Electrical Drives, Motors and Actuators
Electrical Drives, Motors and Actuators
This domain covers motor types and their control: DC motors, three-phase induction motors, single-phase motors, steppers, servo drives and variable-frequency drives, plus starting methods, braking and protection. Calculations on synchronous speed, slip and step angle are common.
Core rules
- DC motor: back EMF Eb = V − IaRa; torque is proportional to flux and armature current, and speed falls as load rises. Field weakening raises speed.
- Induction motor synchronous speed Ns = 120f/p. Slip s = (Ns − N)/Ns. Rated slip is typically 2–6 %.
- VFD control keeps voltage proportional to frequency (V/f) below base speed to hold flux; above base speed the voltage is fixed and flux falls (field weakening), so torque drops.
- Starting: DOL suits small motors; star-delta needs a six-lead motor rated for delta and reduces current to one third; soft starters ramp voltage; an overload relay protects against sustained overcurrent, not short circuit.
- Stepper: step angle = 360°/(steps per revolution); microstepping divides the step further. Servo motors give closed-loop position control with high dynamic response; IE classes rank motor efficiency.
Worked example
A 4-pole, 50 Hz induction motor runs at 1440 rpm. Synchronous speed = 120 × 50/4 = 1500 rpm. Slip = (1500 − 1440)/1500 = 0.04, i.e. 4 %. On a VFD at 25 Hz, synchronous speed becomes 750 rpm. A 1.8° stepper has 200 steps/rev; with 8 microsteps, one revolution needs 1600 pulses, and 4 revolutions need 6400.
Traps
Using rated speed instead of synchronous speed for slip; applying star-delta to a motor connected in star internally; assuming constant torque above base frequency; forgetting the holding torque falls as step rate rises.