Motor-test without rotor 8

[zlatkodo]

Some rewinders make the test of three-phase motors without rotor with reduced voltage on the stator.
I would like to know a little bit more about this test: what is the purpose of this test, what voltage is applied, how to interpret the results of measurements etc (here I am not talking about the polarity-test).
Zlatkodo

 

[waross]

The old school test was to energize the stator and throw in a bearing ball. If the stator is connected properly the ball will spin around inside the stator. If a stator coil is reverse connected it will throw the ball out. Is this the test you mean?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

Ball bearings would be the polarity test.

IEEE 112 mentions this removed-rotor test to determine stray losses.

5.7.2.1 Stray-load loss at fundamental frequency
The stray-load loss occurring at fundamental frequency is determined by applying balanced polyphase
voltage to the stator-winding terminals with the rotor removed. The electrical input minus the stator I2R loss
at test temperature is equal to the fundamental frequency stray-load loss

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(2B)+(2B)' ?

 

[electricpete]

I also remember, with full voltage applied and no rotor, the current is around full load current. I vaguely recall someone mentioning that when they design a brand new winding, they measure no-rotor current as a double check of their design. I don't really remember exactly what they were checking, but trying to recreate the thought process... maybe it would be stator leakage reactance.

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(2B)+(2B)' ?

 

[starkopete]

waross, I'm old school also and I still require a ball test or equivalent (we have some mini/false rotors that serve the same function) on all of our rewinds.

epete, I will have the guys in the shop do a full voltage/no rotor test in the next day or two on some rewinds. If you are correct, I'll probably add it to our spec for all rewinds. I'm a big fan of full voltage/full current tests. I'll let you know what we find.

Thanks

 

[YorkMech1]

Starkopete;
When you run your next ball-bearing/false rotor test on a stator, could you please explain to this lowly scribe why in the world the ball bearing goes in the "Opposite" direction of the false rotor. While the false rotor will always give the true rotation of an installed rotor, the lowly ball bearing always goes in the "Opposite" the direction of an installed rotor.
This has bugged me for many a year..

 

[jraef]

When you have a rotor, the rotating magnetic field in the stator induces a voltage onto the cage of the rotor, which then produces its own counter rotating magnetic field. It is the interaction of those fields that becomes the rotation of the rotor. When you use a false rotor or a ball bearing, you are only seeing the stator field rotation.

Side note on the ball bearing test if you've never done it:
Make sure the stator is level when you do this! If there is a slight pitch to it, the bearing spins around and works it's way to the end, then comes flying out of there like a bullet in some random direction.

Murphy's law states it always finds the fluorescent lamp that is most difficult to replace as well...!

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[YorkMech1]

Thank you jraef for that information..For over 40 years this has bothered me. While I seldom ever post on this site, I do however read each & every post here with interest.

 

[starkopete]

I have some results:

50 HP / 6 pole / 460 volt / 62.3 FLA, No rotor test: 62 amps @ 62.3 volts

400 HP / 6 pole / 460 volt / 456 FLA, No rotor test: 456 amps @ 99 volts

2000 HP / 16 pole / 4000 volt / 282 FLA, No rotor test: 282 amps @ 1200 volts

Zlatkodo, after thinking about this I remember that I had a customer who required this test. His primary concern was current balance.

 

[zlatkodo]

Hi , Starkopete,
I think you are right. It seems to me that the main purpose of this test is to check the balance of currents (perhaps also the phase-coil resistance). It is hard to imagine that this test can provide an estimate of no-load current (with rotor) for example.
Zlatkodo

 

[electricpete]

What I was suggesting above is it provides data the the OEM winding designer can use to validate his calculations. It is not a comparison against a fixed number like FLA or NLA.... it is a comparison against his model.

As a customer, I can think of no reason whatsoever that a customer would prefer to test current balance before assembly compared to after assembly. It is the motor shop and OEM that has an interest in finding problems at an earlier stage of assembly to avoid having to repeat assembly steps.

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(2B)+(2B)' ?

 

[starkopete]

electripete, I couldn't agree with you more, but as the saying goes - the customer is always right.

 

[electricpete]

Yes. I guess customers do ask for some crazy things sometimes.

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(2B)+(2B)' ?

 

[zlatkodo]

Starkopete,
By the way, obviously you have an impressive testing equipment. Can you give us a brief technical description (maybe pictures too)? It is interesting to many who do not have such opportunities.
Zlatkodo

 

[wolf39]

Zlatko:

Current balance seems to me the most important reason for such tests on motor stators because proper sense of rotation can easily be achieved by switching two phase connections.

