×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Single Phase electric motor problem (No load high current and heating)
2

Single Phase electric motor problem (No load high current and heating)

Single Phase electric motor problem (No load high current and heating)

(OP)
Hi 
I have a single phase electric motor. I got it from a friend that did not use it for a while. I cleaned it up and it sounds that all components are ok. I removed all dusts and add some oils to the bearings. 
bearings are ok. I cleaned the connectors and checked them all and all sounds good. 
I did a beep test to find the connections between coils and wires. what I find is two separated coil, one with 2.5 Ohm resistance (blue - Yellow) and second one with 3.1 ohm (Black-Red).
I far as i understood, the 3.1 ohm coil is the starter coil as it goes to starter capacitor. the motor runs well and no noise comes out from motor. it turns well and the torque sounds good. 
Here is the problem:
the full load current is 6.4 A based on the motor characteristics label but when I run the Motor with No load, its current reach o 7.2 A !!!!
after 5 to 10 min operation, the motor case get hot. as it is class B motor, it can reach to 80 C.  But I think there might be something wrong as it took too much current under no load test and the generated heat sounds weird.

I though it might be due to centrifugal switch mechanism and electrodes that do not well operate.I tried to check if the centrifugal switch. I can hear that it operates and due to good start, it should be fine but I can not check if the start coil well disconnect or not (should be ok)   

some photos is attached to see the motor conditions. the schematic is based on what I tracked on wire management board and it sounds ok for me.

I did all I can based on my knowledge and experiences (Not too much) but I do not know what could be the problem.
any comments or idea is highly appreciated.
Waiting for your feedbacks


RE: Single Phase electric motor problem (No load high current and heating)

First, this is a professional forum, not a DIY forum.
Second:
Find out why the starting winding is staying in the circuit.
That is the issue 98% of the time.

Quote:

I can hear that it operates and due to good start,
That is trying to prove a negative.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Single Phase electric motor problem (No load high current and heating)

> as it is class B motor, it can reach to 80 C.

just as a point of clarification, class B insulation is rated for a continuous 80C temperature rise above ambient temperature of 40C...so 120C.

=====================================
(2B)+(2B)' ?

RE: Single Phase electric motor problem (No load high current and heating)

(OP)
Thanks for your reply.
I am an electronic engineer and trying to change my filed. that's why I sent my post here to ask from professionals.
how can I check if that is the fault of start coil.
is it possible to disconnect the starting Capacitor and make it run by turning the shaft and then measuring the input line current?

thanks

RE: Single Phase electric motor problem (No load high current and heating)

Without knowing the amount of potential applied to the motor during the bench test, the measured 7.2 amps is likely insignificant.

Guessing the motor is rated somewhere around a 1/3rd horsepower and may even be an OEM device built for a specific purpose
noting the style of die-cast end-bracket barely visible off to right of the rotor in the photos.
The device vaguely appears to have a threaded shaft end as well. (It's not likely a pool pump motor for they would have a locked drive-end
bearing and photos do not show a bearing retainer behind the front bearing.)

Thank you for the very clear photos. A photo of the nameplate would have been the most useful.

I don't believe anything is wrong with the motor.

John

RE: Single Phase electric motor problem (No load high current and heating)

(OP)
Hi
thanks for your replies. Actually, it is a CGE electric motor. internal condition sound good to me and based on what I have found on theories, it sounds good except the No-Load current and generated heat.
I didn't expected too much current without Load, even higher than its rated full Load current mentioned on the nameplate.
I run this motor on 115-120 V AC and I know very well the Ohm laws. but based on my knowledge it should take too much current.
based on the nameplate, it does not sounds to be an OEM motor but if it is as you said, you are right and it can take what ever it want.

I attached the nameplate photo just for more info, if it helps to clarify more issues.

RE: Single Phase electric motor problem (No load high current and heating)

Measure the capacitor current.
It should drop to zero Amps if the centrifugal switch is working properly.
The second possibility is that the start and run windings are interchanged.
No load current on a small motor is often more than expected due to a poor power factor.
No load current (7.2 Amps), greater than rated FLA (6.4 Amps) is too much.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Single Phase electric motor problem (No load high current and heating)

(OP)
I did some more investigation. Now I can confirm that the Red coil (Red-Black wire) is the start coil and the yellow coil, yellow-Blue wire is the running coil.
I assembled the motor and measure the Cap current when the motor was running and it gave 0.065 A.
still the No load current is around 7 to 7.2 A.
I measured the motor case temp with a direct touch sensor and after almost 10 min it reached to 65 C.
the Cap current confirms that the centrifugal switch works fine and I really can not understand why the No load current is high like this while on the nameplate it is mentioned FLA 6.4 A. is it possible that they gave wrong value?
for me 7.2 Amp is high for a 1/3 HP motor.

