Braking resistor is a carrier used to consume regenerative energy of motor in the form of heat energy. It includes two important parameters: resistance value and power capacity. Generally, corrugated resistance and aluminum alloy resistance are widely used in engineering. The surface vertical corrugated resistance is conducive to heat dissipation and reduce parasitic inductance. High flame retardant inorganic coating is used to effectively protect the resistance wire from aging and prolong its service life. Aluminum alloy resistance is easy to be installed tightly and heat sink is easy to be attached. The aluminum alloy outer box with beautiful appearance and high heat dissipation is fully enclosed Structure, with strong vibration resistance, weather resistance and long-term stability; small size, high power, convenient and stable installation, beautiful appearance, widely used in highly harsh industrial environment.

1、 What is the use of frequency converter with brake resistor

1. When the motor decelerates, the excessive inertia of the equipment will turn the motor into a generator, which is due to the power generation operation state. The motor supplies power to the inverter in reverse direction, which will cause the inverter over-voltage alarm. In order to release this energy, the method of increasing the resistance power (reducing the resistance value appropriately) is adopted. There are also reverse power supply to the power circuit, which is more used in the common DC bus frequency conversion system, which can save energy. The braking resistance and power generation effect are the same, which can prevent the converter from deceleration overvoltage, reduce the deceleration distance and improve the dynamic performance.

The built-in brake of the motor is generally used for the final parking brake, but not for deceleration braking, which is essentially different from the resistance braking, because the resistance braking only works during the deceleration of the motor, and has no effect after the motor stops. The brake must be used to keep the motor stationary (with potential energy load).

2. When the motor decelerates, the excessive inertia of the equipment will turn the motor into a generator, which is due to the power generation operation state. The motor supplies power to the inverter in reverse direction, which will cause the inverter over-voltage alarm. In order to release this energy, the method of increasing the resistance power (reducing the resistance value appropriately) is adopted. There are also reverse power supply to the power circuit, which is more used in the common DC bus frequency conversion system, which can save energy. The braking resistance and power generation effect are the same, which can prevent the converter from deceleration overvoltage, reduce the deceleration distance and improve the dynamic performance.

The built-in brake of the motor is generally used for the final parking brake, but not for deceleration braking, which is essentially different from the resistance braking, because the resistance braking only works during the deceleration of the motor, and has no effect after the motor stops. The brake must be used to keep the motor stationary (with potential energy load).

3. For example, you need to make the motor stop immediately when it is running at high speed, but the motor has inertia, especially when the high-power motor drives some equipment with large momentum, it is difficult to stop quickly. At this time, the kinetic energy caused by inertia can be released in the form of heat energy. The component that consumes these heat energy is the brake resistance.

Many of the built-in brakes of the motor are completed in the similar structure with the brake pads, and some of them are achieved by changing the external magnetic field of the motor rotor, and the braking principle of the braking resistance is basically not adopted.

2、 Why is brake resistor connected to DC bus bar

Is it necessary to convert the remaining power of the motor into direct current and then consume it during the braking process? Even so, what about the voltage already existing on the bus bar?

When and how to calculate the function of Inverter Braking Resistor?

This is what I understand: when the frequency converter decelerates, the frequency converter output frequency decreases, but when the motor changes from high speed to low speed, the motor is programmed to generate power by the electric state, and the electricity generated will return to the DC bus through the IGBT switch, so the DC bus voltage will rise when braking.

When the brake resistor is used, it should cooperate with the brake unit. When the brake unit is put into operation, if the DC bus voltage is too high, it will control the IGBT in the braking unit to conduct discharge (similar to the variable frequency output, intermittent conduction). When the voltage drops below the set value, the trigger will be stopped.

3、 Calculation of braking resistance

The selection of braking resistance is limited by the maximum allowable current of energy consumption braking unit for frequency converter, and there is no clear corresponding relationship with the braking unit. The resistance value is mainly selected according to the required braking torque, and the power is determined according to the resistance value and utilization rate of the resistance. There is an inviolable principle for the selection of braking resistance value: the current IC flowing through the braking resistor shall be ensured to be less than the maximum allowable current output capacity of the braking unit, i.e., R > 800 / IC.

Among them: 800 -- the maximum DC voltage that may appear on DC side of frequency converter. IC -- maximum allowable current of braking unit.

In order to make full use of the capacity of the special brake unit for frequency converter, it is usually the most economical to select the value of braking resistance close to the minimum value calculated in the above formula, and at the same time, the maximum braking torque can be obtained. However, this requires larger braking resistance power. In some cases, large braking torque is not required. At this time, a more economical way is to select a larger value of braking resistance, thus reducing the power of braking resistor, thus reducing the cost of purchasing brake resistor. The cost is that the capacity of brake unit is not fully utilized.

4、 Calculation of braking resistance power

After the resistance value of the braking resistor is selected, the power value of the braking resistor should be determined. The selection of the braking resistance power is relatively complicated, which is related to many factors.

The instantaneous power consumed by braking resistor is calculated as follows: P instantaneous = 7002 / R

The power value of braking resistor calculated according to the above formula is the power value that the braking resistor can work continuously for a long time and dissipate. However, the braking resistance is not uninterrupted work. This selection has great waste. In this product, the utilization rate of braking resistor can be selected, which specifies the short-term working ratio of braking resistor. The actual power consumed by the braking resistor is calculated as follows:

P = 7002 / R × RB% RB%: utilization rate of braking resistance.

In actual use, the brake resistance power can be selected according to the above formula, and the utilization rate that the braking resistance can bear can be calculated according to the selected braking resistance value and power, so as to correctly set and avoid damage caused by overheating of brake resistance.

5、 Calculation of brake resistance power

The utilization rate of braking resistor specifies the service efficiency of braking resistor, so as to avoid damage caused by overheating of braking resistor, which will affect the braking effect of braking unit. The lower the utilization rate of the braking resistor is, the lower the heating degree of the resistor is, the less energy is consumed on the resistor, and the worse the braking effect is. At the same time, the capacity of brake unit is not fully utilized.

Theoretically speaking, when the utilization rate of braking resistor is 100%, the capacity of braking unit is the most fully utilized, and the braking effect is the most obvious. However, this requires the cost of larger braking resistance power, which should be comprehensively considered by users. On the premise that the resistance value and power of braking resistance have been determined, for the large inertia load with slow deceleration, selecting a lower resistance utilization rate will achieve better results. For the load that needs to be shut down quickly, a larger utilization rate of braking resistance should be selected.