Rotor Current (Induction Motor)

Xbr is the reactance when the rotor is blocked, it's based upon one of the many ways of modeling an induction motor. It's a bit odd that you'd be interested in the rotor current since on most machines it can't be measured and is usually referred to in terms of the stator current. The exception is if you're modeling a wound rotor induction motor which does have the rotor winding brought out to a variable resistance via slip rings and cn therefore be measured..

Hi.

Text books: 1. Electrical machines by P S bimbra

2. Electrical machines by IJ Nagarath & kothari.

Dear Mr. hari520,

I want to supplement to your answer.

Electrical Book by B.L.THERAJA.

Also we can get much details from http://electrical-engineering-portal.com

DHAYANANDHAN.S

Sir, do you know the famous pastime of IIT Electrical Engineering Students? Picking up mistakes in BLT.

These books are only examination & syllabus oriented. They do not discuss the subject in-depth. A professional electrical engineer can point out millions of conceptual mistakes in such books.

Rather " A Theory of Alternating Current Machines" by M. G. Say would be the best to understand AC Machines.

Dear Mr.electricalexpert65,

Thanks for your response and I agree with your views.

Mistake/errors are there. The subject matter was dealt with in a broad manner. What I felt, from understanding point of view it will be OK.

Dhayanandhan.s

This formula is correct one if we take into consideration the fact that both rotor resistance and reactance are strongly influenced by skin effect and are changing with rotor frequency-that means the slip. For protection setting calculation purpose there are some appreciations as following:

"The positive- and negative-sequence rotor resistances are

given by the linear functions of slip S:

R1 = (RM - RN )S+ RN

R2 = (RM - RN )(2 -S) + RN

where: RM is the rotor resistance at locked rotor

RN is the rotor resistance at rated speed "

See: From the Steinmetz Model to the Protection of High Inertia Drives by Stanley E. Zocholl Schweitzer Engineering Laboratories, Inc.

How do you define slip S? Usually it's (synchronous speed - actual speed)/synchronous speed. But on that basis it = 100% with locked rotor. If the motor is then unlocked S falls as the motor accelerates, indicating rising I_R, but in practice current falls as speed rises.

CM, Correct, at standstill (blocked rotor) slip is 100%. Take a second look at the equation, slip appears in both the numerator and denominator, therefore as the motor accelerates, slip falls from 1.0 (100%) to around 0.03 (3%), and Ir does indeed fall.

Here's a link to a good slide presentation that shows the derivation (you'll have to do some algebra to get the same exact form) and a plot (pg. 8) of the rotor current as a function of both slip and speed.

You're right, I hadn't spotted that. My mistake