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I'm trying to figure out an experiment in a lab that we did with a three phase motor connected in a single phase power supply. The schematic is as below:

enter image description here

I measured all currents in the motor windings, and they seemed somewhat equal. (around 3 A, the motor was almost at full load)

As I believe that's not really possible because in this configuration the windings are at different potentials, I measured the winding resistance and inductance and made a simulation in multisim:

enter image description here

Turns out currents are very much different (even though this is simulated for startup conditions) I believe that with the motor spinning the currents should still be different or am I missing something? I studied some possible cases with different values of the capacitor, but I concluded that even if the reactive current in L2 was fully compensated by the capacitor, currents should still be very different.

I checked the connections I made 3 or 4 times and I'm 99% sure they were correct.

Can someone help me in this? Thanks

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  • \$\begingroup\$ Cute trick. What was the size of the motor? What was the motor driving? Your simulation values should reflect correct values. \$\endgroup\$
    – StainlessSteelRat
    Commented Oct 27, 2021 at 21:39
  • \$\begingroup\$ You also have to model the motor back EMF. (Essentially it's only back EMF on the W phase, but it's generated EMF on the other two phases. ) What you have built there is called a "rotary phase convertor". \$\endgroup\$
    – user16324
    Commented Oct 27, 2021 at 22:39
  • \$\begingroup\$ Your simulation is missing a vital component, the MOTOR acting as a generator! It's not simply 3 isolated windings, they are coupled by the motor's rotation. Try measuring the phase voltages and currents while you vary the motor load, you'll find they vary. \$\endgroup\$
    – Neil_UK
    Commented Oct 28, 2021 at 5:00
  • \$\begingroup\$ @StainlessSteelRat it's called the Steinmetz connection (given that's the guy which invented the induction motor it shouldn't surprise). For small motors it works, obviously with reduced performance. \$\endgroup\$
    – Lorenzo Marcantonio
    Commented Oct 28, 2021 at 6:24
  • \$\begingroup\$ @LorenzoMarcantonio Thanks. And that's what I was going for. Had to be small motors and performance would suffer. \$\endgroup\$
    – StainlessSteelRat
    Commented Oct 28, 2021 at 10:03

1 Answer 1

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(even though this is simulated for startup conditions)

The current for startup conditions should be expected to be much higher than for running. Also you may not have properly determined the equivalent circuit component values. The values are normally determined using no-load and locked-rotor test results in addition to determining the DC winding resistance.

You should not model the motor based on back EMF. You should use the Steinmetz equivalent circuit. Tesla and others invented the induction motor. Steinmetz developed the mathematical methods for analyzing it, greatly improving the design process. See https://electronics.stackexchange.com/q/364318

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