Motor Control

  • Within a motor, there are 3 outputs that one seeks to regulate:
    • Torque (the turning force of the moment)
    • Speed
    • Position (the location of the wheel with respect to its shaft)
  • Examples of why:
    • Torque: In an elevator, too much torque can mean too much force which can cause people to fall over.
    • Speed: Too much speed could cause too rapid of an air pressure change in an elevator
    • Position: Elevators need to stop at the right floor.
  • There are two ways to do this, Analogue control and Digital/Switching control

Analogue control

  • As the force on a motor is given by $F = BIl \sin{\theta}$, we can vary the torque of most motors by altering the current inside the circuit.
  • Through Ohm's Law of V=IR, this can be done through a variable resistor.
Type of motor Controlled Input Controlled Output Notes
DC Motor I (Current via Resistance) Torque Max speed is limited by back emf
Synchronous Motor I (Current via Resistance) Torque Speed is set by supply of AC to stators
Induction Motor V (Voltage via a VariableVoltageVariableFrequency machine) Torque Max speed affected by supply frequency

Digital control

  • For Switching, it's merely a logic gate sequence (binary) that gives feedback which controls the motors.
  • Pulse Width Modulation (PWM) is a concept that works with super fast switching.
    • By switching something off and on at certain intervals, it creates a relative net ratio of the times spent on and off.
    • For example, if a light was switched on for 1 millisecond and off for 3, repeating this patter would make the light appear a quarter as bright.
    • This ratio is known as the duty cycle and is measured in percentage:
\begin{align} Duty Cycle = \frac{time on}{(time on)+(time off)} \times 100 \end{align}
  • The ratio itself can also be altered with time to create specific patterns. (called Time varying pulse width modulation.
  • In terms of Voltage, it appears to be a way to control motors.