I'm having a difficult time trying to understand what effect the frequency of a power supply has on the power of a synchronous motor.

For example, if the supply frequency were to increase from 50Hz to 60Hz (with the supply voltage remaining constant), what effect does that have on the motor power? I understand that the synchronous speed of the motor will increase to match the supply frequency. Does the power increase proportionally by drawing more current at the same voltage?


1 Answer 1


The current that is due to the mechanical load will increase if the mechanical power also increases. Given that increasing the frequency also proportionately increases the speed (and more-than-likely also increases the torque to the mechanical load), it is likely that a 6:5 increase in speed delivers a 36:25 increase in mechanical power hence, for a given supply voltage, the current (due to load) will rise by 1.44 times.

Also remember that there is magnetization current into the stator and this will decrease with an increase in frequency (voltage remains constant) so, the net effect will be somewhat less than a 1.44 times increase in overall current but somewhat more than simple a 6:5 (1.2 times) increase in current.

  • \$\begingroup\$ Hello Andy and thank you for your reply. That makes sense, so the mechanical motor power increasing by 1.44 is due to the motor speed and torque increasing proportionally by 1.2 (1.2 * 1.2 = 1.44). P = 2πTN / 60, where T = torque, N = speed \$\endgroup\$
    – sammas9
    Commented Oct 31, 2018 at 13:56
  • \$\begingroup\$ @sammas9 yes, that's how I see it. \$\endgroup\$
    – Andy aka
    Commented Oct 31, 2018 at 14:31

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