3 Ways to Protect a VFD Braking Resistor from Short-Circuit Failure

Energy overload situations can cause braking resistor failure and lead to performance issues, equipment failures, and, in some cases, safety concerns. That’s not all: the cost of replacing or repairing a braking resistor, including labor, could run into the thousands. Fortunately, these failures can be easily prevented with the right equipment and system design.

In this article, we’ll explain the cause of overload conditions and present three methods that will help you address this common problem. After all, you want to protect your VFD equipment in order to prevent unhappy customers and co-workers who experience a system failure.

How VFD Brake Resistors Work

Energy from electric motors flows into the VFD’s DC bus. VFD brake resistors help control that setup to prevent excessive energy levels that would otherwise cause damage or faulty performance.

How does the brake resistor control this energy flow? The VFD will monitor voltage and activate the brake chopper when voltage exceeds a certain threshold. This “chopper” switches the brake resistor on and off. That process allows for energy to become safely absorbed. Generally speaking, it’s about controlling deceleration to protect the drive.

 
Article: When and How Should I Select a Braking Resistor?
 

Understanding Brake Choppers

Under normal operation, the brake resistor is driven by a brake chopper transistor when excess energy is returned to the VFD. The brake chopper continuously turns the brake resistor on and off until the extra energy is dissipated. The resistors are sized for use with a brake chopper, not for direct, continuous connection to the VFD’s DC bus.

Brake Resistor Failure Causes

Brake resistors can sometimes fail due to bad brake chopper performance. A brake chopper failure occurs when the braking transistor in a VFD can no longer properly control regenerative energy on the DC bus.

In most cases, the chopper fails in a shorted state. During a short-circuit failure, the braking resistor may be connected to the DC bus without normal switching control.

As mentioned earlier, the brake resistor is sized to be used with a chopper circuit. When correctly sized, the resistor can safely absorb regenerative energy without exceeding thermal limits.

However, a direct connection to the DC bus overloads the brake resistor, can cause external damage, and can also become a fire hazard. That could mean extensive costs for inspections, production losses, and part replacements, as well as labor.

Another potential cause of brake resistor failure is a failure in ventilation or cooling. When airflow is restricted, trapped heat can prevent a braking resistor from dissipating energy effectively. Such situations may lead to gradual insulation breakdown or even thermal failure.

You may also see common causes for resistor failure, like general mechanical or installation issues, exposure to elements, and excessive braking.

3 Methods to Protect Braking Resistors From Overload

There are several options to prevent a braking resistor from failing due to overload.

KEB suggests at least one of these options:

• Brake-Chopper Transistor Monitoring Board
• Installation of Brake Resistors with a Thermal Switch
• Intrinsically-Safe Brake Resistors

1. Brake-Chopper Transistor Monitoring Board

KEB offers an optional monitoring board for various sizes of F6 VFDs. This board monitors for short-circuit failures within the brake chopper circuit. When a brake chopper malfunction is detected, a drive fault is generated, triggering a dedicated form-C relay. The control system should be designed so that power is removed from the drive or resistor when this relay is opened.

2. Brake Resistors with a Thermal Switch

Brake resistors can be equipped with a thermal switch to detect a potential thermal overload condition. When a resistor is overloaded, it attempts to dissipate more heat than it was designed to do.

In this case, the thermal switch will open when the resistor overheats. KEB recommends interrupting one of the high voltage supplies to either the VFD or the resistor. Refer to the next 2 diagrams on how this can be implemented.

Above: The thermal switch opens from excessive heat. The control voltage used to close the mains contactor is interrupted. This opens the main contactor and removes power from the VFD.

Above: The thermal switch opens from excessive heat. The control voltage used to close the contactor to the braking resistor is interrupted. This opens the contactor and removes power from the braking resistor while the VFD remains powered.

3. Intrinsically-Safe Braking Resistors

KEB also offers braking resistors that are intrinsically safe. These resistors come equipped with internal overload protection that will open, similar to a fuse, when overloaded. This option does not require any additional hardware or control devices as the protection is built-in to the resistor.

Above: The resistor’s internal safety mechanism has opened due to overload. This safety feature behaves similarly to a fuse. No extra hardware is required for this type of resistor.

To learn more about braking resistor protection, check out this full article on the subject.

Choosing the Right Protection Method

Your choice in protection methods for VFD brake resistors will depend on what failures you’re trying to prevent.

• If you’re concerned about a shorted shortened brake chopper, then your setup may call for a transistor monitoring board.
• Using a brake resistor with a thermal switch provides protection against overheating.
• For simplicity, your best option may be braking resistors that are intrinsically safe. These have built-in overload protection.

Why Generic Brake Resistors Fail More Often

VFD brake resistors need the utmost care when it comes to optimizing their build. Generic brake resistors may be vulnerable to overheating from reduced cooling efficiencies. They can sometimes even use lower-grade insulation and terminals, which may result in oxidation.

Other potential issues include limited or no built-in protection and inferior resistor elements.

Frequently Asked Questions

How often do brake choppers fail?
It sometimes depends on the quality of the materials and design of the system. Usually, brake choppers operate smoothly for several years at a time.

Can I add protection to an existing system?
Yes, typically you can add protection to an existing VFD system, and it’s quite common.

Are there code requirements for brake resistor protection?
Yes, there are electrical codes and safety standards regarding brake resistors. Examples include the NFPA 70 and UL 508A.

What is the typical cost difference between methods?
An intrinsically safe resistor would be your most affordable prevention method. Coupling a resistor with thermal switches will result in moderate cost, while chopper monitoring boards may be on the pricier side (though sometimes recommended).

How do I calculate if my resistor is properly sized?
A brake resistor is properly sized when it meets the drive’s electrical limits and your application’s energy demands.

 
Video: How Are Braking Resistors Sized?
 

Protect Your VFD Investment

Interested in KEB technology for your industrial automation needs? We’ve engineered machine products like VFD brake resistors for over 50 years. That includes everyday motor and drive solutions as well as custom solutions tailored for client needs. Connect with our people to learn how we can help.