Electronic – How to calculate RPM using frequency

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).

A variable frequency drive (VFD) will generally try to maintain a constant magnetizing current in the motor with any frequency and load. The results will vary depending on the VFD design and set-up. That will result in an operating power factor that is generally similar to the pf vs load plot provided by the motor manufacturer for rated voltage and frequency sine wave operation.

To calculate output power, motor efficiency must also be considered. That will vary with speed.

You can not accurately determine the output mechanical power from the motor using the method that you propose. If the VFD provides a torque display, that is likely to be more accurate than any determination made by measuring motor current. A power determination using DC bus current of the VFD might also be more accurate. There are clamp-on wattmeters on the market that can accurately measure VFD output power. Of course, that still leaves the problem of estimating motor efficiency. I have some empirical data that might help, but I don't have time to post it right now.

Here is the data that I mentioned above. It is calculated from motor design data, not empirical test data.

I believe this data was calculated in the 1970's by Alberto Abbondanti of Westinghouse, but I can't find where it was originally published. I received it in class notes for a tutorial.