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)


1 Answer 1


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.

  • \$\begingroup\$ In that case I should measure the voltage between the phase and the neutral pin of the transformer right? I was measuring between phases, that will take me nowhere. I also learn from your answer that the equivalent circuit is the same whether the motor is running in wye or delta connection, correct? Please tell me if that is not it. \$\endgroup\$
    – rmarques
    Commented Apr 14, 2016 at 5:13
  • \$\begingroup\$ All measurements are done at the motor terminals. You will measure the line-to-line voltage, but divide by the square root of three to get the voltage for the equivalent circuit. You assume that the motor is wye connected even if you know it is delta connected. The equivalent circuit is the wye circuit that is equivalent to the actual circuit whether it is wye or delta. \$\endgroup\$
    – user80875
    Commented Apr 14, 2016 at 12:51
  • \$\begingroup\$ The method I am using uses the angle between voltage and current, so I need the phase to neutral voltage, measured in the transformer, or at least this is the way I understand. \$\endgroup\$
    – rmarques
    Commented Apr 14, 2016 at 16:59
  • \$\begingroup\$ You don't need to do it that way, but if you have access to the transformer neutral, and that is the method that you understand, you can do it that way. \$\endgroup\$
    – user80875
    Commented Apr 14, 2016 at 18:20

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