Electric brake of three-phase asynchronous motor
4.2 Electrical braking of three-phase asynchronous motor
d. Electronic brake
Electronic braking can be achieved with just one speed controller with a brake resistor. In this case, the asynchronous motor is used as a generator, and the mechanical energy is dissipated into the brake resistor without increasing the loss in the motor.
For more information, please refer to the electronic speed control device in the chapter on motor starter equipment.
e. Braking through supersynchronous operation
In such a system, the load of the motor is driven above the synchronous speed, making it behave like an asynchronous generator and generating a braking torque. Except for a small amount of dissipation, the energy is completely recovered by the power source.
In the case of a lift motor, this operation is equivalent to a drop in load at rated speed. Braking torque accurately compensates for the torque of the load and operates the motor at a constant speed (rather than deceleration).
On slip ring motors, all or part of the motor resistors must be short-circuited to prevent mechanical damage when the motor is driven more than the rated speed. This system can well limit the drive load:
●The speed is stable, independent of the driving torque.
● Energy can be restored to power energy.
However, it has only one speed, which is similar to the rated speed. The super-synchronous braking system is also used in multi-speed motors to speed up from fast to slow. Super-synchronous braking is easily achieved with an electronic speed controller that automatically triggers the system when the frequency setting is reduced.
f. Other brake systems
Single phase braking systems are still used in some applications. This system drives the motor between the two power phases and connects the unused terminals to one of the other two terminals connected to the power supply.
The braking torque is limited to 1/3 of the maximum motor torque. This system cannot brake the entire load and must be fully functional with the help of a reverse current braking system. This system creates a serious imbalance and a large loss.
Another system is braking by eddy current deceleration. In addition to mechanical brakes (electric retarders), this system works like a system used in industrial vehicles. Mechanical energy is dissipated in the reducer. The brake function is controlled by only one field winding. However, one disadvantage of this system is that it significantly increases inertia.
