How to Size a Braking Transistor for a VFD Choosing the Right Dynamic Braking Transistor for Overvoltage Protection
A braking transistor helps protect a VFD from overvoltage faults by monitoring the DC bus and switching regenerated energy into a braking resistor when needed. Proper sizing depends on system voltage, peak braking current, duty cycle, stopping time, minimum resistance, and braking resistor requirements.
In This Article
Use this guide to understand what a braking transistor does, what sizing information is required, and how to choose between small-drive, standard-duty, heavy-duty, and complete braking kit options.
Bonitron Products
Bonitron braking transistor products are used with AC drives to dissipate regenerated energy, reduce overvoltage faults, and support controlled braking.
What Is a Braking Transistor?
A braking transistor is an overvoltage protection device used with a VFD and braking resistor. During deceleration, lowering, or overhauling load conditions, a motor can act like a generator and return energy to the drive DC bus. If that energy is not managed, the DC bus voltage can rise high enough to cause an overvoltage fault.
The braking transistor monitors the drive DC bus. When the DC bus rises to the transistor's turn-on level, the transistor switches regenerated energy into a braking resistor. The resistor converts that energy into heat, helping prevent the DC bus from rising too high.
When Do You Need a Braking Transistor?
A braking transistor is commonly needed when the application regenerates enough energy to raise the VFD DC bus during stopping, lowering, deceleration, or overhauling load conditions. Without a braking path, the drive may trip on overvoltage before the motor can stop as commanded.
Some drives include an internal braking transistor. Others require an external braking transistor. Even when an internal transistor is available, the application may still need an external solution if braking current, duty cycle, resistor requirements, or application severity exceed the drive's internal braking capability.
Information Needed to Size a Braking Transistor
Braking transistor sizing starts with the drive and the application. The goal is to select a transistor that matches the DC bus voltage, peak braking current, braking duty cycle, and resistor requirements without exceeding the transistor or resistor ratings.
Drive Voltage
Check the AC input voltage and expected DC bus voltage range before selecting a braking transistor.
Motor Size
Horsepower is a starting point, but it should not be the only sizing factor.
Stopping Time
Shorter stops usually require higher braking power and may increase peak current demand.
Load Inertia
High-inertia loads can return significant energy to the DC bus during deceleration.
Braking Frequency
Repeated stops may require a higher-duty braking transistor and resistor package.
Minimum Resistance
The braking resistor must not be lower than the transistor's minimum resistance rating.
Peak Current and Minimum Resistance
Two of the most important braking transistor ratings are peak braking current and minimum resistance. The selected resistor must not be lower than the braking transistor's minimum resistance rating. A resistor that is too low can demand more current than the transistor is designed to handle.
The selected braking transistor must be able to handle the peak current required during the braking event. The braking resistor must also be able to absorb the energy and dissipate the average heat created over the braking cycle.
Bonitron Engineering Note
Always check the braking transistor and resistor together. A correctly rated transistor paired with the wrong resistor can still result in excessive current, overheating, nuisance faults, or reduced braking performance.
Braking Duty Cycle
Duty cycle describes how much of the operating cycle requires braking. A light-duty application may brake occasionally. A severe application may brake frequently, brake for long periods, or operate with an overhauling load.
For smaller or standard applications, a 20% duty braking transistor may be appropriate. For high-horsepower, frequent braking, or continuous overhauling applications, a heavy-duty solution may be required.
Bonitron's M3675T and M3575T products are positioned for 20% braking duty applications, while the M3452 heavy-duty product family is positioned for higher-duty and high-horsepower braking applications.
Bonitron Braking Transistor Options
Bonitron offers multiple braking transistor product families depending on drive size, system voltage, braking current, duty cycle, and whether the application needs a separate resistor or a complete transistor-and-resistor package.
M3675T
Best fit for small-drive applications where the drive has no internal braking transistor. Supports small-drive and microdrive applications up to 10 HP with 20% braking duty.
M3575T
Standard-duty braking transistor for applications up to 600 A DC with braking duty cycles up to 20%. Designed for drives with DC bus field terminals.
