Control Panels, Motor Starters and Protection Devices
Starters and protection
Direct-on-line (DOL) starting is used for small motors, typically up to about 5 kW on 415 V. Star-delta reduces starting current and torque to one-third, at the cost of a changeover timer and two contactors plus a main contactor. Autotransformer, soft starter and VFD are used where controlled acceleration is needed.
Control circuit logic
- Start push button: normally open (NO); stop: normally closed (NC); thermal overload: NC in the control circuit.
- An auxiliary NO contact of the contactor is wired in parallel with the start button to hold the circuit after release.
- Star-delta needs electrical and mechanical interlocking so the star and delta contactors cannot close together.
- Protection functions: short circuit (MCB/MCCB/fuse), overload (thermal relay), earth fault (RCCB/ELR), phase failure and undervoltage.
Worked example
A 15 kW, 415 V, three-phase motor has full-load current I = P/(√3 × V × η × cos φ). Taking η = 0.88 and cos φ = 0.85, I = 15000/(1.732 × 415 × 0.88 × 0.85) = 27.9 A. Set the thermal overload relay to about 28–30 A and select a contactor rated above that current.
Traps
Wiring the stop button NO so the motor will not stop, omitting the holding contact, and adjusting the overload to the starting current rather than full-load current. Never bypass an overload to make a motor run.