XNUMX. The difference in structure:
1. Higher insulation requirements
Generally, the insulation class of the frequency conversion motor is F or higher. Strengthen the insulation to the ground and the insulation strength of the turns, and especially consider the ability of the insulation to withstand impulse voltage.
2. The vibration and noise requirements of variable frequency motors are higher
The frequency conversion motor must fully consider the rigidity of the motor components and the whole, and try to increase its natural frequency to avoid resonance with each force wave.
3. Different cooling methods for variable frequency motors
Frequency conversion motors generally use forced ventilation cooling, that is, the main motor cooling fan is driven by an independent motor.
4. Different requirements for protection measures
Bearing insulation measures should be adopted for variable frequency motors with a capacity exceeding 160KW.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.
5. Different cooling systems
The frequency conversion motor cooling fan uses an independent power supply to ensure continuous heat dissipation.
XNUMX. The difference in design
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 frequency conversion 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, the 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 pair The adaptability of non-sine wave power supply.
2. Structural design
When designing the structure, it is also necessary to consider the impact of non-sinusoidal power supply characteristics on the insulation structure, vibration, and noise cooling methods of the variable frequency motor.Ordinary asynchronous motors are designed according to constant frequency and constant voltage, which cannot fully meet the requirements of variable frequency speed regulation.Forcibly installing frequency converters on ordinary motors will bring many disadvantages. The following is the influence of frequency converters on the motors.
1. The efficiency and temperature rise of the motor:
The frequency converter can generate different degrees of harmonic voltage and current during operation, so that the motor runs under non-sinusoidal voltage and current. The high-order harmonics inside will cause the motor stator copper loss, rotor copper loss, iron loss and additional loss to increase. , It is the copper loss of the rotor. These losses will cause the motor to generate extra heat, reduce the efficiency, and reduce the output power. The temperature rise of ordinary motors generally increases by 10%-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 thousand Hz, 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 bear relatively The harsh test.
In addition, the rectangular chopping impulse voltage generated by the PWM frequency converter 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 an ordinary asynchronous motor is powered by a frequency converter, 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 excitation 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 the noise.Because the motor has a wide operating frequency range and a large 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.
