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:
(1)
\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