The frequency converter is equipped with dynamic resistance, mainly to consume part of the energy on the DC bus capacitance through the braking resistance, so as to avoid the high voltage of the capacitance. Theoretically, if there is more energy stored in the capacitor, it can be used to release the drive motor to avoid energy waste, but the capacity of the capacitor is limited, and the withstand voltage of the capacitor is also limited. When the voltage of the bus capacitor reaches a certain level, it may damage the capacitor, and some can also damage the IGBT, so it is necessary to release the electricity through the braking resistor in time. This release is a white wave What is spent is an approach that has no way.

Bus capacitance is a buffer zone with limited capacity

After all three-phase alternating current are rectified, the capacitor is connected. When the bus is in full load operation, the normal voltage of the bus is about 1.35 times, 380 * 1.35 = 513v. This voltage will certainly fluctuate in real time, but the minimum voltage can not be lower than 480v, or the under voltage alarm protection will occur. The bus capacitance is generally made up of two sets of 450V electrolytic capacitors in series. The theoretical withstand voltage is 900v. If the bus voltage exceeds this value, the capacitance will explode directly, so the bus voltage can not reach 900v in any case.

In fact, the voltage withstand value of the three-phase 380V input IGBT is 1200V, which is often required to work within 800V. Considering that if the voltage increases, there will be an inertia problem, that is, you will immediately let the brake resistor work, and the bus voltage will not decrease quickly, so many frequency converters are designed to let the brake resistor work through the brake unit at about 700V, so that The bus voltage shall be reduced to avoid further impulse upward.

Therefore, the core of braking resistance design is to consider the withstand voltage of capacitor and IGBT module, so as to avoid that the two important components are damaged by the high voltage of bus, and if these two components are damaged, the frequency converter will not work normally.

Quick stop requires braking resistance, and instant acceleration also requires

The reason why the bus voltage of the frequency converter will become high is that the frequency converter makes the motor work in the electronic braking state, and makes IGBT pass through a certain conduction sequence. The large inductance current of the motor can not be suddenly changed, and the high-voltage bus capacitance will be generated for charging in an instant, at this time, the speed of the motor will be reduced quickly. If there is no braking resistance to consume the energy of the bus in time, the bus voltage will continue to increase and threaten the safety of the converter.

If the load is not very heavy, and there is no requirement for quick stop, this platform does not need to use the brake resistor. Even if you install the brake resistor, the working threshold voltage of the brake unit is not triggered, and the brake resistor will not be put into operation.

In addition to the need to increase braking resistance and braking unit for quick braking in the case of heavy load deceleration, in fact, if it meets the requirements of heavy load and very fast starting time, it also needs the braking unit and braking resistance to cooperate for starting.

There is also a special case, that is, vector control field, the torque of the motor is opposite to the speed direction, or it works in the field with zero speed and 100% torque output. For example, when the crane drops a weight and stops in the air, torque control is needed in the winding and rewinding situation, so the motor needs to work in the state of the generator, and the continuous current will be backfilled into the bus capacitance. Through the braking resistance, the These energy can be consumed in time to keep the bus voltage balanced and stable.

Many small frequency converters, such as 3.7KW ones, often have built-in braking units and braking resistors. It should be considered that the reason for the bus capacitance to be reduced is right, while the small power resistors and braking units are not so expensive.