The difference between ordinary asynchronous motor and variable frequency motor

The difference between ordinary asynchronous motor and variable frequency motor

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

　　 The following is the influence of the inverter on the motor.

　　 1. The efficiency and temperature rise of the motor

　　 No matter which type of frequency converter, it will produce different degrees of harmonic voltage and current during operation, making the motor run under non-sinusoidal voltage and current.

According to the information, taking the 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, after the high-order harmonic voltage cuts the rotor bar with a large slip, a large rotor loss will occur.

　　 In addition, the additional copper loss due to the skin effect needs to be considered. These losses will cause the motor to generate extra heat, reduce efficiency and output power.

　　 If an ordinary three-phase asynchronous motor is operated under the condition of a non-sinusoidal power supply output by the frequency converter, the temperature rise will generally increase by 10% to 20%.

　　2, the insulation strength of the motor

　　 At present, many small and medium-sized inverters are controlled by PWM.

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, making the motor’s turn-to-turn insulation more resistant. The harsh test.

　　 In addition, the rectangular chopping impulse voltage generated by the PWM inverter is superimposed on the motor operating voltage, which will pose 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 large speed variation range, 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 for rapid braking, in order to achieve frequent starting and braking. The conditions are created so that 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 problem at low speed

　　 First of all, the impedance of the asynchronous motor is not ideal. When the frequency of the power supply is low, 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 in the 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, but the key issue to be solved is how to improve the motor Adaptability to non-sine wave power supplies. The method is generally as follows:

　　 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.

　　 In order to suppress the higher harmonics in the current, it is necessary to increase the inductance of the motor appropriately. However, the rotor slot leakage reactance is large, the skin effect is also large, and the copper loss of higher harmonics is also increased. Therefore, the magnitude of the motor leakage reactance must take into account the rationality of impedance matching in the entire speed range.

　　The main magnetic circuit of the variable frequency motor is generally designed to be unsaturated. One is to consider that higher harmonics will increase the saturation of the magnetic circuit; the other is to increase the output voltage of the inverter in order to increase the output torque at low frequencies.

　　2, structural design

　　 In the structural design, the main consideration is also the influence of non-sinusoidal power supply characteristics on the insulation structure, vibration, noise cooling method of the variable frequency motor, etc. Generally pay attention to the following issues:

　　 Insulation grade, generally F grade or higher, strengthen the insulation to ground and the insulation strength of the turns, especially considering the ability of the insulation to withstand impulse voltage.

　　 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.

　　 Cooling method: Generally, forced ventilation cooling is adopted, that is, the main motor cooling fan is driven by an independent motor.

　　Measures to prevent shaft currents. For motors with a capacity exceeding 160kW, bearing insulation measures should be adopted. The main reason is that the asymmetry of the magnetic circuit is easy to occur, and 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.

　　 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.

　　 Three, synchronous motor

　　 1. Features

　　The power factor is advanced, and the general rated power factor is 0.9, which is conducive to improving the power factor of the grid and increasing the grid capacity.

　　The operation stability is high. When the grid voltage suddenly drops to 80% of the rated value, the excitation system can generally automatically adjust the forced excitation to ensure the stable operation of the motor.

　　 The overload capacity is larger than the corresponding asynchronous motor.

　　High operating efficiency, especially for low-speed asynchronous motors.

　　2, start method

　　 Asynchronous start method. Most synchronous motors are equipped with a start winding similar to the cage winding of an asynchronous motor on the rotor. Connect an additional resistance approximately 10 times the resistance of the field winding in the excitation circuit to form a closed circuit. Connect the stator of the synchronous motor directly to the power grid to start it as an asynchronous motor. When the speed reaches sub-synchronous speed (95%), Then cut off the additional resistance.

　　 Frequency conversion start. Start with inverter, so I won’t repeat it.

　　 3. Application

Masters who have done power saving in oil fields know that the pumping unit motors in oil fields require a large starting torque. Engineers generally design the motors to be very large when designing. This causes the phenomenon of "big horse-drawn carts", such as 55kW. After the balance weight is basically adjusted for the pumping unit motor, its actual active power is generally more than a dozen kilowatts, sometimes still small.

　　 Someone has made such a transformation, changing the 55kW asynchronous motor of the pumping unit to a 22kW synchronous motor, and then controlled by a frequency converter. Of course, it can also be automatically controlled according to the discharge volume or other signals, and the power saving rate can reach 40%.

　　 Therefore, asynchronous motors, synchronous motors, and variable frequency motors have their own characteristics. They mainly depend on the working environment under control. Of course, according to the project cost, asynchronous motors can be used as much as possible.