When designing an electric motor control system, it is not constantly clear if a braking resistor is required as well as, if it is, how to continue in selecting a braking resistor. This post is meant to simplify that procedure so it is clear when as well as exactly how to select a braking resistor.
Why are braking resistors needed?
Braking resistors are introduced into a motor control system in order to stop equipment damage and/or nuisance faults in a VFD. They are called for since in particular operations, the motor regulated by the VFD is working as a generator as well as power is receding in the direction of the VFD, instead than towards the motor. A motor will work as a generator whenever there is a revamping load (e.g. keeping a consistent speed as gravitational forces try to accelerate a lift as it relocates down) or the drive is being utilized to decrease the electric motor. This triggers the drive’s DC bus voltage to increase and also will certainly bring about over voltage mistakes in the drive if the produced energy is not dissipated.
If there are sections of the responsibility cycle where the power created is as well excellent for the drive alone, then a braking resistor can be introduced. The braking resistor will certainly dissipate the excess energy by converting it to warmth throughout a resistor component. If the regenerated energy from the electric motor is constant or the duty is high, after that it might be extra useful to make use of a regen system instead than a braking resistor.
What should be considered when choosing a braking resistor?
Once it has actually been made a decision a braking resistor is needed for the application, there are two main variables when selecting the resistor: the resistance value as well as the power dissipation capability of the resistor.
Minimum Resistance Worth
VFDs that make use of a brake resistor will certainly additionally have a “chopper circuit” or brake transistor. When the DC bus voltage obtains as well high, the brake transistor shunts existing from the DC bus throughout the brake resistor This brake transistor wiring has present constraints and also the VFD maker will typically list an optimum current worth as well as task cycle.
The brake transistor manages the present across the brake resistor.
Given That V= IR, if the voltage is constant then a smaller resistance will cause a bigger current. Therefore if limit voltage is recognized to be the KEB over voltage degree of 840VDC, it is feasible to compute the minimal resistance that would keep the existing value below the braking transistor’s max present score. While the minimum resistance value doesn’t influence the operation of the resistor or its capacity to dissipate power, it is critical to ensure it works correctly with the VFD.
Power Dissipation Capacity
The second factor to consider when choosing a braking resistor is its ability to dissipate power. The KEB 10BR100-1683 resistor can safely dissipate up to 2200W for one stretch of 6 seconds over the training course of two mins or instead might do two cycles of 3 seconds each over the training course of two mins.
KEB Braking resistors reveal different power dissipation ratings depending upon responsibility.
Currently that it is recognized what resistance values will securely function with the VFD and the power dissipation abilities of the numerous resistors, it should be considered exactly how much power will be generated back in the direction of the drive that will require to be dissipated. More info on performing these calculations can be discovered in the braking resistor manual. Since of that, it is commonly essential to figure out the appropriate power dimension of the braking resistor through a screening approach.