Motors, Controls, and Utilization Equipment
What this domain tests
Motor circuit design, motor protection and starting, contactor and starter selection, and rules for utilisation equipment such as socket outlets, water heaters and fixed appliances.
Core rules
- Two protections are needed: short-circuit protection (MCB or fuse, rated for starting current and fault level) and overload protection (thermal overload relay set to motor FLC, or 1.05 times the service factor current).
- The overload relay sits in the line conductors, never the neutral, and must suit the starter configuration.
- Contactors are rated by utilisation category: AC-1 for resistive loads, AC-3 for squirrel-cage motors, AC-4 for plugging and jogging. Check the AC-3 rating at the actual enclosure ambient.
- Starting methods: DOL draws roughly 5–8 times FLC; star-delta reduces starting current and torque to one third; soft starters and VFDs limit current and control acceleration.
- Local isolation must be provided at the motor, and emergency stop devices must latch and require deliberate reset before restart.
Worked example
An 11 kW, 400 V three-phase motor with efficiency 0.91 and power factor 0.86. FLC = 11 000/(1.732 × 400 × 0.91 × 0.86) = 20.3 A. The overload relay is set at 20.3 A. With DOL starting at 6 × FLC = 122 A, a star-delta starter would cut the line current to about 41 A, at the cost of one third of starting torque.
Traps
- Setting the overload to the cable or contactor rating instead of the motor FLC.
- Sizing a contactor from its AC-1 rating when it switches a motor.
- Assuming star-delta suits any load: it needs a six-terminal motor with enough starting torque.
- Forgetting shielded cable and correct EMC earthing for VFD-fed motors.