Is it possible to drive a small synchronous motor with permanent magnetized rotor to other speeds it is designed for?

I tried to drive a small clockwork AC motor which is symmetrically build but mechanicaly constrained to one direction. I tried to drive it at different frequencys, while some lowers (eg. 20 Hz) seem to work to some point, the motor seem to stall at higher frequencys above 100 Hz it seems. Also I could not test at lower frequencys so far.

The voltage it needs to spin up rises with the frequency it seems. At 20 Hz, it works from 40 V, while it almost need the 220 V it's rated to spin up at 50 Hz.

Is it possible to spin it at 1/10 or 10 times of the design frequency if the voltage is adjusted, or is the frequency limited by design?


In theory, yes, it is possible to operate the motor at different frequencies if you keep the ratio of the voltage to frequency constant. This is often called "V/Hz" or "scalar" operation.

In practice, at low frequencies, additional voltage is often added due to the effects of winding resistance. This is called "voltage boost".

Operation at high frequencies is limited by the motor's insulation system's limits and/or mechanical considerations (e.g. balance of rotor/load, bearing alignment, increased windage losses, etc.).

The maximum speed for a motor (generator) is a standard design input. As such, it can be exceeded only to the degree that the design has additional margin. Since this margin adds cost to the machine/system, it is typically small.

Note also, that prolonged operation at higher speeds will reduce the life of the motor.

In general, provided the motor has sufficient cooling, there should be no lifetime issues with operating at lower speeds. Cooling may be an issue in motors that were designed with a shaft mounted cooling fan and the assumption of operation at rated speed.

In your case, your observed limit of ~2x speed on your motor is not unexpected and I suspect may be due to the windage loss torque reaching the the motor's peak torque output. For low frequency operation, I don't expect cooling to be an issue since I'd be surprised if this motor had an integral cooling fan.

  • 2
    \$\begingroup\$ As with their close cousins stepper motors, operation at higher frequencies may see winding inductance play a role in limitation, too. This would have the effect of increasing the voltage required as desired speed increases. \$\endgroup\$ Aug 28 '12 at 6:32
  • \$\begingroup\$ in a synchronous motor, the current is nominally a sine wave and so the winding inductance's effect is to effectively increase the slope of the V/Hz line beyond the BEMF component. Thus it has an impact, but doesn't form a limit on frequency. In a stepper motor, the current is closer to a square wave and so the current slew rate provides an effective upper limit to frequency. The slew rate is approximately the applied voltage divided by the winding inductance. Thus, the stepper (or other switched current) motor has an upper frequency limit based on winding inductance... \$\endgroup\$ Aug 28 '12 at 15:02
  • \$\begingroup\$ There is no real distinction there beyond convenience. Stepper motors get driven by square waves for simplicity rather than necessity, and fine positioning drives may actually use approximate sine waves in quadrature for fraction stepping and overall smoothness. The hard limitation is the frequency at which the inductance (at the fundamental frequency of the commutation rate) makes the required drive voltage for a given torque exceed the power supply voltage (which might be limited by the driver semiconductors amongst other factors) \$\endgroup\$ Aug 28 '12 at 15:32
  • \$\begingroup\$ @ChrisStratton: An important distinction is that stepper motors require two or more out-of-phase signals to be applied, while many synchronous motors do not. Clockwork motors often have the shapes of their cores tweaked so that some parts will have the magnetic field change more quickly than others. The extent of that deliberately-engineered effect will be frequency-dependent. \$\endgroup\$
    – supercat
    Aug 28 '12 at 17:19
  • \$\begingroup\$ @supercat - the "no real distinction" was a response to claim about sine vs square waves for the respective motor types \$\endgroup\$ Aug 28 '12 at 17:56

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