RE: Single Phase electric motor problem (No load high current and heating)

Hi,AR.

Unlike three-phase motors, single-phase motors are not kind of general-purpose motors.
Each of them is designed to work on a strictly defined machine and for a specific load.
Theoretically, an ideal single-phase motor should have a different winding and capacitance for each particular value of load.
That's why it is not recommended to run the motor at no-load or at small loads at all.
In short, in your case it is perfectly normal for the NLA to be larger than the FLA.
Don't run this motor at no-load conditions.

More on Winding Design and Motor Repair

RE: Single Phase electric motor problem (No load high current and heating)

Quote:

In short, in your case it is perfectly normal for the NLA to be larger than the FLA.
Don't run this motor at no-load conditions.
I gotta throw the BS flag on this.
There is not that much difference between the theory of a running single phase induction motor and a running three phase induction motor.
From the pictures and the nameplate that appears to be a general purpose motor.
Despite the resistances, the start and run windings may be interchanged.
Check the windings by energizing with a small battery and testing with and a compass or hack saw blade.
The motor may be failing.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Single Phase electric motor problem (No load high current and heating)

(OP)
I already checked the coil and it seams that they are really separated. I did beep test and tested with a power supply.
it is a general purpose motor and based on the pulley that I founded on it, it should be used as a blower fan.
there was a kind of V-belt pulley already installed on the motor.
I have not if I can hack it with a table saw but the torque sounds ok for me while I have never tested it with load.
As I am going to use it for a belt sander (needs some modifications to adapt it to machine) I preferred to be sure about it functionality and they doing the modifications.
At this time I am nut sure if I can use it or not.
I searched almost all over the internet, consulting with you and others as experts, checked all the error sources as much as possible but still it is runs at high current with lo load.
Based on my knowledge, it is strange except if the running coil took too much current or have been changed and does not respect the nameplate (which should not be happened if I trust what I hear from the guy that bring it to me).

I am still working on it but honestly I have no more ideas what is wrong with it!!!

RE: Single Phase electric motor problem (No load high current and heating)

Have you checked the applied voltage?
Your voltage may be too high.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Single Phase electric motor problem (No load high current and heating)

(OP)
as I live in Canada, the residential power voltage is 120 V. I will check it today but I would say it is is 120 V.

RE: Single Phase electric motor problem (No load high current and heating)

120 Volts is fine but some wire service providers push the voltage up to and past the limit to maximize the KWHrs and thus their profit.
If the voltage is running much above 120 Volts, your motor may be saturating.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Single Phase electric motor problem (No load high current and heating)

Hi, Waross,
I am sorry for this BS flag.
I don't think I deserved it.
It seems to me that you haven’t dealt much with single-phase motors.
Otherwise you would know that the NLA of single-phase motors is very often close to or equal to or higher than the FLA. If higher than the FLA, as the load increases to the rated value, the current drops to the FLA value.
That is why it is not good for such a motor to run with no-load, because the winding can burn out.
It’s not that hard to check it.

Visit our web site

RE: Single Phase electric motor problem (No load high current and heating)

(OP)
Thanks Zlatkodo for your explanation.
Would you please let me know what is the reason for such a reaction from Motor.
it does not make scene. on theory, the motor is designed for a specific RPM with some limits.
what defines the limits are the coil wiring and some other factors. increasing the load should increase the AMP to keep the motor RPM at its nominated RPM.
why should it run at such high current during no load test?
also would you please let me know how could we define the proper load for a general use motor?

I will try to add some load on shaft and see how the current changes.

RE: Single Phase electric motor problem (No load high current and heating)

> Otherwise you would know that the NLA of single-phase motors is very often close to or equal to or higher than the FLA.

> Would you please let me know what is the reason for such a reaction from Motor

Jumping in on that question. The ratio of NLA / FLA in general depends on speed (lower speed have higher ratio) and size (smaller motors have higher ratio) again in general. One of the reasons for the variation with size is that when you scale down motor size, the airgap does not shrink in proportion to the rest of the motor.

