Inverters: Using braking resistors

Theory and Operation

The vast majority of "standard" frequency inverters (variable frequency drives) and servos without an active front-end do not have the capability to regenerate power back to the grid during deceleration or when the motor is driven by the application. During heavy deceleration, the internal DC bus voltage will rise, and to avoid damaging the drive, this power must be dissipated through a braking resistor.

Internally, the vast majority of modern frequency inverters and servos feature a braking transistor/"chopper" whose task is to direct power away from the DC bus and onto an external braking resistor. The most important factor when selecting a braking resistor is to choose an ohmic value higher than the drive's minimum allowable resistance value, combined with a power rating high enough to dissipate the required energy for your application.

Also consider the application's duty cycle: what percentage of the time do you spend braking the load vs the time the motor is driving the load? The duty cycle indicates what percentage of a given time unit the drive is braking. If you brake for 25 seconds within a 100-second period, the duty cycle is 25%. If you brake for 5 seconds, the duty cycle is 5%, and so on.

The graph below illustrates how the internal DC bus voltage (yellow) inside the frequency inverter rises during braking (output-speed= purple). When the DC voltage exceeds a defined threshold (typically 760V on a 400V drive), the braking unit enables the use of the braking resistor until the DC voltage drops again. This often occurs in cycles, as shown below.

How to Calculate Resistors: Series and Parallel

It is not always possible to find a single braking resistor that meets your specific requirements for both ohmic value and power rating. Sometimes you must combine two or more resistors to achieve the desired total resistance and/or power capacity.

Ideally, the resistance should be as close to the frequency inverter's minimum allowable resistance value as possible, but NEVER below it, as this will damage the inverter. Use standard calculation rules for series and parallel circuits to find the combination of resistors that meets your requirements.

• Remember: If you brake for more than 10% of the time, you should consider a separate braking unit, even on small inverters, as their internal braking transistors are not designed for continuous or heavy-duty braking cycles.

• The power rating in watts of a braking resistor should typically be between 5% to 30% of the motor's power rating in watts.

• If you only need to take the edge off a deceleration cycle (such as on a saw with infrequent stops), the energy requirement is low and the duty cycle will be small; in this case, 5% to 15% of the motor power may be sufficient. However, demanding applications involving suspended loads may require up to 30% of the motor power. Keep in mind that gearboxes and mechanical components also affect and can assist in braking the application.

NOTE! A braking resistor can become extremely hot. Always mount braking resistors on the exterior of the control enclosure to ensure heat is dissipated as effectively as possible.

An actual example: Braking Resistor Requirement for a 750W G5 Servo

Checking the G5 tab in the documentation, we see that the minimum resistance value for the drive is 25 ohms. We therefore choose to use two 50-ohm, 80W resistors connected in parallel.

This results in a total resistance of 25 ohms (two 50-ohm resistors in parallel) and a total capacity of 160W (2x80W). This corresponds to 20% of the nominal motor power of the drive (160 / 800 = 0.2), making it well-suited for a generic application.

Minimum connectable resistor values for MX2 

Minimum connectable resistor values for M1

Minimum connectable resistor values for R1

Minimum connectable resistor values for G5

Minimum connectable resistor values for 1S

Minimum connectable resistor values for C6