Electric Motor Torque 2

[lanier]

Taking a sample of 10 electric motors (2Hp, 208/230V, 3600rpm, single phase, capacitor start)and coupling them to a pump we tried to find the Pull/Up Torque of each motor. The motors were designed to have a minimum Pull/Up Torque of 90 oz-ft. All of the motors are from the same batch off the production line ( i.e same date codes). Eight of the motors produced approx. 90 oz-ft of torque +/- 2. Two of the motors, however, developed 43 oz-ft of torque. All of the motors were tested under the same conditions and with the same pump. Initial analysis has not indicated any differences between the motors. Any ideas on what it could be?

 

[busbar]

You've probably checked this already, but is there any possibility of undersized/defective start capacitors in the two weak units?

 

[lanier]

busbar,
i have already checked the capacitors and they checked out fine. i have also checked the resistances of the start windings as well and took measurements of the rotor and bearings. everything appears to check out.

 

[electricpete]

Well it sounds like you have looked at the likely things so we have to start thinking about far-out possibilities.

Is the condition reproduceable? ie the same two motors always test lower than the others. If so then of course you'll look closely at those motors. It sounds like you have looked at stator pretty closely. Maybe a close look at stator connections. Rotor might also cause those symptoms.

If the condition is not reproduceable then of course you look at test conditions:

Next time you run the test you might want to measure voltage directly at the motor terminals to make sure there isn't something in the supply system creating a problem. Intermittent open circuit or loose connection, contactor problem etc.

Are you sure you can trust your torque measurement equipment. Is there a redundant indication avialable?

 

[jbartos]

Suggestion: Check the whole circuit, including the switch.

 

[Marke]

Could be a rotor problem. These will be pressure diecast aluminium rotors and it is possible that some bars are not properly formed. Mark Empson

http://www.lmphotonics.com

 

[UKpete]

Is it possible to swap rotors between a good and bad motor to see if the problem follows the rotor?

 

[THooper]

If your local motor repair facility has a small dynamometer, have the motors load tested. This data, along with electrical data taken measuring such parameters as current, voltage, power (watts) frequency, slip, winding resistance, ambient temperature and motor temperature rise can be used to determine motor performance characteristics under various load conditions to verify the integrity of these motors.

 

[jbartos]

Suggestion: A set of tests mentioned in the previous posting could even reveal any rotor bar noticeable irregularity. Power quality analysis may be added.

 

[RUSO]

My experience has been that the bearing system is some what out of alignment. We use a low voltage start criteria on our shaded pole motors to make sure that all is free in the motor.

 

[3b2]

Seems like a QA problem with the motor. Check the motor start current. Verify the capacitor mfd.

3b2

 

[CB2]

Check the centrifugal switch in the capacitor start circuit, sound like it is not engaging if the capacitors are good.