But how would the NLA/FLA ratio ever get above 1? That's a head scratcher. I guess if there were a run capacitor then the motor could end up with a lower power factor (leading) at no load than at full load which would be a factor to nudge the current in the direction of higher at no load... but that factor still has to overcome the increase in load component of current which seems unlikely to me (but you never know).

Are there capacitors in this motor, and how many?

=====================================
(2B)+(2B)' ?

RE: Single Phase electric motor problem (No load high current and heating)

(OP)
yes, there is only one start Cap on this motor which is 400-480 MFD - 110 VAC.
I measured the current passing trough it and after running it drops to 0 A (0.06 exactly)

RE: Single Phase electric motor problem (No load high current and heating)

ok thanks, sorry I missed that. Then I'm with Bill, I don't see any mechanism to expect current being higher at no-load than at full load on this motor.

=====================================
(2B)+(2B)' ?

RE: Single Phase electric motor problem (No load high current and heating)

Let's take a look at the components of motor current.
There is always magnetizing current.
In the first instance the magnetizing current is largely dependent on the air gap.
While the air gap is fixed, the effective air gap is mot fixed.
As the motor is loaded, the magnetic field is stretched and the effective air gap increases.
The magnetizing current or reactive current may be expected to increase slightly under load.
Real current. The real current is proportional to the load and the losses.
The real current may be expected to increase roughly proportionally to the load.

What combination of factors may cause an abnormally high no load current that drops under load?
Saturation.
A high applied voltage may drive a motor into partial saturation.
The subject motor has a running winding of 2.5 Ohms, and 6.4 Amps.
115 Volts/ 6.4 Amps gives a Volts per Amp ratio of 18.
In full saturation, the current increase is limited mainly by the resistance of 2.5 Ohms.
In saturation, current increase is limited mainly by the 2.5 Ohm resistance.
1 Volt/2.5 Ohms gives a Volts per Amp ratio of 0.4 Volts per Amp.
In full saturation, a 2 Volt increase will result in an 8 Amp increase in current.

How can the addition of load current drop the total current?
How much voltage drop will 8 Amps of reactive current cause on a 120 Volt circuit with an assumed feeder resistance of 1 Ohm?
The simple answer is 8 Volts but that 8 Volts is at 90 degrees to the applied voltage.
√(120V^2 - 8V^2) = 19.73 or a drop of 0.27 Volts.
Even one or two Amps of real current will drop the voltage enough to avoid saturation and lower the magnetizing current.

Where will we see this?
In NEMA land it may occur in rural areas.
Even with primary voltage regulators, the voltage on long rural distribution circuits varies quite a bit and in some locations will exceed the recommended maximum voltage.
An additional factor is the length of the secondary circuits at 120/240 Volts, or service drops.
It is not unusual for rural service drops to run to several hundreds of feet.
Overvoltage is not uncommon.
In IEC land, with harmonization, a legacy motor rated for the lowest pre-harmonization voltage on a legacy system running at the highest recommended pre-harmonization voltage may be at or near saturation.

Note: While manufacturers do not publish no load current information for motors, they do publish load/current graphs and voltage current graphs for typical motors.
Occam's razor tells us that a combination of high voltage and voltage drop due to real current is the explanation for this symptom of a healthy motor.
Or
The motor may be failing.
or
The windings are interchanged.
Physically, the larger diameter wire will be the run winding.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Single Phase electric motor problem (No load high current and heating)

Don't forget some uncontrolled solar on the end of a rural run can easily crank the voltage way up.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Single Phase electric motor problem (No load high current and heating)

Actually the problem is worse closer to the voltage regulators where the voltage is high to allow for line drop to the customer 5 or 10 miles down the road.

As for solar, I can't say too much due to confidentiality issues, but I saw a case where the opposite was true.
The voltage was so high that some solar inverters could not drive full allowable KW output to the grid.
The utility installed an expensive voltage regulator on the 120/240V service and left the voltage high for another customer on the same transformer.
On a sunny day the voltage to the neighbour would be quite high.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Single Phase electric motor problem (No load high current and heating)

