Why use a bypass on a Variable Frequency Drive?

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At SAKO, we believe in continual learning and sharing knowledge with our partners. As the general manager of foreign trade in the SAKO VFD department, I find it exciting to delve into the depths of motor control systems. One aspect that often piques interest is the use of a bypass on a Variable Frequency Drive (VFD). The question is, why do we use a bypass on a VFD, and what difference does it make?

In simple terms, a bypass on a VFD is an alternate motor starting method designed to ensure motor operation even when the VFD is unavailable or needs maintenance. It essentially provides a backup plan, enhancing system reliability and minimizing operational downtime.

Continue reading as we delve into this intriguing aspect of VFDs, discussing why incorporating a torque bypass can be a game-changer for your motor control operations.

What is a Bypass in a VFD System?

In a VFD system, equipment with a bypass, sometimes referred to as a “bypass contactor” or “bypass circuit,” acts as an alternate pathway for the electric current to reach the motor if the VFD fails or requires maintenance.

The flexibility provided by a bypass also adds value. With a bypass in place, you have the flexibility to choose between energy-efficient VFD control units and full-speed DOL operation depending on your needs.

What are the Advantages and Disadvantages of Using a Bypass?

As with most engineering choices, the use of a bypass in a VFD system comes with its pros and cons.

The primary advantage is increased system reliability. With a bypass in place, your motor operations won’t come to a halt even if the primary VFD fails, or is down for maintenance. This reliability can be crucial, especially for systems where downtime can result in substantial losses.

Moreover, a bypass allows for flexibility in motor control. Depending on the application requirements, you can switch between the energy-efficient VFD and the full-speed DOL operation. This feature makes it possible to adapt motors well to varying operational demands.

However, a bypass does add complexity and cost to the system. The initial setup cost is higher due to additional components, and maintenance requirements increase with system complexity. Moreover, running the motor on DOL through the bypass does not offer the energy efficiency and precision control that a VFD bypass provides.

How Does a VFD Without Bypass Impact System Operations?

To fully comprehend the repercussions of a VFD without bypass, we must first understand the general function of a bypass in a motor control setup.

A bypass arrangement, often referred to as a “bypass circuit” or “bypass contactor,” serves as an alternative path for electric current to flow if the VFD faces unexpected issues, enabling the motor to continue running in direct line power. However, the bypass does add to the system’s complexity and cost. So, what happens when we remove this bypass from the equation?

A VFD without bypass simplifies the motor control setup. Instead of a dual method (VFD and DOL), the motor operation is solely regulated by the VFD. This simplicity often translates into cost savings, reduced maintenance needs, and enhanced efficiency.

Is a VFD With Bypass Right For You?

Whether or not to use a bypass with your VFD depends largely on your specific needs and the criticality of uninterrupted operation in your application.

If your system can afford occasional downtime or if you’re looking to keep the setup simple and cost-effective, a VFD without a bypass could suffice. But, if operational continuity is of paramount importance in your application, a VFD with bypass may be the right choice.

As always, it’s advisable to consult with professionals when making such decisions. At SAKO, we’re always ready to assist you in evaluating your needs and making the best choice.

Three different types to implement a bypass in your VFD system:

1. Manual Bypass:

This is the simplest and most cost-effective method. It involves the use of a manual switch that physically disconnects the motor from the VFD and connects it directly to line of power supply. The switch can be a simple toggle switch or a more complex selector switch.

However, this method lacks the sophistication of an automatic bypass. The manual nature of the operation means the switch needs to be physically operated by someone, and any error in the undertaking can lead to motor damage.

2. Automatic Bypass:

An automatic bypass uses sensors and control logic to detect a VFD failure and automatically switch the motor control from the VFD to line power. This method ensures that the work of the motor continues without any human intervention, enhancing the reliability of your system.

Typically, An automatic bypass is more complex and expensive than a manual bypass, but the enhanced reliability and ease of operation often justify the additional cost.

3. Smart Bypass:

A smart bypass takes the functionality of an automatic bypass one step further. In addition to detecting a VFD failure, a smart bypass unit also monitors the motor load conditions and can switch between VFD control and DOL operation depending on the load requirements.

This method is the most complex and expensive of the three, but it offers the highest level of flexibility and control, making it suitable for more demanding applications.

HVAC VFDs: 2-Contactor Bypass or 3-Contactor Bypass, Which One is Better?

When discussing bypass configurations in HVAC applications, we often encounter the debate between 2-contactor bypass and 3-contactor bypass. Both configurations have their distinct advantages and use cases. Let’s dive a bit deeper to understand the differences and help you make the right choice for your HVAC system.

2-Contactor Bypass

In a 2-contactor bypass configuration, there are two contactors present: one for the VFD output and one for the bypass. When the bypass mode is activated, the VFD output contactor opens, and the bypass contactor closes, allowing the motor to run directly on line power. When the system switches back to VFD control, the opposite happens.

The 2-contactor bypass configuration is simpler and more cost-effective than the 3-contactor bypass option. However, the downside is that the VFD is directly connected to the line power when in bypass mode, which could pose a risk if the VFD has an internal short circuit.

3-Contactor Bypass

A 3-contactor bypass configuration includes an additional contactor to isolate and disconnect the VFD from the line power when the system is in bypass mode. This setup provides an extra layer of safety, protecting the VFD from potential damage.

While the 3-contactor bypass offers enhanced safety and protection features for the VFD, it comes at a higher cost and complexity compared to the 2-contactor bypass.

Which One is Better?

The choice between 2-contactor bypass and 3-contactor bypass largely depends on your specific requirements and budget.

If cost and simplicity are your primary concerns, a 2-contactor bypass could be a suitable choice. However, if safety and VFD protection are paramount, investing in a 3-contactor bypass would be the better decision.


In conclusion, using a bypass on a VFD enhances system reliability, offering an alternate path for motor operation when the primary VFD itself is unavailable. While this setup does add some complexity and cost, the reliability and flexibility it brings can be a game-changer for critical applications.

Remember, there’s no one-size-fits-all solution in engineering, and the decision should be based on your specific needs. Don’t hesitate to reach out to us at SAKO for expert advice tailored to your unique situation. We’re here to help!


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