Motor speed controller

[mark641]

The local Men's Shed has some machine tools, all 1 phase, typically 1 KW, ~~ 20 years
old. A member asked me to see if I could make them run at variable speed. At present
you can only change speeds by moving a belt from one pulley to another, which is
inconvenient. I found some variable speed units which say they use rheostats. Please:

Do these units work with any type of motor ?
Is there a danger that they will get hot at slow speeds?

Thank you

 

[James]

I would steer away from trying to do variable speed on 20year old single phase motors.

at the moment, when you move the belt to give a lower speed, you get a higher torque because you have changed the gear ratio.
if you leave the belts at high speed ratio and use any form of speed control on the motor, you have a much higher torque to overcome and the fact the motor is being run at reduced voltage and frequency means there is less torque available from the motor.

to give good speed control at low speeds and be able to provide a similar amount of torque you could end up needing a much bigger motor than the original.
similar projects I have done, a 1kw motor ended up being changed to a 4kw motor to give the right performance.

 

[Lucien Nunes]

Agree with the above. Variable speed on conventional induction motors is not straightforward nor always satisfactory. It requires a good understanding of the motor, its load and the drive electronics to achieve predictable results. With single-phase motors the task is more complex and the options more limited because their behaviour is less than ideal and their starting arrangements have to be accommodated. For example, a capacitor-start motor with a centrifugal switch cannot be run below about 75% of nameplate speed otherwise the switch will remain closed and the capacitor or motor will overheat.

If people want variable speed drives, then a 3-phase motor and single-to-3-phase variable speed inverter drive is the way to go, although as mentioned by @James, typically a larger motor will be needed and the cost may be out of proportion to the value of the machine. Also, a risk assessment and competent design input will be needed for the user controls and safety features.

Just out of interest, paste a link to the speed control product you are looking at. You mention 'rheostats' and if that term is included in the description of the product, I think it might be the wrong kind of thing altogether, as in a phase angle control for a series unversal motor rather than for an induction motor. Rheostats themselves (i.e. power variable resistors) have not been used for speed control generally since the 1960s.

 

[mark641]

Thank you, you have persuaded me that this was a bad idea.