3 PHASE AC INDUCTION MOTORS & CURRENTS DRAWN

[cginamdar]

please help me for -

i have an induction motor rated for 3 phase / 415 v /50 hz AC supply.
if i connect it to 3 phase / 380VAC/ 60 HZ SUPPLY at rated load, then what will be the current drawn by the motor.

regards
[sig][/sig]

 

[jbartos]

Approximately:
I2=(415/380)xI1 in amps, where
I1=(746 x HP)/(sqrt3 x 415Volts x Power Factor x Efficiency)
The missing data should be on the motor nameplate or within the catalog cut. [sig][/sig]

 

[jbartos]

It is tacitly assumed that the efficiency is related to 60HZ and 415V. The approximation for I2 is very approximate. The lower frequency is, the lower losses are (Eddy Currents, and Steinmetz Law (Fink D.G. and Beaty W.H. "Standard Handbook for Electrical Engineers," 14th Ed., McGraw-Hill Book Co., pp 2-46 and 2-47).

 

[cginamdar]

dear mr. jbartos,
thanks a lot,
i am more interested in combined effect of increased frequency and reduced supply voltage.
can you help further
regards
c.g.inamdar

 

[jbartos]

The increased frequency will result in slightly increased losses due to Eddy Currents and Steinmetz Law covering remagnetization losses. The increased current will increase WattLosses=R x I**2 that increase the motor heat source. Therefore, one might possibly prefer to derate the shaft load or verify that the motor shaft load is below the rated shaft load.

 

[CP]

additionally to what jbartos explained, you can also look into other derating factors like your cabling method (grouping) and ambient temperature of your installation. refer to the BS7679 table 4. also take into your cable sizing the motor starting method. as the typically calculation on the current drawn relates to your worse case amplitude.

 

[peterb]

It seems that the motor may not develop full nameplate output or rated torque with this configuration. The magnetic field strength will be lower, due to the decreased Volts/Hz ratio involved -
415V/50 Hz = 8.3 V/Hz
380V/60 Hz = 6.33 V/Hz

If the motor were connected to a 480V, 60 Hz supply, the result would be 8.0 V/Hz, which would provide closer to the rated value.
Also note that the cooling fan loss will be higher at the higher shaft speed.

 

[jbartos]

Suggestion: Notice that if the motor has a constant shaft HP and the motor terminal voltage decreases the motor input current must increase.