Variation In No Load Amperage Of Similar Rated 3- Phase Motors - Appliances - DIY Chatroom Home Improvement Forum

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11-11-2011, 08:23 PM   #1
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## Variation in no load amperage of similar rated 3- phase motors

The status of the 3 similar rated 3-phase motors [16 KW, 27 amps, 440V] is as follows:
Motor A] No-load = 14 amps On-Load = 37 amps

Motor B] No-load = 8 amps On-Load = 40 amps
Motor C] No-load = 2 amps On-Load = 40 amps

We are at a loss to comprehend the variation in no-load amperage of 3 similar rated motors.

11-11-2011, 09:23 PM   #2
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these motors are used for what purpose? what is the load?

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 The Following User Says Thank You to hardwareman For This Useful Post: valentine (11-13-2011)
 11-12-2011, 06:39 PM #3 Member   Join Date: Aug 2011 Posts: 729 Rewards Points: 500 Are these motors the same horsepower? Similar ratings doenst mean they are the same motor. They could have different service factors or rpm's. Did you wire all of the windings the same way?
 The Following User Says Thank You to Master of Cold For This Useful Post: valentine (11-13-2011)
11-12-2011, 08:21 PM   #4
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Quote:
 Originally Posted by valentine We are at a loss to comprehend the variation in no-load amperage of 3 similar rated motors.
The no-load current may not be that important to the end user so it is not controlled by the manufacturer. Also, at no load, perhaps a lot of parasitic effects dominate the motor performance that are not noticed at full load.

But what troubles me is that 14A vs. 37A implies that this motor is 60% efficient which is very low for a motor this large.

Last edited by Yoyizit; 11-13-2011 at 10:17 AM.

 The Following User Says Thank You to Yoyizit For This Useful Post: valentine (11-13-2011)
 11-13-2011, 01:40 PM #5 Newbie   Join Date: Nov 2011 Posts: 9 Rewards Points: 10 These motors are used for driving an AC blower via pulley & v-belt transmission.
 11-13-2011, 01:42 PM #6 Newbie   Join Date: Nov 2011 Posts: 9 Rewards Points: 10 We do not have the HP of these motors. These motors are old & have been in service for a long time. They could have been rewound during their life till date.
 11-13-2011, 01:46 PM #7 Newbie   Join Date: Nov 2011 Posts: 9 Rewards Points: 10 Star connection is used. However delta connection was also tried out. With delta connection - the load current increased by 3 amps when the motor was in operation.
11-13-2011, 01:50 PM   #8
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Quote:
 Originally Posted by Yoyizit The no-load current may not be that important to the end user so it is not controlled by the manufacturer. Also, at no load, perhaps a lot of parasitic effects dominate the motor performance that are not noticed at full load. But what troubles me is that 14A vs. 37A implies that this motor is 60% efficient which is very low for a motor this large.
Is the Thanks your response. Kindly enlighten if the motor drawing the lowest amperage on no-load condition is the healthiest motor among the 3 motors?? How do we determine which is the best motor to use in this case??

 11-13-2011, 01:53 PM #9 Member   Join Date: Aug 2011 Posts: 729 Rewards Points: 500 Is there a y-delta starter ?
11-13-2011, 01:57 PM   #10
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Quote:
 Originally Posted by valentine the motor drawing the lowest amperage on no-load condition is the healthiest motor among the 3 motors?? which is the best motor to use in this case??
Almost certainly it is the most efficient, if not the healthiest.
Motor health equals insulation health plus other factors and insul. health can be checked with a Megger.
I guess I'd also listen to the bearing noise with a stethoscope as you spin the shaft by hand.

If you can define 'best' I can try to steer you to an optimal choice. It depends on your 'utility function.'
If I'm paying the electric bill for a motor that sometimes runs with no load then I have a high utility [=it is very useful to me] for an efficient motor.

A 100% efficient motor pulling 16 kw = 21 hp so the motor rating is less than 21 hp.

Last edited by Yoyizit; 11-13-2011 at 02:01 PM.

