DOL

What is a DOL starter?

A DOL (Direct-On-Line) starter is a motor starter that switches a three-phase induction motor straight onto full mains voltage in a single step, without limiting voltage or current during acceleration. It is the simplest and cheapest starting method: a contactor, an overload relay and short-circuit protection are all it takes. The trade-off is an inrush current of six to eight times rated current and a torque shock on the driven machine.

DOL starters sit in almost every MCC and control cabinet. They switch pumps, fans, compressors and conveyors wherever the supply is strong enough and the machine can tolerate a hard start.

🧠 How does a DOL starter work?

A DOL starter combines three functions in one main circuit, plus a simple control circuit:

• Short-circuit protection - fuses or a motor protection circuit breaker disconnect on a short circuit in the cable or motor.
• Contactor - the coil (terminals A1/A2) pulls in the main contacts, putting the motor directly on 400 V.
• Overload relay - thermal or electronic protection tolerates the brief starting current but trips on sustained overcurrent.

The classic control circuit has a start button (normally open), a stop button (normally closed) and a hold-in (latching) contact: an auxiliary contact of the contactor wired in parallel with the start button that keeps the coil energised. If the supply fails, the contactor drops out and the motor stays off until someone starts it again. That behaviour is exactly the restart protection machinery standards ask for. In modern installations a PLC issues the start command via an output on an IO module, while the overload contact and the auxiliary contacts are read back as feedback. Where the start command passes through an interposing relay, the same principle applies.

⚡ What current and torque does a direct start produce?

At the moment of switch-on the rotor is stationary and the motor behaves like a short-circuited transformer. It therefore draws its locked rotor current until it reaches speed. IEC 60034-12 describes this behaviour for cage induction motors. Design N (normal starting torque) is explicitly intended for direct-on-line starting, from 0.12 to 1,600 kW. Design NE may have a higher locked rotor current in order to reach a higher efficiency class.

The starting current follows from the locked rotor apparent power ratio Sₗ/Pₙ, which IEC 60034-12 caps per power band: Iₗ = (Sₗ/Pₙ) × Pₙ / (√3 × Uₙ). The advantage is that you can estimate the starting current early in a project from rated power and voltage alone. Torque varies with the square of the voltage. The standard therefore allows for 10 % undervoltage at the motor terminals during acceleration, at which the motor still delivers just over 80 % of its starting torque.

🔄 How does DOL compare with other starting methods?

DOL has no power electronics, so no harmonics, no heat losses in semiconductors and no firmware. The motor delivers its full torque immediately, which helps with loads that are hard to start. Star-delta, soft starters and frequency drives trade that simplicity for a lower supply burden and a gentler start. For an overview of drive options, see also Drives.

🏭 When should you choose a DOL starter?

A DOL starter is the logical choice when three conditions hold:

• The supply is strong enough - the voltage dip during starting does not disturb other loads. On a plant’s own transformer, motors of tens of kilowatts are routinely started direct. On a small connection to the public low-voltage network the practical limit is often around 5.5 kW, and the network operator may set requirements.
• The machine tolerates the torque shock - couplings, gearboxes and belts absorb the shock without slipping or damage.
• A fixed speed is acceptable - after starting, the motor always runs at rated speed.

For equipment connected to the public network, IEC 61000-3-3 (up to 16 A per phase) and IEC 61000-3-11 (16 to 75 A, conditional connection) limit the voltage fluctuations and flicker a device may cause. A motor that starts direct and frequently can hit those limits first.

Less suitable are long pipelines where a hard start and stop cause water hammer, conveyors carrying loose products, and applications with many starts per hour. IEC 60034-12 itself notes that the number of starts should be minimised, because every start heats the winding and shortens motor life.

🔧 How do you size a DOL starter?

1. Read the nameplate - rated current, voltage (400 V Δ or 230/400 V), efficiency class and, from the datasheet, the Iₗ/Iₙ ratio.
2. Select the contactor on AC-3 - a contactor to IEC 60947-4-1 with an AC-3 rated current at 400 V of at least the motor current. Use the AC-3e rating where the manufacturer states one for IE3 and IE4 motors with higher starting current.
3. Set the overload relay - to the nameplate current, with trip class 10 for a normal start and class 20 or 30 for a heavy start.
4. Choose the short-circuit protection - preferably a combination tested by the manufacturer for type 2 coordination.
5. Check the voltage dip - especially for large motors or a weak supply. Consult the network operator where needed.

