Electrical – operating a motor with a undersized vfd

Induction motors are called "asynchronous" because they don't go an exact speed based on the ac frequency, not because they can be easily speed controlled using voltage/current limiting instead.

• For a thorough explanation of the motor type, the Wikipedia Article is a great place to start.
• In general terms, induction motors can run with fair efficiency/control between 90-100% of "synchronous speed" (at 50hz, that's 3000rpm divided by the number of coil 'poles' in your motor, which is likely 1), and act as generators between 100-110% of "synchronous speed."
  • The further from 100% of synchronous speed the motor is turning/being turned, the more current it will draw/generate (without external current limiting) to produce torque towards recovering to its synchronous speed.

Thus, for best speed control of an induction motor, you'll need some form of VFD. Luckily, however, being single-pole & only 100W, you could use a rectifier+capacitor to store the power you'll need, then a timing circuit & "Logic-Level Gate" Power MOSFET to generate the needed frequency quite easily (could use as few as 4 total components added to your circuit).

You should be able to find a VFD that will be capable of making the motor produce at least 150% of rated torque at any speed from zero to rated speed. The current required to produce that torque with the VFD should not exceed 150% of rated current by very much. The selected VFD should have a "sensorless vector" or "direct torque control" control strategy. Operating in the V/Hz control mode will probably not be sufficient. Also a VFD that is only rated for fan and centrifugal pump loads will probably not be sufficient. Inquire locally to find a VFD that has a good reputation. There are many manufacturers that have very capable VFD designs>

For more information about how a VFDs can start a motor without supplying high motor current, look at my answer to this question.

Note that you should not use a VFD model for a motor that is smaller than the smallest motor recommended by the manufacturer for that particular model. For the best performance, the VFD must be "tuned" to the motor design parameters. The VFD can do that automatically, but only for the range of motor ratings for which it is designed. The VFD also has built in overload protection for the motor that is limited to a range of motor ratings.