Braking resistors work in VFD applications by slowing down the motor to the precise speed set by the drive. They are particularly useful for the rapid deceleration of electric motors. The braking resistor can also feed any excess energy into the VFD to boost the voltage on the DC bus.
When the voltage is properly handled, the inverter will not be damaged and the system can operate normally. Machine builders must choose the right braking resistor size and type to get the best results.
For example, some resistors can generate more heat and stop the motor faster than others. That's why it's so important to choose the right braking resistor.
Braking resistors are introduced into the motor control system to prevent VFD hardware damage or interference failures.
In some operations, a motor controlled by a VFD acts as a generator and power flows back to it instead of the motor. Whenever there is an access load or drive used to slow down the motor, the motor will act as a generator.
This causes the DC bus voltage of the drive to rise, and if the energy generated is not dissipated, it can lead to an overvoltage failure.
There are a few basic ways to handle the energy generated by a motor.
First of all, the drive itself can absorb some energy in a very short period of time. This is typical when an overhaul load is not present and a rapid deceleration is not required. If the energy generated is too large for the drive, a braking resistor can be introduced. The braking resistor will dissipate the excess energy by converting it into heat on the resistive element.
Finally, if the regenerative energy of the motor is continuous, it may be beneficial to use a regenerative unit instead of a braking resistor. This still protects the VFD from hardware damage and interference failures, enabling users to capture and reuse energy instead of dissipating it.
The two main factors in choosing a braking resistor are the minimum resistance value and the power dissipation capability.
Minimum resistance value
Drives that use braking resistors will also have "chopper circuits" or braking transistors. When the DC bus voltage is too high, the braking transistor shunts the current from the DC bus to the braking resistor. This brake transistor circuit has a current limit. As a result, VFD manufacturers often list the maximum current value and duty cycle.
The braking transistor controls the current across the braking resistor
Since V=IR, if the voltage is constant, a smaller resistance will result in a larger current. For example, suppose the maximum voltage is the 840VDC overvoltage level of KEB. The user can then calculate the minimum resistance to keep the current value below the maximum rating of the braking transistor. The minimum resistance value does not affect the operation of the resistor or its ability to dissipate power.
However, knowing this value is useful to ensure that it works properly with VFD.
It is important to know which resistor values are safe to use with VFD, as well as the power dissipation of the resistor. From here, you should consider how much energy will be produced back to the drive that needs to be dissipated. This ensures that the braking resistor has enough capacity to safely dissipate the energy generated by the motor.
The first method is through calculations. You can calculate the power produced by a motor if you know the following:
The mass moment of inertia of the motor and the load.
Motor torque. Speed changes.
Slowdown time. You can find more information about these calculations in the Braking Resistance Handbook. However, it is difficult to calculate all these values in practical applications. Therefore, it is usually best to determine the appropriate power size of the braking resistor by test method.
The general rule is as follows: the greater the load, the faster the deceleration, and the more power needs to be dissipated.
However, with the oscilloscope function in the Combivis 6 software, it is possible to record the DC bus voltage of the driver throughout operation. This gives us a more accurate use of the braking resistor.
With an oscilloscope, you can monitor whether you need resistors with different power dissipations or sizes. In the latter case, the operation can be adjusted to improve performance, such as speeding up deceleration.
The final consideration when choosing a braking resistor is installation. If the braking resistor is not installed in accordance with UL standards, the circuit may fail and create a fire hazard. Learn how to protect your VFD braking resistors.
In addition to conventional resistors, KEB also sells intrinsically safe braking resistors. These solutions fail like a fuse, protecting the system during a short-circuit failure.
The installation environment is also important. For example, installations involving flammable fibers, such as textiles or sawdust, require special consideration.
Interested in learning more about how EAK braking resistors can benefit your application? Contact an application engineer at EAK today!