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I like to drive a small (150W) single phase induction motor by an existing three phase inverter by removing the capacitor and just connecting the two windings to the inverter in an incomplete triangle circuit.

I've done that with very small (15W) motors before, which run well, despite a little bit more noisey at low frequencys. I guess this is because the single phase motor uses a 90 degree offset between the main and the capacitor generated phase, while the inverter provides a 120 degree phase offset.

So I like to know what problems (despite the noise) may occur by this abuse if driving very low torque and speed in respect to the motor capabilities.

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  • \$\begingroup\$ Are you interested in the answer for theoretical reasons, or are you just trying to drive the motor? If you just want to drive the motor, I think it might be easier to use one of the 3 phase legs from your inverter to drive the single-phase motor without modification. Obviously, check the voltage first. Some larger motors only use the capacitor for starting. They actually have a relay that cuts out the capacitor once the motor speed exceeds some threshold. \$\endgroup\$
    – mkeith
    Dec 2, 2014 at 8:04
  • \$\begingroup\$ I like to drive the motor for real. It is an induction motor, that is permanently fed by the capacitor. It has two windings I guess. Driving the motor with the capacitor is only possible if it is driven in it's natural frequency (50Hz). I like to drive extreme low frequencys, eg. 1 Hz. At this point, the capacitor would not provide a helper phase any more, but something totally out-of-phase for the second winding. So I decided to drive the two windings without a capacitor. \$\endgroup\$
    – dronus
    Dec 11, 2014 at 11:29

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I did some experimentation on topic with a ceiling fan motor (wings detached) with rated power 80W stamped on the device. The motor is a permanent-split-capacitor motor intended for 230V / 50Hz. My digital multimeter displayed exactly this voltage for the mains connection. The voltage measured across the auxiliary winding was 226V while running the motor normally in the original shape. The drive was a Siemens Micromaster 410 rated for 0,37kW.

  • I measured the temperatures of the motor rotating drum with the original setup (using an IR meter). The temperatures varied between about 50 centigrades on the outer edge of the rotating external drum and about 55 degrees near the axis after running the motor for more than an hour

  • I then removed the capacitor and connected the main winding and the auxiliary winding at one end to two output connectors of the drive and the common point of the windings to the third connector so that the direction of rotation did not change

  • I measured the temperatures after running the motor again for more than an hour with the three-phase capacitor-less drive-based setup; the measured temperatures remained the same, and at least, the temperatures were not higher than with the original setup

Other observations:

  • the temperatures with the three-phase drive-based setup was highest with the highest rotation speed and frequency tested (50Hz)

  • the noise with the drive-based three-phase setup was higher, but mainly due to noise from the switching frequency (2kHz in this case)

  • the starting torque was considerably stronger with the three-phase drive-based setup than with the original single phase capacitor-split setup; the torque difference was even larger at lowest frequencies tested (20Hz)

Overall, the idea seems plausible for further testing with the power range of typical ventilation fans used in single residence houses. The most critical problem that might arise is the PWM switching noise. Also, the voltage stress caused by the spiky PWM waveform might need consideration depending the motor. The tested setup would solve the problem of very low starting torque related with single-phase permanent-split-capacitor asynchronous motor run at reduced frequency by an inverter drive. An ideal drive for the purpose would allow adjustment of effective voltage between at least two output terminal pairs and the phase angle difference between the same two, which standard drives do not support.

Update (21st December 2020):

I tested the same setup (three-phase inverter drive for a single-phase run capacitor ceiling fan motor) with the modification for true 90 degree two-phase circuit. The latter circuit was made up of two identical isolation transformers 400V/230V. The primary windings were connected mirror symmetric between the connectors feeding the main winding and the common 3rd connecter of the drive. The secondary windings were connected in series. Since the drive outputs the effective voltage of 230V, the transformer cores have a wider margin for magnetic saturation than with the specified primary voltage (which is not necessary, but might provide further options for selection of drive frequency vs. voltage curves for enchanced low rpm torque control). Thus, the result voltage for the starting winding is also about 230V at nominal frequency (50Hz).

I did the same temorature measurements as above. This time the temperature of the motor casing remains well below 45 centigrades. This is lowest of the three cases tested, which is no wonder, since the motor is now driven with ideal two-phase alternating voltages of 230V plus minus something with the phase angle separation of 90 degrees at any frequency produced by the inverter drive (the true effective voltages will follow the frequency as usual).

So I concluded to adopt this circuitry for driving the ventilation fans of my house with synchronised but variable speed. This is now a true two-phase system. I used to have the drive to feed a single-phase voltage with a variable frequency to the run-capacitor split-phase fan motors, but I was dissatisfied with the approach, since the starting torque was very low especially at reduced rpm and the capacitors tended to degrade within a few years creating even worse behavior. Now, I can remove the capacitors of trhe fan motors and feed the auxiliary coil directly with this phase-shifted voltage. True three-phase motors are difficult to get for such relatively small house-oriented ventilation fans. I prefer asynchronous fan motors over EC ones due to fewer problems with wear-induced tonal noise and lower cost, even though power consumption is somewhat higher.

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I believe you can use a standard wiring diagram from something like a Commander SK which anticipates what you want to do. I can't remember the circuit off the top of my head. I use them for three phase motors.

You can change the phase angle by changing the capacitor. I believe the torque will be very low.

I had to recently turn a single phase motor fan into a variable fan and we used a single phase chopper like is used to slow down a (series wound) drill. It worked fine and rattled a lot at low speed. That's all. The fan has a very low start up load and at low speed the magnetic field just slips a lot - no problem.

It works by "trying" to be synchronous but the voltage is not high enough so it slips and therefore slows down. You are far better off using the correct frequency so it tries to keep up, but can't because of low power, than to try to be synchronous and go at a low frequency/speed using a motor that is inherently unable to do that.

The only issue you will have with the former case is you may want to manually switch out the starter windings if it is running below the cut out speed. It might not be necessary. Just see if it starts to overheat.

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Inverters have a pulse width modulated, high frequency output coming from a dc bus which requires that the motor be able to withstand a higher breakdown voltage across windings and a higher insulation class. On this account, check the insulation class of the motor or if it may be 'inverter duty rated'before applying an inverter to it. Otherwise, the motor windings will break down and fail early.

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If you try to connect a single phase motor to a 3 phase terminal no current will flow through the neutral therefore the motor won't rotate if you use Y configuration. So you must use Delta configuration. However I am trying to see how current will not go through immediately from one pole to another...

There is a reason some devices work with 3 phase current they simply have 3 loads!However it is not possible to power a single phase device from a 3 phase current unless you use only 1 phase of the 3 phase current or 2 phases using a delta configuration.

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