Dear MR AR. (Electrical
A. "....Here is the problem: the full load current is 6.4 A based on the motor characteristics label but when I run the Motor with No load, its current reach o 7.2 A !!!!... ".
C. I go through the various learned advices and make a summery:
1. Name-plate rating: 1/3 Hp, 115V 60Hz, 6.4A . Seems to be Ok.
i.e. 746 x Hp = V x I x k let k=pfxeff (which shall be <1)
k = (746 x 1/3 )/ (115 x 6.4) = 0.33.
2. You had assumed Blue-Yellow 2.5 ohm to be the run winding , Black-Red 3.1 ohm to be the starting winding; are Ok. Remark: Usually the (run winding) is with [lower ohmic value].
3. Motor rated 115V 60Hz 6.4A. Tested on 120V 60Hz , no-load current reads 7-7.2A is NOT ok.
Possible fault: run winding short between turns. Note: it does not show out in the resistance test.
4. All general motors can be run on no-load, very low load and up to full-load without any restriction.
5. At no-load, the NLA close to FLA is usual and OK. But NLA > FLA is NOT ok.
6. Bearing, Centrifugal switch + capacitor seems OK.
7. Guessing: see above 3.
Che Kuan Yau (Singapore)

RE: Single Phase electric motor problem (No load high current and heating)

(OP)
Hello everybody,
first, I would like to thank everybody for taking time and helping me to solve my issue.
Here is another brief of what I have done last weekend:I disassembled the Motor again, check all the mechanical issues.

1- I used multimeter beep test and ohm test to check the wires for running and starting coils. they are as before and no short between coils (run and start) and between coils and Motor case or stator.
2- I checked the Start Cap current after motor is running, normal operation and there is no current passing through the cap.
3- when the motor was disassembled, I tried to find if the centrifugal switch works fine or not and when the motor rotate and switch acts, it release the running coil and it sounds ok.
4- I visually check the centrifugal switch operation mechanism when was running (after assembling it for last test) and it was obvious that it release the contact and was able to see it when motor was running.
5- I checked the voltage when motor was running and it varies around 121 to 123 V AC.
6- Bearings and other parts sounds good to me.

Unfortunately I have no more hypothesis to check. I can not say that the nameplate gives the wrong info as it is CGE motor.
I do not want to give up but I have no more ideas.

thanks

RE: Single Phase electric motor problem (No load high current and heating)

Have you tried running it with some sort of load?

Even a fan would tell you if the amps come down a bit as load goes up.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Single Phase electric motor problem (No load high current and heating)

If a turn to turn short has developed in the run winding, the Amps will be high.
With a shorted turn, the turn acts as the secondary of a transformer with a short on the terminals.
This will add considerably to the run current.
The shorted turn will be generating excess heat.
The fault may be expected to progress.
The motor may fail soon.

I may have mentioned this;
From the pictures, it appears that the black wire connects to the run winding, not to the start winding.
The run winding is the much larger and heavier winding.

Quote:

Now I can confirm that the Red coil (Red-Black wire) is the start coil
How did you confirm this?

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Single Phase electric motor problem (No load high current and heating)

(OP)
Thanks Waross.
First I should say, based on the schematic that I attached (which I found by tracking the connectors and connection inside the Motor), I found that the black wire goes to the centrifugal switch.
this means it is for start coil. Also, another piece of existing black wire goes to Cap. moreover, the type of the connectors used for the wires are different and this black and Red Wire both have the same connector (please refer to photos) witch connects to s specific position on power board. those positions are for centrifugal switch and cap connector.
in order to be sure that I didn't do any mistake, I scratched the brown coil (thinner wire which assumed to be start coil) and removed the isolation (size of a small dote). I did the beep test and found the connection between Black - RED and dark brown coil.

that's why I confirm that the thinner coil (darker coil and thinner - as it should be ) id the start coil.
for the turn to turn connection, as I said I tried to visually check if the connection released after start or not and what I saw sounds good to me, means the centrifugal switch works fine.

RE: Single Phase electric motor problem (No load high current and heating)

Thinking outside the box on a free, used motor.
The motor may have the wrong rotor or someone may have tried to clean up the surface and reduced the diameter.
Lay the rotor in the stator without the end bells and look at the air gap.
It should be as little as possible.
A large part of the magnetizing current is creating the field in the air gap.
A few thousandths of an inch increase in the air gap will cause a noticeable increase in the magnetizing current.

