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surge due to "bump" test for motor rotation?
13

surge due to "bump" test for motor rotation?

surge due to "bump" test for motor rotation?

(OP)
During motor retest following motor removal/replacement, we confirm the direction of rotation by momentarily  energizing the motor just long enough to get it rotating.

A fairly knowledgeable engineer recently told me that this can cause excessive surge stresses to the motor. Specifically, the full starting current is still flowing at the time the breaker is re-opened.  He said that the interruption of this higher-than-normal current caused excessive voltage surges which could over time damage the insualtion.  v=L*di/dt and the i in this case is ~ 6xFLA.

By the way we have mechanical circuit breakers.

I don't know enough about surges and circuit interruption to evaluate this concern.  I am skeptical for the following reasons: I was under the impression that this bump test was fairly common practice.  Therefore if it were creating severe surges, I would think I would have heard more about it.

What do you guys think?

RE: surge due to "bump" test for motor rotation?

(OP)
By the way, I realize it is a fairly simple resolution to just allow the motor enough time to come up to speed... but I still want to know whether our existing practice is really creating a problem.

RE: surge due to "bump" test for motor rotation?

2
Pete,

How often is this practice applied to any given motor ...

once an hour ?

once a day ?

once a week ?

once a month ?

once a year ?

etc....etc....etc ..


Consider that the junk yards would be full of failed motors if it was really a problem to infrequently 'bump' a motor. And motor manufacturers, NEMA, EASA etc...  would publish a warning about such practice.

Kr... jO

RE: surge due to "bump" test for motor rotation?

Since you need to do it only once
and since it may happen anytime by accident
  -- a motor have to be able to survive it
So yes, you get large voltage spike but I don't think the risk is significant.

<nbucska@pcperipherals.com>

RE: surge due to "bump" test for motor rotation?

(OP)
It may be a few times during the life of the motor winding. Definitely after initial installation. Definitely after refurbishment of motor which we sometimes do at 10-20 years.  Possibly when motor is removed to access pump.  I think at various other times when motor is de-term'd re-term'd in place for test or troubleshooting, we just label the leads carefully to ensure they are reconnected the same way.

RE: surge due to "bump" test for motor rotation?

While it is true that any switching on inductive load could creat a voltage 'spike' or surge, things has to be put in perspective.

You should not be worried, imho.
Because:

1. Presumably you have had no problems so far so why try to fix a problem which does not exist.

2. Bumping a motor on no load (even starting), the inrush current spike does not last much beyond first half cycle or even say 2-3 cycels. Even fastest manual breaker closing and opening takes more than 1 second (60 cycles) so you are not interrupting any high currents while opening a unloaded motor after a 'bump'.

3. Since you are not repeating this thing several times on a motor in a short period of time, there definitely is no threat to the motor.

4. If any, the concern shall be with the circut breaker contacts. As long as you use a adequately sized breaker you should not have any promblem. Overtime, I am sure, you can surely afford to replace a breaker or two if any problem occurs.

RE: surge due to "bump" test for motor rotation?

(OP)
thanks rb.

You mention that inrush current only lasts a matter of cycles.  I agree from the exponentially-decaying dc componentn of the current... but I think the high ac starting current ~ 5*LRC persists until the motor is near full speed, somewhere in the range 0.5 - 2 seconds.

RE: surge due to "bump" test for motor rotation?

Electricpete,
     I deal with 75 hp motors on a daily basis that we "jog" or "bump" on a several times a day. They are always jogged under load.
     If you are going to have a machine with this function I strongly recommend you overate the contactors. A good rule of thumb I have seen is about 200% for NEMA contactors and 300% for IEC contactors.

Regards,
Afterhrs

RE: surge due to "bump" test for motor rotation?


There are those goofy little $100 phase-sequence testers that operate over 20mV-600V, and can be used on an idle motor to indicate motor phasing with a decoupled shaft spin.  The line-side {energized} connections should be limited to fused PT-secondary connections for anything over low-voltage/low-current applications.  [http://www.rke.com/mrt.pdf]
  

RE: surge due to "bump" test for motor rotation?

electricpete:

The engineer who answered you is absolutely correct. The fact is that, during start, the motor has not yet built any back EMF - which would otherwise limit the voltage surge during disconnection from the power line. Instead, you will have a huge Ldi/dt voltage spike across the motor terminals.

