What is an electrical brake? How to control?
1. Brake structure and control principle
There are generally two types of braking methods: mechanical braking and electrical braking.
A. Mechanical brake
The method of using mechanical devices to quickly stop the motor after disconnecting the power is called mechanical braking.
Commonly used method: electromagnetic brake.
1. The structure of electromagnetic brake:
It is mainly composed of two parts: brake electromagnet and brake shoe brake.
The brake electromagnet is composed of iron core, armature and coil. Brake shoe brake includes brake wheel, brake shoe and spring, etc. The brake wheel and the motor are installed on the same rotating shaft.
2. Working principle:
The motor is powered on, and the electromagnetic brake coil is also energized. The armature pulls in and overcomes the tension of the spring to separate the brake shoe from the brake wheel and the motor runs normally. When the switch or contactor is disconnected, the motor loses power and the electromagnetic brake coil is also loses power. The armature is separated from the iron core under the action of the spring tension, and the brake shoe of the brake tightly embraces the brake wheel, and the motor is braked and stopped. turn.
3. Characteristics of electromagnetic brake
Mechanical braking mainly adopts electromagnetic brake and electromagnetic clutch braking, both of which use electromagnetic coils to generate magnetic field after energization, so that the static iron core generates enough suction to attract the armature or moving iron core (the moving iron core of the electromagnetic clutch is Pull in, separate the dynamic and static friction plates), overcome the tension of the spring and meet the requirements of the work site. The electromagnetic brake is to brake the brake wheel by the friction plate of the brake shoe. The electromagnetic clutch uses the friction force between the dynamic and static friction plates to make the motor brake immediately after the power is cut off.
Advantages: electromagnetic brake, strong braking force, widely used in lifting equipment. It is safe and reliable and will not cause accidents due to sudden power failure.
Disadvantages: The electromagnetic brake is large in size, the brake is severely worn, and vibration occurs during rapid braking.
4. Adjustment method of motor brake gap
① Shut down. (Confirm the mechanical and electrical shutdown, release the pressure and lock the power, and hang the warning signs of "under maintenance" and "no start".)
② Remove the fan blade cover;
③Remove the fan retainer and remove the fan blade;
④Check the remaining thickness of the brake lining (minimum thickness of the brake lining);
⑤Check the protection disc: if the edge of the protection disc has touched the mark of the positioning pin, the brake disc must be replaced;
⑥Adjust the air gap of the brake: Tighten the three (four) bolts until the air gap is zero, and then loosen the bolts at an angle of 120°. Use a feeler gauge to check the brake gap (at least three points). Uniform and in line with the specified value; please re-adjust if it is wrong; (Note: the brake type is different, the reverse loosening angle and the brake gap are also different).
⑦Manual operation, the brake action sound is clear, the stopping position is accurate and effective.
⑧ 6S standard cleaning on site.
B. Electric brake
1. Energy consumption braking
①The principle of dynamic braking:
After the motor cuts off the AC power supply, the rotor continues to rotate due to inertia, and direct current is immediately applied to the two-phase stator windings to generate a static magnetic field in the stator. The guide bar in the rotor cuts this static magnetic field to generate an induced current, which is subjected to electromagnetic force in the static magnetic field. The torque generated by this force is opposite to the rotor inertial rotation direction, called braking torque, which forces the rotor speed to drop. When the rotor speed drops to 0, the rotor no longer cuts the magnetic field, the motor stops, and the braking ends. This method uses the energy of the rotor to cut the magnetic flux to generate braking torque. In essence, the kinetic energy of the rotor is consumed on the resistance of the rotor circuit, so it is called dynamic braking.
②Characteristics of dynamic braking:
Advantages: strong braking force, stable braking, and no big impact; the application of energy-consuming braking can make the production machinery stop accurately, and it is widely used in production machinery such as mine lifting and crane transportation.
Disadvantages: DC power supply is required, and braking torque is small at low speed. When the power of the motor is large, the investment in the braking DC equipment is large.
2. Reverse connection brake
①Power reverse connection brake
The power is reversed, the rotating magnetic field is reversed, and the direction of the cutting magnetic field of the rotor winding is opposite to the motor state, which acts as a brake. When the speed drops to close to zero, the power is immediately cut off to avoid the motor from reversing. The characteristics of reverse braking: the advantage is that the braking force is strong, the stop is fast, and no DC power supply is needed; the disadvantage is that the braking process has a large impact and consumes a lot of power.
②Resistance reverse connection braking
The wire-wound asynchronous motor does not change the wiring of the power supply when lifting heavy objects. If the resistance of the rotor circuit is continuously increased, the rotor current of the motor will decrease, the electromagnetic torque will decrease, and the speed will continue to decrease. When the resistance reaches a certain value, the speed will be 0. When the resistance is increased, the motor reverses.
Features: Large energy loss.
2. Control method of holding brake
There are many control methods for the control of the brake, such as relay or contactor logic interlock control, PLC programming control, and internal brake logic control of the inverter.
Generally, it is realized by the control function of the inverter itself. When braking is required, the inverter outputs 24VDC to the relay, and the relay drives the contactor to control the brake coil. When a signal is output, the motor brake is opened, and it is in the braking state without output. The advantage is that the motor speed controlled by the inverter will only act when it can be manually set and reached accurately. The normal operation of the drive equipment is satisfied.
This method is very simple, but it relies too much on the inverter itself. From this point of view, the safety and reliability are worse. Now in some key occasions, such as when the brake does not participate in the operation of the control equipment, but serves as a safety guarantee, try to separate the brake from the control of the inverter or even the PLC directly as an independent relay control loop for control. But in general occasions, the logic control function of the inverter itself is quite practical for industries such as lifting.
In the actual project, the brake is divided into two types: normally closed and normally open. Except for cranes, cranes and other special occasions, which are normally closed (the brake is in a tight state when not working), our applications are generally normally open , That is, hold tightly when the brake circuit is connected.
It is also a way to control the brake through network communication between the inverter and the PLC. PLC is used to control the start, stop, speed setting, and switch of the brake.
After the inverter feeds back the operating parameters to the PLC, the PLC controls the brake through the obtained real-time parameters of the inverter. In addition, it is more complete that the PLC and the inverter side should also do a communication test.
The PLC can also perform closed-loop control through device-side speed or position detection. The PLC obtains feedback signals, analyzes and judges the real-time position and speed, and calculates the output signal to the brake, making the control more reliable and the safety interlocking more tight.
More important occasions can also directly connect the signal to the inverter enable signal, the inverter emergency stop signal, and even make a completely independent brake control loop to safely open and close the brake to prevent accidents in production, equipment and personal accidents. .
3. Timing of opening and closing of the brake
I personally think that the most important principle in brake control is that the brake force cannot conflict with the motor torque, or the conflict is small enough to be acceptable, and the motor cannot reverse under external force. In the end, it must be set according to process requirements.