Working in a large sawmill several generations ago, we overhauled a DC motor.
We sent the armature over to the machine shop to have the commutator turned.
The armature came back, we undercut the com and reassembled the motor.
Back in service, the motor ran hotter than it did before.
Eventually our foreman asked the machine shop foreman if they had seen anything out of the way when they had the armature.
The reply;
"I put my best man on it.
He even took a little cut on the rotor surface to clean it up for you."
Great. There was the trouble.
He had increased the air gap.
There is an old method of checking power factor that you can use to separate the load current from the magnetizing current.
You will need a second motor rated capacitor, a clamp-on ammeter, a drafting board and a large compass.
You are free to improvise on the drafting board and compass.
This will help separate air gap issues from shorted turn issues.
Let me know if you are interested.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Single Phase electric motor problem (No load high current and heating)

(OP)
Thanks a lot for your feedback. Well I would say that I am agree with you. while I could not find any issue (visible parts) inside the motor, but I am agree with you.
I also checked the air gap and it sounds good to me. I will upload a photo here to show it (may be be helpful for someone else).
but honestly I do not know what is the problem and it could be anything.

unfortunately I do not have all required equipment to launch your proposed test. it would be great if at least I could find what was the reason for no load high current on this motor even if I should put it aside.
I can not check the coils conditions and even the quality of used material for stature or rest, that's why I think I an done with this Motor.
But I will do my best to find its problem whenever I have enough time, equipment and ideas

RE: Single Phase electric motor problem (No load high current and heating)

Returning to the idea of shorted turns, I agree with waross it is very unlikely that an ac winding of any type continues to operate with a shorted turn because there is an auto-transformer effect that tends to increase the current through the group of turns that are shorted and it typically escalates, possibly with additional shorted turns along the way, until there is an open circuit or a ground fault trip or perhaps overcurrent trip.

With all that said, I've heard a few anecdotal reports of random wound motors (like yours) that continued to operate with shorted turn (maybe something about random wound makes the situation less predictable than form wound). In such situation you might see some visual evidence of discoloration in the endwingdings or on a wedge above the slot but I gather you didn't see that. In a motor shop they might try a growler type of test for further troubleshooting to find shorted turns (or resistance bridge for that matter, but I'll bet you don't have one). I imagine the growler test is beyond the amount of trouble you want to go to for this motor (the test doesn't fix anything, it only potentially answers a question of what's going on).


=====================================
(2B)+(2B)' ?

RE: Single Phase electric motor problem (No load high current and heating)

You may substitute squared graph paper for the drafting table.
The test:
Connect the test capacitor line to line.
Measure the current ahead of the test capacitor. (Yest capacitor current plus motor current)
Measure the motor current after the test capacitor. (Motor current alone)
Measure the test capacitor current.
Use the three values to construct a scale triangle.
Start with the test capacitor current drawn vertically. This current will be reactive and at 90 degrees to the real current.
Lay out the total current from the top of the vertical line.
Lay out the motor only current from the bottom of the vertical line.
You should now have a scale triangle representing the three current measurements.
Interpretation:
From the point where the total current vector meets the motor only current, construct a horizontal line to intersect the vertical line or an extension of the vertical line. Mark that point X.
From the horizontal line you may scale the real component of the motor current.
From point X you may scale the reactive component of the motor current.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Single Phase electric motor problem (No load high current and heating)

If you want to test the higher load - lower current theory put your meter on it, put on some gloves, turn it on, and grab the shaft. More squeeze -> more load. Don't try that with motors over 1hp.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Single Phase electric motor problem (No load high current and heating)

There is nothing wrong with the motor.

Acknowledging long time dwellers here are professing 7.2 amps is too high.

How does one politely explain the current is not excessive?
Typing again, 7.2 amps is not too high given the type of motor it [is].

The motor is in a 33 frame configuration making it kind of an odd ball.
It was almost certainly made for the original end user's environment and its unique application.
No one reading this forum likely knows what this motor was originally designed for.
I state this because of the concern about the heat being generated as excessive.

The OP stated the measured potential connected during a test was as high as 123 volts.
The motor is labeled and rated for 115 volts A.C.

One hundred and twenty-three volts applied to the motor is a tad past 7 percent in excessive potential.
This means the measured current is going to be a tad higher as well.

The discussion makes no mention to the integrity of the device(s) being used in measuring the current and potential.
Are we talking laboratory grade instruments that have been calibrated and tagged with a certificate?

One can fiddle around with a clamp-on style meter and get varying readouts.

Looking at the beautiful photos provided by the OP, they clearly show the Class B slot wedges and cuffed slot liners are in pristine condition.

Yes the winding is dirty. But it's not showing any tell-tale indication of heat stress.

Another tid-bit of observation.

The Electrical Apparatus Service Association has published a little engineer's handout for decades.

