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

[AR.]

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

 

[electricpete]

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)' ?

 

[waross]

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!

 

[itsmoked]

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

 

[waross]

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!

 

[che12345]

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)

 

[AR.]

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

 

[LittleInch]

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.

 

[waross]

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.

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!

 

[AR.]

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.

 

[waross]

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!

 

[AR.]

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

 

[electricpete]

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)' ?

 

[waross]

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!

 

[itsmoked]

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

 

[dArsonval]

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

 

[itsmoked]

Sold!


Thanks dArsonva.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[electricpete]

> 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:
[ul]
[li]Small motor can have no-load current not too far below full load labelplate current to begin with, even at nominal voltage[/li]
[li]Bump it up due to high voltage and saturation and no-load current might be higher than labelplate.[/li]
[li]as you say the current measurement may not be completely accurate[/li]
[/ul]

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)' ?

 

[zlatkodo]

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

 

[waross]

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:

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!

 

[zlatkodo]

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.

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