Introduction
Some of our customers run production processes with AC induction motors. These motors come with speed controllers based on thyristors turning on part of the AC cycle. By turning on later in the AC cycle, a reduced amount of power can be delivered to the induction motor, which in turn, reduces the speed.
Intermittent Controller
One of these AC motor speed controllers intermittently stopped running. Intermittent failures can be quite difficult to fix. Here is the controller:
The speed is controlled by a knob which presents a voltage from the attached variable resistor to the ARM based microcontroller's analog to digital converter.
Based on the A/D input and the motor speed, the microntroller calculates the turn on time for the thyristor, which feeds the AC to the induction motor.
In turn, the tachogenerator in the induction motor tells the microcontroller its angular velocity.
Two additional pins selects the clockwise, or counter clockwise rotation of the induction motor. These come with pull up resistors to VCC, so the motor does not run, if, for example, the clockwise pin is not grounded.
The Fix
A quick measurement reveals that the ground plane for pulling down the clockwise/counter clockwise pins were in fact, not connected to ground, intermittently. By ensuring the connection to ground, the induction motor speed controller works perfectly once again. Another piece of industrial equipment has been saved from the landfill!
