more power in case of loss of one phase than the wye connected transformer on its primary. It needs to be recalled that the induction motor is somewhat a special case of the transformer. Whoever, can perform laboratory measurements will see the difference.\\\

RE: Detecting Regenerated Voltages

Suggestion to electricpete (Electrical) May 19, 2002 marked ///\\\
gjones
you say "the slip increases" while jbartos says that "we lose torque".
///Please, would you point to my posting where I stated "we lose torque."\\\
 I say you're both right.

RE: Detecting Regenerated Voltages

buzzp— I think it's generally recognized that any (upstream) loss at motor terminals has to be reflected at the voltage-unbalance relay terminals.  If they are not the same, all bets are off.  

The advertisements for a number of voltage-sensing/phase-loss devices claim responding to about ±15% voltage imbalance.  I may be misinterpreting the advertising claims but, that seems to be an ineffective level of protection if allowed to remain beyond a very short interval.  

In many cases, it seems that a persistent voltage imbalance of less than 15% could yield unacceptable motor heating and consequent coil-to-ground and/or turn-to-turn insulation damage.
  

RE: Detecting Regenerated Voltages

(OP)
Busbar- most unbalance relays I have seen usually have an adjustable unbalance setting from 5-15% or sometimes lower. The ones with fixed unbalance trip levels are usually set to 6% (the ones I am familiar with). They usually have a trip time of 2 seconds for unbalance. Also, unbalance percent does not have a sign attached to it.

RE: Detecting Regenerated Voltages

gjones -

I am fully convinced that if you spin your motor at full speed you may get a high voltage approaching full line voltage.

I am not at all convinced that this provides an explanation for the voltage generated on the open phase of a motor. (although as I've said many times I cannot provide any alternative explanation myself for this regenerated voltage that apparently does exist).

Your residual magnetism motor amounts to a syncronous permanent magnet generator. The voltage you generate will correspond to the speed of the rotor.  [As an unrelated side discussion...it would be interesting to see if you could sustain your voltage under load. An experiement would be to short the stator windings and spin it.... then stop spinning and remove the short, spin again and see if voltage remains].

If you want to consider the rotor during the one-phase open case as a permanent magnet, then you would come to the conclusion that the generated voltage is "syncronous" to the rotor speed. But it cannot possibly be syncronous to the rotor speed (under load conditions) and also syncronous to the power frequency, since these will differ depending on slip. There is no way that we could get a third induced phase 120 degrees apart from the 2 applied power phase  because the speed/frequency would be different... phase is undefined.

The solution to the above contradiction is that we cannot view the rotor as a simple permanent magnet.... the rotor field is not stationary with respect to the rotor... it moves at slip speed with respect to the rotor. More importantly the heart of the matter is that the rotor field is NOT the only field we need to consider.  It is a FACT that in a normal-condition induction motor with leakage reactance neglected for simplicity, the air-gap flux is independent of the rotor flux. The reason is that a load-component of the stator current flows to create a field which is equal and opposite to the rotor field (as I have mentioned a few times .  Whether this cancellation holds when only 2 of 3 phases are energized (open-phased why motor) is open to debate.

Likewise there is no direct comparison between the open-phased motor and an induction generator. The induction generator draws it's excitation from either external supply or external capacitors.  The open-phase motor cannot draw any excitation on the open phase.  

One more distinction to draw...Your residual magnetism generator is also different than an induction generator.... one is induction and one is syncronous.  I believe residual rotor magnetism plays an important role only in establishing the initial field of the standalone induction generator with caps. I don't believe residual rotor magnetism can possibly play a role during steady state under-load operation of an induction generator because the frequency of any static residual field would not match the terminal frequency.  

RE: Detecting Regenerated Voltages

Suggestion: Reference:
M.G. Say "Alternating Current Machines," John Wiley & Sons, Inc., 1978 Paragraph Single-Phasing on page 331
It stresses, if single-phasing occurs, a loaded motor may stall because of disconnection of the stator terminal introduces a negative phase sequence torque. Figure 8.58 Normal and single-phasing performances show current versus load curves for three-phase and single-phase conditions and torque versus speed relationships for 3-phase and 1-phase conditions. Clearly, the 1-phase condition torque is about (1/sqrt3) x 3-phase torque and the single-phase rotor or stator current is approximately 2 times the rated 3-phase condition stator or rotor current at rated load 1.0 per unit. However, there are the motor stator winding connections not mentioned.

RE: Detecting Regenerated Voltages

Small encore: Above mentioned Reference Figure 8.58 shows curves from no load to 1.25 pu load and from 0 speed to 1.0 p.u. speed. This is somewhat useful to this thread since some postings deal with light shaft load conditions. E.g. 1-phase condition torque at zero speed is equal to zero and the three phase condition torque at zero speed is equal to 1.0 pu.
Three phase stator current at no load is about 0.3 pu and one phase condition stator current at no load is about 0.45 pu.

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