M3452
Heavy-duty braking transistor for high-horsepower braking, with models rated up to 1600 A per unit and higher-duty braking applications.
M3452 Complete Braking Kit
Combines the braking transistor and braking resistor in one enclosure to reduce wiring, simplify installation, and support quicker deployment.
M3452-S100
Specialty braking transistor option for 72 VDC servo drive applications. Use this when the application requires a low-voltage servo braking solution.
Braking Resistors
The transistor must be paired with a properly selected braking resistor. Resistor value, wattage, enclosure, and duty cycle must match the application.
Braking Transistor Selection Guide
Use this table as a starting point for selecting the right braking transistor family. Final sizing should be based on the drive, DC bus voltage, stopping requirements, peak braking current, duty cycle, resistor value, and thermal load.
| Application Situation | What to Check | Likely Product Path | Why |
|---|---|---|---|
| Small drive without internal braking transistor | Voltage class, horsepower, resistor value, and braking duty. | M3675T | Designed for small-drive and microdrive applications up to 10 HP and 20% braking duty. |
| Standard VFD braking application | DC bus terminals, peak current, minimum resistance, and 20% duty requirement. | M3575T | Standard-duty option for applications up to 600 A DC and braking duty cycles up to 20%. |
| High horsepower or frequent braking | Peak current, RMS current, resistor capacity, braking frequency, and diagnostics needs. | M3452 Heavy Duty | Heavy-duty product family with models up to 1600 A per unit for severe braking applications. |
| Need transistor and resistor together | Voltage class, horsepower, duty cycle, space, wiring, and installation time. | M3452 Complete Braking Kit | Includes transistor and resistor in one enclosure, reducing separate wiring and footprint. |
| Low-voltage servo braking application | Servo drive DC voltage, braking current, resistor requirements, and application duty. | M3452-S100 | Specialty option for 72 VDC servo drive braking applications. |
| Continuous overhauling load | Whether braking energy is occasional or continuous, and whether heat dissipation is practical. | Review M3452 or line regeneration | Severe regenerative loads may require heavy-duty braking or a regenerative solution instead of standard-duty braking. |
Common Braking Transistor Sizing Mistakes
Sizing by Horsepower Alone
Horsepower does not tell the full story. A low-horsepower application with frequent stops may be more demanding than a larger drive that only brakes occasionally.
Ignoring Minimum Resistance
The braking resistor must not be lower than the transistor's minimum resistance rating. Too little resistance can cause excessive braking current.
Undersizing for Duty Cycle
A transistor that can handle one short stop may not be able to handle repeated stops or long braking events. Always check duty cycle and heat dissipation.
Forgetting the Resistor
The braking transistor and resistor must be selected together. A properly sized transistor with the wrong resistor can still lead to faults, overheating, or poor braking performance.
Using Braking Where Regeneration May Be Better
Dynamic braking dissipates energy as heat. Applications with frequent or continuous regenerative energy may be better suited for line regeneration or common DC bus energy sharing.
Frequently Asked Questions
What does a braking transistor do?
A braking transistor monitors the VFD DC bus and switches regenerated energy into a braking resistor when the DC bus voltage rises too high. This helps prevent overvoltage faults during braking or overhauling load conditions.
How do I know what size braking transistor I need?
Start with system voltage, drive horsepower, peak braking current, minimum resistance, stopping time, braking frequency, and duty cycle. Then compare those requirements against the product ratings for the braking transistor and resistor.
Can I use any braking resistor with a braking transistor?
No. The resistor must meet the transistor's minimum resistance requirement and must also be rated for the energy and heat generated by the braking cycle.
When should I choose M3575T instead of M3452?
The M3575T is a standard-duty option for applications up to 20% duty. The M3452 is a better fit for high-horsepower, higher-current, or higher-duty applications.
When should I use a complete braking kit?
A complete braking kit is useful when you want the braking transistor and resistor packaged together in one enclosure. This can reduce separate wiring, simplify installation, and save panel or installation time.
Need Help Sizing a Braking Transistor?
Bonitron can help review your drive voltage, stopping time, peak braking current, resistor requirements, and duty cycle to determine the right braking transistor solution.
Contact Bonitron