Question on torque for Induction Motors

Discussion in 'Homework Help' started by Kayne, Mar 28, 2010.

1. Kayne Thread Starter Active Member

Mar 19, 2009
105
0
Hi,

I have been given the following information for the question the motor is a 50Hz, 8 pole induction motor with 4% full load slip. Rotor resistance is .001Ω per phase and a stand still reactance of .005Ω per phase.

I need to find the ratio of max to full load torque

I have been able to work out a few things for the problem but I have been looking at it now and not sure what the next step is.

I want to find the torque but the formulas need either a voltage, EMF or a current. I think that I am heading in the right direction just need a push.

The working so far are in the attachment.

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2. t_n_k AAC Fanatic!

Mar 6, 2009
5,448
789
The slip at maximum torque will be

$s_{T_{max}}=\frac{R_{2}}{\sqrt{R_{2}^{2}+X^{2}}}$

and

$\frac{T_{max}}{T}=\frac{\frac{s_{T_{max}}}{s} + \frac{s}{s_{T_{max}}}}{2}$

You have the full load slip as 4% at torque T. R2 & X are also given.

Kayne likes this.
3. Kayne Thread Starter Active Member

Mar 19, 2009
105
0
I will have to try and find these formulas in my study books and do some more questions. The answers that i obtained in this question are close to the TB answers.

Thanks for the help again

4. t_n_k AAC Fanatic!

Mar 6, 2009
5,448
789
Actually I notice the formula I gave for sTmax is wrong

It should be

$s_{T_{max}}=\frac{R_{2}}{\sqrt{R_{1}^{2}+X^{2}}}$

where R1 & R2 are the values for the standard induction motor equivalent circuit at steady state conditions. It seems in your question R1 is assumed to be either zero or a value you have to determine from other information ......?

Last edited: Mar 31, 2010