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In the frequency conversion speed regulation system, the deceleration and shutdown of the motor is realized by gradually reducing the frequency, that is, deceleration to stop. At this time, the power tube is not turned off, and the car still stops according to the deceleration time.

There is a problem here. When the frequency decreases, the speed of the motor decreases. However, due to the existence of inertia, the actual speed of the motor cannot rotate at the given frequency of the inverter, so that after the power tube is turned off , the motor is still spinning for a while.

This point has been tested on site. It takes about 30 seconds for a pump to go from zero frequency output to the complete stop of the motor. If it is a fan, it can still run for more than a few minutes.

That being the case, frequency conversion parking is very important. During the stop process, if the synchronous speed is lower than the actual speed of the motor, then the motor is in the state of generating electricity, and the torque on the motor shaft becomes the braking torque.

The electric energy generated in the power generation state must be returned to the DC bus through the full-wave rectification of the freewheeling diode. On the DC bus, there is no other use except for charging the filter capacitor, and it cannot be fed back to the grid. When the DC bus voltage rises, the will be inevitable. If the deceleration stop time is set too short, it is not surprising that DC overvoltage is reported.

In order to solve this problem, the brake circuit of the frequency converter appears. The key is to consume the electric energy generated by the motor when it is in the power generation state. So, how to consume it? It is realized by adding a discharge resistor on the DC bus, consuming the generated electric energy on the discharge resistor, converting the electric energy into heat energy and dissipating it.

There are built-in or external discharge resistors. Generally, low-power inverters are built-in, and high-power inverters require external resistors. Of course, there are some, which depends on the site conditions.

With the resistor, there is still a "switch" in front of it, which is the brake unit called in the instruction manual of the inverter. It is equivalent to a switch, which determines when to turn on or off according to the conditions. That is to say, light braking is not allowed, and the DC bus voltage still needs to be maintained. If the DC bus voltage exceeds the set limit, the switch will be turned on and the discharge resistor will be turned on. If the bus voltage returns to within the limit, it will still be turned off. Yes, it is an automatic process. Therefore, the brake unit includes a power tube, a voltage sampling comparison circuit and a drive circuit.

The DC bus voltage limit is set by inverter parameters. Low-voltage inverters are generally around 700V. If it exceeds this value, it will pose a threat to the breakdown of the filter capacitor.

At the end of this article, I will focus on the composition and application of the brake circuit of the inverter. It feels very rough and not very popular, but it is still interesting to chew slowly. Inadequacies, but also please give advice!

The discharge resistor is very simple, usually a green cylindrical body with protrusions, which belongs to the corrugated resistor, and the other is an alloy resistor, these two are the most common frequency conversion energy consumption resistors. When you see them, you will judge that the inverter has a brake circuit. When choosing, pay attention to the two parameters of resistance and power, because it is an optional part, and the specific parameters still need to be mastered.