Why can’t ordinary motors be used as variable frequency motors? What is the difference between them?

Why can’t ordinary motors be used as variable frequency motors? What is the difference between them?

 

Ordinary motors are designed according to constant frequency and constant voltage, which cannot fully meet the requirements of frequency converter speed regulation, so they cannot be used as frequency conversion motors.

1. The influence of the frequency converter on the motor is mainly in the efficiency and temperature rise of the motor

The frequency converter can generate different degrees of harmonic voltage and current during operation, so that the motor runs under non-sinusoidal voltage and current. The higher harmonics inside will cause the motor stator copper loss, rotor copper loss, iron loss and additional loss to increase The most significant is the copper loss of the rotor. These losses will cause the motor to generate extra heat, reduce efficiency, and reduce output power. The temperature rise of ordinary motors generally increases by 10%-20%.
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2, the insulation strength of the motor

The carrier frequency of the inverter is from several thousand to ten thousand Hz, so that the motor stator winding has to withstand a high rate of voltage rise, which is equivalent to imposing a steep impulse voltage on the motor, which makes the inter-turn insulation of the motor to withstand a more serious test .

3. Harmonic electromagnetic noise and vibration
When an ordinary motor is powered by a frequency converter, the vibration and noise caused by electromagnetic, mechanical, ventilation and other factors will become more complicated. The harmonics contained in the variable frequency power supply interfere with the inherent spatial harmonics of the electromagnetic part of the motor to form various electromagnetic excitation forces, thereby increasing noise. Due to the wide working frequency range of the motor (public number: pump housekeeper) and the wide range of speed change, the frequency of various electromagnetic force waves is difficult to avoid the natural vibration frequency of the various structural parts of the motor.

4, the cooling problem at low speed

When the frequency of the power supply is low, the loss caused by the higher harmonics in the power supply is relatively large; secondly, when the speed of the flexible motor is reduced, the cooling air volume decreases in proportion to the third power of the speed, which causes the motor heat to not be dissipated and the temperature rises sharply. Increase, it is difficult to achieve constant torque output.
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5. In view of the above situation, the inverter motor adopts the following design:

Minimize the stator and rotor resistance and reduce the fundamental copper loss to compensate for the increase in copper loss caused by higher harmonics

The main magnetic field is not saturated design, one is to consider higher harmonics will deepen the saturation of the magnetic circuit, and the other is to consider increasing the output voltage of the inverter in order to increase the output torque at low frequencies.

The structural design is mainly to improve the insulation level; fully consider the vibration and noise of the motor; the cooling method adopts forced ventilation cooling, that is, the main motor cooling fan adopts an independent motor drive mode, and the role of the strong cooling fan is to ensure that the motor is at a low speed Cool down.

The distributed capacitance of the coil of the variable frequency motor is smaller, and the resistance of the silicon steel sheet is larger, so that the influence of the high frequency pulse on the motor is smaller, and the inductance filtering effect of the motor is better.

Ordinary motors, power frequency motors, only need to consider the starting process and the working conditions of one point of the power frequency, and then design the motor; while variable frequency motors need to consider the starting process and the working conditions of all points within the frequency range, and then design the motor.

In order to adapt to the PWM widened wave analog sine alternating current output by the inverter contains a lot of harmonics, the specially made variable frequency motor can actually be understood as a reactor plus an ordinary motor.
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Post time: Jul-27-2020

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