Why can't ordinary motor be used as frequency conversion motor?
Ordinary motor is designed according to constant frequency and constant voltage, which can not fully meet the requirements of frequency converter speed regulation, so it can not be used as frequency conversion motor.
1、 The influence of frequency converter on motor is mainly in the efficiency and temperature rise of motor
The frequency converter can produce different degrees of harmonic voltage and current in operation, so that the motor operates under non sinusoidal voltage and current. The higher harmonic inside will cause the increase of stator copper loss, rotor copper loss, iron loss and additional loss. The most significant one is rotor copper loss. These losses will make the motor extra heat, reduce efficiency, reduce output power and ordinary motor temperature In general, it will increase by 10% - 20%.
2、 Insulation strength of motor
The carrier frequency of the frequency converter ranges from several thousand to more than ten kilohertz, which makes the stator winding of the motor bear a very high voltage rise rate, which is equivalent to imposing a steep impulse voltage on the motor, which makes the turn to turn insulation of the motor withstand a more serious test.
3、 Harmonic electromagnetic noise and vibration
The vibration and noise caused by electromagnetic, mechanical, ventilation and other factors will become more complex when the ordinary motor is powered by frequency converter. The harmonics in the frequency conversion power supply interfere with the inherent space harmonics of the electromagnetic part of the motor, forming various electromagnetic exciting forces, thus increasing the noise. Because the motor has a wide frequency range (official account: Pump housekeeper) and the speed range varies greatly, the frequency of various electromagnetic waves can hardly avoid the natural vibration frequency of each structural part of the motor.
4、 Cooling problem at low speed
When the frequency of the power supply is low, the loss caused by the high-order harmonics in the power supply is large; secondly, when the speed of the flexible motor is reduced, the cooling air volume is proportional to the third power of the speed, which makes the heat of the motor unable to emit, the temperature rise increases sharply, and it is difficult to achieve constant torque output.
5、 In view of the above situation, the variable frequency motor adopts the following design:
1. The resistance of stator and rotor should be reduced as much as possible, so as to reduce the copper loss caused by higher harmonics
2. In the design of main magnetic field unsaturation, one is to consider that the higher harmonic will deepen the saturation of magnetic circuit; the other is that the output voltage of frequency converter can be appropriately increased in order to improve the output torque at low frequency
3. The structure design is mainly to improve the insulation level; to fully consider the vibration and noise problems of the motor; the cooling mode adopts forced ventilation cooling, that is, the cooling fan of the main motor adopts the independent motor driving mode, and the function of the forced cooling fan is to ensure the cooling of the motor at low speed.
4. The coil distributed capacitance of variable frequency motor is smaller, and the resistance of silicon steel sheet is larger, so the influence of high frequency pulse on the motor is small, and the inductance filtering effect of motor is better.
5. Ordinary motor, that is, power frequency motor, only needs to consider the starting process and the working condition of a power frequency point, and then design the motor; while the variable-frequency motor needs to consider the starting process and the working conditions of all points within the frequency range, and then design the motor.
6. In order to adapt to the PWM width modulation wave output from the frequency converter and simulate the sinusoidal alternating current containing a large number of harmonics, the special variable frequency motor can actually be understood as reactor plus ordinary motor.