What is a motor starter? What are the classifications and functions of motor starters?

A motor starter is a device used to assist in starting a motor. It can start the motor smoothly, reduce the impact on the power grid, and also achieve soft stopping, braking, overload and phase loss protection for the motor.

1. Magnetic starter

1. The magnetic starter is a combined control appliance with an AC contactor as the main body and a thermal relay as the combination. It can control the start and stop of small and medium-sized three-phase squirrel cage asynchronous motors through button operation. It has two structures: reversible and irreversible.

2. The magnetic starter itself can achieve voltage loss protection and overload protection. In order to achieve short-circuit protection, a fuse must be added. A knife switch for isolation should be installed in front of the starter.

3. The magnetic starter is composed of a steel stamping shell, a steel bottom plate, an AC contactor, a thermal relay and corresponding wiring. When in use, it should be equipped with a start-stop button switch and the manual signal cable should be correctly connected. The metal shell and bracket of the magnetic starter should be grounded or grounded.

4. Working principle of magnetic starter

When the start button is pressed, the AC contactor coil inside the magnetic starter is energized, the armature drives the contact group to close, the motor power is turned on, and the auxiliary contacts are self-locked. When the stop button is pressed, the internal AC contactor coil loses power, the contacts are disconnected, the motor power is cut off and unlocked.

5. According to different control requirements, the magnetic starter can also be flexibly wired to achieve functions such as inching and phase change. The built-in thermal relay provides overload protection for the controlled motor, and the setting current of the thermal relay should meet the power requirements of the motor.

6. Magnetic starter belongs to full voltage direct starting, and is used when both the grid capacity and load allow full voltage direct starting. The advantages are convenient operation and control, simple maintenance, and relatively economical. It is mainly used for starting small power motors, and this method is not suitable for motors greater than 11kw.

2. Pressure Reducing Starter

For squirrel cage motors with a capacity of more than 10kW or a capacity that accounts for more than 20% of the transformer capacity, due to the large starting current, a pressure reducing starter should be used to reduce the voltage applied to the stator winding. After the motor is started, the voltage is raised to the rated value to enable normal operation.

1. Y-△ Launcher

1) Ya-△ start

For squirrel cage three-phase asynchronous motors whose stator windings are connected in a △ shape during normal operation, if the stator windings are connected in a Y shape during starting and then in a △ shape after starting, the starting current can be reduced and the impact of the motor on the power grid can be reduced. This starting method is called star-delta decompression starting, or Y-△ starting for short.

2) Working principle of Y-△ startup

As shown in the figure above, when the button SB1 is pressed, the time relay KT and contactor KM3 are energized, KM3 is energized to connect the motor winding into a Y shape, and at the same time, its normally open auxiliary contact is closed to energize KM1, and the motor is started in a Y-shaped connection. After a certain delay, when the motor is started, the normally closed contact of the time relay KT is disconnected with a delay, causing KM3 to lose power and release, and at the same time, the normally closed auxiliary contact of KM3 is closed to energize KM2, and the motor is changed to a △-shaped connection for operation.

3) Advantages of the Y-△ startup method

(1) The starting current is 1/3 of that of direct starting, which has little impact on the power grid.

(2) The Y-△ starter has a simple structure and is cheap.

(3) When the load is light, the motor can be operated in a Y-shaped connection to achieve a match between the rated torque and the load, thereby improving the operating efficiency of the motor.

(4) There is no need to purchase additional starting equipment, and control equipment such as starting switches and AC contactors can be used to achieve this.

4) Disadvantages of the Y-△ startup method

(1) Can only be used for motors with delta connection.

(2) Only applicable to small asynchronous motors of 4 to 100 kW.

(3) When the stator winding is connected in Y-shape, the starting torque is reduced to 1/3 of that in △-shape connection, so it cannot be started under heavy load.

2. Resistance pressure reducing starter

1) Resistance pressure reduction start

When the motor starts, an appropriate resistor is connected in series in the motor circuit. Due to the voltage drop across the resistor, the voltage applied to the stator winding is lower than the power supply voltage. When the motor accelerates, the motor current decreases, the voltage drop across the resistor also decreases, and the motor terminal voltage increases. Therefore, the motor voltage and torque gradually increase, and the motor accelerates smoothly. When the motor is close to the rated speed and has completed the start, the series resistor is short-circuited, and the motor terminal voltage becomes the line voltage, that is, it operates normally at the rated voltage. This starting method is called resistor pressure reduction starting.

