Can an induction motor be run safely with a 2:1 step down transformer? They don't work well with triacs because of increased rotor slippage, for example, but if I understand correctly that's because of the chopped up non-sinusoidal power waveform.

What are the implications of undervolting? Would it affect slip or efficiency?

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    \$\begingroup\$ Details. What kind of motor? Single or three phase? Pure induction or PM/Synchronous? Next question... Why do you want to run it at half voltage? Most motors, by single phase and three phase, can typically be wired for high voltage or low. Third, just randomly changing any operating condition will have a negative effect on slip, efficiency, power output, heating, and operation unless you do it intelligently (if possible).. As in with a Motor Drive. \$\endgroup\$ – R Drast Mar 24 '17 at 10:22
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    \$\begingroup\$ What is the goal you want to achieve? \$\endgroup\$ – Marko Buršič Mar 24 '17 at 10:39
  • \$\begingroup\$ It depends on your goal. If you have a 110V motor and want to run it from a 220V line, then it is quite common to do so. Many machines use multi-tap transformers to make them run in different countries. If it's for some other reason... see other answers here. \$\endgroup\$ – Trevor_G Mar 24 '17 at 12:18
  • \$\begingroup\$ If you reduce the voltage, you must adjust the frequency as well such that the ratio V/f remains constant. Otherwise, the motor core saturates resulting in inefficient performance. \$\endgroup\$ – Sâu Jun 13 '18 at 10:48

If the voltage is reduced, the torque capability will be reduced at any given slip is proportion to the square of the voltage. If the voltage is reduced to half, the torque at a given slip will be 25% of the previous torque. The slip at any given load torque will increase accordingly as shown below.

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The motor will operate at the intersection of the torque capability curve (blue) and the torque demand characteristic curve of the load (red). The torque characteristic curve of a fan is shown because this type of speed control is sometimes used for fans. Loads that require the torque to remain constant at reduced speeds or have less reduction in torque requirement are rarely operated this way.

The losses due to slip are proportional to the load torque multiplied by the percent slip.

This can not be done with a single-phase, capacitor-start motor because the motor needs to be operating near full speed for the centrifugal switch to disconnect the capacitor.

  • \$\begingroup\$ This is a great answer. I've been running a fan undervolted (60v from 120v) for about 18 months now, and I'm still not sure if it's a good idea (don't know what kind of specific kind of induction motor it is,) but it has thermal protection and I haven't had any trouble yet. Slip must be higher now, and therefore efficiency is lower, but it's quiet now and that's what I needed. Thanks Charles. \$\endgroup\$ – nmr Oct 23 '18 at 15:41

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