No load current

[maypot]

I recently replaced a 250 kW 400 V slip ring motor with a similar one same size, voltage , though of different make.The no-load current on the motor is 47 % of the full load current.
I am told that the no-load current on the previous motor was only 25 % of FLC and the loading current was less than 50 % of FLC.
The new motor has not yet been tested with load.
I am now a bit worried.

Bob

 

[Marke]

Hello maypot

The no load current of an induction motor is primarily the magnetising current of that motor. A high magnetising current will result in a high iron loss.

It is common for very small motors to have mag currents in the order of 40 - 60% and large motors to have a mag current of 20 - 30%.

You will find a range of magnetising currents for motors of a similar size and rating. You will find that the trend is for increasing size to be associated with reducing mag current. There is also a trend for increasing mag current with reducing motor speed. i.e. An 8 pole motor will have a higher magnetising current than a 2 pole motor.

An extra high magnetising current usually indicates that the motor is connected to a supply that has a higher than design voltage, or lower than design frequency.

I have found this commonly with motors designed for 440Volt 60Hz operating off 400volt 50Hz. I even worked on one motor that had a 50Hz nameplate but was wound for 60Hz.

Check your voltage and frequency against the nameplate ratings.

Run for a period of time under no load conditions and monitor the temperature rise. If the stator temperature rise is reasonable, the motor is probably OK.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[maypot]

Thanks Mark for your prompt and intelligent reply. The previous motor built in the early 1970 was designed star connected. The voltage and frequency are suitably rated, 400 V and 50 Hz.
The difference between the two configurations lie in the open circuit rotor voltage, the old one is rated @ 515 V , 300 A and the new one 638 V , 238 A. But I trust that it has nothing to do with the high magnetising current.
I understand the fact that the old motor, star connected, was dawing less current at no load. But the full load power absorbed by the two motors should not differ much.

Thanks

Bob

 

[aolalde]

No load current of motors depends on the manufacturer design criteria and no standard or regulations limit that parameter.
Identical power, voltage, frequency and speed motors made by different manufacturers regularly have totally different no load currents. However a well designed and constructed motor will work and perform properly into the “Nameplate Parameters”

 

[Marke]

Hello Maypot

I would not expect the open circuit secondary voltage to affect the amgnetising curent of the motor. It will however, alter the starting characterisitcs of the motor if the same secondary resistance starter is employed.
To match the start characteristics, you should alter the resistors.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[maypot]

Hello Marke,

I am running the 250 k W motor with a secondary starter. What parameters should I monitor to ascertain that the rotor is not overloaded at starting.

Thks.

Bob

 

[Marke]

Hello maypot

I do not believe that the problem will be overloading the rotor circuit, rather a change in starting characteristics.

The resistive steps are chosen to provide maximum torque points at intervals along the speed curve. The different rotor characteristics will offset those maximum torque points to different speeds. In many cases, you will see little if any difference. If the motor still accelerates to full speed with no problem, and the start current is acceptable, then I would say that all is well. The most critical stage is the final stage. If the maximum torque point is reduced too low in the final resistve stage, there will be a large current step in the stator circuit when that final stage is shorted out.

Best regards,

Mark Empson

http://www.lmphotonics.com