Selection and Advantages and Disadvantages of Common Starting Mode of Three-phase Asynchronous Motor
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1. Direct startup
When the motor starts, the voltage of the distribution bus is no less than 85% of the nominal voltage of the system. Usually, as long as the rated power of the motor does not exceed 30% of the rated capacity of the power transformer, it can start at full voltage. Detailed calculations are required only when the estimates are marginal.
The advantages are less equipment needed, simple start-up mode and low cost.
The disadvantage is that it is mainly used for starting small power motors.
2. Starting with Self-Coupled Transformer
Autotransformer step-down starting: autotransformer high voltage side to the grid, low voltage side to the motor. When starting, autotransformer taps are used to reduce the voltage of the motor. When the speed rises to a certain value, the autotransformer is automatically cut off, and the motor is connected with the power supply. The motor runs normally under full voltage.
In this starting method, the tap position of the autotransformer can be selected to adjust the terminal voltage of the motor, and the starting torque is larger than that of the star-triangle step-down starting.
However, the autotransformer investment is large and frequent start-up is not allowed. It is only suitable for star or triangle connected motors with large capacity.
3. Y-depressurization start-up
When starting, the stator winding is connected into a star shape, so that the voltage of each phase winding is 1/3 of the phase voltage in normal operation. After starting, the motor can be restored to a triangular connection, and then the motor can run normally under full voltage.
Its advantages are low cost of starting equipment and easy operation.
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 or no-load starting.
4. Start-up of rotor series resistance or frequency sensitive resistance
Series resistance startup
The stator winding series resistance starting is carried out at the sacrifice of starting torque. It is only suitable for starting under light load or no load.
When heavy load starting is needed, the method of three-phase rotor series symmetrical resistance can be used. Winding three-phase asynchronous motor, the rotor winding is connected with resistance through sliding ring.
Generally, the starting resistance is connected to a star by stages. When starting, all the starting resistance is connected first. With the starting process, the speed of the motor is raised, and the starting resistance of the rotor is shortened in turn. At the end of starting, all the resistance is shortened.
Series resistance startup increases rotor resistance and reduces startup current. The power factor and starting torque of the rotor increase significantly when the resistance of the rheostat is selected properly.
If we want to get good start-up characteristics, we usually need more start-up stages, more electrical appliances, complex control circuit, large investment in equipment and inconvenient maintenance. At the same time, because of the step-by-step removal of resistance, there will be a certain mechanical impact.
Frequency-sensitive rheostat is widely used in industrial and mining enterprises to control the starting of winding asynchronous three-motor instead of starting resistance.
Frequency-sensitive rheostat is a static contactless electromagnetic element whose impedance varies with frequency (sensitive to frequency). It is essentially a three-phase reactor with very large core loss. When the motor starts up, the frequency-sensitive rheostat is connected in series with the rotor winding. Because 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 frequency-sensitive rheostat can be used to start the motor smoothly.
The inadequacies of series frequency-sensitive rheostat start-up: due to the existence of inductance, the power factor is low, and the start-up torque is not very large. It is only suitable for light-load startup, not for heavy-load startup.
5. Soft starter startup
Soft starter uses three-phase reverse parallel thyristor as voltage regulator, which is connected between power supply and motor stator. This kind of circuit is like three-phase full-controlled bridge rectifier circuit. When starting the motor with soft starter, the output voltage of thyristor increases gradually, and the motor accelerates gradually until the thyristor is fully switched on. The motor works on the mechanical characteristics of the rated voltage to achieve smooth start, reduce the starting current and avoid starting overcurrent tripping.
When the motor reaches the rated speed, the start-up process ends, and the soft starter automatically replaces the completed thyristor with bypass contactor to provide rated voltage for the normal operation of the motor, so as to reduce the heat loss of the thyristor, prolong the service life of the soft starter, improve its working efficiency and avoid harmonic pollution of the power grid.
Soft starter also provides soft parking function. Soft parking is contrary to soft starting process. Voltage decreases gradually and speed decreases gradually to zero to avoid the impact of torque caused by free parking.
Soft starter also has built-in protection functions, such as stall and stop-up test, phase balance, under-load protection, under-voltage protection and over-voltage protection, which play a further role in motor protection.
6. Frequency Converter Start-up
Frequency converter is a kind of electric control equipment which uses frequency conversion technology and microelectronics technology to control AC motor by changing the working power frequency of the motor. Frequency converter is mainly composed of rectifier (AC to DC), filter, inverter (DC to AC), brake unit, drive unit, microprocessor unit of detection unit, etc.
Frequency converter mainly adopts AC-DC-AC mode (VVVVF or vector controlled frequency conversion). First, power frequency AC power supply is converted into DC power supply through rectifier, and then DC power supply is converted into AC power supply with controllable frequency and voltage to supply motor.
Frequency converter adjusts the voltage and frequency of the output power supply by interrupting the internal IGBT, and provides the required power supply voltage according to the actual needs of the motor, so as to achieve energy saving and regulation.