After cutting off the power supply, the motor still needs to rotate for a period of time before it stops due to its own inertia. In actual working conditions, some loads require the motor to stop quickly, which requires braking control of the motor. The so-called braking is to give the motor a torque opposite to the direction of rotation to make it stop quickly. There are generally two types of braking methods: mechanical braking and electrical braking.
Mechanical braking uses a mechanical structure to complete braking. Most of them use electromagnetic brakes, which use the pressure generated by springs to press the brake pads (brake shoes) to form braking friction with the brake wheels. Mechanical braking has high reliability, but it will produce vibration when braking, and the braking torque is small. It is generally used in situations with small inertia and torque.
Electric braking generates an electromagnetic torque that is opposite to the steering during the motor stopping process, which acts as a braking force to stop the motor. Electric braking methods include reverse braking, dynamic braking, and regenerative braking. Among them, reverse connection braking is generally used for emergency braking of low-voltage and small-power motors; regenerative braking has special requirements for frequency converters. Generally, small and medium-power motors are used for emergency braking. The braking performance is good, but the cost is very high, and the power grid must be able to accept it. Energy feedback makes it impossible to brake high-power motors.
According to the position of the braking resistor, energy-consuming braking can be divided into DC energy-consuming braking and AC energy-consuming braking. The DC energy-consuming braking resistor needs to be connected to the DC side of the inverter and is only applicable to inverters with a common DC bus. In this case, the AC energy-consuming braking resistor is directly connected to the motor on the AC side, which has a wider application range.
A braking resistor is configured on the motor side to consume the energy of the motor to achieve a quick stop of the motor. A high-voltage vacuum circuit breaker is configured between the braking resistor and the motor. Under normal circumstances, the vacuum circuit breaker is in the open state and the motor is normal. Speed regulation or power frequency operation, in an emergency, the vacuum circuit breaker between the motor and the frequency converter or the power grid is opened, and the vacuum circuit breaker between the motor and the braking resistor is closed, and the energy consumption braking of the motor is realized through the braking resistor. , thereby achieving the effect of quick parking. The system single line diagram is as follows:
Emergency Brake One Line Diagram
In emergency braking mode, and according to the deceleration time requirements, the excitation current is adjusted to adjust the stator current and braking torque of the synchronous motor, thereby achieving rapid and controllable deceleration control of the motor.
In a test bed project, since the factory power grid does not allow power feedback, in order to ensure that the power system can stop safely within a specified time (less than 300 seconds) in an emergency, an emergency stop system based on resistor energy consumption braking was configured.
The electrical drive system includes a high-voltage inverter, a high-power double-winding high-voltage motor, an excitation device, 2 sets of braking resistors, and 4 high-voltage circuit breaker cabinets. The high-voltage inverter is used to realize variable frequency starting and speed regulation of the high-voltage motor. Control and excitation devices are used to provide excitation current to the motor, and four high-voltage circuit breaker cabinets are used to realize the switching of frequency conversion speed regulation and braking of the motor.
During emergency braking, high-voltage cabinets AH15 and AH25 are opened, high-voltage cabinets AH13 and AH23 are closed, and the braking resistor starts to work. The schematic diagram of the braking system is as follows:
Braking system schematic diagram
The technical parameters of each phase resistor (R1A, R1B, R1C, R2A, R2B, R2C,) are as follows:
- Braking energy (maximum): 25MJ;
- Cold resistance: 290Ω±5%;
- Rated voltage: 6.374kV;
- Rated power: 140kW;
- Overload capacity: 150%, 60S;
- Maximum voltage: 8kV;
- Cooling method: natural cooling;
- Working time: 300S.
This technology uses electrical braking to realize the braking of high-power motors. It applies the armature reaction of synchronous motors and the principle of energy consumption braking to brake the motors.
During the entire braking process, the braking torque can be controlled by controlling the excitation current. Electric braking has the following characteristics:
- It can provide the large braking torque required for rapid braking of the unit and achieve high-performance braking effect;
- The downtime is short and braking can be performed throughout the process;
- During the braking process, there are no mechanisms such as brake brakes and brake rings that cause the mechanical braking system to rub against each other, resulting in higher reliability;
- The emergency braking system can operate alone as an independent system, or it can be integrated into other control systems as a subsystem, with flexible system integration.
Post time: Mar-14-2024