GH

RE: surge due to "bump" test for motor rotation?

That's an interesting concern that I've never heard voiced.  My experience is that bumping motors to verify rotation is standard practice every place I've ever been.

 

RE: surge due to "bump" test for motor rotation?

pete,

isn't twice the rated voltage that is the maximum that could occur in a normal ac motor during switching ? if so, bump test couldn't be any worse than normal switching operations ? what do you think ?

kumar

RE: surge due to "bump" test for motor rotation?

Pete,

My experience of this is that it kills contactors if carried out too frequently. If it is a frequent occurence, rather than a DoR test after maintenance, consider uprating the contactor to a larger type or one rated for a more severe duty.


RE: surge due to "bump" test for motor rotation?

Suggestion: Normally, motors are designed to survive a start in the wrong direction with a possible motor protector, i.e. circuit breaker, overload relay or fuse, trip, if the motor is excessively loaded in the wrong direction. There is usually a bigger concern about the motor load being started in incorrect direction, whether or not it will not be damaged.

RE: surge due to "bump" test for motor rotation?

(OP)
Well, it seems there is some difference of opinion within responses here and I really don't know the answer.

For the future to be on the safe side I will just ask our operators to wait 5 seconds between starting and stopping during the bump test.  I really don't know if it's required but certainly is easy to do. I am still curious (and skeptical) though.

edison - The doubling that you refer to occurs during energization.  During deenergization it's a little harder for me to picture what's going on.

I guess I can vaguely envision a scenario that if one contact opens before the others, it creates a sudden increase a v=Ldi/dt across that open contact.  If the increase in V occurs suddenly it creates a traveling wave?

I do know that some people consider vacuum breakers create surges which can be damaging to motors. Is it during the closing or interruption?  If it is during the interruption, then I can see maybe it is because they interrupt very suddenly, increasing the di/dt.  So by same logic, increasing the current would also increase the di/dt and create similar effect.

There can be even higher at the time of current interruption during a phase-to-phase fault.  (or phase-to-ground fault for solidly grounded power systems I am not familiar with).  In that case, I would expect that if current trips on phase-to-phase fault we might sometimes expect to find shorted line-end-turns during subsequent surge testing.  I have not heard of this... has anyone?

As a coincidence,  I was just involved in reviewing investigation of a motor which tripped on ground fault in a resistance-grounded power system.  In addition to the fault to ground 4 coils in, two of the line end coils failed surge testing at the normal maintenance-level test voltage.  But they were not physically near the failed coil... no reason to connect them to the fault. The motor had not been surge tested since manufacture.  As I have alluded, the ground fault current was limited by resistance grounding system and there was no phase-to-phase fault possibility based on examination of damage and relays.... so the hi Ldi/dt scenario doesn't seem to apply.  

RE: surge due to "bump" test for motor rotation?

pete,

It has been emprically proven all switching surges (in or out) cannot exceed 2 x rated voltage especially with VCB's. This is the reason new or rewound machines are ac hipotted and surge tested at 2 rated voltage + 1 KV. If you talking about one pole of breaker not opening or closing, then it is single phasing  with its disastrous consequences.

RE: surge due to "bump" test for motor rotation?

-bump test is  done for very samll duratuion in comparision to the life of motor.
-the sil/bil is supposed to take care of the surge if any during bump test.
-what if an unintentional bump test occour i.e motor closed /tripped during normal operation.
- a motor has to be designed taking this fact in to account.
-it is correct that there will be surges during bump test , but what about the magnitude and duration.

RE: surge due to "bump" test for motor rotation?

Edison123:

Consider that the insulation test by means of the highpot method... is typically done with a sine wave source. The NEMA standard for insulation, as you stated, is (2 x rated) + 1000v.  

However, in a bump test, you have maximum (6x)....or near maximum..... current flowing into the inductance of the stator... at the just before you interrupt this current, the field around the conductors is very, very strong, and, there is a lot of force or torque stress on the conductors (not to be confused with shaft torque)... So the wires are straining to move... and sometimes are able to cause insulation failure when they encounter a metal edge and there is movement of the conductor...  