I'm familiar with this test when carried out on large hydro generator stators.

An adjustable three-phase voltage source of rated frequency is connected to the stator terminals. Then the voltage is raised until rated current is obtained. The respective voltage divided by the generator rated voltage is equivalent to the per unit (p.u.) value of the so-called sum of stator leakage plus bore reactance. The stator bore leakage reactance can be calculated with great accuracy and after deduction of this figure from the measured value you get the stator leakage reactance x1sigma. This x1sigma is a very important and common reactance for the determination of synchronous reactances, transient reactances and subtransient reactances.

Wolf

http://www.hydropower-consult.com

 

[starkopete]

Zlatkodo,
I’m attaching a couple of brochures. Our unit is several times larger than the one depicted in the brochure. Our previous test set used SCR’s and produced extremely distorted power. When we decided to upgrade we opted for this particular design because with the tapped transformers we get much cleaner power. The downside is that it requires quite a bit of maintenance.

Thanks

 

[starkopete]

and:

 

[rbulsara]

Interesting! Learning never stops.

Rafiq Bulsara

http://www.srengineersct.com

 

[rhatcher]

First, do not apply full voltage to a stator with no rotor installed. This will result in overcurrent and stator winding failure.

Wolf39's 21 Apr 11 13:19 post is, in my opinion, the closest to answering the question at hand. By 'closest correct answer', I am not implying that what he says is wrong. My opinion is quite the opposite. Wolf39's response contains detailed information that is absolutety (star quality) correct. However, zlatkodo is looking for an similar answer that is on a different level.

An adjustable three-phase voltage source of rated frequency is connected to the stator terminals. Then the voltage is raised until rated current is obtained. The respective voltage divided by the generator rated voltage is equivalent to the per unit (p.u.) value of the so-called sum of stator leakage plus bore reactance.

In a motor rewind shop this is called an 'open stator impedance test.' The reason that I say that wolf39's answer is on a different level is that in a motor rewind shop you are not normally concerned with design aspects of the winding such as 'synchronous reactances, transient reactances and subtransient reactances.'

The primary concern in a motor rewind shop is to successfully copy the existing winding or to make a redesign based on the existing winding.

In this case, the 'open circuit stator impedance test' is a test that is used to evaluate whether the winding design is valid. It is not an 'absolute' test with a definite answer. Instead, it is a general test that is similar to a 'no load run test.'

Specifically, as wolf39 describes, the variable source of three phase voltage is applied to the stator and the voltage is increased until the stator current is equal to nameplate current.

The ratio of applied voltage to nameplate voltage (applied volts/nameplate volts) is then calculated. The result is a comparison of the apparent impedance of an 'open stator' with the apparent impedance of a fully loaded motor.

The correct answer is a ratio in the range of 15-30%, much like a the way that a no load test run results in a 'no load amps/full load amps' ratio of similar values. Like a no load test run, a ratio in the desired range is not a guarantee of a proper winding but, it is one check among many that the winding is correct. Also, like a 'no load test run,' a ratio outside of the 15-30% range is not an indication of a guaranteed fault but it is an indication that you should take another look at the winding to make sure it is correct.

This test is performed after the winding is fully connected and tied down but before the winding is varnish treated. This is an 'in process' test that is normally performed on a rewind but is not normally performed on an otherwise good winding that is being reconditioned (ie. varnish treated alone) unless a problem is indicated.

Other checks that are performed at the same time are the 'dummy rotor' test and the open stator amp balance check that others have mentioned. All three of these tests are equally important in determining the winding condition before proceeding with varnish treatment and motor assembly. The idea is that this combination of tests will give an indication of a problem before further work is performed.

The test sequence for a new winding would be: meggar test, high potential test, winding impedance test (bridge test, not open stator test), surge test, open stator impedance test, open stator amp balance test, and dummy rotor test. If these tests all give good indications then you would proceed with varnish treating of the winding. More tests follow the varnish treatment and then more tests are performed after motor assembly.

 

[Motorwinder]

We call it a phase balance test. We use it on redesigns and wound rotor rewinds.

Basically we apply 90 volts for a 460 volt machine, and 30 volts for a 230 volt motor. Your amps should be close to FLA. If not, you made a mistake on your calculations.

On rotors I use it to confirm my connection. I once wound a 1250hp rotor, and was one bar off on my connection. It surge tested good, but on the phase balance test it wouldn't spin the test rotor.