Commonly Used Starting Methods for Three-Phase Asynchronous Motors

  1. Use auto-transformer step-down start

Auto-transformer step-down start: connect the high-voltage side of the auto-transformer to the power grid, and the low-voltage side to the motor. When starting, use the auto-transformer tap to reduce the voltage of the motor. When the speed rises to a certain value, the auto-transformer is automatically cut off, and the motor is connected to the power supply and operates normally under full voltage.

In this starting method, the tap position of the auto-transformer can be selected to adjust the terminal voltage of the motor, and the starting torque is larger than that of the star-delta step-down starting.

However, the auto-transformer has a large investment and does not allow frequent starting. It is only suitable for star or delta connection and larger capacity motors.

  1. Y-step-down start

When starting, the stator windings are first connected into a star shape, so that the voltage of each phase winding is 1/√3 of the phase voltage during normal operation. After starting, it is restored to a delta connection, and the motor enters full voltage. normal operation.

The advantage is that the cost of starting equipment is low, and the method is simple and easy to operate.

Although the starting current of this method is reduced to 1/3 of the full-voltage starting, the starting torque is only 1/3 of the rated torque, so this method is mostly used for light-load or no-load starting.


  1. Voltage Maintenance

Upon initiation of the motor, the power distribution bus must maintain a voltage no less than 85% of the system’s designated voltage. Typically, if the motor’s power rating falls within 30% of the power transformer’s capacity, it can be fully powered on without the need for intricate calculations. Only in scenarios where extreme conditions are anticipated should detailed calculations be conducted. The benefits of this approach include a reduced equipment requirement, a straightforward starting process, and minimal cost implications.

When the motor is started directly, the voltage of the power distribution bus is not lower than 85% of the nominal voltage of the system. Generally, as long as the rated power of the motor does not exceed 30% of the rated capacity of the power transformer, it can be started at full voltage. Detailed calculations are only required when the estimates are edge cases.

The advantage is that less equipment is required, the starting method is simple, and the cost is low.

The disadvantage is that it is mainly used for starting small power motors.


  1. Rotor series resistance or frequency sensitive resistance start

Series resistance start

stator winding series resistance start is carried out under the condition of sacrificing starting torque, which is only suitable for starting under light load or no load.

When heavy load starting is required, the method of connecting symmetrical resistors in series with the three-phase rotor can be used. Wound three-phase asynchronous motor, the rotor winding is connected to the resistor through a slip ring.

Generally, the starting resistors are graded and connected into a star shape. When starting, all the starting resistors are first connected. As the starting progresses and the motor speed increases, the rotor starting resistors are shorted in turn. At the end of the starting, all the resistors are shorted. catch.

Starting with a series resistance increases the rotor resistance and reduces the starting current. Select the appropriate rheostat resistance value, the power factor of the rotor will increase significantly, and the starting torque will also increase greatly.

If you want to obtain good starting characteristics, you generally need more starting stages, more electrical appliances, complex control circuits, large equipment investment, and inconvenient maintenance. At the same time, a certain mechanical impact will be generated due to the step-by-step removal of resistors.

In industrial and mining enterprises, frequency-sensitive rheostats are widely used instead of starting resistors to control the start-up of wound-type asynchronous three-motors.

 Frequency-sensitive rheostat

It’s a static non-contact electromagnetic element whose impedance value changes significantly with frequency (sensitive to frequency). It is essentially a three-phase reactor with very large core loss. When the motor is started, the frequency-sensitive rheostat is connected in series in the rotor winding. Since the equivalent impedance of the frequency-sensitive rheostat decreases with the decrease of the rotor current frequency, the purpose of automatic rheostat is achieved. Therefore, only one stage of frequency-sensitive rheostat is needed to start the motor smoothly.

Disadvantages of starting with frequency-sensitive rheostats in series: due to the existence of inductance, the power factor is low and the starting torque is not very large. Only suitable for light load start, not suitable for heavy load start.


  1. Soft Starter

The soft starter uses a three-phase parallel thyristor as a voltage regulator, which is connected between the power supply and the motor stator. Such a circuit is a three-phase fully controlled bridge rectifier circuit. When using a soft starter to start the motor, the output voltage of the thyristor gradually increases, and the motor gradually accelerates until the thyristor is fully turned on, and the motor works on the mechanical characteristics of the rated voltage to achieve smooth start, reduce the starting current, and avoid starting over-current tripping.

When the motor reaches the rated speed, the starting process is over, and the soft starter automatically replaces the thyristor that has completed the task with a bypass contactor to provide the rated voltage for the normal operation of the motor, so as to reduce the heat loss of the thyristor and prolong the service life of the soft starter. Improve its work efficiency, and make the power grid avoid harmonic pollution.

The soft starter also provides a soft stop function. The soft stop is the opposite of the soft start process. The voltage gradually decreases and the speed drops to zero to avoid torque impact caused by free stop. The soft starter also has built-in protection functions, such as stall and stall test, phase balance, underload protection, under-voltage protection and over-voltage protection, etc., which play

a further protective role for the motor

Frequency conversion technology and microelectronics technology control the power control equipment of AC motors by changing the frequency of the motor’s working power supply. The inverter is mainly composed of rectification (AC to DC), filtering, inverter (DC to AC), braking unit, drive unit, detection unit and micro-processing unit.

The frequency converter mainly adopts the AC-DC-AC mode (VVVF frequency conversion or vector control frequency conversion). electric motor.

The inverter adjusts the voltage and frequency of the output power supply by switching off the internal IGBT, and provides the required power supply voltage according to the actual needs of the motor, thereby achieving the purpose of energy saving and speed regulation.

Frequency converters generally have overheat protection, overload protection, overcurrent protection, and some even have phase loss protection.

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