possible performance change due to stator rewind? 5

[electricpete]

It seems to me that a standard assumption during rewind of a stator is that none of the motor characteristics will be changed: Torque speed characteristics, LRC, FLA, ability to safely start its load at 80% voltage

If anything, sometimes load test is requested performed to verify thermal performance and FLA. But nothing to verify those other parameters.

We are rewinding a nuclear safety-related motor and a reviewer of my rewind specification questioned why I did not include any testing or calculation to verify these performance parameters (I stated they had to remain the same as original, with no request for documentation).

So what do you think? Is there a possibility that a rewind can change torque characteristics, current characteristics, or starting performance? Or is the assumption of no changes justified?

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[aolalde]

Hi Electricpete.

A rewinding will change the motor performance if any winding parameter is changed. Some parameters like number of turns per coil, coil span or connection will result in big changes. Some others like copper cross section area or coil mean length will affect only the efficiency and operating temperature.

Normally a report of the winding parameters as found and the winding parameters as rewound is part of the quality assurance process. Winding resistances and No-load test parameters will back-up duplicate condition.

 

[TheBlacksmith]

In the stated application "nuclear safety-related motor", sometimes assume is not good enough. Testing also verifies quality of rewind for a critical application.

Blacksmith

 

[motorspert]

aolalde is correct, generally the rewind company will use same number of turns, coil pitch etc and for HV motors check if the wedge material is 'magnetic'. Thus the fundamental performance will be unchanged. A no-load test will show the magnetic current and losses are the same as before the failure and should suffice. However, the amount of insulation/copper size and coil geometry could be changed and may have a minor effect on the motor performance.

Tests for motor starting current/torque are generally subject to extrapolation and interpolation and the relevenace if the test should be established before specifying it.

First, identify which parameters are important and then perform a risk assessment to see how much they could vary and if that affects the safety of the installation.

I would recommend always specifying a no-load test

 

[UKpete]

Pete, in the case of a major repair such as a full rewind, you could base your acceptance spec on the level of testing (and the associated acceptance criteria) the motor receives when new.

In your type of high-safety environment, after rewind there may be an argument to specify at least the same level of testing as the works-test when new.

I say at least, because:
1. judged against the OEM factory the repair shop is probably less likely to give out consistent work and also risks causing uncorrected damage or other deterioration during strip-down (e.g. due to excessive burn-out temperature)
2. because the level of OEM works tests can be a bit lean (e.g. most new motors never receive a load test).

 

[raisinbran]

Electricpete
On of the "variables" when a motor is rewound is the condition of the stator iron. If the older insulation is not baked out properly, or the iron was damaged due to a winding failure, the iron may be questionable and may be re-used. Although the iron can be tested, it is not always black and white, and you still can have a "gray" area. This would affect some of the parameters that you spoke of - LRA, temperaure rise, efficiency, etc.

Another is the conditon of the rotor bars - are they tested thoroughly for cracks, breaks, etc. This would affect any of the torque parameters.

Depending on the size of the motor, we (mining industry) try to run a full load test on the motor to determine FLA, full load torque, and full load speed. We also try to run a locked rotor test at reduced voltage (usually half voltage).
Good luck,
Raisinbran

 

[aolalde]

To take care of the "stator core stack" loop magnetic tests are run at 85 kL/sqin flux density in the back iron.
This test will define the watts/Lb loss and short circuited laminations as hot temperature spots. Thermo graphic pictures will show the temperature map of the core.
These tests are run at least before the stator winding is stripped and then after the winding is removed and the stator core is ready for winding assembly.

 

[petronila]

Hello Electricpete,

I am agree with aolalde´s explanation, This pages contains additional information about the Ansi/Easa AR 100-2001 and Guidelines for maintainning motor efficiency during rebuilding:

http://www.easa.com/indus/ar100-2001

http://www.easa.com/indus/eemtr_rpr999

Regards

Petronila

 

[petronila]

Hello all,

For an Induction Motor,the Torque is proportional to Current times Flux.When you perform changes in the winding This parameter change.
The Back emf is in opposition to the applied voltage in each phase of the motor and is given by: E= 4,44Fn?CfDf where E is back emf, F=frecuency, n= Turns in serie per phase, ?= Flux per Pole,Cf= Chord Factor and Df= Distribution factor. For Rewound propuoses ( Not for redesign), E and F remains constant and Df depends of Pole,Phase and Slot numbers and remains constant too unless the phase changes.So the rewinder have 3 diferent variables to control: n, ? and Cf and the product of this 3 variables most remain constant in order to satisfy the above Back emf per phase. Due to the flux per pole decrease or incresa if you increase or decrease the other two variables or remains unchanged if this two variables changed in inverse proportion.

To maintain the motor performance (Eficiency and Torque) is necessary to control the flux per pole and it most remain unchanged. Another important factors to control are: Mechanicall Stress (When you replace the core Iron),stator or Rotor core damage, ELECTROMAGNETIC CHANGES,thermal damage to the interlamination insulation. (PERFORMING CORE TEST and not overheating the cores above 350 C ).

Regards
PETRONILA

 

[motorspert]

The effect of the stator burn out process on the stator iron is an interesting one. On a 7MW stator core we measured the losses before and after burn out (400C)and the losses were lower after the burnout. Not quite what we expected, we think thet the heating process stress relieved the cut edges of the laminations reducing the localised losses.

I agree that testing a motor fully after repair can save time and heartcahe at site.