There are many ways to start electric motor, combined with our usual experience, today I will talk about how to start the asynchronous electric motor is more reasonable.
Electric Motor starting is the process of electric motor energization from standstill to rated speed.
Although the electric motor starting time is relatively short, due to the high motor current during this process, it is easy to cause winding heating problems, thus it is necessary to ensure the normal starting and running needs of the electric motor through necessary parameter adjustment and suitable starting methods.
When the starting current is too large, a large voltage drop will be generated on the line, which will affect the normal work of other loads on the same line, and in serious cases, the motor may not start because the starting torque is too small. For the motor that needs to be started frequently, it often causes the winding to heat up and the insulation to age, thus shortening the service life of the motor.
In the starting process, the motor should have enough starting torque to shorten the starting time as much as possible; the starting current should be as small as possible under the premise of ensuring enough starting torque.
Please refer to another article on how to realize the starting measures of large torque and small current, which will not be repeated here.
From the aspect of simplification, economy and reliability of the equipment, the choice should be made with the characteristics of the motor and the actual working conditions.
Direct start
Direct start, which applies rated voltage to the electric motor stator winding, can be used for cases where the electric motor capacity is not too large and the grid capacity is large.
When the motor power is less than 7.5kW, or the motor starting does not cause too much fluctuation of the grid voltage (not more than 10%), direct starting can be used.
When direct starting, no additional starting equipment is needed, and the cost is lower and more convenient, but not for high power motors.
Reduced voltage starting of cage motors
Commonly used buck starting are autotransformer buck starting, star-delta buck starting, extended-edge triangle starting and stator series-connected resistance buck starting.
The star-delta conversion method is suitable for no-load or light-load starting.
The advantages of this starting method are simple equipment, low cost, and automatic switching for convenience.
Delayed-edge triangle starting method, is in the motor starting part of the stator winding according to the star connection, another part according to the triangle connection, for this reason the stator winding more taps, poor operability.
Stator winding series resistance starting, is to start in series on the stator winding resistance to produce voltage drop and share the stator winding voltage, when the electric motor starts and then short the resistance, the cost of the method is large, the use of very little.
Starting of wire-wound rotor motors
There are two types of winding rotor starting methods: series resistor and frequency sensitive resistor.
In the rotor circuit in series resistance way, not only reduces the starting current, but also has a larger starting torque, can realize the electric motor in heavy load conditions.
This starting method is used more in lifting equipment, winding equipment.
But this starting method is relatively large investment, starting equipment is also more complex, the starting process is not smooth.
When using a frequency sensitive resistor to start, at the beginning of the start, the electric motor speed is low, the rotor frequency is high, and the frequency sensitive resistor resistance is large, which effectively limits the starting current and improves the starting torque; as the motor speed increases, the rotor frequency rises, the frequency sensitive resistor resistance becomes smaller, and the change is smoother, and when the rated motor speed is reached, the frequency sensitive resistor is cut off.
This starting method is simple in structure, easy to use, and has a long life, but the power factor is relatively low, and the starting torque is not very large, which is not suitable for heavy load starting.
