Why can't ordinary motor be used as variable frequency motor? Mar 09,2021

Why can't ordinary motor be used as variable frequency motor?

General motors are designed according to constant frequency and constant voltage, so it is impossible to completely get used to the requirements of frequency converter speed regulation.


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

Inverter can produce different degree of harmonic voltage and current in the operation, making the motor run in non-sinusoidal voltage and current. The high order harmonic inside can cause copper loss of stator, iron loss and copper loss of rotor, as well as additional loss. The most significant is the rotor copper loss, which will cause high temperature rise by 10% to 20%, and obvious power decrease.


2. Insulation strength of motor

The carrier frequency of converter ranges from thousands to more than ten kHz, which makes the winding of the stator endure a very high rate of voltage rise. This is equivalent to imposing a very steep impulse voltage on the motor, and makes the inter-turn insulation of the motor endure serious test.


3. Harmonic electromagnetic noise and vibration

When general motor uses frequency converter power supply, the vibration and noise caused by electromagnetic, mechanical, ventilation and other factors will become more chaotic. The harmonics contained in the variable frequency power supply and the inherent space harmonics of the electromagnetic part of the motor interfere with each other, and constitute a variety of electromagnetic excitation force, and then increase the noise. Because the operating frequency scale of the motor is wide and the speed change scale is large, the frequency of various electromagnetic force waves is difficult to avoid the natural oscillation frequency of each structural part of the motor.


4. Cooling problem at low speed

When the frequency of the power supply is relatively low, the loss caused by the higher harmonics in the power supply is large. Secondly, when the speed of the variable frequency motor decreases, the cooling air volume decreases in direct proportion to the cubic power of the speed, which causes the heat of the motor to be emitted and the temperature rise to increase sharply, making it difficult to achieve constant torque output.


Differences Between Ordinary Motor And Variable Frequency Motor

1. Higher insulation grade requirements

The insulation grade of variable frequency motor usually needs to reach F or higher. The ground insulation and turn insulation should be strengthened. The resistance ability to impact of voltage of insulation should be particularly considered.


2. Higher requirements of vibration and noise of variable frequency motor

The design of variable frequency motor should take the rigidity of the motor components and the whole into account. The natural frequency should be improved to avoid resonance with various force waves.


3. The cooling method of variable frequency motor is different

Frequency conversion motors generally adopt forced ventilation cooling, that is, the main motor cooling fan is driven by an independent motor.


4. Different requirements for protection

The bearing insulation method should be adopted for variable frequency motors with power exceeding 160kW. The main reason is that magnetic circuit asymmetry is easy to occur, and the shaft current will also occur. When the current generated by other high frequency weights is combined with the action of the shaft current, the shaft current will be greatly added, and then lead to bearing damage, so the insulation method is generally adopted.

For constant power variable frequency motor, when the speed exceeds 3000r/min, special high-temperature resistant grease should be selected to compensate the temperature rise of the bearing.


5. Different cooling systems

Independent power supply should be chosen for variable frequency motor radiating fan to ensure continuous radiating capacity.  

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