DOL starter: contactor plus overload relay
A direct-on-line (DOL) starter connects the motor straight to the supply. It is built from a contactor that switches the motor and a thermal overload relay that opens the control circuit when the motor draws too much current for too long. Short-circuit protection comes from a breaker or fuses upstream. Contactors, overload relays and starters are covered by IEC 60947-4-1.
DOL is simple and cheap, but the motor draws several times its full-load current at start (often around 6 to 8 times, see the motor data) and starts with full torque, which can stress pipes, belts and the supply.
Motor protection circuit breaker (MPCB)
An MPCB combines an adjustable thermal overload and a magnetic short-circuit trip in one device, with a rotary or push-button operator. With a contactor it makes a compact starter that saves space and wiring. Set the thermal dial to the motor full-load current from the nameplate. Check its breaking capacity against the fault level at the panel, as with any breaker.
Contactor ratings: AC-3 and AC-4
Contactors are rated by utilisation category under IEC 60947-4-1:
- AC-3: squirrel-cage motors, starting and switching off while running. This is the rating for normal motor duty.
- AC-4: squirrel-cage motors with plugging, inching or reversing by switching while the motor is still starting. The contactor must break the high starting current, so its rating is much lower than for AC-3.
- AC-1: non-inductive or slightly inductive loads such as heaters, not motors.
Choose the contactor by its AC-3 current (or kW) at your voltage, and the coil voltage of your control circuit.
Overload trip classes
The trip class tells you how long the relay allows a high current before tripping. IEC 60947-4-1 defines the maximum tripping time from cold at 7.2 times the setting current: class 10 within 10 s, class 20 within 20 s, class 30 within 30 s (class 10A within 10 s, with a lower minimum time). Use class 10 for normal starts, and class 20 or 30 for heavy loads that take longer to run up, such as large fans, crushers and loaded conveyors, so the relay does not trip during a healthy start.
Reducing the starting current
| Method | How it works | Points to check |
|---|---|---|
| Star-delta | Starts the motor in star, then switches to delta; starting current and torque fall to about one third of DOL values | Motor must be rated for delta at the supply voltage, with 6 terminals; there is a current peak at changeover |
| Soft starter (IEC 60947-4-2) | Ramps up the voltage electronically for a smooth start and stop | No speed control in running; usually bypassed by a contactor once at speed |
| VFD (IEC 61800 series) | Changes frequency and voltage; full torque at low current, plus speed control | Higher cost; harmonics, motor cable length and screened cable need attention |
When a VFD is worth it
A variable frequency drive pays back where the speed must change or the flow is throttled: pumps, fans and compressors running at part load. For centrifugal pumps and fans, the affinity laws say power varies roughly with the cube of speed, so in theory running at 80 % speed needs about half the power. Real savings depend on the system curve and running hours. A VFD also gives the softest start, which helps long conveyors and weak supplies. For a motor that runs at constant full load, a DOL or star-delta starter is often the simpler choice. Our VFD sizing tool helps match the drive to the motor and the load type.
Coordination type 1 or type 2
IEC 60947-4-1 defines what happens to the starter after a short circuit. Type 1: no danger to people or the installation, but the contactor and relay may be damaged and need repair or replacement. Type 2: no damage except light contact welding that can be easily separated, and the starter is usable after the fault. Makers publish tested combinations of breaker or fuse, contactor and overload for each type at a given fault level; for process plants where downtime is costly, type 2 is the usual specification.
What to send us for a quotation
Motor kW, voltage and full-load current from the nameplate, the load (pump, fan, conveyor, compressor), the starting method, control voltage, the fault level at the panel and the coordination type required. For a VFD, add whether you need speed control and the cable length to the motor. We quote the complete starter or drive with breakers, cable and accessories.
Common questions
What is the difference between a contactor and an overload relay?
The contactor switches the motor on and off. The overload relay senses excessive current over time and opens the contactor's control circuit to protect the motor.
What does AC-3 mean on a contactor?
It is the IEC 60947-4-1 utilisation category for starting squirrel-cage motors and switching them off while running. Size motor contactors by their AC-3 rating.
Does a VFD always save energy?
No. Savings come where speed can be reduced, mainly pumps and fans at part load. A motor running at constant full speed saves little and the drive adds its own losses.
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This guide is general information. Final selection must follow the project design, the applicable standards and the maker's data.