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Bypass Soft Starters vs Online-type: Key Differences

2024-12-13

When choosing a soft starter for your motor control system, you may come across two primary types: bypass soft starters and online-type soft starters. Each type has its advantages and is suitable for different applications, depending on your specific needs. In this guide, we’ll delve into the key differences, advantages, and disadvantages of these two types of soft starters, as well as provide practical tips for making an informed decision.

What is a Soft Starter?

Before we dive into the details of bypass and online-type soft starters, let’s quickly review what a soft starter does. A soft starter is an electronic device used to reduce the inrush current when starting an electric motor. This helps protect the motor and the electrical system from damage due to sudden power surges. Soft starters gradually ramp up the voltage supplied to the motor, allowing it to start smoothly without unnecessary wear or stress on the system.

Now, let's compare the two main types: bypass-type soft starters and online-type soft starters.

1. External Bypass Soft Starters

A bypass soft starter combines the advantages of both thyristor-based and contactor-based motor control. The key to understanding this type lies in the use of an external bypass contactor.

How Does a Bypass Soft Starter Work?

In a bypass soft starter, a thyristor is used during the soft-start phase to control the motor’s voltage. After the motor starts and reaches its normal operating speed, the thyristor is bypassed by a contactor, which then supplies full voltage to the motor for efficient running. This allows the motor to operate without the energy losses that occur in continuous thyristor operation.

Advantages of Bypass Soft Starters

  1. Reduced Power Loss and Harmonics: Since the thyristor is only used during startup, the bypass soft starter reduces power losses and harmonic distortion, which are common in online-type soft starters.
  2. Compact Design: Bypass soft starters don’t need large heat sinks or forced cooling, making them more compact and easier to install compared to online-type soft starters.
  3. Lower Operating Costs: With a shorter operating time for the thyristor, bypass soft starters consume less energy, reducing your long-term operating costs.
  4. Motor Protection: These starters offer protection against overcurrent, phase failure, and other issues that could potentially damage your motor. The system will automatically cut off the power if any abnormalities are detected.
  5. Maintenance and Reliability: Bypass starters typically have lower maintenance needs and a longer service life due to reduced thermal stress and component wear.

Example Applications of Bypass Soft Starters

  • HVAC Systems: Bypass soft starters are ideal for heating, ventilation, and air conditioning systems that require smooth motor startups without excessive power draw.
  • Pumps and Compressors: These motors often run continuously, making the bypass design ideal as it reduces heat dissipation issues.
  • Conveyor Systems: In systems where motor startups need to be smooth but energy efficiency is also a priority, bypass soft starters offer a balanced solution.

2. Online-type Soft Starters

An online-type soft starter, also known as a thyristor-based soft starter, keeps the thyristors engaged for the entire duration of the motor’s operation, including the startup, running, and shutdown phases. Unlike bypass starters, there is no contactor that bypasses the thyristor once the motor reaches full speed.

How Does an Online-type Soft Starter Work?

In this type of soft starter, the thyristors are used to gradually increase the voltage supplied to the motor during startup. Once the motor is up and running, the thyristor remains in the circuit, continuously controlling the voltage supplied to the motor. This can lead to more consistent control of the motor’s performance.

Advantages of Online-type Soft Starters

  1. Smooth and Precise ControlSince the thyristor is continuously regulating the motor's voltage, online-type soft starters offer more precise control of motor performance, especially in applications requiring fine-tuned speed and torque management.
  2. Simple DesignOnline-type soft starters have a simpler design without the need for external contactors. This can make installation easier for certain applications.

Disadvantages of Online-type Soft Starters

  1. Higher Power Consumption: The primary downside to online-type soft starters is that the thyristor remains engaged throughout the motor’s operation, leading to higher energy consumption. This can result in higher operating costs over time, especially for larger motors.
  2. Increased Harmonic Distortion: The continuous operation of the thyristor in online-type soft starters leads to more harmonic distortion in the power supply. This can reduce the overall power factor of the system and increase the need for additional power conditioning equipment.
  3. Higher Heat Dissipation: Since the thyristor is continuously active, online-type soft starters produce more heat. This means the devices require larger heat sinks and, in some cases, forced cooling to prevent overheating, increasing their size and complexity.

Example Applications of Online-type Soft Starters

  • Crushers and Mills: Equipment that operates under varying loads and requires precise control of motor speed will benefit from the consistent performance of online-type soft starters.
  • Elevators: These systems require smooth starts and stops, and online-type soft starters can help in achieving this level of control.
  • Fans and Blowers: When fine control of the motor’s speed is critical, online-type soft starters provide the necessary regulation for smooth operation.

3. Internal Bypass Soft Starters: The Best of Both Worlds

For engineers looking for the benefits of both types, internal bypass soft starters offer a hybrid solution. These starters combine the simplicity of the bypass contactor with the precise control of the thyristor in a single unit.

How Do Internal Bypass Soft Starters Work?

In an internal bypass soft starter, the bypass contactor is integrated into the soft starter itself. The thyristor operates during the startup phase, while the contactor kicks in when the motor reaches full speed, switching to full-voltage operation. This design helps reduce energy losses and harmonics while maintaining precise control.

Advantages of Internal Bypass Soft Starters

  1. Space-Saving Design: By integrating both the thyristor and contactor into one unit, internal bypass soft starters save valuable space, making them ideal for tight installations.
  2. Improved Efficiency: The internal bypass mechanism ensures that the motor runs efficiently after startup without consuming unnecessary energy.
  3. Longer Lifespan: These starters are designed to reduce the thermal and mechanical stress on components, leading to a longer service life and greater reliability.

Comparison: Bypass vs. Online-type Soft Starters

To help you make an informed decision, here's a comparison of bypass soft starters, online-type soft starters, and internal bypass soft starters based on key criteria:

Feature/Criteria

External Bypass Soft Starter

Online-type Soft Starter

Internal Bypass Soft Starter

Size

Compact, space-saving

Larger, more complex design

Moderately compact

Power Efficiency

Very efficient

Higher energy consumption

Highly efficient

Harmonics

Minimal harmonic distortion

Higher harmonic distortion

Low harmonic distortion

Heat Generation

Minimal heat production

Significant heat generation

Low heat production

Maintenance

Low maintenance

Requires regular maintenance

Low maintenance

Durability

Long lifespan

Shorter lifespan

Very long lifespan

Tips for Choosing the Right Soft Starter

  • For Energy Efficiency: If minimizing power consumption is your priority, a bypass soft starter is the better option.
  • For Precision Control: If your application requires more precise control over motor speed and torque, consider an online-type soft starter.
  • For Space-Saving Design: If you’re tight on space but want the benefits of both types, an internal bypass soft starter is your best choice.

Conclusion

Choosing the right soft starter for your motor is essential to ensure long-term efficiency, performance, and cost-effectiveness. Bypass soft starters are the most common and cost-effective option for most industrial applications, offering significant benefits in terms of energy efficiency, compact design, and reliability. Online-type soft starters are suitable for applications where precise control is crucial, but they come with higher energy consumption and maintenance requirements. Internal bypass soft starters provide the best of both worlds, offering a compact, efficient, and reliable solution with precise motor control.

Ultimately, the choice depends on your specific needs, including the size of the motor, application requirements, and long-term operating costs.