What is the difference between a variable frequency motor and an ordinary motor?

Introduction: ​The difference between variable frequency motors and ordinary motors is mainly reflected in the following two aspects: First, ordinary motors can only work near the power frequency for a long time, while variable frequency motors can be seriously higher than or lower than the power frequency for a long time. Work under the condition of power frequency. Second, the cooling systems of ordinary motors and variable frequency motors are different.

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

  The difference between variable frequency motor and ordinary motor is mainly reflected in the following two aspects:

  First, ordinary motors can only work for a long time near the power frequency, while variable frequency motors can work for a long time under conditions that are seriously higher or lower than the power frequency; for example, the power frequency in our country is 50Hz. , if the ordinary motor is at 5Hz for a long time, it will soon fail or even be damaged; and the appearance of the variable frequency motor solves this deficiency of the ordinary motor;

  Second, the cooling systems of ordinary motors and variable frequency motors are different. The cooling system of an ordinary motor is closely related to the rotational speed. In other words, the faster the motor rotates, the better the cooling system is, and the slower the motor rotates, the better the cooling effect is, while the variable frequency motor does not have this problem.

  After adding the frequency converter to the ordinary motor, the frequency conversion operation can be realized, but it is not a real frequency conversion motor. If it works under the non-power frequency state for a long time, the motor may be damaged.

Inverter motor.jpg

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

  The inverter can generate different levels of harmonic voltage and current during operation, so that the motor runs under non-sinusoidal voltage and current. , the most significant is the rotor copper loss, these losses will make the motor extra heat, reduce the efficiency, reduce the output power, and the temperature rise of ordinary motors generally increases by 10%-20%.

  02 The insulation strength of the motor

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

  03 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 space harmonics of the electromagnetic part of the motor to form various electromagnetic excitation forces, thereby increasing the noise. Due to the wide operating frequency range of the motor and the wide range of rotational speed variation, it is difficult for the frequencies of various electromagnetic force waves to avoid the natural vibration frequency of each structural member of the motor.

  04 Cooling problems at low rpm

  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 motor decreases, the cooling air volume decreases in direct proportion to the cube of the speed, resulting in the heat of the motor not being dissipated and the temperature rising sharply. increase, it is difficult to achieve constant torque output.

  05In view of the above situation, the frequency conversion motor adopts the following design

  Reduce the stator and rotor resistance as much as possible and reduce the copper loss of the fundamental wave to make up for the increase in copper loss caused by higher harmonics.

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

  The structural design is mainly to improve the insulation level; the vibration and noise problems of the motor are fully considered; the cooling method adopts forced air cooling, that is, the main motor cooling fan adopts an independent motor drive mode, and the function of the forced cooling fan is to ensure that the motor runs at a low speed. cooling down.

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

  Ordinary motors, that is, power frequency motors, only need to consider the starting process and the working conditions of one point of power frequency (public number: electromechanical contacts), 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 conversion range, and then design motor.

  In order to adapt to the PWM width modulated wave analog sinusoidal alternating current output by the inverter, which contains a lot of harmonics, the function of the specially made variable frequency motor can actually be understood as a reactor plus an ordinary motor.

  01 The difference between ordinary motor and variable frequency motor structure

  1. Higher insulation requirements

  Generally, the insulation grade of the frequency conversion motor is F or higher, and the ground insulation and the insulation strength of the turns should be strengthened, especially the ability of the insulation to withstand impulse voltage.

  2. The vibration and noise requirements of variable frequency motors are higher

  The frequency conversion motor should fully consider the rigidity of the motor components and the whole, and try to increase its natural frequency to avoid resonance with each force wave.

  3. The cooling method of the variable frequency motor is different

  The frequency conversion motor generally adopts forced ventilation cooling, that is, the main motor cooling fan is driven by an independent motor.

  4. Different requirements for protection measures

  Bearing insulation measures should be adopted for variable frequency motors with a capacity exceeding 160kW. The main reason is that it is easy to produce asymmetrical magnetic circuit, and also produces shaft current. When the currents generated by other high-frequency components work together, the shaft current will increase greatly, resulting in bearing damage, so insulation measures are generally taken. For constant power variable frequency motor, when the speed exceeds 3000/min, special grease with high temperature resistance should be used to compensate for the temperature rise of the bearing.

  5. Different cooling systems

  The variable frequency motor cooling fan is powered by an independent power supply to ensure continuous cooling capacity.

  02 The difference between ordinary motor and variable frequency motor design

  1. Electromagnetic Design

  For ordinary asynchronous motors, the main performance parameters considered in the design are overload capacity, starting performance, efficiency and power factor. The variable frequency motor, because the critical slip is inversely proportional to the power frequency, can be started directly when the critical slip is close to 1. Therefore, the overload capacity and starting performance do not need to be considered too much, but the key problem to be solved is how to improve the motor pair. Adaptability to non-sinusoidal power supplies.

  2. Structural Design

  When designing the structure, it is also necessary to consider the influence of the non-sinusoidal power supply characteristics on the insulation structure, vibration, and noise cooling methods of the variable frequency motor.


Post time: Oct-24-2022