So maybe it doesn't happen the first time... but every time the motor is DOL started, mechanicl stress occurs in the conductors (winding, etc..) of the stator.

At some point in time... failure will occur... and along the way, insulation withstand is reduced.

Now, to the point... when the energy in the inductance of the stator windings, is suddenly interrupted, the voltage spike consequent to the L di/dt.... is not a gentle sinewave... but rather a steep wavefronted voltage, which can cause the insulation in the motor to fail where under a highpot test...it would not...

An oscilloscope used to capture the voltage transient thusly generated would substantiate that the characteristic is not that of a sinewave.

The insulation test for such occurrences, is not that of a highpot or of a meggar type test.... rather, the windings should be subjected to a pulse injection (surge)type test, whereby the rise time and amplitude of the pulse can be selected.  

Typical would be a Baker Instruments Surge & HiPot tester. These Surge and HiPot testers verify insulation integrity by finding weaknesses in Turn-to-Turn, Phase-to-Phase, and Coil-to-Coil insulation systems within the motor.

Many motor manufacturers apply such a test to the stator before the rotor is installed as a quality check. I've seen it done on random wound and form wound stators.

The surge test is also a good means for determining the motor's ability to withstand the stress of dv/dt that is seen when the motor is sourced from a VFD and is suggested for old motors newly being applied to a VFD.

A friend of mine who sits on the NEMA MG-1 committee... tells me that the hipot is not meant to protect against high dv/dt stress, but rather.... to accommodate line transients..... not unlike those caused by the switching of capacitors on the mains.

RE: surge due to "bump" test for motor rotation?

(OP)
meenakhi brings up a good point. We have one motor that has on three different occasions tripped upon starting for unknown reasons over a 2 year period. Now has not happened in 8 months.  It would be the same transient.

edison - I can easily see where the voltage doubling applies during closing.  It is simple reflection of voltage wave (nominal  voltage) upon seeing a change of impedance.  I don't see where it applies on opening.  The scenario that I tried to paint only assumed a slight difference in timing of contacts opening...just because it is easier to imagine where the voltage appears (across the single open contact).  It does seem credible that whatever transient exists during opening is proportional to current if we assume simple linear circuit based on dv/dt.  I think the nature of transients upon opening are much more complex than I understand.  I know there is a lot of literature on circuit interruption but I havent read much of it.  Terms like restrike, reverse recovery voltage etc... I have heard them but don't really understand them.  Once again... is it not true that vacuum breakers impose a worse voltage transient than than mechanical breakers during interruption?  If so that suggests the di/dt term is important and will depend upon magnitude of I.  (Just arguing the devil's advocate position here)

jO - Your point out that in the bump scenario there is surge stress at the same time as mechanical stress. Interesting point.

RE: surge due to "bump" test for motor rotation?

pete,

I agree that the inrush current of the cage motor during starting (or bump testing) may be 5 to 6 times, but it does not automatically follow that the surge volatges across windings will also reach to 5 to 6 times the rated voltage (regardless of whether you make or break the circuit). If it were so, I am sure most of the motors will fail during starting since they are not designed to withstand 5 to 6 times rated voltage. If you have any theory about surge voltages exceeding twice rated voltages, I'll be interested to know.

RE: surge due to "bump" test for motor rotation?

From a surge point-of-view switching a motor off is similar to switching a motor on - the surge just changes sign. Motors have a surge capability whose magnitude is a function of the surge rise time - 1 pu at .02 usec and 4.5 pu at 5 usec (Red Book Fig 45 which refers to old motors, newer insulation systems are even better). So as long as you don't exceed this envelope, you do not stress the motor. In most case, there is enough capacitance in the cable feeding the motor to keep the surge voltage at safe levels - I have seen a paper and done a study that show about 50m of cable is usually adequate.

RE: surge due to "bump" test for motor rotation?

(OP)
Thanks Gord. Your comments that closing surge and opening surge strengthen's edison's comments. I don't get the comment about the opening being a minus of the closing and it doesn't seem follow from simple wave behavior: O->VL increasing front reflects/doubles and becomes 2VL (forward wave +VL, reflected wave +VL).  VL->0 decreasing front reflects/doubles and becomes 0 (incident wave is 0 and reflected wave is -0).   But the source voltage proposed during opening  was affected by the di/dt.  Maybe this di/dt voltage plays no role at all?

