What is DOL (Direct-On-Line) starting?
DOL (Direct-On-Line) is the simplest and most widely used method of connecting a three-phase electric motor directly to the mains. When switched on, the motor is immediately energised at full mains voltage, which provides a fast start but also a high inrush current.
DOL starters are widely used in installations with small to medium-sized motors where high starting torque is desirable and the current peak is not a problem.
🧠 How does a DOL starter work?
- Direct mains connection
- When the main switch or contactor closes, the motor is connected directly to 400 V (3-phase).
- Control and protection
- Switching via a contactor with coil (A1/A2)
- Thermal overload relay protects against overheating
- Start/stop control via pushbuttons or PLC
- Current and torque
- Inrush current: up to 6–8× the rated current
- Full starting torque available from the start
DOL is simple, inexpensive and robust, but causes mechanical and electrical peaks.
🏭 Use in industrial installations
- Fans, pumps, compressors, conveyors
- Motors <5.5 kW (up to 10–15 kW in some installations)
- Where short start-up is not a problem
- In an MCC (Motor Control Center) or control cabinet
- Controlled via PLC or IO module
Suitable where mains pollution or mechanical load are less critical.
🔍 DOL vs. other starting methods
| Starting method | Inrush current | Starting torque | Complexity | Application |
|---|---|---|---|---|
| DOL | 6–8× rated | 100% | Simple | Small motors, robust |
| Star-delta | 2–3× rated | ~33% | Moderate | Medium motors |
| Soft starter | Adjustable | Adjustable | Medium | Pumps, conveyors |
| Variable frequency drive | Low to rated | Fully controllable | Complex | Variable speed required |
🔐 Safety considerations
- Thermal protection against overload
- Short-circuit protection via fuse or circuit breaker
- Motor starters are often equipped with emergency-stop functionality
- Watch for voltage drops: restart protection needed during mains outages
- Combine with Access Control and PLC for safe operation
Some applications are subject to grid code or installation requirements limiting maximum inrush currents.
📌 In summary
DOL is a simple and reliable method for starting electric motors, ideal for smaller power ratings and robust applications. It is cost-effective, but requires attention to inrush current and peak loading.