Motors
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.
page revision: 2, last edited: 21 Jun 2011 13:32