How to start an asynchronous motor?

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What are the starting methods of asynchronous motors, and what are their advantages and disadvantages?

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The starting methods of asynchronous motors mainly include full voltage direct starting, star-delta starting, autotransformer step-down starting, soft starting, variable frequency starting, etc.

Next, we will introduce the characteristics of various starting methods.

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1. Full voltage direct starting

Full voltage direct starting refers to connecting the motor to a power source and directly applying rated voltage and frequency to start the motor. This is the simplest starting method, suitable for low-power motors.

Full voltage direct starting will result in a starting power of 4-7 times the rated power, requiring the power supply (transformer, inverter, etc.) to have sufficient power and overload resistance. Therefore, high-power motors are generally not suitable for full voltage direct starting.

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2. Star-delta starting

For motors with delta connection, they can be changed to star connection first, and then connected to the power supply. This way, the voltage obtained by each phase winding is the phase voltage, and the starting current is small. After the motor reaches a certain speed, it can be switched to delta connection to allow the winding to obtain the rated voltage and then increase to the rated speed.

In applications, star-delta starting can be achieved through the following circuit.

Close the power switch QF, press button SB2 to power on KM1 and lock it, then power on KM3. Connect the motor in a star shape and connect it to a three-phase power supply for voltage reduction starting. At the same time as KM3 is powered on, the time relay KT is powered on. After a period of delay, KT's normally closed contact opens, KM3 loses power, and KT's normally open contact closes. KM2 is powered on and self-locking, and the motor winding is connected to a triangular full voltage operation. When KM2 is powered on, its normally closed contacts disconnect, causing KT to lose power and avoiding long-term operation of the time relay. KM2 and KM3 normally closed contacts are interlocked to prevent power short circuits caused by simultaneous star and triangle connections.
The circuit for star-delta starting is relatively simple, easy to operate and control, but it is still not suitable for high-power motors.

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3. Autotransformer step-down starting

Autotransformer starting is a method used for starting asynchronous motors, which gradually adjusts the voltage of the motor through an autotransformer to achieve smooth starting. The following is the basic process of starting an autotransformer:

① Initial stage: At the initial start-up of the motor, the connection of the autotransformer is in the starting position. At this position, a portion of the transformer winding (autotransformer winding) is connected to the power supply through one end of the transformer, while the other portion is connected to the motor winding.

② Gradually increasing voltage: During the start-up process, the power supply voltage of the motor is gradually increased by rotating the connection point of the autotransformer. Autotransformers provide a means of gradually increasing voltage, thereby gradually increasing the torque and speed of the motor.

③ Transformer switching: As the motor gradually accelerates, when a certain speed or torque is reached, the connection point of the autotransformer is switched to the final position. At this position, the motor is directly connected to the power supply, while the function of the autotransformer is completed. The motor is currently operating at rated voltage and frequency.

Self coupling voltage reduction starting can be applied to high-power motors. At the same time, the structure of the autotransformer is mainly a coil without electronic components, so its reliability is relatively high. Therefore, it can be used in situations such as fire pumps that require high system reliability.

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4. Soft starting

The soft starter is connected in series between the power supply and the motor, and thyristors or other controllable silicon can be used in the main circuit of the soft starter.

① When the soft starter is powered on for the first time, a large resistance is generated, which limits the starting current in the circuit.

② Then gradually reduce this resistance until it is fully conductive, so that the motor can obtain the rated voltage.

If this state is maintained during operation, it is called an online soft starter. Due to the long-term need for current to pass through it, the lifespan of an online soft starter is relatively short.

If the soft starter is disconnected and the bypass contactor is closed after the motor is started, it is called a bypass type soft starter. Obviously, the structure of a bypass type soft starter is more complex and costly, but it has a longer lifespan and is easier to maintain.

According to the opposite control method, soft parking of the motor can also be achieved.

Soft starters can be used for starting and stopping high-power motors, but they cannot adjust the frequency of the power supply, so they cannot be used as speed control devices for motors.

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5. Variable frequency starting

The variable frequency starting of motors is usually achieved by a VFD, and the main circuit of the VFD is AC-DC-AC.

① Rectify the input voltage of the power supply into DC (pulsating DC).

② Then use capacitor filtering to adjust the pulsating DC power to a DC power with smaller voltage fluctuations.

③ By outputting AC power through an inverter, the output frequency can be adjusted by changing the switching frequency of the switching transistor. At the same time, the output voltage can also be adjusted by changing the duty cycle of the switching transistor. Therefore, the output voltage can be lower than the input voltage but cannot be higher than the input voltage.

When the VFD is powered on for the first time, it first outputs a lower voltage and frequency to the motor, and then gradually increases to the set voltage and frequency. The current during the entire starting process will not exceed the rated current. The soft start of a motor (different from the soft start of a soft starter) is not the main function of a VFD, but rather the regulation and reliable control of the motor. We will provide a detailed introduction in the next issue. But frequency conversion is still the best solution for motor starting, as it can fully meet the VF characteristics of the motor, providing the best control and protection for the motor.

The VFD can achieve soft parking and free parking of the motor. For scenarios with high braking requirements, the braking resistor can be increased to convert the electrical energy generated during the deceleration and parking process of the motor into heat energy, such as cutting operations, and the energy recovered from braking, such as new energy vehicles and high-speed trains.

The disadvantage of a VFD is that it generates a large amount of harmonics. For applications with high requirements for power quality, it is necessary to use equipment such as input reactors, output reactors, DC reactors, filters, etc. Single phase motors are also not suitable for variable frequency control.