2) Working principle of resistance pressure reduction starting

As shown in the figure above, press the start button SB1, contactor KM1 and time relay KT are energized, KM1 is attracted and self-locked, and the motor stator winding is connected in series with a resistor to start the motor. After a certain delay, when the motor is started, the normally open delayed closing contact of the time relay KT is closed, KM2 is energized, its main contact is closed to short-circuit the resistor, the motor stator winding is added with the full power supply voltage, and the starting process is completed.

3) Resistance pressure reducing starter has manual control, contactor control and time relay control. The starting resistor is generally cast iron resistor, because of its high power, it can pass a large current and generate enough torque to make the motor start to rotate from rest.

4) Because the larger the current, the greater the voltage drop on the starting resistor, so the voltage drop on the motor at the moment of starting is relatively large, which greatly reduces the starting torque of the motor. Therefore, the resistor pressure reducing starter is only suitable for light-load starting motors.

3. Auto-coupling pressure reducing starter

1) Auto-transformer pressure reducing starter, also known as compensator, uses autotransformer to reduce starting voltage and limit starting current.

2) Working principle of auto-coupling pressure reducing starter

As shown in the figure above, when starting the motor, push the knife handle to the starting position. At this time, the three-phase AC power supply is connected to the motor through the autotransformer. After the start is completed, move the knife handle to the running position to cut off the autotransformer, so that the motor is directly connected to the three-phase power supply, and the motor operates normally. At this time, the pull-in coil KV is energized and the knife handle is kept in the running position through the interlocking mechanism. When stopping, press the SB button.

3) The auto-coupling pressure reducing starter consists of auto-transformer, protection device, contact system and handle operating mechanism. The tap voltage of the auto-transformer is 65% and 80% of the power supply voltage, which can be selected according to the load size at the time of starting. The coil is designed for short-time power supply and can only be started twice in succession.

4) The autotransformer pressure reducing starter is equipped with overload protection and undervoltage protection.

(1) Overload protection is provided by a bimetallic thermal relay.

(2) Undervoltage protection uses a pressure-loss release. The pressure-loss release consists of a coil, an iron core and an armature, with the coil connected across two phases. When the power supply voltage is normal, the coil is energized to make the iron core attract the armature; when the power supply voltage drops below 85% of the rated voltage, the iron core suction force decreases, the armature falls, and the compensator trips through the operating mechanism, cutting off the power supply to the motor to prevent the motor from burning due to too low voltage. When the power is suddenly cut off, the compensator will also trip to prevent the motor from starting at full voltage automatically when the power is restored.

3. Frequency sensitive resistor

1. Frequency sensitive resistor start

Frequency-sensitive rheostat starting is a method of starting by inserting a frequency-sensitive rheostat in series in the rotor circuit of a wound-rotor motor. The frequency-sensitive rheostat is actually a reactor with very large core loss, and its core is made of several 30-50mm thick cast iron plates or steel plates. The three-phase winding of the frequency-sensitive rheostat is generally connected in a Y shape and connected to the rotor of the motor through the terminal.

2. Frequency sensitive rheostat starting working principle

As shown in the figure above, press the start button SB2, the time relay KT is energized and attracted, and its normally open contact makes KM1 energized and self-locked, the motor stator winding is connected to the power supply, and the rotor is connected to the frequency-sensitive resistor to start. As the motor speed rises steadily, the impedance of the frequency-sensitive resistor gradually decreases. When the speed rises to close to the rated speed, the time relay KT delay time is reached, and its delayed closed normally open contact is closed, so that KM2 is energized and self-locked, and the frequency-sensitive resistor is short-circuited, and the motor enters normal operation; at the same time, the delayed disconnected normally closed contact of KT is disconnected, and the normally closed contact of KM2 is disconnected, cutting off the self-locking circuit before KT, so that KT is de-energized and released.