Things to consider when designing variable frequency motors
Release time:
2021-03-04 16:26
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The motor designed for the frequency converter is called a special variable frequency motor; the motor can achieve different speeds and torques driven by the frequency converter to adapt to the changing needs of the load. The general variable frequency motor is derived from the traditional squirrel cage motor; the traditional motor self-cooling fan is usually replaced by an independent fan, and the corona resistance performance of the motor winding insulation is improved. In some situations where the output characteristics of the motor are not high, such as low power and when the operating frequency is near the rated frequency, ordinary squirrel cage motors can be used instead.
1. Electromagnetic design For ordinary asynchronous motors, the main performance parameters to be considered during redesign are overload capacity, starting performance, efficiency and power factor.
Since the critical slip ratio is inversely proportional to the power supply frequency, the variable frequency motor can be started directly when the critical slip ratio is close to 1. Therefore, there is no need to consider too much overload capacity and starting performance, but there are still some problems that need to be solved. The key issue is how to improve the adaptability of non-sinusoidal power supplies of variable frequency motors.
The method is generally as follows:
(1) Reduce the resistance of the stator and rotor as much as possible. Lowering the stator resistance can reduce the basic copper losses to compensate for the increased copper losses caused by higher harmonics.
(2) In order to suppress higher harmonics in the current, it is necessary to appropriately increase the inductance of the variable frequency motor. However, the leakage resistance of the rotor slot is greater, the skin effect is greater, and higher harmonic copper losses are also increased. Therefore, the size of the leakage reactance of the variable frequency motor must consider the rationality of the impedance matching within the entire speed adjustment range.
(3) The main magnetic circuit of the variable frequency motor is usually designed to be in an unsaturated state. One is to consider that high-order harmonics will deepen the saturation of the magnetic circuit, and the other is to consider appropriately increasing the frequency converter. output voltage to increase the output torque at low frequency.
2. In the structural design, the impact of non-sinusoidal power supply characteristics on the insulation structure, vibration and noise cooling methods of the variable frequency motor is also considered. Generally, the following issues should be paid attention to:
(1) Insulation grade, usually F grade or higher, can enhance the insulation strength to the ground and turns, especially the ability to withstand insulation pulse voltage.
(2) Regarding the vibration and noise of the variable frequency motor, the rigidity of the variable frequency motor components 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: Forced ventilation cooling is generally used, that is, the main motor cooling fan is driven by an independent motor.
(4) Measures to prevent shaft current. Motors with a capacity exceeding 160KW should take bearing insulation measures. The main reason is that asymmetry of the magnetic circuit may occur, and shaft current will also be generated. When the currents generated by other high-frequency components are combined, the shaft current will increase greatly, causing bearing damage, so insulation measures are usually 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 increase in bearing temperature.
The variable frequency motor can run for a long time in the range of 0.1HZ-130HZ. Ordinary motor 2-pole can operate in the range of 20-65Hz. 4-pole can operate in the range of 25-75hz for a long time. 6-pole can operate in the range of 30-85hz for a long time. 8 poles can operate for a long time in the range of 35-100hz.
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