Motor Control Circuits and Starting Methods
This domain tests why starters exist, how each method limits current, and how the control logic prevents dangerous conditions.
Starting methods
- A squirrel-cage motor at direct-on-line draws 6–8 times full-load current. DOL is simplest and suits small motors, roughly up to 5–7.5 kW, where the supply permits.
- Star-delta starts the motor in star, so phase voltage is 1/√3 of line; starting current and torque fall to one-third of DOL values, with changeover by timer. The motor needs six terminals and must be delta-rated for the line voltage.
- Auto-transformer starting with 50/65/80% tappings reduces current in proportion to the square of the tapping.
- Soft starters and VFDs give stepless control; a VFD also saves energy on variable-torque loads such as fans and pumps.
Control logic essentials
- Self-holding contact around the start button.
- NC stop and NC overload contact in series with the coil.
- Forward/reverse uses two contactors with NC electrical interlock plus a mechanical interlock, and a short delay before reversing.
- Star-delta uses three contactors (main, star, delta), a timer and an interlock so star and delta can never close together.
Worked example
A 15 kW, 415 V motor has FLC ≈ 28 A. DOL starting current ≈ 6 × 28 = 168 A. Star-delta starting current ≈ 168/3 = 56 A, with starting torque also one-third of DOL. If the driven load needs more than about one-third of full-load torque to break away, star-delta may stall the motor and a different starter is required.
Traps
- Timer set too short (motor never accelerates) or too long (star contactor overheats).
- No interlock between star and delta contactors, causing a phase-to-phase short.
- Applying star-delta to a motor wound only for delta.
- Overload relay set above the motor's full-load current, or with the wrong trip class.