I'll have to think about it some more.

RE: surge due to "bump" test for motor rotation?

(OP)
Once again: isn't it true that vacuum breakers (without suppresion) create high di/dt upon opening which can produce dangerous surges?

If so, then we expect any similar effect will be lower on mech breakers due to longer arcing time.  But also we then expect that the magnitude of current at time of opening is important in determining the opening surge.

RE: surge due to "bump" test for motor rotation?

(OP)
I did find some literature (epri 4862v1) which seems to address this questions.

For vacuum circuit breakers during interruption, multiple restrikes can occur resulting in escalating voltage which can reach 10 p.u.   (1 p.u. = VLL* sqrt(2)/sqrt(3) = peak line-to-neutral voltage).

For air mechanical circuit breakers, the interrupting medium does not have as high dielectric strength and the same multiple restrike scenario is not possible.

From the whole context of their discussion I think they are saying that for air breakers, the only surge of concern occurs during closing, but for vacuum breakers there are surges of concern on both opening and closing.

RE: surge due to "bump" test for motor rotation?

(OP)
just to clarify,

for vacuum breakers (only), the magnitude of the surge on opening IS heavily dependent upon the current flowing prior to opening.  People with vacuum breakers should probably avoid the quick bump if at all possible (let it run for 5 seconds).  

for air breakers, any opening surge should be far less than 2 pu.



RE: surge due to "bump" test for motor rotation?

EP:

Many decades ago, when I was a young inexperienced lad working on the test floor at a Westinghouse Electric plant, a wiring error on a control panel inadvertently connected the load side of the panel input circuit breaker to the line side of the L3 terminal of the breaker... Since the wiring was bundled... it wasn't obvious.

When 460/60/3 power was applied, the 4/0 cable connecting the panel to the distribution board came about 2-feet off the concrete floor..... before the breakers tripped.

It was at that moment that I learned about the torque force produced by the field around the wire resulting from the short circuit current passing thru it.

Motor engineers will tell you that the same torque forces exist within motors when subjected to DOL starting currents. Part of the varnishing and lacing applied to the windings, is to restrict their movement under such conditions. Now, while you can lace the wires outside the slots, you cannot lace the wires within the slots.... and they will find a way to move... ..

Interesting things that go on inside the motor housing that we don't see  ... and don't think about .... until a failure occurs.... and we try to find a reason...

Kind regards,

jO

RE: surge due to "bump" test for motor rotation?

Suggestion to the previous posting: Apparently, there are different qualities of electrical motors around. One type of electrical motors has conductors in slots placed at random, other type will have the winding consisting of conductors carefully laid without any room for movements and beginning turn far away from the ending turn. Visit
http://www.lincolnmotors.com/pdf/tb-100.pdf
etc. for more info

RE: surge due to "bump" test for motor rotation?

JB,

The only motors that I know of that use an ordered winding pattern were Lincoln Electric.... They used machines that looked like a sewing machine lying on its side that was made by a company in Milwaukee that is no longer in business. The coils were actually wound into the stator slots.

The vast majority of motor manufacturers use a random wound method for producing stator coils. And then they pound them (force them) into the slots.

As to your claim about them not being able to move..... BullPuckey.... !

jO

RE: surge due to "bump" test for motor rotation?

(OP)
jO
Today form wound motors are widely used for medium voltage (2kv and above), random wound for low voltage (<600vac).

But your comments were useful... I did not specify which type of motor I was considering.

RE: surge due to "bump" test for motor rotation?

Peter,

Your correct. My comments were predicated upon low voltage type motors.

FYI ..

ABB uses form wound in their larger low voltage motors (HXR type).

It's more expensive as you can imagine.... but there are
benefits as well that justify the added cost.

Form wound stators rarely, if ever, experience dv/dt failures when supplied from power darlington and IGBT type inverter bridges of the PWM and vector type inverter bridges.

RE: surge due to "bump" test for motor rotation?

JB,

What LOW VOLTAGE VFD manufacturers are presently using IGCTs in their products today ?

