# Three phase induction motor problem

Discussion in 'Homework Help' started by EngineeringStudent, Jun 11, 2014.

1. ### EngineeringStudent Thread Starter New Member

Jun 11, 2014
3
0
Hello everyone im recently new to this forum and i would like help on the following question:

'A three phase cage induction motor running at full load draws a stator current of 60A at a power factor of 0.8 lagging from a 415v, 50hz supply. Under the following conditions the stator loss is 3.6 kw, the rotor loss is 1.3kw and the percentage slip is 4.5%. The shaft power is 29.4kw calculate the following':
- Shaft output torque
- Efficiency
- Mechanical friction/windage loss

Could someone explain and show me how to work out the above, thanks you!

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
784
This is a relatively simple problem for a student of induction machine theory. You need to show your working thus far to receive assistance.

3. ### EngineeringStudent Thread Starter New Member

Jun 11, 2014
3
0
Could you tell me the formulas to use i have no workings outs yet, i have only just started recently learning about electrical principles hence why i required help on this 'simple' question.

Last edited: Jun 11, 2014
4. ### crutschow Expert

Mar 14, 2008
16,228
4,340
Do you know how to calculate three-phase power from an AC line given the current, voltage, and power factor?

Given the input power from above, the given internal power losses, and the output shaft power, you can determine the friction/losses since that's what's left.

To calculate output torque from the output power you need the number of motor poles to determine the motor speed, which you didn't state.

5. ### EngineeringStudent Thread Starter New Member

Jun 11, 2014
3
0
The question did not state the amount of motor poles given, is there a way to work that out as well as efficiency?

6. ### crutschow Expert

Mar 14, 2008
16,228
4,340
You need the number of poles to determine the motor speed, and in turn, the motor torque. Perhaps the assumption is that it's a 2-pole motor, but that's a big assumption.

The efficiency is determined by dividing the output power by the input power.

Also this information should be provided in your course work. Have you studied that?