Electrical Machines, Motors and Motor Controls in Vehicles
Electrical Machines, Motors and Motor Controls in Vehicles covers DC motors, permanent-magnet and series motors, stepper and brushless motors, and the relays, H-bridges and PWM controllers used in vehicles. The test checks motor behaviour, current draw and safe control.
Core rules
- A DC motor draws high inrush current; stall current can be many times running current.
- Permanent-magnet motors are common for wipers, windows and blowers; series motors produce high starting torque in older starters.
- Speed control uses PWM; average voltage = duty cycle × supply voltage. Direction reversal uses an H-bridge or reversing relay.
- Inductive loads need suppression or freewheeling diodes to protect switches and ECUs.
- Stepper motors move in fixed steps; BLDC motors use electronic commutation and Hall sensors.
Worked example
A window motor is supplied 12 V through a PWM controller at 40% duty. Average voltage ≈ 0.40 × 12 = 4.8 V, so speed is roughly 40% of full speed, assuming constant load and no back-EMF complications. If the motor stalls, current may rise from 8 A running to 25 A; the fuse and wiring must withstand locked-rotor current for the specified time, and the controller must limit or protect.
Traps
Sizing a fuse only for running current and ignoring stall. Forgetting a suppression diode across a relay coil. Reversing a permanent-magnet motor without a proper H-bridge. Assuming a motor is faulty when the linkage is seized.