Mechanism Analysis of Variable Frequency Motor Shaft Voltage and Shaft Current
When the motor is driven by a sine wave power supply, the shaft voltage is generated by the alternating flux linkage of the motor shaft. These flux linkages are caused by magnetic flux imbalance caused by the rotor and stator slots, the connection between the separated core pieces, the orientation properties of the magnetic material, and the imbalance of the power supply [1]. In the 1990s, when the PWM inverter with IGBT as the power device was used as the motor drive power source, the motor shaft current problem was more serious, and the generation mechanism was completely different from that of the sine wave power supply. The literature [1] pointed out that an IGBT inverter with a high carrier frequency (for example, above 10 kHz) causes the bearing of the motor to be damaged faster than the inverter with a low carrier frequency. Busse analyzed the relationship between bearing current generation and bearing current density and bearing damage in detail [2], and established the bearing current circuit model under PWM drive, but the model failed to reflect the bearing current and inverter. The relationship between switching frequencies. In order to discuss the generation mechanism of motor shaft voltage and shaft current when high frequency PWM pulse voltage is driven, this paper analyzes the condition and form of shaft current generation based on the model of shaft voltage and shaft current circuit, and the output voltage of inverter The characteristic voltage changes and the presence or absence of overvoltage at the motor end, through simulation analysis, the shaft voltage and bearing current waveform under different conditions are obtained.
In suppressing bearing current, the method given in [1] uses a sine-wave filter to convert the PWM voltage into a sine wave voltage, so that the motor works in a sinusoidal power supply state, but the method has large inductance and slow dynamic response. At the same time, the voltage drop across the inductor and power consumption increase. In this paper, a small inductor is connected to the output of the inverter and supplemented by an RC absorption network, which can effectively suppress the shaft current generated by the PWM inverter.
2 common mode voltage and shaft voltage
It is generally believed that magnetic circuit imbalance, unipolar effect and capacitive current are the main reasons for generating shaft voltage in the motor [3]. In ordinary motors powered by the power grid, people generally pay more attention to the influence of magnetic circuit imbalance. However, in the inverter-powered motor, the shaft voltage is mainly generated by the voltage imbalance, that is, the zero-sequence component of the power supply voltage. Due to the imbalance of circuit, component, connection and loop impedance, the supply voltage will inevitably produce zero drift, which will produce zero sequence current in the system, and the bearing will be part of the motor zero sequence loop. When the sine wave power supply is driven, it is known by calculation that =0. Under the PWM inverter drive, the value depends on the inverter switch state, and the change period is consistent with the inverter carrier frequency. In fact, it is only a form of common mode voltage. Due to electrostatic coupling, there are distributed capacitors of different sizes between the various parts of the motor, thus forming a zero sequence loop of the motor. According to the transmission line theory, a distributed parameter circuit can be replaced by an equivalent lumped parameter π network model with the same input-output relationship.
3 bearing model and bearing current generation
Due to the presence of the distributed capacitance and the excitation of the high frequency pulse input voltage, a coupled common mode voltage is formed on the motor shaft. In fact, the appearance of the shaft voltage is not only related to the above two factors, but also directly related to the bearing structure. The front and rear ends of the rotor are supported by a bearing, and its structure is shown in Figure 3.
4.1 Change the rise time tr
4.2 Change coupling parameters and bearing parameters
5 inhibition methods
