The Difference between Common Motor and Frequency Converter Motor and Its Working Principle!

Ordinary asynchronous motors are designed according to the constant frequency and constant voltage, and can not fully meet the requirements of variable frequency speed regulation.


One, the following is the influence of the frequency converter on the motor.


1. The problem of the efficiency and temperature rise of the motor


Regardless of the form of the converter, in operation, produce different degree of harmonic voltage and current, so that the motor operates under non sinusoidal voltage and current. In the case of the current commonly used sine wave PWM frequency converter, the low subharmonic is basically zero, and the remaining harmonic components with more than one times larger than the carrier frequency are: 2u+1 (U is the modulation ratio).


Higher harmonics will increase the copper consumption, rotor copper (aluminum) consumption, iron loss and additional loss of the motor stator. The Zui is the copper consumption of the rotor significantly. Because the asynchronous motor rotates at the synchronous speed corresponding to the fundamental frequency of the fundamental wave, the high harmonic voltage can cause a large rotor loss after cutting the rotor guide with a larger difference. In addition, additional copper consumption due to skin effect should also be considered. These losses will make the motor extra heat, reduce the efficiency and reduce the output power. For example, the temperature rise of the common three-phase asynchronous motor is generally increased by 10%--20% under the condition of the non sinusoidal power supply.


2. The problem of electric motor insulation strength


At present, many small and medium frequency converters are controlled by PWM. The frequency of his carrier is about several thousand to ten thousand kHz, which makes the stator windings of the motor bear a high rise in voltage, which is equivalent to the high steepness of the motor, which makes the interturn insulation of the motor suffer a more severe test. In addition, the rectangular chopper impact voltage produced by the PWM converter is superimposed on the operating voltage of the motor, which poses a threat to the ground insulation of the motor, and will accelerate the aging of the ground insulation under repeated impact of the high voltage.


3, harmonic electromagnetic noise and vibration


When induction motors are powered by frequency converters, vibration and noise caused by electromagnetic, mechanical, ventilation and other factors will become more complicated. Each time harmonic contained in the variable frequency power supply is interfered with the inherent space harmonics of the electromagnetic part of the motor to form various electromagnetic exciting forces. When the frequency of the electromagnetic force wave is consistent with or close to the natural vibration frequency of the motor body, a resonance phenomenon will occur, thus increasing the noise. Due to the wide operating frequency range and large speed range, the frequency of various electromagnetic waves can hardly avoid the natural vibration frequency of each component of the motor.


4. Adaptability of Motor to Frequent Starting and Braking


Since the frequency converter is used, the motor can be started at a low frequency and voltage without shock current, and can be fast braking by various braking modes supplied by the frequency converter, which creates conditions for the realization of frequent starting and braking, so the mechanical and electromagnetic systems of the motor are in the cyclic alternating force. The fatigue and accelerated aging problems of mechanical structure and insulation structure are brought about.


5, the problem of cooling at low speed


First, the impedance of the asynchronous motor is not ideal. When the power frequency is lower than the bottom, the loss caused by the higher harmonics in the power supply is larger. Secondly, when the rerotating speed of the ordinary asynchronous motor is reduced, the cooling air volume and the three times of the speed are reduced, which causes the low speed cooling of the motor and the increase of the temperature rise, which is difficult to realize the constant torque output.


Two. Characteristics of frequency conversion motor


1. Electromagnetic design


For ordinary asynchronous motors, the main performance parameters in redesign are overload capacity, starting performance, efficiency and power factor. The frequency conversion motor, because the critical transfer ratio is inverse to the power frequency, can start directly when the critical transfer rate is close to 1. Therefore, the overload capacity and starting performance are not considered too much, and the key problem to be solved is how to improve the adaptability of the motor to the non sinusoidal power supply.


The general way is as follows:


1) reduce the stator and rotor resistance as much as possible. Reducing the resistance of the stator can reduce the copper consumption of the fundamental wave to compensate for the increase of copper consumption caused by the high harmonics.


2) in order to suppress the higher harmonics in the current, the inductance of the motor should be appropriately increased. However, the larger leakage effect of rotor slot is, the larger the skin effect is, and the higher harmonic copper consumption is also increased. Therefore, the size of motor leakage should take into account the rationality of impedance matching in the whole speed range.


3) the main magnetic circuit of the variable frequency motor is generally designed as an unsaturated state. One is that the high order harmonic will deepen the saturation of the magnetic circuit, and the two is to improve the output voltage of the inverter in order to improve the output torque at low frequency.


2. When the structure design is redesigned, it is mainly concerned with the influence of the non sinusoidal power characteristics on the insulation structure, vibration and noise cooling mode of the variable frequency motor.


1) the insulation grade is generally F or higher, and the insulation strength and insulation strength of the wire to the ground should be strengthened, especially the ability of insulation to withstand impulse voltage.


2) for the vibration and noise of the motor, we should take full consideration of the rigidity of the motor component and the whole, and try to improve its natural frequency, so as to avoid the resonance phenomenon of every force wave.


3) cooling mode: forced ventilation cooling is commonly used, that is, the main motor cooling fan is driven by an independent motor.