How to Choose the brake resistor for inverter?

The motor speed reduction and shutdown are achieved by gradually reducing the frequency in the variable frequency speed regulation system. The motor speed lowers at the moment of the frequency reduction due to inertia,but the rotor speed doesn’t change.

The phase of the rotor current changes almost 180 degrees when the speed is less than the rotor speed.

The motor changes from the electric state to the power generation state. At the same time, the torque of the motor becomes the braking torque which causes the motor speed to 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 rectifier diode.

Since the electric energy of the DC circuit cannot be fed back to the grid through the rectifier bridge, it is only absorbed by the capacitor of the inverter itself. This causes the capacitor a short-term charge accumulation and increases the DC voltage.

1. The type of braking resistor

The two types of resistors that are usually used are corrugated resistors and aluminum alloy resistors.

Corrugated resistors use vertical corrugations on the surface which is conducive to heat dissipation and reduces parasitic inductance. Highly flame-retardant inorganic coatings effectively protect the resistor wire from aging and extend its service life.

Aluminum alloy resistors have beautiful appearances and fully sealed aluminum alloy shells with high heat dissipation.They have strong vibration resistance, weather resistance and long-term stability. They are widely used in highly harsh industrial environment with small size, high power, easy installation.

2. How to choose suitable braking resistor?

We will take the corrugated resistor as example.

(1) Calculation of braking resistor value

There is an inviolable principle in selecting the braking resistor value: the current Ic flowing through the braking resistor should be less than the maximum allowable current output capacity of the braking unit, that is: R > 800/Ic.

(800 - the maximum DC voltage that may appear on the DC side of the inverter. Ic - the maximum allowable current of the brake unit)

The selection of the brake resistor value is the most economical when it is close to the minimum value calculated by the above formula, and the maximum braking torque can be obtained to fully utilize the capacity of the selected inverter-specific brake unit

Braking torque or brake resistor calculation (380V series):

92% R=780/motor KW

100% R =700/motor KW

110% R=650/motor KW

120% R=600/motor KW (greater than 7.5KW motor)

R=400/motor KW (less than 7.5KW motor)

Note:

①The smaller the resistance value, the greater the braking torque and the greater the current flowing through the brake unit;

②The working current of the brake unit cannot be greater than its maximum allowable current, otherwise the device will be damaged;

③The braking time can be selected manually;

(2)Resistor power calculation method

Resistor power = Inverter power* (10%--20%)

-General load W(Kw)= Inverter power*10℅

-Frequent braking (more than 5 times per minute) W(Kw)= Inverter power*15℅

-Long-term braking (more than 4 minutes each time) W(Kw)= Inverter power*20℅

Generally, the selection of the braking resistor should ensure that the braking current  does not exceed the rated current of the inverter and the maximum power Pmax of the braking resistor should be less than 1.5 times the inverter power. And then multiplied by the overload factor. The overload factor is related to the deceleration time and the continuous braking time.