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induction motor control


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hello all< I have been reading all of your comments with interest and am becomeing more confused with each post. my problem is I have a 3/4 hp 240 v single phase ac induction motor for a hobby woodturning lathe and I would like to control the speed with out having to change the belt on the pulleys does anyone have a circuit diagram to build one or should I give up and bite the bullet and buy one from someone like RS components, only trouble is they cost £200 sterling upwards many thanks to all who read or respond;d;
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Hello Stuart.


Unfortunately, there is generally no easy way to vary the speed of a standard single phase induction motor.

Three phase motos can be easily speed controlled using a variable speed drive (inverter), but the single phase motor is a different story. The best solution is to replace the motor with a three phase motor of the same size and voltage and use a single phase input, three phase output variable speed drive. This is not the sort of thing that you will build yourself!!


You may find single phase controllers available that are variable voltage controllers. These are designed to be used with special single phase motors that have a high resistance rotor. These motors are typically "fan" motors and are often an integral part of a fan assembly. They do not control the speed as such, but limit the torque that the motor can produce and thereby the fan is slowed because the motor does not produce enough torque to get to full speed.

If you apply this technology to a standard motor, you will damage the motor.


Best regards,

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