RE: surge due to "bump" test for motor rotation?

JB,

That's my point; IGCTs were developed for use in medium voltage drives.  

So, when you made your comment: " Suggestion: Also, IGCTs are more suitable for the motor controls....."

You lost me on that one... because I couldn't see any relevance to the discussions in this thread.

JB, there is nothing in the previous posts about IGBTs ... VFDs....etc..... that would seem to warrant your comment.

So, please tell me why you felt compelled to make that comment about IGCTs and how it relates to the discussion in this thread which, unless I missed something,  is about bumping a motor, and what harm can be caused by doing so....


Thanks.

p.s.

FYI JB, medium voltage drives usually have a filter in the output that knocks down the dv/dt... and renders as close as possible, a sine wave to the motor.

Without such filtering .... lots of motor failures.
Fast switching and medium voltage = very high dv/dt rates.

RE: surge due to "bump" test for motor rotation?

Hi,

Just a silly question, In motor junction box (where you connect the motor lead to cable feeeder) in MV motors usually have some kind of arrester to to protect motor from switching surge, is that count on this thread ?

RE: surge due to "bump" test for motor rotation?

Suggestions to jOmega (Electrical) Sep 26, 2003 marked ///\\\
JB,
That's my point; IGCTs were developed for use in medium voltage drives.  
///Yes, agreed by ABB.\\\
So, when you made your comment: " Suggestion: Also, IGCTs are more suitable for the motor controls....."
///Since the original posting is not limited to any voltage level.\\\
You lost me on that one... because I couldn't see any relevance to the discussions in this thread.
///Please, just recall the "excessive voltage surges" context in original posting with respect to IGCTs that were designed to reduce those by somewhat slower switching with respect to IGBTs.\\\
JB, there is nothing in the previous posts about IGBTs ... VFDs....etc..... that would seem to warrant your comment.
///Please, see my previous statement.\\\
So, please tell me why you felt compelled to make that comment about IGCTs and how it relates to the discussion in this thread which, unless I missed something,  is about bumping a motor, and what harm can be caused by doing so....
///Addressed above.\\\
Thanks.
p.s.
FYI JB, medium voltage drives usually have a filter in the output that knocks down the dv/dt... and renders as close as possible, a sine wave to the motor.
///Yes, true. I am familiar with those.\\\
Without such filtering .... lots of motor failures.
///Yes, agreed.\\\
Fast switching and medium voltage = very high dv/dt rates.
///Yes, that is why there are IGCTs invented thanks to ABB.\\\

RE: surge due to "bump" test for motor rotation?

This is a common practice for large motors as well as small motors where I used to work. The nuclear electrical industry motors ranged anywhere from 480V up to 6.9KV  We bumped the motors for rotation uncoupled. I empahasize UNCOUPLED. We found the effects which you have spoken of to be negligable. I would not recommend bumping a motor for roation check while coupled to its work output. Especially if the leads had been removed and relanded.

RE: surge due to "bump" test for motor rotation?

This is a common practice for large motors as well as small motors where I used to work. The nuclear electrical industry motors ranged anywhere from 480V up to 6.9KV  We bumped the motors for rotation uncoupled. I empahasize UNCOUPLED. We found the effects which you have spoken of to be negligable. I would not recommend bumping a motor for roation check while coupled to its work output. Especially if the leads had been removed and relanded.

RE: surge due to "bump" test for motor rotation?

(OP)
thx cerpico. That is the main purpose of our bump test... to verify rotation BEFORE we couple.

We did have one instance of pump somehow got coupled and started with motor leads reversed. It wasn't pretty.

RE: surge due to "bump" test for motor rotation?

Suggestion: The motor bumping can be so considerate to the motor that the motor barely makes one revolution. If on no load there is practically no impact on the motor except a voltage spike. Then, transient voltage suppressors might be used for the motor bumping to protect the motor winding insulation.

RE: surge due to "bump" test for motor rotation?

... and JB, the consequence of /b]NOT making one revolution is  WHAT ????

Suggestion:  Instead of a motor, JB, think of it in terms of a three phase transformer.

Suggestion:  re-read my post of 20 & 21 Sept. to learn of other stresses that occur within the motor every time the windings are connected to line power when the rotor is in a dead-stop state.

