Motor control circuits
Direct-On-Line Starting Circuit for an Induction Motor
What is direct-on-line starting?
Direct-on-line (DOL) starting connects an induction motor straight to full mains voltage through a contactor. It is the simplest, cheapest and most common motor starting method. At the first moment the motor draws several times its rated current and reaches normal speed in a short time.
Where is direct-on-line starting used?
It is used for small and medium motors: pumps, fans, extractors, compressors, conveyors, mixers and workshop machines. Wherever the starting current does not strain the supply, DOL is the first choice. For larger motors, star-delta starting, a soft starter or a frequency drive is used.
Power circuit and control circuit
In the power circuit the motor circuit breaker, the contactor's main contacts and the overload relay are in series; the overload relay protects the motor against overcurrent. The control circuit contains the stop button, the start button, the overload contact and the contactor coil. This start-stop control circuit is the basis of nearly every motor panel on site.
What to watch out for
The overload relay is set to the rated current on the motor nameplate. The contactor and circuit breaker are sized for the motor power; a motor-type breaker is preferred so it does not trip during starting. Because the starting current is high, large motors are not started direct-on-line on weak supplies.
Parts used
- Motor 3~
- M1
Run it in the simulator.
This circuit is ready in the CoilPath library. Open it, press the buttons and follow the current. It also comes with a wiring view and a panel layout.
Related circuits
- Induction Motor Jog (Inching) Circuit
- Seal-in Circuit (Continuous Running of an Induction Motor)
- Seal-in Circuit (Continuous Running of an Induction Motor with a Relay)
- Jog Button with Momentary Running and Seal-in Cancel Circuit
- Seal-in Circuit with Start–Stop Control from Three Locations
- Forward–Reverse Direct-On-Line Starting Circuit