Electrical Control and Motor Starting
Core Concepts
Understand control circuits: start/stop pushbuttons, contactors, relays, overload relays, timers, and limit switches. Know motor starting methods: direct-on-line (DOL), star-delta, auto-transformer, soft starter, and variable frequency drive (VFD). Understand forward/reverse control and interlocking. Know the difference between a contactor and a relay (contactor for high power, relay for control). Understand ladder logic diagrams and symbols. Know protection: overload, short circuit, phase failure, and under-voltage.
Traps in Questions
- Star-delta starter reduces starting current to one-third and starting torque to one-third.
- DOL starting current is typically 6-8 times full load current.
- Interlocking prevents simultaneous operation of forward and reverse contactors.
- Overload relay protects against sustained overload, not short circuit.
- A contactor is not a circuit breaker; it does not provide short-circuit protection.
Worked Example / Rule of Thumb
Example: A 15 kW, 400 V, 3-phase motor has full load current ≈ 28 A. DOL starting current ≈ 6×28 = 168 A. With star-delta, starting current ≈ 168/3 = 56 A. Select a contactor rated at least 28 A (AC-3) and an overload relay set to 28 A. Rule of thumb: For star-delta, the contactor in the delta path carries phase current (I_L/√3), so it can be smaller; the main and star contactors carry line current.