Direct On Line Starter
A Direct-On-Line (DOL) Starter, also called an across-the-line starter, is the simplest motor starting method that connects a three-phase induction motor directly to full supply voltage via a main contactor and overload relay, delivering full starting torque instantly without limiting inrush current.
On the shop floor, DOL starters are used primarily for small motors (typically ≤5 HP or ≤7.5 kW) where the power grid can tolerate high inrush current. They provide immediate full-speed operation for pumps, fans, and conveyors that do not require gradual acceleration. Additionally, DOL starters often serve as a redundant bypass for Variable Speed Drives (VSDs) during VSD failure, ensuring continuous operation.
- High inrush current (6–8× rated) causes voltage dips, thermal stress on windings, and premature contact wear in the contactor.
- Instant full-torque application creates shock loads on couplings, belts, and gearboxes, leading to shaft fatigue, bearing failure, and mechanical misalignment.
- Repeated DOL starts on motors >10 HP cause supply system disturbances, overload relay burnout, and nuisance tripping if grid capacity is insufficient.
Why is DOL unsuitable for motors >10 HP?
Because inrush current (6–8× rated) exceeds grid tolerance, causing voltage dips and excessive thermal/mechanical stress that damage motor windings and connected mechanics.
How does DOL protect against overload?
Via an overload relay that monitors current; if current exceeds safe limits for a set duration, it trips the contactor to stop the motor.
What is the starting torque of a DOL starter compared to Star-Delta?
DOL delivers 100% full starting torque, whereas Star-Delta reduces torque to ~33% at startup to limit current.
When is DOL used in a VSD bypass circuit?
As a redundant backup to maintain full-speed continuous operation if the VSD fails or requires maintenance.