Motor Turn Stall Detector

Schematic Diagrams      Comments Off on Motor Turn Stall Detector

In single phase AC
induction motors, often used in fridges and washing machines, a start
winding is used during the starting phase. When the motor has reached a
certain speed, this winding is turned off again. The start winding is
slightly out of phase to the run winding. The motor will only start
turning when the current through this winding is out of phase to that of
the run winding. The phase difference is normally provided by placing a
capacitor of several µF in series with the start winding. When the
motor reaches a minimum speed, a centrifugal switch turns off the start

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

Circuit diagram:

Motor Turn Stall Detector Circuit

Motor Turn Stall Detector Circuit Diagram

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
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

Most Wanted Diagrams: