Electric brake of three-phase asynchronous motor
4.2 Electrical braking of three-phase asynchronous motor
a. Introduction
In many systems, the motor is stopped by natural deceleration. The time of stopping depends only on the inertia and resistance torque of the machine driven by the motor. However, in many cases it is necessary to shorten this time, and electric braking is a simple and effective solution. Compared to mechanical and hydraulic brake systems, it has the advantage of being relatively stable and does not require the use of any wear parts.
b. Reverse current braking principle
The motor is isolated from the power supply while it is running and then reconnected to the power supply via another channel. This is a very efficient brake system, the torque is usually higher than the starting torque, so it must be braked as early as possible to prevent the motor from starting in the opposite direction.
When the speed is close to zero, the stopping process is controlled by several automatic devices:
● Friction stop detector, centrifuge stop detector.
●Precise timing equipment.
● Frequency measurement or rotor voltage relay (slip ring motor).
Squirrel cage motor
The thermal stress during braking is more than three times higher than the thermal stress generated during acceleration. At braking, the current and torque peaks are significantly higher than the starting values.
To ensure smooth braking, each stator phase is often connected in series with a resistor when switching to reverse current. In this way, the torque and current can be reduced as in the case of the stator starting. Reverse current braking in squirrel cage motors has significant drawbacks, so this system is only used for some low power driven motors.
2. slip ring motor
After the current is turned on, the actual rotor voltage is twice as long as the pause, and in some cases the corresponding insulation measures are required. As with the squirrel cage motor, a large amount of energy is released into the rotor circuit. It is completely dissipated (except for a small amount of loss) in the resistor.
The motor can be automatically stopped by means of one of the above devices or a voltage or frequency relay in the rotor circuit. This system keeps the drive load at medium speed. Its characteristics are very unstable (as long as the torque changes slightly, the speed will change a lot).
c. By injecting a DC current braking system
The rectified current produces a fixed magnetic flux in the air gap of the motor. In order to generate the proper flux and ensure the correct braking effect, the current must be 1.3 times larger than the rated current. A slight overcurrent can cause excess heat loss, which is compensated by a pause after braking.
Since the current value is completely set by the stator winding resistance, the voltage at the rectified source is low. The rectified source is usually supplied by a rectifier or speed controller. They must be able to withstand transient voltage surges that are just emerging from windings that are disconnected from an AC source such as the 380V rms. There is a slip between the motion of the rotor and the fixed magnetic field (the magnetic field rotates in the opposite direction to the reverse current system). The behavior of the motor is similar to that of a synchronous generator that discharges within the rotor. Compared to reverse current systems, the characteristics of rectified injection systems vary widely:
• Less energy is dissipated in the rotor resistor or cage. It is only equivalent to the mechanical energy released by the moving material. The kinetic energy obtained from the power source is only used to start the stator.
● If the load is not driving the load, the motor will not start in the opposite direction.
● If the load is driving the load, the system will brake frequently and keep the load at a low speed. This is a deceleration brake instead of a standstill brake. Its characteristics are much more stable than the reverse current.
In the case of a slip ring motor, the speed-torque characteristic depends on the resistor selected. In the case of a squirrel-cage motor, the system can easily adjust the braking torque by operating the DC starting current. However, if the motor is running at high speed, the braking torque will be lower. In order to prevent severe overheating, the relevant equipment must be used to cut off the current in the stator at the end of the braking.
