Why is a split phase or capacitor start motor typically not a varialbe speed motor?What would have to be done to operate a split phase or capacitor start motor as a variable speed motor?
When you run a current through a wire, it will induce a magnetic field. A motor works on the principle that when you place a magnet N to N or S to S, it will repel. By inducing a magnetic field that constantly 'flips', constant torque is applied to the shaft. On a DC motor, this 'flip' is done using a physical device called a commutator. Basically, it flips the + and - leads on the coil around every half turn, so the N and S poles of the fields are always in the reigon where they repel, causing circular motion. When you increase the current capacity, the motor will experience greater torque and will turn faster, and the commutator will flip the polarity as quickly as needed, so you can control a DC motor by reducing the current capacity of the input, by reducing the voltage or by inserting a variable resistor. On an AC motor like the ones you describe, this 'flip' is part of the waveform itself. You can't reduce the speed of the motor by reducing the current capacity(using a rheostat or resistor) because the torque the motor has at a certain shaft angle is related to the voltage it sees. The way AC motors are made to be variable speed is to change the frequency of the input waveform using something called a Variable Frequency Drive or Variable Speed Drive. It will take an input AC signal, turn it into DC, then use power transistors and computer controls to create a new waveform, whose frequency can be easily controlled.
most of these motors have a start winding that is disconnected by a centrifugal switch near operating speed. If the desired speed is less than the speed needed to keep the switch open the start winding would then be reconnected and soon burn out. The starting capacitor would most likley burn out first. Shaded pole motors can have multi speeds by varying voltage but are limited to smaller outputs like fans ...The shaded pole motor has many positive features, but it also has several disadvantages. Its low starting torque is typically 25 to 75% of full load torque. It is a high slip motor with running speed 7 to 10% below synchronous speed. Also, it is very inefficient, usually below 20%... at second link Depending on application it may be possible to use a variable speed pully such as the one at the 3rd link (tb woods)