In those little booklets, they republished Single-Phase Full-Load currents lifted from the National Fire Protection Association's tables.

NFPA 70-1981, NFPA 70-1987, Table 430.248 (2011 National Electrical Code) and so on.
I don't have the latest publication in front of me, and know this is no excuse for typing a mini essay in an Internet forum.

But, guess what all those tables indicate as rated full load current for a 1/3 HP single phase motor [is].

Seven Point Two amps. ( 7.2 Amps )

I know there are higher math minds among the crowd that will dispute all the resistances, the turns in the winding, the enormity of it all.

But, it's time to put this thread to bed. There is nothing wrong with the motor.

John

RE: Single Phase electric motor problem (No load high current and heating)

Sold!


Thanks dArsonva.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Single Phase electric motor problem (No load high current and heating)

> I know there are higher math minds among the crowd that will dispute all the resistances, the turns in the winding, the enormity of it all.
> But, it's time to put this thread to bed. There is nothing wrong with the motor

Well if your goal is to try to belittle the other responders, then congratulations on a job well done.

No-one highlighted that the measured voltage was higher than nameplate, it's good you noticed that. I don't buy trusting a table more than the actual labelplate, but I can buy your argument that the motor is probably ok:
  • Small motor can have no-load current not too far below full load labelplate current to begin with, even at nominal voltage
  • Bump it up due to high voltage and saturation and no-load current might be higher than labelplate.
  • as you say the current measurement may not be completely accurate
Loading up the motor in the intended application certainly seems like the next logical step as many suggested before. There's nothing to lose, and probably it will operate fine.

=====================================
(2B)+(2B)' ?

RE: Single Phase electric motor problem (No load high current and heating)


Hi, dArsonval
Thank you for confirming my claims from 2 Mar 21 18:03.

RE: Single Phase electric motor problem (No load high current and heating)

I am having a lot of trouble understanding how the current of a motor may drop as the load increases.
I am aware of magnetizing current and real current or load dependent current.
Is there another current factor of which I am unaware.
Magnetizing current:

Quote (Induction Motor Magnetizing Inductance Modelling as a Function of Torque)

Abstract —The torque dependency of the magnetizing inductance
of an induction motor is studied. The magnetizing inductance
is calculated at different load conditions using instantaneous
rotor and stator currents as well as the air-gap
flux linkage obtained from the finite-element analysis. It is
shown that the magnetizing inductance of an induction motor
decreases as a function of torque. The calculation results are
compared with the measurements.
Link
As the magnetizing inductance drops under increased torque (load) so does the inductive reactance with an increase in magnetizing current.
Short version: Magnetizing current increases with increased load.
Similarly, the major part of the real current is proportional to the load.

We are all familiar with the effect of magnetizing current causing fairly high no-load current.
However, for no-load current to be greater than full load current, some component of the current must devrease under load.
Please explain what that component is and by what mechanism does it reduce under load?
I have shown explanations of the effects of both magnetizing current and of real current.
I am met with contradictions and unsupported statements.
Please explain by what theory or effect a motor current will drop under load.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Single Phase electric motor problem (No load high current and heating)

This is one of the most important disadvantages of single-phase motors.
At low loads, the power factor drops to very small values.
This drop is much larger than in the case of three-phase motors.
A low power factor causes a higher current to be drawn. This results in higher copper losses.

Visit us

RE: Single Phase electric motor problem (No load high current and heating)

You are confusing PU values with real values.
The real current drops considerably at low or no load.
The reactive current drops slightly at no or low load.
At full load, both real current and reactive currents are at maximum.
As the load decreases, the real current drops at a much greater rate than the reactive current.
As a result, the power factor drops, but the drop in power factor reflects a change in the ratio between the components of the current.

Quote:

A low power factor causes a higher current to be drawn. This results in higher copper losses.
A misleading statement.
The power factor is a result, not a cause.
The power factor is a result of the ratio between real current and reactive current.
The power factor is what it is and by itself does not cause anything.

As the load on a motor is reduced, the apparent current is reduced.
As the load is reduced, the real current drops faster than the reactive current.
As a result, the PF drops.
Result, not cause.

The reason for the low power factor of a single phase motor is related to the low losses of an unloaded single phase motor.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Single Phase electric motor problem (No load high current and heating)

(OP)
First, I really appreciate for all comments and even dArsonval comments even the way it was explained.
Well, As I tried to well understand most proposed ideas, I tried to add some loads on the motor and see the results.

