×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

3 PHASE AC INDUCTION MOTORS & CURRENTS DRAWN

3 PHASE AC INDUCTION MOTORS & CURRENTS DRAWN

3 PHASE AC INDUCTION MOTORS & CURRENTS DRAWN

(OP)
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

RE: 3 PHASE AC INDUCTION MOTORS & CURRENTS DRAWN

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.

RE: 3 PHASE AC INDUCTION MOTORS & CURRENTS DRAWN

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).

RE: 3 PHASE AC INDUCTION MOTORS & CURRENTS DRAWN

(OP)
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

RE: 3 PHASE AC INDUCTION MOTORS & CURRENTS DRAWN

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.

RE: 3 PHASE AC INDUCTION MOTORS & CURRENTS DRAWN

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.

RE: 3 PHASE AC INDUCTION MOTORS & CURRENTS DRAWN

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.

RE: 3 PHASE AC INDUCTION MOTORS & CURRENTS DRAWN

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources