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electric braking of three phase induction motor

1. Reverse brake:

When the motor is disconnected from the power supply, in order to make the motor stop quickly, using the power control method in motor power supply coupled with the normal operation of power supply phase at this time, the direction of rotation of the rotation direction of the motor rotor and the motor rotating magnetic field in electromagnetic force at the moment is generated by the motor brake torque, motor speed reducer. At this time, the main magnetic field in the motor remains unchanged, and the armature continues to rotate due to inertia. This moment is the braking torque, so that the motor speed decreases until it stops.

The reverse brake has one of the biggest disadvantages, that is, when the motor speed is 0, if the power supply is not removed in time, the motor will reverse. There are two ways to solve this problem: 1. Add a time relay to the control loop of the motor's inverting power supply. When the inverter is braking for a period of time, it will disconnect the power after inverting, so as to avoid motor reversing. However, because the braking time is difficult to be estimated, the braking effect is not accurate. 2, add a speed relay to the control loop of the motor's inverting power supply. When the sensor detects the motor speed of 0, cut off the inverter power of the motor in time.

Two. Energy consumption brake:

A fixed magnetic field is produced in the stator winding through direct current. At this point, the rotor cuts the magnetic force line in the direction of rotation, thus producing a braking torque. Because this braking method is not like regenerative braking, the energy generated by braking is fed back to the power grid, but it is consumed by motor alone, so it is called energy consumption braking. Because of the direct current braking in the stator winding, the energy consumption brake is called the DC injection brake.

Energy consumption braking is simply relying on the motor to consume kinetic energy to achieve the purpose of parking, so the braking effect and precision are not ideal. In some occasions when the braking time is short and the braking effect is good, the braking method is generally not used.

Three. Regenerative braking:

The regenerative braking and the two kinds of braking methods are different. Regenerative braking is only a working condition of motor in special cases, and the above two methods are applied to the motor in order to achieve the purpose of rapid shutdown.

The principle of regenerative braking is that when the rotor speed of the motor exceeds the rotation speed of the synchronous magnetic field, the rotation direction of the electromagnetic torque generated by the rotor windings is opposite to that of the rotor, when the motor is in the braking state. The reason is that the state of this time is called regenerative braking, because at this point the motor is in the state of electricity, that is, the kinetic energy of the motor is converted into electrical energy. At this time, some measures can be taken to give back the generated electricity to the power grid. To achieve the purpose of saving energy. Therefore, regenerative braking is also called power generation brake.

Regenerative braking will appear in the following two situations: 1. When the crane weight drops, the rotor speed of the motor rotor may exceed the synchronous speed under the gravity gravity manual operation. At this time, the motor is in regenerative braking state. At this time, the braking torque of the motor is to prevent the fall of the weight, until the braking torque is equal to the torque formed by gravity, and the weight will cease to fall. 2. When the frequency conversion is reduced, the synchronous speed also decreases when the frequency converter reduces the frequency. However, the rotor speed will not decrease immediately due to the load inertia. At this time, the motor will also be in the regenerative braking state until the speed of the drag system decreases.

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