The air gap between the router and magnets are rally small and less than 1 mm. I contacted the seller and he confirmed that he never did any machining on the motor and the reason he sold the motor was that he never used it.

there was a label on the motor case saying, it is a utility motor (somehow a general used motor) so it was not designed for a very specific application.
That's why I wanted to use it for custom table belt-disk sander machine. this weekend I added some loads (they I decided to use it - a V-Belt and my sander mechanism) the current drop to 6.9 A.
still way from the FLA but it drops a bit. I thought that the load is not high enough , so I tried to add more load manually, results was another 100 mA drop.
getting clos to FLA but it gets hot quicker than before (which sounds normal - More load more heat).
But at any cases, what I do not understand is why the NLA, is higher that the FLA. the motor is designed and rated for 6.4 FLA and normally it should not exceed its FLA.

dArsonval, I tried to verify the coils and they are really not dirty, they was some black spots on coils and I found that those are color. the case was sprayed with black color.

hope this help

RE: Single Phase electric motor problem (No load high current and heating)

> But at any cases, what I do not understand is why the NLA, is higher that the FLA. the motor is designed and rated for 6.4 FLA and normally it should not exceed its FLA.

When you say FLA, if you're comparing to labelplate, then you have a bunch of potential errors to consider (is nameplate wrong, is nameplate based on a different voltage as it was in your earlier data where you measured current when voltage was higher than nameplate, and is there an inaccuracy in your measurement).

> That's why I wanted to use it for custom table belt-disk sander machine. this weekend I added some loads (they I decided to use it - a V-Belt and my sander mechanism) the current drop to 6.9 A.
still way from the FLA but it drops a bit. I thought that the load is not high enough , so I tried to add more load manually, results was another 100 mA drop.
getting clos to FLA but it gets hot quicker than before (which sounds normal - More load more heat).
But at any cases, what I do not understand is why the NLA, is higher that the FLA

That's a stranger situation (current decreasing as load increases) and harder for me to understand. Voltage measurements at terminals might fill out part of the picture.

Here is a good EASA discussion of no-load current for THREE Phase motors. It doesn't show any mechanism for current to decrease as load increases.

Try analysing induction motor with equivalent circuit. Add a model of the power source as ideal voltage source with series R/L impedance before it gets to the motor (I know the actual power system could be a lot more complex but I think that captures relevant aspects for qualitative discussion). Make two assumptions: 1 - THREE PHASE MOTOR; 2 - LINEARITY. With these assumptions then the only thing that changes as you increase load is that s increases and R2/s decreases and since you are decreasing the impedance of one element within an R/L network than current must increase.

If indeed current is going the other direction (decreasing as load increases), then it would have to be explained by violating one of those two assumptions:
1 - non-linearities are playing an effect. As you increase load as Waross pointed out you may be decreasing terminal voltage and pushing magnetizing branch into saturation. Even if terminal voltage doesn't change, as similar load-dependent voltage drop appears across the stator leakage reactance (especially where it represents the endturns) and could have the same effect of pushing the magnetizing branch further into saturation.
2 - Single phase motor might act way different than 3-phase motor. It's stator produces not only forward rotating field but also reverse rotating field. As I recall there is a modification of the equivalent circuit to account for this. I'd have to go back to my textbooks to refresh my memory on that one *

* EDIT Here is single phase induction motor equivalent circuit.
Let's say as you load the motor, s increases from 0.001 to 0.05. Then the effect on R/(2s) (which decreases by factor of 0.05/0.001= 50) is going to be much more pronounced than the effect on R2/[2*(2-s)] (which increases by a factor of 1.999/1.95 ~1.025) which might make us think current will increase with load. IF these two resistances were DIRECTLY in series THEN it's certainly obvious that the small fractional change in the smaller resistance (the reverse resistance) would be insignificant compared to the large fractional change in the larger forward resistance. But as a complicating factor, these two resistances are not directly in series, we have the series reactances X2/2 and the parallel reactances Xm/2 in parallel with each. Since R2/[2*(2-s)] << R2/(2s), the R2/[2*(2-s)] may tend to play a bigger role in determining the impedance of the reverse rotor circuit than the R2/(2s) plays in determine the impedance of the forward rotor circuit, which could possibly tilt the conclusion in the opposite direction (toward decreasing current as load increases). As an extreme example if X2~0 and we imagined that R2/[2*(2-s)] << |Xm/2| << R2/(2s) then the change in resistive component of the forward circuit would tend to be irrelevant and the change in resistive component of the reverse circuit would dominate (resulting in decreasing current as load increases). I don't think things are that extreme but I'll think about it some more (I'm open to comment).



