In the variable frequency speed regulation system, the motor speed reduction and shutdown is achieved by gradually reducing the frequency, at the moment of frequency reduction, the synchronous speed of the motor decreases, and because of mechanical inertia, the rotor speed of the motor does not change. When the synchronous speed is less than the rotor speed, the phase of the rotor current changes almost 180 degrees, and the motor changes from an electric state to a power generation state, at the same time, the torque on the motor shaft becomes a braking torque, so that the motor speed drops rapidly, and the motor is in a regenerative braking state. The electric energy regenerated by the motor is fed back to the DC circuit after full wave rectification by the rectifier diode. Because the electric energy of the direct current circuit can not be returned to the power grid through the rectifier bridge, only by the capacitor absorption of the inverter itself, although other parts can consume electric energy, but the capacitor still has a short time of charge accumulation, forming a "pump voltage", so that the DC voltage rises.
Brake resistance is a carrier used to consume the regenerative energy of the motor as heat energy, which includes two important parameters: resistance value and power capacity. When the motor or other inductive load driven by the inverter is stopped, it is generally achieved by energy consumption braking, which is to consume the kinetic energy of the motor and the magnetic energy in the coil after the stop through a other energy-consuming component, so as to achieve rapid stop.
Generally, two kinds of corrugated resistance and aluminum alloy resistance are used more in engineering:
Corrugated resistance using vertical surface corrugated is conducive to heat dissipation to reduce the parasitic inductance, and the selection of high flame retardant inorganic coating, effectively protect the resistance wire from aging, extend the service life. Aluminum alloy resistance is easy to install tightly, easy to attach radiator, beautiful appearance, high heat dissipation aluminum alloy shell, fully sealed structure, with strong vibration resistance, weather resistance and long-term stability; Small size, large power, easy to install stable, beautiful appearance, widely used in highly harsh industrial environment.
Ripple resistor
When the inverter decelerates, the output frequency of the inverter decreases, but when the motor changes from high speed to low speed, the motor changes from electric state to power generation state, and the generated electricity returns to the DC bus through the IGBT switch, so the braking time will lead to the DC bus voltage increase, and the brake resistance should be used with the brake unit, when the brake unit is put in, When the DC bus voltage is too high, it will control the IGBT conduction (similar to the frequency conversion output, intermittent conduction) in the brake unit to discharge, and when the voltage drops below the set value, the stop trigger.
Calculation of resistance value of brake resistance
The selection of brake resistance is limited by the maximum allowable current of the special energy consumption brake unit of the inverter, and there is no clear corresponding relationship with the brake unit, its resistance value is mainly selected according to the size of the braking torque required, and the power is determined according to the resistance value and utilization rate of the resistor. The selection of the brake resistance value has an inviolable principle: it should be ensured that the current Ic flowing through the brake resistance is less than the maximum current output capacity allowed by the brake unit, that is, R > 800/Ic.
Where: 800 - The maximum DC voltage that may appear on the DC side of the inverter. Ic - The maximum allowable current of the brake unit.
In order to make full use of the capacity of the selected inverter special brake unit, the selection of the brake resistance value close to the minimum value calculated above is usually the most economical and can also obtain the maximum brake torque.