Crane specific frequency converter supplier reminds you that brake resistors can always be seen in the daily industrial control of cranes. Some people also call it brake resistors. What specific function does it play in the electrical system of cranes? And some cranes also use a braking unit (braking chopper), what is the relationship between it and the braking resistor? Today we will talk about the functions and working principles of braking resistors and braking units in detail.
Crane energy consumption braking method
Braking resistor, to summarize its function in one word: "heat generation". To put it professionally, it is to convert excess electrical energy into thermal energy and consume it.
There are many types of brake resistors in terms of structure, including corrugated brake resistors, aluminum shell brake resistors, stainless steel brake resistors, and so on. The specific choice depends on the work environment. Each has its own advantages and disadvantages.
We can also summarize its function in one word: 'switch'. Yes, it is actually a more advanced switch. Unlike ordinary switches, it is internally a high-power transistor GTR. It can pass a large current and can also be turned on and off at a high operating frequency, with an operating time in milliseconds.
After gaining a general understanding of the braking resistor and braking unit, let's now take a look at their wiring diagram with the frequency converter.
Crane energy consumption braking method
Generally, low-power inverters have the braking unit built into the inverter, so you can directly connect the braking resistor to the terminals of the inverter.
Let's first understand two knowledge points.
Firstly, the normal bus voltage of the frequency converter is around DC540V (AC 380V model). When the motor is in the generating state, the bus voltage will exceed 540V, with a maximum allowable value of 700-800V. If this maximum value is exceeded for a long time or frequently, the frequency converter will be damaged. Therefore, braking units and braking resistors are used for energy consumption to prevent excessive bus voltage.
Secondly, there are two situations in which the motor can transition from an electric state to a generating state:
A、 Rapid deceleration or too short deceleration time for high inertia loads.
B、 Always in power generation mode when the load is lifted and lowered.
For the lifting mechanism of a crane, it refers to the time when the lifting and lowering deceleration stops, and the time when the motor is in the power generation state during heavy load lowering. You can think about the translation mechanism yourself.
The action process of the braking unit:
a、 When the electric motor decelerates under external force, it operates in a generating state, producing regenerative energy. The three-phase AC electromotive force generated by it is rectified by a three-phase fully controlled bridge composed of six freewheeling diodes in the inverter section of the frequency converter, which continuously increases the DC bus voltage inside the frequency converter.
b、 When the DC voltage reaches a certain voltage (the starting voltage of the braking unit, such as DC690V), the power switch tube of the braking unit opens and the current flows to the braking resistor.
c、 The braking resistor releases heat, absorbs regenerative energy, reduces motor speed, and lowers the DC bus voltage of the frequency converter.
d、 When the DC bus voltage drops to a certain voltage (braking unit stop voltage such as DC690V), the power transistor of the braking unit is turned off. At this time, no braking current flows through the resistor, and the braking resistor naturally dissipates heat, reducing its own temperature.
e、 When the voltage of the DC bus rises again to activate the braking unit, the braking unit will repeat the above process to balance the bus voltage and ensure the normal operation of the system.
Due to the short-term operation of the braking unit, which means that the power on time is very short each time, the temperature rise during the power on time is far from stable; The interval time after each power on is longer, during which the temperature is sufficient to drop to the same level as the ambient temperature. Therefore, the rated power of the braking resistor will be greatly reduced, and the price will also decrease accordingly; In addition, due to the fact that there is only one IGBT with a braking time of ms level, the transient performance indicators for power transistor turn-on and turn off are required to be low, and even the turn off time is required to be as short as possible to reduce the turn off pulse voltage and protect the power transistor; The control mechanism is relatively simple and easy to implement. Due to the above advantages, it is widely used in potential energy loads such as cranes and in situations where rapid braking is required but for short-term work.