The difference between variable frequency motor and power frequency motor
First, ordinary asynchronous motors are designed according to constant frequency and constant voltage, and it is impossible to fully adapt to the requirements of frequency conversion speed regulation.
The influence of the inverter on the motor
1, the efficiency of the motor and the problem of temperature rise
Regardless of the form of the frequency converter, different levels of harmonic voltage and current are generated during operation, so that the motor operates under non-sinusoidal voltage and current. Rejecting the data introduction, taking the sinusoidal PWM type inverter commonly used at present as an example, the lower harmonics are basically zero, and the remaining higher harmonic components which are about twice as large as the carrier frequency are: 2u+1 (u For the modulation ratio). Higher harmonics cause an increase in the stator copper loss, rotor copper (aluminum) consumption, iron loss and additional losses, most notably the rotor copper (aluminum) consumption. Since the asynchronous motor rotates at a synchronous speed close to the fundamental frequency, the high-order harmonic voltage will cause a large rotor loss after cutting the rotor bar with a large slip. In addition, the additional copper consumption due to the skin effect needs to be considered. These losses will cause the motor to generate extra heat, reduce the efficiency, and reduce the output power. For example, if the ordinary three-phase asynchronous motor is operated under the non-sinusoidal power supply output of the inverter, the temperature rise will generally increase by 10%-20%.
2, motor insulation strength problem
At present, many small and medium-sized inverters use PWM control. His carrier frequency is about several thousand to ten kilohertz, which makes the stator winding of the motor to withstand a high voltage rise rate, which is equivalent to applying a steep shock voltage to the motor, so that the inter-turn insulation of the motor is more resistant. A harsh test. In addition, the rectangular chopper surge voltage generated by the PWM inverter is superimposed on the operating voltage of the motor, which poses a threat to the insulation of the motor to the ground, and the ground insulation will accelerate aging under the repeated impact of high voltage.
3. Harmonic electromagnetic noise and vibration
When the ordinary asynchronous motor is powered by the inverter, the 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 interferes 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 coincides with or is close to the natural vibration frequency of the motor body, a resonance phenomenon occurs, thereby increasing noise. Since the operating frequency range of the motor is wide and the range of the rotational speed is large, the frequencies of various electromagnetic force waves are difficult 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
Since the inverter is powered, the motor can be started with no inrush current at very low frequency and voltage, and can be quickly braked by various braking methods provided by the inverter, in order to achieve frequent starting and braking. The conditions are created, so that the mechanical system and the 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 speed
First of all, the impedance of the asynchronous motor is not ideal. When the power frequency is lower, the loss caused by the higher harmonics in the power supply is larger. Secondly, when the normal asynchronous motor is reduced in speed, the cooling air volume is proportional to the cube of the rotational speed, which causes the low-speed cooling condition of the motor to deteriorate, and the temperature rise sharply increases, making it difficult to achieve constant torque output.
Second, the characteristics of variable frequency motor
1, electromagnetic design
For ordinary asynchronous motors, the main performance parameters considered in the redesign are overload capability, starting performance, efficiency, and power factor. The variable frequency motor, because the critical slip ratio is inversely proportional to the power supply frequency, can be directly started when the critical slip rate is close to 1. Therefore, the overload capability and the starting performance do not need to be considered too much, and the key problem to be solved is how to improve the motor pair. The ability to adapt to non-sinusoidal power supplies. The method is generally as follows:
1) Reduce the stator and rotor resistance as much as possible. Reduce the stator resistance to reduce the copper loss of the fundamental wave to compensate for the increase in copper consumption caused by higher harmonics
2) In order to suppress higher harmonics in the current, it is necessary to appropriately increase the inductance of the motor. However, the rotor slot leakage resistance is large, and the skin effect is also large, and the high-order harmonic copper consumption is also increased. Therefore, the size of the motor leakage resistance should take into account the reasonableness of impedance matching within the entire speed range.
3) The main magnetic circuit of the variable frequency motor is generally designed to be unsaturated. One is to consider the higher harmonics to deepen the saturation of the magnetic circuit, and the other is to increase the output voltage of the inverter in order to increase the output torque at low frequencies.
2, structural design
When designing the structure again, it is mainly considering the influence of the non-sinusoidal power supply characteristics on the insulation structure, vibration and noise cooling mode of the variable frequency motor. Generally pay attention to the following problems:
1) Insulation grade, generally F grade or higher, to strengthen the insulation strength of the ground and the insulation of the coil, especially the ability of the insulation to withstand the impact voltage.
2) For the vibration and noise of the motor, the rigidity of the motor component and the whole should be fully considered, and the natural frequency should be increased as much as possible to avoid resonance with each force wave. 3) Cooling method: Generally, forced air cooling is adopted, that is, the main motor cooling fan is driven by an independent motor.
4) Measures to prevent shaft current. For bearings with a capacity exceeding 160 kW, bearing insulation measures should be adopted. Mainly due to the asymmetry of the magnetic circuit, the shaft current is also generated. When the currents generated by other high-frequency components work together, the shaft current will increase greatly, resulting in bearing damage, so insulation measures are generally taken.
5) For constant power variable frequency motor, when the speed exceeds 3000/min, special high temperature resistant grease should be used to compensate for the temperature rise of the bearing.
The variable frequency motor can be operated for a long time in the range of 0.1HZ--130HZ. The ordinary motor can be used in:
2-pole long-term operation in the range of 20--65hz.
4 poles for long-term operation in the 25--75hz range.
6 poles for long-term operation in the 30--85hz range.
8-pole long-term operation in the 35--100hz range.
