The difference between ordinary asynchronous motor and variable frequency motor
1. The influence of the frequency converter on ordinary asynchronous motors
1. The efficiency and temperature rise of the
motor 2. The insulation strength of the motor
3. Harmonic electromagnetic noise and vibration
4. The adaptability of the motor to frequent starting and braking
5. Low speed cooling problem when
two, frequency characteristics of the motor
1, the electromagnetic design
an ordinary induction motor constant frequency and voltage are by design, can not fully meet the requirements of the inverter. The following is the influence of the frequency converter on the motor:
1. The efficiency and temperature rise of the motor.
Regardless of the type of frequency converter, different degrees of harmonic voltage and current are generated during operation, so that the motor is under non-sinusoidal voltage and current. run. Refusal to introduce the information, take the currently commonly used sine wave PWM inverter as an example, its low-order harmonics are basically zero, and the remaining high-order harmonic components that are about twice the carrier frequency are: 2u+1 (u Is the modulation ratio).
High-order harmonics will cause the increase of stator copper loss, rotor copper (aluminum) loss, iron loss and additional loss, the most significant is the rotor copper (aluminum) loss. Because the asynchronous motor rotates at a synchronous speed close to the fundamental frequency, the high-order harmonic voltage cuts the rotor bar with a large slip, which will cause a large rotor loss. In addition, the additional copper loss due to the skin effect needs to be considered. These losses will cause the motor to generate additional heat, reduce the efficiency, and reduce the output power. For example, if an ordinary three-phase asynchronous motor is operated under the condition of a non-sinusoidal power supply output by the frequency converter, its temperature rise will generally increase by 10% to 20%.
2. The insulation strength of the motor
At present, many small and medium-sized inverters adopt PWM control methods. Its carrier frequency is about several thousand to ten kilohertz, which makes the motor stator winding to withstand a very high rate of voltage rise, which is equivalent to imposing a steep impulse voltage on the motor, which makes the motor’s turn-to-turn insulation more resistant. The harsh test. In addition, the rectangular chopping impulse voltage generated by the PWM frequency converter is superimposed on the motor operating voltage, which poses a threat to the motor’s ground insulation, and the ground insulation will accelerate its aging under repeated impacts of high voltage.
3. Harmonic electromagnetic noise and vibration When
ordinary asynchronous motors are powered by frequency converters, the vibration and noise caused by electromagnetic, mechanical, ventilation and other factors will become more complicated. The time harmonics contained in the variable frequency power supply interfere with the inherent spatial 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, resonance will occur, thereby increasing noise. Because the motor has a wide operating frequency range and a wide range of speed changes, it is difficult for the frequency of various electromagnetic force waves to avoid the natural vibration frequency of each component of the motor.
4. The ability of the motor to adapt to frequent starting and braking.
After the inverter is used for power supply, the motor can be started at a very low frequency and voltage without inrush current, and various braking methods provided by the inverter can be used. Fast braking 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 the insulating structure.
5. Cooling problems at low speeds
First of all, the impedance of the asynchronous motor is not ideal. When the power supply frequency is lower, the loss caused by the higher harmonics in the power supply is relatively large. Secondly, when the rotation speed of an ordinary asynchronous motor decreases, the cooling air volume decreases in proportion to the third power of the rotation speed, resulting in the deterioration of the low-speed cooling condition of the motor and the sharp increase in temperature rise, making it difficult to achieve constant torque output.
2. Features of variable frequency motor
1. Electromagnetic design
For ordinary asynchronous motors, the main performance parameters considered when redesigning 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 do not need to be considered too much, but the key issue to be solved is how to improve the motor The adaptability of non-sine wave power supply. The method is generally as follows:
1) Reduce the stator and rotor resistance as much as possible.
Reducing the stator resistance can reduce the fundamental copper loss to compensate for the increase in copper loss caused by higher harmonics.
2) In order to suppress the higher harmonics in the current, it is necessary to appropriately increase the inductance of the motor. However, the rotor slot leakage reactance is large, the skin effect is also large, and the copper loss of high-order harmonics is also increased. Therefore, the size of the motor leakage reactance should take into account the rationality of impedance matching in the entire speed range.
3) The main magnetic circuit of the variable frequency motor is generally designed to be unsaturated. One is to consider that higher harmonics will deepen the saturation of the magnetic circuit, and the other is to appropriately increase the output voltage of the inverter in order to increase the output torque at low frequencies.
2. In structural design
and restructuring design, the main consideration is the influence of non-sinusoidal power supply characteristics on the insulation structure, vibration, noise cooling method and other aspects of the frequency conversion motor. Generally, the following issues should be paid attention to:
1) Insulation level, generally F level or higher , Strengthen the insulation to the ground and the insulation strength of the turns, especially considering the ability of the insulation to withstand impulse voltage.
2) For the vibration and noise of the motor, it is necessary to fully consider the rigidity of the motor components and the whole, and try to increase its natural frequency to avoid resonance with various force waves.
3) Cooling method: Forced ventilation cooling is generally used, that is, the main motor cooling fan is driven by an independent motor.
4) Measures to prevent shaft currents, bearing insulation measures should be adopted for motors with a capacity exceeding 160KW. The main reason is that the asymmetry of the magnetic circuit is easy to occur, and the shaft current is also generated. When the currents generated by other high frequency components work together, the shaft current will greatly increase, resulting in damage to the bearing, so generally insulation measures must be taken.
5) 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 rise of the bearing.
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