18/12/2025
Here is a clear and practical explanation of the difference between an online (in-line) soft starter and a bypass soft starter — useful for electrical, automation, and motor-control work.
✅ 1. Online (In-Line) Soft Starter
Definition
The soft starter stays in the motor power path at all times, both during starting and normal running.
How it works
Power flows through the soft starter’s SCRs continuously.
SCR heat losses remain throughout operation.
The soft starter handles start, run, and stop modes.
Advantages
Simple wiring.
Full control available at all times (soft stop, current limit, protections).
Disadvantages
Generates heat continuously → requires larger heat sink.
More losses = lower efficiency.
Higher cost for cooling and long-term stress on SCRs.
When used
Applications requiring continuous torque control.
Where soft stop is necessary (e.g., pumps, conveyors that need controlled stopping).
✅ 2. Bypass Soft Starter
Definition
The soft starter is used only during starting.
Once the motor reaches full speed, an internal or external bypass contactor closes, routing current around the SCRs.
How it works
SCRs ramp the motor up.
At ~90–100% speed, bypass contactor closes.
Motor then runs direct-on-line, not through SCRs.
Advantages
Much lower heat loss after start.
Higher efficiency during running.
Smaller enclosure needed.
SCRs have longer life.
Disadvantages
No continuous control (torque, voltage) once bypass is engaged.
Soft stop is often unavailable unless the bypass is opened again.
When used
Most general-purpose applications: fans, pumps, compressors.
Where you only need soft start, not continuous control.
