Cause analysis of braking resistor used in elevator frequency conversion control

In common variable frequency speed regulation systems, the speed reduction and shutdown of the motor are realized by gradually reducing the frequency. At the moment of frequency reduction, the synchronous speed of the motor decreases, while the rotor speed of the motor does not change due to mechanical inertia. When the synchronous speed is less than the rotor speed, the phase of the rotor current almost changes by 180 degrees, and the motor changes from electric state to power generation state; At the same time, the torque on the motor shaft becomes the braking torque, which makes the motor speed drop rapidly, and the motor is in the regenerative braking state. The electric energy regenerated by the motor is fed back to the DC circuit after full wave rectification by the freewheeling diode. Since the electric energy of the DC circuit cannot be fed back to the power grid through the rectifier bridge, it can only be absorbed by the capacitor of the frequency converter itself. Although other parts can consume electric energy, the capacitor still has a short-term charge accumulation, forming a "pumping voltage", which increases the DC voltage. Excessive DC voltage will damage all parts of the devices.
Therefore, necessary measures must be taken to deal with this part of regenerative energy when the load is in the power generation braking state. Methods for handling regenerative energy: energy consumption braking and feedback braking. These two methods include the use of braking resistance.

1.Working mode of energy consumption braking

The method of energy consumption braking is to add a discharge resistance unit component at the DC side of the frequency converter to consume the regenerative electric energy on the power resistance to realize braking. This is the most direct way to deal with the regenerative energy. It consumes the regenerative energy on the resistance through a special energy consumption braking circuit and converts it into heat energy. Therefore, it is also called "resistance braking", which includes two parts: braking unit and braking resistance.

Brake unit
The function of the braking unit is to connect the energy dissipation circuit when the voltage UD of the DC circuit exceeds the specified limit (such as 660V or 710v), so that the DC circuit can release energy in the form of heat energy after passing through the braking resistance.

Braking resistance
The braking resistance is a carrier used to consume the regenerative energy of the motor in the form of thermal energy. It includes two important parameters: resistance value and power capacity. Generally, corrugated resistance and aluminum alloy resistance are widely used in engineering.

2. Process of energy consumption braking

Braking resistance box

1. When the motor decelerates and reverses (including being dragged) under the action of external force, the motor will run in the power generation state, and the energy will be fed back to the DC circuit to increase the bus voltage;
2. When the DC voltage reaches the on state of the braking unit, the power pipe of the braking unit is turned on, and the current flows through the braking resistance;
3. The electric energy consumed by the braking resistor is heat energy, and the motor speed decreases, so does the bus voltage;
4. When the bus voltage drops to the value that the braking unit is to be turned off, the power pipe of the braking unit is cut off, and no current flows through the braking resistance;
5. The bus voltage value is sampled, and the braking unit repeats the on/off process to balance the bus voltage and make the system operate normally.

Post time: Jun-20-2022