Motor Starters Explained: DOL, Star-Delta and Autotransformer Starting
At the instant a three-phase induction motor is switched on, the rotor is stationary, slip is 1, and the motor behaves like a transformer with a shorted secondary. It draws a starting current typically six to eight times its full-load current. For small motors this is harmless, but for larger motors it causes voltage dips that flicker lights, trip other equipment and stress the supply transformer. A motor starter controls this starting process and also protects the motor during running.
This article explains the control circuit common to all starters, then compares DOL, star-delta, autotransformer and soft starting with a worked example for a 15 kW motor.
What Every Starter Does
- Switching: A contactor, operated by a coil, connects and disconnects the motor.
- Overload protection: A thermal or electronic overload relay trips the contactor if current stays above the set value for too long.
- No-volt protection: If supply fails, the contactor drops out and does not restart automatically when supply returns.
- Short-circuit protection: Provided upstream by fuses or a motor protection circuit breaker (MPCB), not by the overload relay.
The basic start-stop control circuit
L ──[Stop NC]──┬──[Start NO]──┬──( K1 coil )──[OL NC]── N
│ │
└──[K1 aux NO]─┘
Start (NO) pressed -> K1 coil energises -> main contacts close
K1 aux NO closes -> holds coil on after Start is released
Stop (NC) pressed -> coil circuit broken -> motor stops
Overload trips -> OL NC opens -> motor stopsThe auxiliary contact in parallel with the start button is called the hold-on or self-holding contact. Because the coil drops out on loss of voltage and needs the start button again to re-energise, this one circuit provides no-volt protection automatically.
DOL (Direct-On-Line) Starter
A DOL starter applies full line voltage to the motor terminals at once. It is the simplest and cheapest starter and gives full starting torque, but also full starting current. It is used for small motors, typically up to a few kW, and wherever the supply is strong enough to accept the inrush. Many DISCOMs specify the type of starter required above certain motor ratings in their supply conditions, so check local rules for pump and workshop connections.
Star-Delta Starter
A star-delta starter needs a motor with all six winding ends brought out and rated for delta operation at line voltage, that is a 415 V delta motor. Three contactors are used: Main (K1), Star (K2) and Delta (K3). At start, K1 and K2 close, so each winding sees 415/√3 = 240 V. After a timer delay, K2 opens and K3 closes, reconnecting the windings in delta at full voltage.
In star, each winding gets 1/√3 of the voltage: Phase current in star = (1/√3) × phase current in delta Line current in star = (1/3) × line current in delta (DOL) Torque ∝ V² = (1/3) × DOL starting torque
Starting current and starting torque are both reduced to one third. This suits fans, centrifugal pumps and machine tools that start lightly loaded, but not loaded conveyors or compressors that need high breakaway torque. The timer is set so the changeover happens when the motor has nearly reached full speed, usually a few seconds; changing over too early produces a second current peak almost as high as DOL.
Worked example: 15 kW motor
A 15 kW, 415 V motor has efficiency 0.90 and power factor 0.85 at full load. Assume DOL starting current is 6 times full-load current.
- Input power = 15000/0.90 = 16667 W.
- Full-load current IFL = 16667/(√3 × 415 × 0.85) = 16667/611 = 27.3 A.
- DOL starting current = 6 × 27.3 = 164 A.
- Star-delta starting current = 164/3 = 55 A.
- Current in each winding in delta running = 27.3/√3 = 15.8 A.
| Component | Carries | Rating basis for 15 kW example |
|---|---|---|
| Main contactor K1 | Winding current | 0.58 × IFL = 15.8 A, choose next standard AC-3 size |
| Delta contactor K3 | Winding current | 0.58 × IFL = 15.8 A |
| Star contactor K2 | Star current | 0.33 × IFL = 9.0 A |
| Overload relay (in winding circuit after K1) | Winding current | Set at 0.58 × IFL = 15.8 A |
Autotransformer, Soft Starter and VFD
An autotransformer starter supplies a reduced voltage from a tap, commonly 50%, 65% or 80%. With tap ratio k, both line current and starting torque reduce to k² times their DOL values; at the 65% tap this is about 42%. It gives more torque per ampere than star-delta and works with three-terminal motors, but is bulkier and costlier.
| Method | Starting current (× DOL) | Starting torque (× DOL) | Notes |
|---|---|---|---|
| DOL | 1 | 1 | Simplest, highest inrush |
| Star-delta | 1/3 | 1/3 | Needs 6-terminal delta-rated motor, transient at changeover |
| Autotransformer, tap k | k² | k² | Selectable taps, higher cost |
| Soft starter | Adjustable ramp | Reduces with voltage | Thyristor voltage ramp, smooth stop for pumps |
| VFD | Close to rated current | Up to rated torque at low speed | Also gives speed control and energy saving |
Troubleshooting and Safety
- Motor runs in star but trips on changeover: timer too short, or star and delta contactors overlapping. Use mechanical and electrical interlocks between K2 and K3.
- Motor hums but does not start: one phase missing (single phasing). Check fuses and contactor contacts.
- Contactor chatters: low control voltage or a broken hold-on contact.
- Overload trips during normal running: wrong setting, overload on the driven machine, or low supply voltage.
Summary
Starters switch, protect and limit starting current. DOL gives full torque with full inrush; star-delta cuts both current and torque to one third; an autotransformer gives k² reduction; soft starters and VFDs give smooth control. Size star-delta contactors and overloads on winding current, about 58% of full-load line current.