The circuit diagram doesn’t show a centrifugal switch; instead it has a triac that is turned on during the staring phase. For clarity, the series capacitor isn’t shown in the diagram. Once the motor turns it will continue to do so as long as it isn’t loaded too much. When it has to drive too heavy a load it will almost certainly stall. A large current starts to flow (as the motor no longer generates a back EMF), which is limited only by the resistance of the winding. This causes the motor to overheat after a certain time and causes permanent damage. It is therefore important to find a way to detect when the motor turns, which happens to be surprisingly easy. When the motor is turning and the start winding is not used, the rotation induces a voltage in this winding.
This voltage will be out of phase since the winding is in a different position to the run winding. When the motor stops turning this voltage is no longer affected and will be in phase with the mains voltage. The graph shows some of the relevant waveforms. More information can be found in the application note for the AN2149 made by Motorola, which can be downloaded from their website at www.motorola.com. We think this contains some useful ideas, but keep in mind that the circuit shown is only partially completed. As it stands, it certainly can’t be put straight to use. We should also draw your attention to the fact that mains voltages can be lethal, so take great care when the mains is connected!
Author: Karel Walraven - Copyright: Elektor July-August 2004