Feedback from the unit supplier reminds you that there are three main electrical braking methods for three-phase induction motors: energy consumption braking, reverse connection braking, and regenerative braking. The induction motor referred to here refers to three-phase asynchronous motors and wound motors.
1. Cut off the three-phase AC power supply of the motor during energy consumption braking and send DC power to the stator winding. At the moment of cutting off the AC power supply, due to inertia, the motor still rotates in its original direction, generating induced electromotive force and induced current in the rotor conductor. The induced current generates torque, which is opposite in direction to the torque generated by the fixed magnetic field formed after the direct current is supplied. Therefore, the motor quickly stops rotating to achieve the purpose of braking. The characteristic of this method is smooth braking, but it requires a DC power supply and a high-power motor. The cost of the required DC equipment is high, and the braking force is small at low speeds.
2. Reverse braking is divided into two types: load reverse braking and power reverse braking.
1) Load reverse braking, also known as load reverse pulling reverse braking. When the rotor of an electric motor rotates in the opposite direction to the rotating magnetic field under the action of a heavy object (when a crane uses an electric motor to lower a heavy object), the electromagnetic torque generated at this time is the braking torque. This torque causes the heavy object to slowly descend at a stable speed. The characteristics of this type of braking are that the power supply does not need to be reversed, specialized braking equipment is not required, and the braking speed can be adjusted. However, it is only suitable for wound motors, and the rotor circuit needs to be connected in series with a large resistor to make the slip rate greater than 1.
2) When the electric motor needs to brake, simply swap the two-phase power lines to make the rotating magnetic fields opposite, and it can quickly brake. When the motor speed reaches zero, immediately cut off the power supply. The characteristics of this type of braking are: fast parking, strong braking force, and no need for braking equipment. However, during braking, due to the high current and impact force, it is easy for the motor to overheat or damage the components of the transmission part.
3. Regenerative braking, also known as feedback braking, refers to the phenomenon where, under the action of a heavy object (when the crane motor lowers the object), the motor's speed exceeds the synchronous speed of the rotating magnetic field. At this time, the rotor conductor generates induced current, which produces anti rotational torque under the action of the rotating magnetic field. The motor enters the power generation state and feeds back to the power grid. This method can naturally enter the feedback braking state and work reliably, but the motor speed is high and requires a variable speed device to slow down.
Although there are not many devices that use brakes, most three-phase motors such as water pumps, fans, and transmission motors are not necessary and can be stopped freely, there are still many specific factory equipment that require braking. The above three braking methods each have their own advantages and disadvantages, and they also have their own applications. The specific one to use depends on the specific equipment.