Worked example

A centrifugal pump at a wastewater treatment plant has a 4-pole IE3 motor of 11 kW, 400 V, with a rated current of about 21 A. With a starting current ratio of around 7, the motor draws some 150 A on a direct start, for one to two seconds. You select a contactor with an AC-3 current of at least 21 A, usually the 11 kW frame size, and set the overload relay to 21 A in class 10. If the pump causes noticeable water hammer in the discharge line, a soft starter with a controlled stop is the better choice.

Since 1 July 2021, new three-phase motors from 0.75 to 1,000 kW must reach at least IE3 under the Ecodesign Regulation (EU) 2019/1781. Replacing an old motor can therefore raise the starting current. Check whether the contactor and short-circuit protection still fit.

🔐 How do you control a DOL starter safely from OT?

• Restart protection - under IEC 60204-1, a machine must not restart unexpectedly when the supply returns if that would create a hazard. The hold-in contact or an edge-triggered start command in the PLC takes care of this.
• Stop category 0 - a contactor that de-energises the motor is an uncontrolled stop under IEC 60204-1. An emergency stop to NEN-EN-ISO 13850 often works exactly this way, via a safety relay that drops out the contactor.
• Hardware interlocking - in a reversing starter, both contactors must never close at the same time. Do not rely on software alone for that.
• Monitor feedback - read the contactor’s auxiliary contact and raise a fault if the motor fails to start or the contactor sticks.
• Restrict remote access - anyone who can start a motor from SCADA or an HMI can also cause damage. Define permissions through access control as part of your overall safety and security approach.
• Use the data - smart motor starters with IO-Link or a fieldbus report current, number of starts and trip causes for condition monitoring.

In the Netherlands the installation itself falls under NEN 1010, and work on the cabinet under NEN 3140.

❓ Frequently asked questions

Why does a motor draw so much current on a DOL start?

On a DOL start the rotor is still stationary, so there is no back-EMF and the motor behaves like a short-circuited transformer. The current is limited only by the winding impedance and reaches six to eight times rated current. As the motor comes up to speed, the current falls back to its normal value.

Up to what power can a motor be started direct-on-line?

There is no fixed legal limit for a DOL start; it depends on the strength of the supply and what the machine can withstand. On a small connection to the public network the practical limit is often around 5.5 kW. In a plant with its own transformer, much larger motors are started direct.

Which contactor does a DOL starter need?

A DOL starter uses a contactor selected on utilisation category AC-3 to IEC 60947-4-1, rated for at least the motor’s full-load current. For IE3 and IE4 motors with a higher starting current, manufacturers sometimes state a separate AC-3e rating. When in doubt, take the contactor from the manufacturer’s coordination table.

Why does the overload relay trip during starting?

An overload relay that trips on a DOL start is usually set to too low a current or has a trip class that is too fast for the acceleration time. A heavy load with high inertia needs class 20 or 30. Never set the relay above the nameplate current to work around the problem.

Can a DOL starter reverse the direction of rotation?

A standard DOL starter runs the motor in one direction only. A reversing starter is a DOL variant with two contactors that swap two phases, with electrical and mechanical interlocking so they never close together.

Does a DOL starter save energy?

A DOL starter does not save energy itself, because the motor always runs at full speed after starting. It does, however, have virtually no losses of its own. For pumps and fans with varying demand, a variable frequency drive delivers substantial savings.

📌 In summary

A DOL starter switches a three-phase motor straight onto full mains voltage: simple, inexpensive and with full starting torque, but with a starting current of six to eight times rated current.

Choose DOL for motors on a strong supply driving a load that can take a hard start. Size the contactor on AC-3 or AC-3e, set the overload relay to the nameplate current and allow for the higher starting current of IE3 and IE4 motors. Where the supply or the machine cannot tolerate the current surge, star-delta starting, a soft starter or a variable frequency drive are the alternatives.