Characteristics of high starting torque three-phase asynchronous motor
2.3 Features
Asynchronous motor start-up requirements:
1. The motor has a large enough starting torque.
2. Under the premise of a certain size of starting torque, the smaller the starting current, the better.
3. The equipment required for starting is simple and easy to operate.
4. The smaller the power loss during startup, the better.
Start of squirrel cage asynchronous motor:
1, start directly
That is, the voltage applied to the stator winding of the motor at the time of starting is the rated voltage. The condition for direct start of three-phase asynchronous motor (satisfying one)
1. Motors with a capacity below 7.5KW can be used.
2. The voltage drop caused by the motor at the moment of starting is not more than 10% of the normal value of the power supply voltage, and can be relaxed to 15% for motors that are not normally started.
3, can use the empirical formula to roughly estimate whether the motor can be started directly
Advantages: The required startup device is simple, the startup time is short, the startup method is simple, reliable, and the required cost is low.
Disadvantages: a certain impact on the motor and the power grid
2, step-down start
The voltage on the stator winding is reduced when the motor is started, and the rated voltage is activated at the end of the start. The step-down start can reduce the starting current of the motor. However, since the torque is proportional to the square of the voltage, the torque of the motor is reduced more during the step-down start, so it is only suitable for no-load or light-load starting.
A, autotransformer (also known as compensator) step-down start
(1) Wiring: The high voltage side of the autotransformer is put into the grid, and the low voltage side is connected to the motor. There are several taps with different voltage ratios to choose from.
(2) Features: The transformation ratio of the autotransformer is K, the primary voltage is U1, the secondary voltage U2 = U1/K, and the secondary current I2 (ie, the line current through the stator winding of the motor) is also proportionally reduced. Also, since I1 = I2/K, the starting current of the power supply motor is 1/K2 times that of the direct start. Since the voltage is reduced by 1/K times, the torque drop is 1/K2 times.
There are 2~3 sets of taps on the secondary side of the autotransformer, such as the secondary voltage being 80%, 60%, 40% of the primary side voltage.
Advantages: The tapping start can be selected by selecting different taps of the autotransformer according to the allowable starting current and the required starting torque, and the stator winding adopts Y or Δ.
Disadvantages: The equipment is bulky and the investment is more expensive.
B, star-triangle (Y-Δ) step-down start
(1) Wiring: When starting, connect the stator to a star, and then start to connect to Δ.
(2) Features: Starting current, power supply current and starting torque are only 1/3 when starting directly.
Advantages: Simple equipment and low price.
Disadvantages: It is only used for Δ connection in normal operation, the pressure reduction ratio is fixed, and sometimes the startup requirement cannot be met.
C, Yanbian triangle start
That is, when starting, a part of the stator winding of the motor is connected into a Y shape, and the other part is connected to a Δ shape.
Features: Start-up, the voltage of each phase winding is larger than that when connected to full star connection, the starting torque is larger, but the winding structure is more complicated and the application is limited.
D, string resistance (or reactance) step-down start
For the squirrel cage asynchronous motor, the series step-down resistor (or reactor) in the stator circuit can be used to start the motor at startup. When the motor starts, the resistor (or reactor) is shorted. This method is simple. However, the stator string resistance starts to consume more energy, and is mainly used for low-voltage and low-power motors; the stator string reactance start-up investment is large, and it is mainly used for high-voltage and high-power motors. Because of the thermal energy loss on the resistor, the reactor is bulky and costly, and this method is rarely used.
