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Control of three-phase AC motor
From: Dongguan (Li Qiang) Li Da mechanical and Electrical Equipment Co., Ltd. Post date: 2018-01-17

1. start control

The three-phase asynchronous motor runs from the power supply, and the speed is gradually rising to the stable operation state. This process is called starting. According to the different starting methods, it can be divided into direct start and step-down start.

The direct starting current is large, and it has great influence on the power supply transformer. The squirrel cage asynchronous motor with large capacity generally adopts the step-down start. The step down start is to increase the power supply voltage and then add it to the stator windings of the motor to start. When the motor starts or ends, it will restore the motor's voltage to the rated value. The main purpose of this is to reduce the starting current, but the starting torque of the motor will also be reduced because of the depressurization. Therefore, the step-down start is only applicable to the no-load or light load start-up.

1) direct start

I. is directly activated by switches. The circuit directly started by switches is only suitable for small capacity motors which do not start frequently. It can not achieve long-distance control and automatic control, nor can it achieve zero voltage, under voltage and overload protection.

Ii. adopts contactor point control: using contactor point control circuit, it can control the motor with larger capacity or start frequently, and achieve the function of "moving at once, loosening".

III. uses contactor long acting control: the contactor long acting motor is controlled, after pressing the start button, the motor starts running, because it has self locking contacts, so if we want to stop the motor, we must press the stop button.

IV. long acting and point mixing control: if the motor is not only dynamic control, but also continuous operation control, it can be combined with three buttons and self locking contacts to achieve point and long operation control.

2) buck start

I. on the stator winding resistance (TCR) step-down start: when the motor is started, the three-phase stator circuit series resistance, reduce motor stator winding voltage to limit the starting current, motor speed to be increased to a certain value, the resistance will be removed, the motor rated voltage in stable operation.

Ii. star triangle step-down start:

The starlike triangle step-down start means that when the motor starts, the stator winding is connected to the star to reduce the starting voltage and limit the starting current. After the motor is started, the stator winding is changed into a triangle to make the motor run from now on. This kind of step-down method can be used in any asynchronous motor with triangular connection made of stator winding in normal operation. When the motor starts, it is connected to Y shape, and the starting voltage added to the stator windings of each phase is only triangular connected. The starting current is triangular connected to 1/3, and the starting torque is only 1/3 connected by triangle. So this method of Buck starting is only suitable for starting with light load or empty load.

III. autotransformer start down:

Auto start is in the startup power supply voltage is added in the primary winding autotransformer, the stator winding of the electric motor is connected with the secondary winding autotransformer, when the motor speed reaches a certain value, the removal of autotransformer, motor directly connected with the power supply, at normal operating voltage.

In the many speed regulating methods of AC asynchronous motor, the performance of frequency conversion speed regulation is the best. Its characteristic is the large speed range, good stability and high efficiency.
  2. positive inversion control

According to the principle of motor science, the inversion can be achieved as long as any two of the three-phase AC power lines connected to the three-phase asynchronous motor are adjusted.

There are four main types of positive and reverse control methods:
1) manual control
2) interlocking control of contactor
3) button interlocking control
4) double interlocking control of contactor and button

3. brake control
After cutting off the power supply, the three phase motor will take a period of time to stop completely because of the inertia. Sometimes, the motor is required to stop running quickly after the power break, which requires the motor to be brake.
The braking method can be roughly divided into two types: mechanical brake and electrical brake. Mechanical braking devices are commonly used two kinds of electromagnetic clutch and electromagnetic brake. The electrical methods include reverse brake, energy consumption braking, regenerative braking and capacitive brake.

1) reverse brake control line
Reverse connect brake motor is power movement in the reverse (i.e. any two line connection exchange) to change the power supply motor stator windings, thereby reflecting rotating magnetic field of stator winding, rotor under braking torque opposite the direction of rotation and quickly stop.
In this type of braking, one problem is worth noting: when the motor speed is close to zero, if the power is not cut off in time, the motor will rotate in reverse. Therefore, the corresponding measures must be taken to ensure that the power supply is quickly cut off to prevent the motor from reversing when the motor speed is braking to near zero. The speed relay is often used for automatic control in the general anti - brake control line.

2) energy consumption braking control line
The energy consumption brake control circuit is when the motor stops, it enters two phase DC power in the stator windings of the motor, making it produce a constant static magnetic field. After cutting the magnetic field by the moving rotor, it will be in the middle of the rotor winding.

The induced current is induced. The current is also affected by the static magnetic field, which produces electromagnetic torque. The direction of the electromagnetic torque is just opposite to the steering of the motor, so that the motor stops rapidly. More application of the transformer bridge rectifier unidirectional operation of energy consumption braking. The advantage of energy consumption braking is that the braking precision energy consumption is small, the impact is small, the shortcoming is that the DC power supply is added, and the braking torque is small.

4. variable frequency speed control
Speed regulation means that the motor can get different speeds under the same load to meet the actual needs. There are three possibilities to change the speed of the motor: one is frequency conversion, the two is variable pole speed regulation, and the three is variable speed difference speed.

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