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Old 11-13-2011, 04:09 PM   #16
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Variation in no load amperage of similar rated 3- phase motors


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Is there a y-delta starter ?
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Originally Posted by Jackofall1 View Post
I have to ask, when you serviced the fan you took the sheaves off both the fan and the motor, are you certain that you put them back on correctly. If you didn't this would explain the high current draw under load, you have sped the fan up resulting in moving more air, just an idea.

As for testing the motors, measure the resistance of the field windings, each set of windings should be close to each other and have low ohms readings. I would select the motor with the most consistant low readings field windings to field windings.

An excerp from a motor testing article http://www.gainesvilleindustrialelectric.blogspot.com/

"To test the windings, use an ohmeter. Make sure any jumpers for voltage selection remain connected. For a nine lead, dual voltage motor connected for 480 volts, these would typically be the 4-7, 5-8, and 6-9 connections. Measure the resistance from 1-2, 2-3, and 3-1. The resistance readings should be between 1-2% of each other - in other words, BALANCED! There is no way to tell how much resistance you should get, but it is typically low. Realize that you are testing with a DC battery. Therefore, you are reading the resistance of the copper wire. In many cases, this will look like a short circuit. GOOD! It's when it doesn't look like a short circuit that you have problems. As long as the readings are low and balanced, you're ready to go to the next step. I can't tell you how many motors are brought to us because "the windings are shorted." Don't make that mistake."

Unless you have a megger you will not be able to test the insulation quality of the motors.

Mark

Thanks your response Mark. We did not touch the blower at first. The initial motor failure was on low insulation. Then we fitted a spare motor with 2 amps no-load amperage & noted that it was drawing 40 amps & overheating / tripping. later we fitted another reconditioned motor of 2 amps no-load amperage & again same drew 37 amps on load & getting heated. We then checked the blower & connecting v-belt & found all in order. hence we suspect motors primarily or it might be the blower as well when running at the high rpm, although no tell tale evidence of same. We have the ohmeter & megger & will check as suggested.

Also the motor that was showing low rinsulation was sent to an electrical workshop for repairs & after coming back, same was checked & found to be drawing 14 amps on no load. Could u suggest some light on this high no-load current. thanks your assistance. Brgds / valentine.

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Old 11-13-2011, 04:22 PM   #17
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Variation in no load amperage of similar rated 3- phase motors


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Originally Posted by valentine View Post
Thanks your response Mark. We did not touch the blower at first. The initial motor failure was on low insulation. Then we fitted a spare motor with 2 amps no-load amperage & noted that it was drawing 40 amps & overheating / tripping. later we fitted another reconditioned motor of 2 amps no-load amperage & again same drew 37 amps on load & getting heated. We then checked the blower & connecting v-belt & found all in order. hence we suspect motors primarily or it might be the blower as well when running at the high rpm, although no tell tale evidence of same. We have the ohmeter & megger & will check as suggested.

Also the motor that was showing low rinsulation was sent to an electrical workshop for repairs & after coming back, same was checked & found to be drawing 14 amps on no load. Could u suggest some light on this high no-load current. thanks your assistance. Brgds / valentine.
A new 20 hp 3 phase 480 volt motor should pull about 10 amps, so 14 amps no load is close.

http://www.baldor.com/products/perfd...ing=40CMB-CONT

Hook it up, if its still pulling high amps (37 amps) your load is too great for the motor.

Mark

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