11-13-2011, 02:10 PM   #11
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Quote:
 Originally Posted by valentine The status of the 3 similar rated 3-phase motors [16 KW, 27 amps, 440V] is as follows: Motor A] No-load = 14 amps On-Load = 37 amps Motor B] No-load = 8 amps On-Load = 40 amps Motor C] No-load = 2 amps On-Load = 40 amps We are at a loss to comprehend the variation in no-load amperage of 3 similar rated motors.
Is the information you are quoting "27 amps" the FLA from the plate on the motors?.

If so your Loaded application is far to large for the motor, a motor with a plated FLA of 27 amps is a 20 HP and your current drawing is suggesting you need to either reduce the load or increase the HP of the motor to 30.

Measuring NL amps of a motor is not really going to tell you much. You are also stating 440 Volts, I didn't think that was a nominal voltage these days, I guess it depends where your are. What is the voltage of the circuit feeding these motors? if its 480 the amperage should be lower not higher.

Mark
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11-13-2011, 02:28 PM   #12
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Quote:
 Originally Posted by Yoyizit Almost certainly it is the most efficient, if not the healthiest. Motor health equals insulation health plus other factors and insul. health can be checked with a Megger. I guess I'd also listen to the bearing noise with a stethoscope as you spin the shaft by hand. If you can define 'best' I can try to steer you to an optimal choice. It depends on your 'utility function.' If I'm paying the electric bill for a motor that sometimes runs with no load then I have a high utility [=it is very useful to me] for an efficient motor. A 100% efficient motor pulling 16 kw = 21 hp so the motor rating is less than 21 hp.
.

Many thanks your response. This motor is used to drive the AC blower of a sea going vessel's accomodation. The vessel is equipped with 3 independant generators which provide the power source. Hence we are not so concerned on the electric bill as fuel for running the generators is on our charterers account.

Best - in terms of reliabilty over a longer period of running as this motor is run 24 x 7 for 365 days in a year. Appreciate your advice - whether the motor drawing 8 Amp or 2 Amp in no-load stage be used. Thanks once again.

11-13-2011, 02:42 PM   #13
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Quote:
 Originally Posted by Jackofall1 Is the information you are quoting "27 amps" the FLA from the plate on the motors?. If so your Loaded application is far to large for the motor, a motor with a plated FLA of 27 amps is a 20 HP and your current drawing is suggesting you need to either reduce the load or increase the HP of the motor to 30. Measuring NL amps of a motor is not really going to tell you much. You are also stating 440 Volts, I didn't think that was a nominal voltage these days, I guess it depends where your are. What is the voltage of the circuit feeding these motors? if its 480 the amperage should be lower not higher. Mark

Thanks Mark. appreciate your inputs. We suspect some fault on the blower unit & have checked same. However same appears to be very free & bearings ok & temperature of bearings ok during running. as motors have been reconditioned - we are trying to figure out if the motor is the problem or the blower is the problem. Until we get a new motor - we are unable to determine this. We are planning to change the bearings of the blower as well. appreciate if u could revert as to which would be the best motor to use in this case if we change the bearings of the blower first - the motor drawing 2 amp or the 8 amp at no-load stage. This motor drives the ac blower of a accomoadtion ac unit of a sea going vessel & runs 24 x 7 for 365 days. Kindly revert with your inputs. Also kindly rvert as to what would be the ideal insulation values??
thanks / valentine

 11-13-2011, 02:43 PM #14 Newbie   Join Date: Nov 2011 Posts: 9 Rewards Points: 10 There is a starter onboard - Will have to check the type & revert. thanks
11-13-2011, 03:11 PM   #15
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Quote:
 Originally Posted by valentine Thanks Mark. appreciate your inputs. We suspect some fault on the blower unit & have checked same. However same appears to be very free & bearings ok & temperature of bearings ok during running. as motors have been reconditioned - we are trying to figure out if the motor is the problem or the blower is the problem. Until we get a new motor - we are unable to determine this. We are planning to change the bearings of the blower as well. appreciate if u could revert as to which would be the best motor to use in this case if we change the bearings of the blower first - the motor drawing 2 amp or the 8 amp at no-load stage. This motor drives the ac blower of a accomoadtion ac unit of a sea going vessel & runs 24 x 7 for 365 days. Kindly revert with your inputs. Also kindly rvert as to what would be the ideal insulation values?? thanks / valentine
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

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