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This is induced motor question

Let's say a three-phase induction motor is rated 50HP, 480V, 60Hz and 1150 RPM.

  1. developed torque
  2. frequency of the rotor current

Is developed torque mean initial power's torque?

How do I find the frequency of the rotor current?

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  • \$\begingroup\$ @optionparty : no; 1150rpm implies a 6 pole motor, with 1200rpm synchronous speed, and 50rpm (5/6Hz) slip frequency. 6 pole motors are not so uncommon at higher power levels \$\endgroup\$ – Brian Drummond Oct 28 '13 at 8:39
  • \$\begingroup\$ @BrianDrummond is correct. You can get a slower motor by adding more poles. I have personally dealt with a 1,000HP motor that ran at 297 RPM (20 poles, 50Hz; synchronous speed 300RPM, 1% slip at full load.) \$\endgroup\$ – Li-aung Yip Oct 28 '13 at 8:47
  • \$\begingroup\$ Thanks for the correction, I should have caught it myself. I'll delete my comment. \$\endgroup\$ – Optionparty Oct 28 '13 at 13:24
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This seems like homework, so I am providing guidance only.

  • With regards to developed torque

    What do you mean by the developed torque?

    What is the difference developed torque and the output torque, i.e. the shaft torque?

    Does this shed any light on how you might calculate the developed torque, given your knowledge of the output torque?

  • With regards to the frequency of the rotor current:

    How many times per second does the stator field spin around the rotor? That is, what is the relative angular velocity of the stator field vs. the rotor field?

    Consider the rotor at standstill (100% slip) and the rotor at synchronous speed (0% slip).

As I noted in your other question, you may find it useful to read a textbook on electric machines.

Specifically, I recommend Mulukutla S. Sarma's Electric Machines (WM. C. Brown Publishers, 1985) section §7.4, Polyphase Induction Machine Performance.

Look for this book, or other texts on electric machines, in your university library.


In passing, you haven't said how many poles your motor has. You have said it is 1150 RPM, which I assume means 1150 RPM at rated load, and not 1150 RPM at some other load. From this I infer it is a 6-pole motor (synchronous speed 1200 RPM.)

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