JB, your proposal about utilizing a suppressor, while technically correct, lacks practical merit for the following reasons:
  • How many JOULEs must it be capable of handling ?
  • How large (physical dimension) a unit would be required ?
  • And how expensive would such a suppressor be as a percentage of original motor cost ?
  • How much cost would such a suppressor add to the installation cost of the motor ?


  • New motors, can certainly withstand an occassional "bump test". Older motors experience insulation and bracing deterioriation as a consequence of heat and time; and so, are more succeptible to experience a failure during the "bump test".

    However, I offer the following for consideration:
      Of the total number of motors in service that are more than 5 years old, what percentage experience internal failures during a "bump test"
      • 10% ?
      • 1% ?
      • 0.1% ?
      • 0.01% ?
      • <0.01% ?


    I think the answer will show why adding a "suppressor" to the motor connections is not a practical consideration. Also, while the suppressor would knock-down du/dt stress to the motor insulation, it does absolutely nothing to releive the mechanical stress that the windings are subjected to.

    RE: surge due to "bump" test for motor rotation?

    Suggestion to the previous posting: I did not mean the permanent voltage suppressor. I meant some portable or mobile one.
    I disagree with the reasoning that covers costs. Often, the motor is engaged in some process that has some value. Any downtime may prove to be costly. Therefore, a device as transient voltage or surge suppressor saving the motor insulation against damage or insipient puncture is very justified and required. Obviously, any fan for a dog house, for example, may or may not need such a suppressor.

    RE: surge due to "bump" test for motor rotation?

    JB,
    You are more than welcome to disagree.

    Perhaps you have not yet learned that the majority
    of such decisions are based in economics. Downtime
    not being at the top of the list UNLESS... there is a high probability of damaging the motor every time it is subjected to a "bump test".

    Question: How many motors do you personally know of that have suffered an insulation failure consequent to a "bump-test" being performed ?

    The TVSS that you propose, even in a portable configuration is not inexpensive, and the COST/BENEFIT RATIO numbers do not justify the expense in the majority of applications. The proof of this claim is in the reality that facilities having a dedicated TVSS for connection to a motor whenever a "bump test" is performed is unheard of.

    Do you have one at your place of employment? Is there a procedure that mandates its use whenever anyone perfoms a "bump test" on a motor ?

    If so,
    • What is the largest size motor you would bump ?
    • What Voltage(s)?and,
    • how many JOULES is the TVSS rated for ?


    Also, as I have said a couple of times in this thread, failure from insulation du/dt stress is not the only failure mode. The mechanical stress that the windings are subjected to also causes failure and would not be prevented by a TVSS.

    Look at the numbers, JB. Most motors are bumped at installation to verify direction of rotation. Of the hundreds of thousands of motors out there.... how many do you know of that have failed ?  Is the number large enough to justify the expense of a TVSS ?   Really ?????

    Also, consider EASA shops; how many motors do you think they have bumped in the last 50 years ?  What percentage do you think have suffered a failure as an immediate consequence thereof ?   Can't be very many; they're still doing it today, and I know they don't connect a TVSS to a motor whenever they do it.

    Can't be much of a concern to those learned folks, you think ? At least not enough to justify the cost of having one in every shop.

    I'd guess they don't disagree.

    RE: surge due to "bump" test for motor rotation?

    Hi Pete,

    It's common practice here in the UK too ! However, in my experience, I uncouple the motor and give it a longer run, about 10 seconds. This is mainly because the contactor can be quite remote from the motor I'm testing and I need time to run from the control panel to it and check the rotation as it is coming to a stop !

    If the contactor is local to the motor then I do a bump just as you do ! never caused me any problems.

    RE: surge due to "bump" test for motor rotation?

    Daazmac:

    1st preference is to bump motor when it is uncoupled from its mechanical load.

    However, there are those, I'm afraid to say, that get lazy or rushed, and choose to ignore such logic.

    The only other time that one could be excused from bumping a motor that is coupled to its load.... is when there is 100% certainty that neither machine nor personnel will be endangered as a consequence there of.

    Can't think of any reason what-so-ever to not allow the motor to run up to speed, and then catch the rotation direction as it decelerates or coasts to a stop.

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