Experimental study on automatic voltage reduction starting control of three-phase asynchronous motor Y - Δ
1. Experimental purpose
⑴ Learn the wiring and operation methods of Y-Δ automatic voltage reduction starting control for three-phase asynchronous motors.
⑵ Understand the concept of Y - Δ automatic voltage reduction starting for three-phase asynchronous motors.
Understand the basic principle of Y - Δ automatic voltage reduction starting for three-phase asynchronous motors.
⑷ Understand the function and operation of the time relay.
2. Preview content and requirements
The Y-Δ conversion starting method for three-phase asynchronous motors is a commonly used voltage reduction starting measure for large capacity motors, but it can only be applied to three-phase asynchronous motors with delta connections. During the starting process, the Y-shaped connection of the winding can be used to reduce the winding voltage and current of the motor, thereby reducing the starting current and minimizing the impact of the motor starting process on the grid voltage. After the starting process of the motor is completed, restore the winding to the delta connection to ensure normal operation of the motor.
⑵ Motor Y - Δ Start Control Principle
① The control circuit and its composition mainly consist of the following components: a. Start button (SB2). Manual button switch can control the starting and running of the electric motor. b. Stop button (SB1). Manual button switch can control the stop operation of the motor. c. Main AC contactor (KM1). The main operating circuit of the electric motor uses a contactor, which starts with the starting current of the motor and runs with the normal operating line current. d. Y-shaped AC contactor (KM3). An AC contactor used for Y-shaped connection during motor starting, which reduces the starting current through the Y-shaped connection and stops working after starting. e. Delta shaped AC contactor (KM2). A contactor used to restore the delta connection for normal operation after the start of an electric motor, through which the phase current of the winding operates normally. f. Time relay (KT). Control the starting process time (motor starting time) of the Y - Δ transformation, which refers to the time required for the motor to start from the start to the rated speed and operate normally. g. Thermal relay (or motor protector FR). The thermal relay is mainly equipped with overload protection for three-phase motors; The motor protector is mainly equipped with overload protection, phase failure protection, short circuit protection, and horizontal protection for three-phase motors.
② The control principle for the Y - Δ conversion starting of a three-phase asynchronous motor is roughly as follows: i. After pressing the start button SB2, the power supply passes through the dynamic disconnection contact of the thermal relay FR, the dynamic disconnection contact of the stop button SB1, and the normally closed auxiliary contact of the Δ - connected AC contactor KM2. The coil of the time relay KT is turned on to activate and delay the start. At this time, although the time relay KT has been activated and the contact should be disconnected, its delay contact is instantly closed and disconnected (disconnected after the delay ends). At the same time, the coil circuit of the Y-connected AC contactor KM3 is connected through this KT delay contact, and the AC contactor KM3 is energized, and its main contact is connected to the three-phase winding, so that the motor is in a Y-connected operating state; Close the KM3 auxiliary normally open contact to connect the coil of the main AC contactor KM1. Ii. After the main AC contactor KM1 is energized and started, its auxiliary contacts perform self holding function (self-locking function); And when the main contact of KM1 is closed to connect the three-phase AC power supply, the starting process of the motor begins. Iii. When the time relay KT delays the disconnection of the contact (dynamic disconnection contact) KT and reaches (or delays until) the end time of the motor starting process, the time relay KT contact will immediately disconnect. After the KT contact of the time relay is disconnected, the AC contactor KM3 loses power. KM3 main contact cuts off the Y-shaped connection circuit of the motor winding; At the same time, the normally closed auxiliary contact of contactor KM3 is closed to connect the coil power supply of the Δ - shaped AC contactor KM2. v. After the AC contactor KM2 is activated, its main contact closes, allowing the motor to operate normally in a delta connection state; The normally closed auxiliary contact of KM2 is disconnected, causing the KT coil of the time relay to lose power and interlocking with the AC contactor KM3. The electric motor is operating normally. After the start-up process is completed, the motor operates normally in a delta shaped connection.