=====================================
(2B)+(2B)' ?

RE: Single Phase electric motor problem (No load high current and heating)

some data points for a few particular single phase motors. All of the following single phase motors show increasing current with load throughout the load range:
I'm pretty sure all testing for these Baldor motors was done at rated voltage with no significant change in terminal voltage as load changed.

=====================================
(2B)+(2B)' ?

RE: Single Phase electric motor problem (No load high current and heating)

There are a couple of measurements missing.

What is the voltage at the panel?
How does the motor terminal voltage change as the motor current changes?

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Single Phase electric motor problem (No load high current and heating)

AR.
Thanks for confirming my claim.
Just to add, in rare cases this happens also with some low-power three-phase motors (especially high pole motors) depending on their design.

RE: Single Phase electric motor problem (No load high current and heating)

I've been following this post for a while now and have no axes to grind.

I would point out to AR that FLA means Full Load Amps as far as I am aware, i.e. the amps, at 115V (not 123V) when the shaft output is 1/3 HP (250W). Nothing there about Amps at any other load and looking at this is it my simple mind here, but input energy is 736 W (115 x 6.4), but output is only 250W? I know there are things to do with Power factor etc, but this looks like s a really inefficient motor to me. Not too surprised it heats up - I assume the fan is working OK?

For things this small I would guess your variance on stated parameters is likely to be in the region of 10 to 15%.

So if it works, doesn't smoke or catch fire and you don't use it all day, then it looks Ok to me....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Single Phase electric motor problem (No load high current and heating)

Motor characteristics would suggest it's not possible for the current to rise as load is removed. Without an explanation, I won't buy that the current can drop as the load is added. No actual explanation has been given yet.

The tests didn't prove anything unless full power measurements were taken. Of course (sadly), it's already been accepted as being absolute proof of a rather dodgey theory.

And all this for a hobbyist asking questions about a motor, a post I REALLY should have red flagged.

RE: Single Phase electric motor problem (No load high current and heating)

LittleInch - Input power of a motor isn't volts x amps. The motor power factor at rated load can have a big effect on that.

RE: Single Phase electric motor problem (No load high current and heating)

(OP)
LionelHuz,
Just want to mention that it is not my hobby. I am an Electronic engineer, tend to change my field. I got involved in electric motors and if you had time to read all the tests that I have done you will find that it is for fun.
from the beginning, all I found was strange for me as I have never seen such results in electronic field. what is interesting is that will all experts here that trying to help me to well understand the situation and also gaining some experience (which a part of it comes from the one who works a lot with motors) still I could not find a complete and logic explanation for this issue.

that's true that I could not do all tests as requested as I do not have all the required equipment, but still I can learn from others (Waross and electricpete) since this might be the golden goal of having these sort of Forums.

I do apologize if it was time consuming for all and wasted your time. I will take the motor to a workshop to see if they can find something weird which explain the problem (or maybe no problem as some other experts said).

RE: Single Phase electric motor problem (No load high current and heating)

Check your voltage:
At the panel,
At the motor on open circuit.
At the motor at no load.
At the motor with some load.

As an example of voltage drop due to reactive current versus real current;
Assumptions for illustration:
Applied voltage at the panel = 125 Volts.
The magnetizing current is causing a 5 Volt drop in the feeder to the motor.
This is the voltage drop measured from the panel end to the motor end of the feeder.
This voltage drop is caused by a reactive current ans is 90 degrees out of phase with the applied voltage.
The voltage at the motor terminals will be √(125V2 minus 5V2) = 124.9 Volts.

An equal magnitude of real current will cause an in-phase voltage drop, and an in-phase voltage drop of 5 volts on a 125 Volt supply will result in a terminal voltage of 120 Volts.
This will drop the magnetic circuit below the saturation knee and drop the magnetizing current noticeably.

Note:
The series of voltage measurements that I asked for are comparative measurements.
We are more concerned with the changes in the voltages than in the actual voltages.
Even if your meter is inaccurate, the readings will provide information that may be valuable.

By the way, if the capacitor is connected in parallel instead of in series with the start winding, the results are unpredictable but that may cause excess current.
And;
Current dropping with load may be a sign that the voltage drop under load is lowering the saturation level and saturation based current.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close