I want to run a 3 phase induction motor on a single phase power supply. I'm aware of the low operating power & low starting torque in this configuration. My load & duty cycles are also not big, so I'm fine with the under performance.

I also have an additional requirement that I need the ability to change the rotation direction of the motor by a 3 position switch (forward, stop, reverse).

I made the below amature circuit based on my understandingMotor Connection

The switch is a 3 position switch the internal connections of which are shown in the below imageSwitch Internal Connections

As you can see, it is a "break before make" type of switch.

Also, we need to apply just enough force to put SW1 in neutral position from any of the 2 extreme positions. If we apply too much force, the SW1 will be pushed to the other extreme position.

This circuit works, but with problems.

  1. I keep SW1 in neutral position, then connect the AC power & turn on the AC mains, I get a humming in the motor. When I push the SW1 to forward position, the motor turns forward.
  2. Same as step 1), but now instead of pushing SW1 forward, I push it Back & the motor turns reverse.
  3. While the motor is turning, if I push SW1 back to neutral, I can see & hear a spark occurring inside SW1. But the motor does not stop after this happens. So, I need to disconnect the mains power to turn the motor off.
  4. While the motor is turning, lets say, forward, if I accidentally push SW1 too much & put it in reverse position, the circuit protection switch in my house gets triggered & I have to disconect the motor & reset it.

Any idea whats happening here?

I suspect the Cap is storing residual voltage, which gets added up to the voltage from the mains & causes a spike. Or, maybe the coils of the motor act as inductors & create a voltage spike when the circuit gets broken.

Please also suggest how I can achieve my desired function using minimal additional components & the setup that I have. For eg, If I get the double pole 3 position switch, will that help?


1 Answer 1


The three coils in the motor, coupled with an energize rotor, can act as both "drivers" and "generators".

Without the cap, there is no rotation of the magnetic field, so the rotor won't turn when you apply power.

As you already appreciate, once you add the cap, the phase shift is enough to cause a rotating field and start the motor. I suspect your cap might be a little big (see this calculator), but not a major problem.

When the motor is running and you remove the cap, it keeps going due to the the already energized rotor and its momentum being enough to generate a continuing torque (single phase cap start motors, where the cap is disconnected by a centrifugal switch are still used). In this state, the unconnected winding will be generating a voltage.

When you flip the switch to the opposite position, large current will flow as the cap lag and generated voltage on the third winding are now out of phase. Hence the spark and possible supply trip.

The simplest fix with a ON-OFF-ON two pole switch is to use the second pole to switch one leg of the mains supply (motor on CO contact, L on the other two contacts). The motor should then deenergize in the middle position, and whilst it won't stop immediately, the overvoltage should be much less, and the motor may well reverse.

If you have an ON-ON-ON two pole switch, then you can still switch the mains on the second pole, but the middle position can be connected to a high power resistor to the N line. In the middle position you will then get an active breaking action, which should stop the motor pretty quickly.

  • \$\begingroup\$ Thanks for the reply. It's hard to understand circuit from textual explanation. So, I've prepared the required items in this circuit drawing/analysing utility (Falstad). Can you use this link & show me the circuit you are describing? You can just click the Draw>Add Wire to draw the wires. I've already setup every other necessary component \$\endgroup\$
    – icyGuy
    Commented Sep 22, 2023 at 17:26

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