What Is A Braking Resistor Design?
A braking resistor solution is needed when there is frequent tripping on the drive due to overvoltage, reduced equipment life or high maintenance costs, or motor and drive damage and/or overheating. In general, we know that resistors consume heat and are used to slow down or stop a mechanical system. The purpose of a braking resistor is to dissipate the energy which a motor generates during deceleration. As the braking resistor dissipates the energy, it helps prevent damage to the drive which could happen due to overvoltage.
May be interested: Dynamic Braking Resistor
How Does A Braking Resistor Work?
If the decelerating motor generates excess voltage, a braking resistor works to stop or slow down the motor. Once the power is removed from a motor, it reduces the load and makes the motor turn at a faster rate. In this way, the motor starts acting as a generator and causes electrical energy, which then flows back into the power circuit.
In case the voltage rises too much, it causes the motor to trip on overvoltage or damage the drive. This is where a braking resistor comes to the rescue by dissipating the energy generated by the slowing motor. A braking resistor prevents damage to the drive by ensuring the voltage rise does not exceed the limit. It also helps extend equipment life by enabling faster braking, lowering the wear on braking components, and eliminating the risk of overheating.
Brake Resistors For Variable Frequency Drives
We know that most DC motors start to behave as generators when they are removed from the power supply. The reason behind it is their permanent magnets. To dissipate the generated energy, a power resistor can be connected as a load.
It is important to understand that these motors have rotating magnetic fields in the stator that induce a magnetic field. That’s why braking resistors help a lot where the motor’s speed exceeds the speed set by VDF or usually when fast deceleration is needed. Braking resistors also provide controlled braking even at an increased torque. In a case when the rotational velocity exceeds the synchronous velocity, it will start acting as a generator. Moreover, the surplus of energy will be then fed into the VFD.
In other words, more energy will be fed back if the speed difference between the motor and the drive is larger. Therefore, experts recommend choosing the right size of a braking resistor. If the ohmic value of a resistor is lower, the faster it will stop the motor (which will eventually generate more heat). To overcome this issue, the braking resistor needs to be bigger. That’s why the designer calculates the resistor’s power rating to ensure it stays under the set temperature limits during braking.
The Bottom Line
Brake resistors have a high power rating and low ohmic values. That’s why they are preferred over friction braking. They control motor voltage with safe levels, help lower wear and tear of components, and offer faster braking of AC and DC motors. Overall, braking resistors are used for small motion systems but they can also be used for large systems like trams or trains.