Motor Starter (DOL / Star-Delta)
Control
Contactor + overload relay packaged to start and protect three-phase motors. DOL slams full voltage in (simple, big inrush); star-delta starts at 58% voltage then switches, cutting the inrush for larger motors.
Schematic symbol
Motor
How motor starter (dol / star-delta) appears in a circuit diagram.
Types & variants
Key specs
Motor rating
kW / HPThe motor size the starter is built for at your voltage.
Coil voltage
VControl circuit voltage: 24/110/230/415 V options.
Overload range
ADial the relay to the motor's nameplate full-load current.
Utilisation category
AC-3 rating covers normal motor switching duty.
Laws that govern it
Star-delta starting is this transformation applied for real: in star each winding sees 1/√3 of the line voltage, cutting starting current and torque to a third.
- Joule's law of heatingH = I² × R × t
The reason to bother: starting current of six to eight times full load heats windings as I²t, and starters exist to limit that.
- Kirchhoff's voltage law (KVL)ΣV(loop) = 0
A DOL starter's control circuit is a latching loop through the hold-in contact and stop button — trace the loop and the wiring makes sense.
Each law is stated in full — with its diagram, variables and worked meaning — on the electrical laws page.
Markings
kW/HP table on the front, overload dial in amps, L1L2L3 / T1T2T3 terminals, A1A2 coil.
Standard values
DOL up to ~7.5 kW; star-delta favoured above that (where the utility demands reduced starting current).
How to choose
DOL for small motors, star-delta or a soft starter above ~7.5 kW or where supply dip matters, reversing starters need mechanical + electrical interlock. Overload set to nameplate FLC, not maximum.
Pinout & package
Panel assembly: incoming supply → contactor(s) → overload → motor; start/stop pushbuttons in the coil circuit with the classic seal-in contact.
Example circuits
- DOL starter with start/stop station for a 3 HP pump
- Star-delta starting a 15 kW compressor
- Reversing starter on a hoist
Common failures
Welded or pitted contactor contacts, humming/chattering from low coil voltage, star-delta timers failing (motor stays in star = no torque), overload trips from actual motor problems being reset repeatedly.
How to test
Electricians only, supply isolated: coil resistance sensible (hundreds of Ω at 230 V), contact continuity when manually pressed, overload trip lever operates the NC contact. Repeated overload trips = investigate the motor, don't bypass.
Substitutes
Same kW/coil-voltage starter; a VFD replaces the whole starter and adds speed control where budget allows.