Is there any connection between frequency converter shutdown and DC overvoltage?

Nov 07, 2023

Some friends are not very clear about the parking method of frequency converters and always confuse them. This article will explain the parking method of frequency converters.

There are usually two parking methods for frequency converters: deceleration parking and free parking. Meanwhile, deceleration parking is also known as oblique wave parking, and free parking is also known as inertial parking.

Let's talk about deceleration parking first. Deceleration parking refers to the frequency converter gradually reducing the output frequency according to the set deceleration time and method after receiving the parking command, causing the motor's speed to decrease accordingly until the motor stops.

In this parking method, the frequency converter does not actually stop. It outputs a certain torque value according to the set speed decrease curve, causing the motor to slow down according to this curve until the motor speed drops to a certain small value. The frequency converter considers it close to "0" speed and blocks the output.

Free parking refers to the process of turning off the output of the frequency converter after receiving a parking command, so that the output voltage is 0. In fact, it cuts off the power supply of the motor, thereby allowing it to stop freely. At this point, the length of motor shutdown time is not controlled and varies depending on the inertia of the traction system.

In fact, this is easy to understand. An asynchronous machine will stop working as long as the power is cut off, but it will not immediately stop because it has inertia. In this case, the load on the fan is most obvious. After the power is cut off, it usually takes a long time to stop.

The previous article mentioned the problem of overvoltage shutdown caused by short deceleration time, which is mainly caused by regenerative braking of the motor during deceleration or deceleration shutdown. When the motor speed exceeds the synchronous speed, the motor will be in a power generation state, charging the capacitor through the freewheeling diode, leading to an increase in bus voltage. So, this situation may occur during deceleration parking, but if it is a free parking or inertia parking, the frequency converter will turn off all IGBT drives, which cannot form a path. Therefore, the DC bus voltage will not increase.

So, by using the shutdown method of the frequency converter, it is also possible to make a judgment on DC overvoltage, which facilitates on-site understanding of the fault.

That's all for this article, focusing on the parking method of frequency converters. If there are any shortcomings, please criticize and correct them.

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