Typical Motor No-Load Currents

[Downstream]

Can someone tell me what a typical amount, percent of nameplate, that a 3 phase 480VAC, induction(Squirrel cage) motor might be?

I have a 20hp motor that when disconnected from the load, draws about 20amps just running. The amps are measured at the leads to the motor with a clip-on ammeter. The amps were measured about 20 seconds after starting. The voltages were normal and held steady. The shaft is not bound and the rotor slowly spins down after the power is shut off. The motor does not overheat. We checked the no-load current because we felt the full-load was close to the nameplate. We have four of these motors all acting the same. They were rewound about 5 years ago.

This seems like an awful lot of amps for speed no-load.

The motor draws about 28amps when connected to the load.

 

[electricpete]

That seems high to me.

What is the speed? Low speed motors draw more no-load current. We have a 22-pole motor and no-load current is 60% of FLA.

Did you check all three phase currents? If different that might suggest a voltage unbalance (most likely) or motor problem (less likely).

Did you check your voltage. For unloaded motor, high voltage may result in high magnetizing current (with more harmonics).

Double check the connection.

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[jbartos]

Comment: NFPA 70 NEC Table 430-150 shows 27A for 20HP, 460V, 3ph squirrel-cage induction motor. The measured 20A would be about (20/27) x 100%= 74%, which is very high current. Some catalogs indicate no load current of motors, e.g. Baldor Motors and Drives Catalog "AC Motor Data Catalog 502" dated Appril 1992, shows several 20HP motors Full Load Amps (FLA) and No Load Amps (NLA)
RPM NLA FLA
1170 9.4 26
1760 12 26
3525 5.2 19
etc.
There may be something wrong with the measuring method or meter since it is highly unlikely that four motors would be defective.

 

[edison123]

Possible reasons for high no-load current:

1. During stator rewind, no. of effective turns per phase were reduced. (acutal reduction of no. of turns or lower pitch factor etc.)

2. Rotor OD was skimmed.

3. Stator core damage (during rewind or due to earlier failure).

4. Rotor core not aligned axially with stator core.

5. High stator voltage with low frequency.

6. Mill duty motors where starting torque is high and hence high flux densities are used resulting in high no-load currents.

7. Star connection changed to delta connection without correspnding change in the winding turns.

 

[jbartos]

Comment on the previous posting: Some of those listed causes are less likely to apply since the motor does not overheat, voltages were normal and held steady, etc.

 

[edison123]

jb,

I mentioned "possible" reasons not positive reasons.

btw, what has high no-load current got anything to do with motor overheat ? (unless the core was damaged).

And, why can't all the 4 four rewound motors have high no-load amps if the repair shop just repeated the same windings (and the same mistakes due to ignorance / lack of feedback etc.)

downstream

Besides all that, a high no-load amp is not going to destroy your motors. (It didn't in the past 5 years). If you want to maintain a good pf, you may have to match your pf correction caps (may be 15 KVAR per motor) for the new no-load current.

 

[aolalde]

20 Amperes with the shaft free is too much for 20 HP at 480 Volts even for 12 or 14 poles motor. Is this a special design? Post the complete nameplate data.

That high current is showing that the motor is defective.

Further analysis has to be done to find the cause of such a high current. If the motor (motors) are left in the present condition a failure will develop in short time.

Send at least one of them to a qualified motor repair shop for test and evaluation. A dynamometer test could be necessary.

 

[Marke]

Hello Downstream

I would expect o motor of that rating to operate with an open shaft current of about 30 - 40% of its rated full load current. Submersible pumps will be more in the 40 - 60% group. If the motor was operating open shaft at that current, I would expect it to run very hot. This is usually indicative of excess flux in the iron, commonly because of a high terminal voltage.

One thing to be sure of is that your meter is not being affected by the field from the motor. Try measuring the current well away from the motor, i.e. at the starter and chech that the reading is the same.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jbartos]

Comment on edison123 (Electrical) Mar 17, 2004 marked ///\\jb,
I mentioned "possible" reasons not positive reasons.
///I mentioned ""Some of those listed causes are less likely to apply since ..."" So what is the big deal?\\
btw, what has high no-load current got anything to do with motor overheat ? (unless the core was damaged).
///See excellent reasoning in Marke's posting regarding overheat related to high motor terminal voltage.\\And, why can't all the 4 four rewound motors have high no-load amps if the repair shop just repeated the same windings (and the same mistakes due to ignorance / lack of feedback etc.)
///Yes, this can happen. That is why I stated ""Some of those listed causes are less likely to apply since ..."".\\downstream
Besides all that, a high no-load amp is not going to destroy your motors. (It didn't in the past 5 years).
///Not, if it is not caused by the motor terminal overvoltage. I saw motors connected to improperly high voltages and they became damaged with a plenty of smoke coming from them.\\ If you want to maintain a good pf, you may have to match your pf correction caps (may be 15 KVAR per motor) for the new no-load current.
///The capacitors have to be selected carefully to avoid excessive oscillations.\\\