Feedback unit supplier reminds you that each frequency converter has a braking unit (low-power is the braking resistor, high-power is the high-power transistor GTR and its driving circuit), low-power is built-in, and high-power is external. Principle of braking unit: When the working machinery requires rapid braking, and within the required time, the regenerative energy of the frequency converter cannot be stored in the intermediate capacitor within the specified voltage range or the internal braking resistor cannot consume it in time, causing overvoltage in the DC part, an external braking component needs to be added to accelerate the consumption of regenerative electrical energy. When the frequency converter drives the motor to be in a braking state (power generation state), such as when the hoist descends or when a load with high inertia stops quickly. Kinetic energy (potential energy) will be converted back into electrical energy and return to the DC bus of the frequency converter, causing a high bus voltage. If your frequency converter has a braking unit, when it detects that the bus voltage is above a certain threshold, it will connect the switch between the braking resistor and the bus, and energy will be consumed through the braking resistor. At this time, the braking resistor will heat up.
Normally, the braking resistor does not generate heat. If the braking resistor generates heat during normal operation, it means that the braking unit is broken or there is a hardware problem that causes the braking resistor to always be connected to the DC bus. Therefore, the operation of your frequency converter is not a major problem, but the energy consumption is definitely high.
When the output of the frequency converter controls the motor in acceleration or constant speed state, the braking resistor does not work. However, when the motor decelerates or stops urgently, due to the regenerative braking state of the motor, the voltage of the DC circuit in the frequency converter will increase, and the braking resistor will consume this increased energy through heating.
The asynchronous motor will be in a regenerative power generation state, generating feedback current. This current returns to the DC circuit through the reflux diodes (D1-D6) and charges the main capacitor, causing the DC voltage to rise. In order to avoid high voltage and damage to the frequency converter, a braking resistor R is connected to the DC circuit side. When the DC voltage exceeds a certain value, the transistor switch TR is turned on and connected to the braking resistor, and the feedback energy is consumed in the form of thermal energy on the resistor R.
During the process of decreasing operating frequency, the braking resistor motor will be in a regenerative braking state, and the kinetic energy of the driving system will be fed back to the DC circuit, causing the DC voltage UD to continuously rise and even reach a dangerous level. Therefore, it is necessary to consume the energy regenerated into the DC circuit to keep UD within the allowable range. The braking resistor is used to consume this energy. The braking unit consists of a high-power transistor GTR and its driving circuit. Its function is to provide a pathway for the discharge current IB to flow through the braking resistor.