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Differences between Ordinary Motor and Variable Frequency Motor

2021-01-25 14:45:38 Weima Electric (Shanghai) Ltd. read

Ⅰ.Ordinary asynchronous motors are designed according to constant frequency and constant voltage, which can not fully meet the requirements of variable frequency speed regulation. The following is the influence of inverter on motor

a. Efficiency and Temperature Rise of Motor

No matter what kind of inverter, it produces various degrees of harmonic voltage and current during running, which makes the motor run under non sinusoidal voltage and current. According to the information materials, taking the commonly used sine wave PWM inverter as an example, its low-order harmonic is nearly zero, and the remaining high-order harmonic component which is about twice the carrier frequency is 2U + 1 (U is the modulation ratio).

High-order harmonics will cause the increase loss of stator copper, rotor copper (aluminum) , iron and additional, especially rotor copper (aluminum) loss. Because the ordinary asynchronous motor rotates at the synchronous speed close to the fundamental wave frequency, the high-order harmonic voltage will produce a large rotor loss after cutting the rotor bar with a large slip. In addition, the additional copper consumption caused by skin effect should be considered. These losses will make the motor become extremely heat, then reduce efficiency and output power. If the ordinary three-phase asynchronous motor is running under the condition of non sinusoidal power outputted by inverter, its temperature rise will increase by 10% ~ 20% generally.

b. Motor Insulation Strength

At present, many small and medium inverters adopt PWM control mode. Its carrier wave frequency is about several thousand to more than ten kilohertz, which makes the stator winding of the motor bear a high voltage rise rate, which is equivalent to imposing a steep impulse voltage on the motor, so that the turn to turn insulation of the motor stands test to its limits. In addition, the rectangular chopper impulse voltage generated by PWM inverter superimposed on the motor operating voltage will constitute a threat to the motor insulation against ground which will accelerate aging under the repeated impact of high voltage.

c. Harmonic Electromagnetic Noise and Vibration

The vibration and noise caused by electromagnetic, mechanical, ventilation become more complex when the common asynchronous motor is powered by frequency converter. Each time harmonic contained in the variable frequency power supply interferes with the inherent space harmonic of the electromagnetic part of the motor to form various electromagnetic exciting forces. When the frequency of electromagnetic force wave is consistent with or close to the natural vibration frequency of motor body, resonance phenomenon will occur which will increase the noise. Due to the wide working frequency range and wide speed variation range of the motor, it is difficult to avoid the natural vibration frequency of various components of the motor.

d. Adaptability of Motor to Frequent Starting and Braking

After the inverter is used for power supply, the motor can be started in the way of no impact current at a very low frequency and voltage, and the various braking modes provided by the inverter can be used for rapid braking, which creates conditions for frequent starting and braking. Therefore, the mechanical system and electromagnetic system of the motor are under the action of cyclic alternating force, which brings fatigue and accelerated aging problems to the mechanical structure and insulation structure.

e. Cooling at Low Speed

First of all, the impedance of ordinary asynchronous motor is not ideal. When the frequency of power supply is low, the loss caused by high-order harmonics is large. Secondly, when the speed of the ordinary asynchronous motor decreases, the cooling air volume and the third power of the speed decrease in proportion, resulting in the deterioration of the low-speed cooling condition of the motor and the sharp increase of the temperature rise, which makes it difficult to achieve constant torque output.

Ⅱ. Characteristics of Variable Frequency Motor

a. Electromagnetic Design

For ordinary asynchronous motor, the main performance parameters considered in redesign are overload capacity, starting performance, efficiency and power factor. Because the critical slip is inversely proportional to the power frequency, the variable frequency motor can be started directly when the critical slip is close to 1. Therefore, the overload capacity and starting performance no longer needs be considered too much. The key problem needs to be solved is how to improve the motor's adaptability to non sinusoidal power supply. The general methods are as follows:

●Reduce the load resistance of stator and rotor as much as possible.

 By reducing the stator load resistance, the fundamental copper loss can be reduced to make up for the increase of copper loss caused by high-order harmonics

●In order to suppress the high-order harmonic in the current, the inductance of the motor should be increased appropriately. But the rotor slot leakage reactance is large, the skin effect is also large, and the high-order harmonic copper consumption is also increased. Therefore, the leakage reactance of motor should consider the rationality of impedance matching in the whole speed range.

●The main magnetic circuit of variable-frequency motor is generally designed to be unsaturated. One is to consider that the high-order harmonics will deepen the saturation of magnetic circuit, and the other is to consider that the output voltage of inverter should be appropriately increased in order to improve the output torque at low frequency.

b. Structural Design

In the structural design, the influence of the characteristics of non sinusoidal power supply on the insulation structure, vibration, noise and cooling mode of variable frequency motor are mainly considered:

●The insulation class is generally F Class or higher, and the strength against ground insulation and wire turn insulation shall be strengthened, especially considering the ability of insulation to withstand impulse voltage.

●For the vibration and noise of the motor, the rigidity of the motor components and the whole should be fully considered, and the natural frequency should be increased as much as possible to avoid the resonance phenomenon with each force wave.

●Cooling methods: forced ventilation cooling is generally adopted, that is, the cooling fan of main motor is driven by independent motor.

●To prevent shaft current, bearing insulation measures shall be adopted for motors with capacity over 160KW. The main reason is that it is easy to produce magnetic circuit asymmetry and it will also produce shaft current. When the current generated by other high-frequency components is combined, the shaft current will increase greatly which results in bearing damage, so insulation measures are generally taken.

●For constant power variable frequency motor, when the speed exceeds 3000 r/ min, special grease with high temperature resistance should be used to compensate the temperature rise of bearing.

Ⅲ. Comparison of Practical Application

a.The efficiency and temperature rise of the motor driven by frequency converter will increase the efficiency by about 10%, while the temperature rise will decrease by about 20%, especially in the low frequency region of vector control or direct torque control.

b.Frequency converter motor is better than ordinary motor for the occasion of frequent starting, frequent speed regulation and frequent braking. 

c.In the aspect of electromagnetic noise and vibration, the frequency converter motor has lower noise and electromagnetic vibration than ordinary motor.

d. As to the insulation strength of the motor, because the inverter motor is specially designed for inverter drive, it can withstand large DU / DT, so the insulation strength of inverter motor needs to be high. Especially in the DTC control mode, the insulation strength of the motor is a great test.

e.The main difference is that the inverter motor has extra heat dissipation (using independent axial fan forced ventilation), and the heat dissipation in low frequency, DC braking and some special applications is much better than that of ordinary AC asynchronous motor.