Why Is a Bypass Contactor Used in Soft Starters?

Understanding the Role of a Soft Starter

A Soft Starter controls motor voltage during startup by using thyristors (SCRs) to gradually increase voltage. This allows the motor to accelerate smoothly, reducing:

• Electrical shock to the power grid

• Mechanical stress on shafts and couplings

• Sudden torque peaks

• Excessive starting current

SOSIAT soft starters are designed to deliver precise Voltage Control, ensuring stable acceleration even under heavy load conditions such as pumps, compressors, conveyors, and fans.

However, once the motor reaches full speed, the soft starter’s job is basically done.

What Is a Bypass Contactor?

A bypass contactoris an electromechanical switch that connects the motor directly to the Power Supply after startup is complete. Instead of current flowing through the SCRs inside the soft starter, it flows through the bypass contactor.

In SOSIAT soft starter systems, the bypass contactor is either built-in or externally controlled for seamless transition after acceleration.

Why Is a Bypass Contactor Needed?

1. Reduce Heat and Power Loss

SCRs create voltage drop and heat during continuous operation. If the motor keeps running through the soft starter:

• Energy Efficiency decreases

• Internal temperature rises

• Component life is shortened

By activating the bypass contactor, SOSIAT soft starters allow current to flow directly to the motor, reducing losses and keeping the system cooler.

2. Extend Equipment Lifetime

Continuous current through power electronics accelerates aging. With bypass operation:

• SCR stress is minimized

• Internal components last longer

• Maintenance intervals are extended

This is why SOSIAT designs its soft starters with optimized bypass logic to protect power modules and improve overall durability.

3. Improve System Efficiency

After startup, the motor no longer needs voltage control. Bypassing the soft starter:

• Eliminates unnecessary voltage drop

• Improves power factor

• Increases overall system efficiency

For large motors, even a small efficiency improvement translates into significant energy savings over time.

4. Enhance Reliability in Industrial Environments

Industrial applications require long continuous operation. With bypass contactors:

• Heat buildup is reduced

• Fault risk is lowered

• The system becomes more stable

SOSIAT soft starters integrate intelligent bypass control to ensure smooth switching without torque shock or electrical disturbance.

5. Allow Compact and Cost-Effective Design

Because the bypass contactor handles steady-state current, the SCRs inside the soft starter mainly manage startup. This allows SOSIAT engineers to design:

• More compact products

• Lower thermal requirements

• Higher current capacity without oversizing

This results in a better balance between performance and cost.

When Does the Bypass Contactor Activate?

In SOSIAT soft starters, the bypass contactor closes automatically when:

• The motor reaches rated speed

• Voltage ramp-up is complete

• Current stabilizes

The transition is smooth and does not affect motor torque or operation, ensuring continuous, safe performance.

Typical Applications Using Bypass Soft Starters

SOSIAT soft starters with bypass contactors are widely used in:

• Water and wastewater pumps

• HVAC fan systems

• Compressors

• Conveyors

• Crushers and mixers

In these applications, the combination of soft start and bypass operation ensures both smooth acceleration and efficient long-term running.

Conclusion

A bypass contactor is not just an accessory—it is a key part of a high-performance soft starter system. It reduces heat, improves efficiency, extends equipment life, and enhances reliability.

With intelligent bypass technology, SOSIAT soft starters deliver the best of both worlds: smooth motor startup and direct-online efficiency during operation. For industries seeking dependable motor control with lower operating costs, SOSIAT’s soft starter solutions provide a smart, future-ready choice.