The motor designed for frequency converter is called frequency conversion special motor; the motor can realize different speed and torque under the drive of frequency converter to adapt to the demand of load change. The general variable frequency motor is derived from the traditional squirrel cage motor; the traditional motor is usually changed from the self cooling fan to the independent fan, and the corona resistance of the motor winding insulation is improved. In some cases where the output characteristics of the motor are not required highly, such as small power and operating frequency near the rated frequency, the ordinary squirrel cage motor can be used instead.
1. For the common asynchronous motor, the main performance parameters to be considered during the redesign are overload capacity, starting performance, efficiency and power factor.
Since the critical slip is inversely proportional to the power frequency, the variable frequency motor can be started directly when the critical slip is close to 1. Therefore, it is not necessary to consider the overload capacity and starting performance too much. However, the key problem to be solved is how to improve the adaptability of non sinusoidal power supply of variable frequency motor.
This method is generally as follows:
(1) Reduce the resistance of stator and rotor as much as possible. Reducing the stator resistance can reduce the basic copper loss to compensate for the increase of copper loss caused by higher harmonics.
(2) In order to suppress the high-order harmonics in the current, it is necessary to increase the inductance of the variable frequency motor. However, the leakage resistance of rotor slot is larger, skin effect is larger, and higher harmonic copper loss is also increased. Therefore, the rationality of impedance matching in the whole speed regulation range must be considered for the leakage reactance of variable frequency motor.
(3) Usually, the main magnetic circuit of variable frequency motor is designed as unsaturated state. One is to consider that the higher harmonic will deepen the saturation of magnetic circuit, and the other is to appropriately increase the output voltage of frequency converter to increase the output torque at low frequency.
2. In the structural design, the influence of non sinusoidal power supply characteristics on the insulation structure, vibration and noise cooling method of variable frequency motor is also considered
(1) The insulation class, usually class F or higher, enhances the insulation strength to the ground and the number of turns, especially the ability to withstand the insulation impulse voltage.
(2) For the vibration and noise of the variable frequency motor, the rigidity of the components and the whole of the variable frequency motor should be fully considered, and the natural frequency should be increased as much as possible to avoid resonance with each force wave.
(3) Cooling mode: forced ventilation cooling is generally adopted, that is, the cooling fan of main motor is driven by independent motor.
(4) Measures to prevent shaft current, bearing insulation measures shall be taken for motors with capacity over 160kW. The main reason is that asymmetry of magnetic circuit may occur, and shaft current will be generated. When the current generated by other high-frequency components is combined, the shaft current will be greatly increased, resulting in bearing damage. Therefore, insulation measures are usually 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 rise of bearing temperature.
Variable frequency motor can run for a long time in the range of 0.1hz-130hz.
The two poles of ordinary motor can operate in the range of 20-65hz.
The 4-pole can operate for a long time in the range of 25-75 Hz.
The 6-pole can operate for a long time in the range of 30-85 Hz.
The 8-pole can operate for a long time in the range of 35-100 Hz.