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Why can’t ordinary motors be used as variable frequency motors?

Date:2023-09-25   Author:XINDA MOTOR

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

01.The impact of the frequency converter on the motor is mainly on the efficiency and temperature rise of the motor

The frequency converter can produce varying degrees of harmonic voltages and currents during operation, causing the motor to operate under non-sinusoidal voltages and currents. The high-order harmonics inside will cause an increase in the stator copper loss, rotor copper loss, iron loss and additional losses of the motor. , the most significant is the rotor copper loss. These losses will cause the motor to generate additional heat, reduce efficiency, and reduce output power. The temperature rise of ordinary motors generally increases by 10%-20%.


02.Insulation strength problem of electric motor

The carrier frequency of the frequency converter ranges from several thousand to more than ten kilohertz, which causes the motor stator winding to withstand a very high voltage rise rate, which is equivalent to applying a steep impact voltage to the motor, putting the motor's inter-turn insulation under severe 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 various harmonics contained in the variable frequency power supply interfere with the inherent spatial harmonics of the electromagnetic part of the motor, forming various electromagnetic excitation forces, thereby increasing the noise. Since the motor has a wide operating frequency range and a large rotational speed variation range, it is difficult for the frequencies of various electromagnetic force waves to avoid the natural vibration frequencies of the various structural parts of the motor.

04. Cooling issues at low speeds

When the power supply frequency is low, the losses caused by high-order harmonics in the power supply are greater; secondly, when the motor speed is reduced, the cooling air volume decreases in proportion to the cube of the speed, causing the motor heat to not be dissipated and the temperature rise to increase sharply. , it is difficult to achieve constant torque output.

05. In view of the above situation, the variable frequency motor adopts the following design:

① Reduce the stator and rotor resistance as much as possible and reduce the fundamental wave copper loss to make up for the increase in copper loss caused by higher harmonics.

② The main magnetic field is not saturated. Firstly, it is considered that high-order harmonics will deepen the saturation of the magnetic circuit. Secondly, it is considered that in order to increase the output torque at low frequency, the output voltage of the inverter can be appropriately increased.

③ The structural design is mainly to improve the insulation level; fully consider the vibration and noise issues of the motor; the cooling method adopts forced ventilation cooling, that is, the main motor cooling fan adopts an independent motor drive method, and the function of the forced cooling fan is to ensure that the motor operates at low temperature. Cooling at RPM.

④The coil distributed capacitance of the variable frequency motor is smaller and the resistance of the silicon steel sheet is larger, so that the impact of high-frequency pulses on the motor is smaller, and the inductance filtering effect of the motor is better.

⑤ Ordinary motors, that is, 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 conversion range, and then design the motor.

⑥In order to adapt to the PWM width modulation wave simulated sinusoidal alternating current output by the frequency converter, which contains a large number of harmonics, the specially made variable frequency motor can actually be understood as a reactor plus an ordinary motor.

How to distinguish between ordinary motors and variable frequency motors?

01.The difference in structure between ordinary motors and variable frequency motors

①Higher insulation level requirements

Generally, the insulation level of variable frequency motors is Class F or higher. The insulation strength to ground and the insulation strength of the wire turns should be strengthened. In particular, the ability of the insulation to withstand impulse voltage should be considered.

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

The variable frequency motor must fully consider the rigidity of the motor components and the entire body, and try to increase its natural frequency to avoid resonance with each force wave.

③Different cooling methods of variable frequency motors

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

④Different requirements for protective measures

Bearing insulation measures should be adopted for variable frequency motors with a capacity exceeding 160KW. Mainly, it is easy to produce magnetic circuit asymmetry, and also produce shaft current. When the current generated by other high-frequency components acts together, the shaft current will greatly increase, resulting in bearing damage, so insulation measures are generally required. For constant power variable frequency motors, when the speed exceeds 3000/min, special high-temperature resistant grease should be used to compensate for the temperature increase of the bearings.

⑤Different cooling systems

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

02.Differences in design between ordinary motors and variable frequency motors

①Electromagnetic design

For ordinary asynchronous motors, the main performance parameters considered during design are overload capacity, starting performance, efficiency and power factor. As for the variable frequency motor, since the critical slip is inversely proportional to the power frequency, it can be started directly when the critical slip is close to 1. Therefore, overload capacity and starting performance no longer need to be considered too much, and the key issue to be solved is how to improve the motor. Adaptability to non-sinusoidal power supplies.

②Structural design

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