Electrical – Measuring R1 in induction motor equivalent circuit and parameter extraction from test

induction motor

I always had this question that I never really found written anywhere and I have the good or bad habit of not believing my teachers when answering this kind of odd questions.

Whenever I read about the induction motor equivalent circuit R1 is always the resistance of the stator winding. Great! I have a stator, how do I get R1?

My first guess is to measure the resistance value between one phase and neutral in Y (star) connection. That or take the resistance value between phases in Delta connection and divide it by 1.5 to get the resistance of only the winding that connects them directly.

Second question:
To extract the parameters of the equivalent circuit, I tested the motor in Delta connection, with the rotor blocked and in free running mode. Is V1 the phase to phase voltage? What about Is? Should I test it in Y instead? I have V(t) (phase to phase) and I(t) that I got using an oscilloscope and a current probe.

Equivalent circuit with variable names:
eq circuit
(source: myelectrical.com)

Best Answer

The equivalent circuit is the line to neutral circuit for one phase of a wye connection. The wye equivalent circuit is used whether the actual motor connection is wye or delta. The component values are adjusted accordingly. R1 is determined from a DC resistance measurement. I believe that the measurement is usually done at the motor terminals with the motor connected as it is connected for the full-load and locked-rotor tests.

You should be able to find on the internet several versions of step-by-step instructions telling how to perform no-load and locked rotor tests. The instructions include the equations used to determine the equivalent circuit values.