Analysis of the Mechanism of Variable Frequency Motor Shaft Voltage and Shaft Current (1)
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/*728*90 created on 2018/5/16*/ var cpro_id = "u3440131"; Home>Bearing knowledge>Analysis of the mechanism of shaft voltage and shaft current of variable frequency motor (1) Analysis of the Mechanism of Variable Frequency Motor Shaft Voltage and Shaft Current (1)
1 Introduction When the motor is driven by a sine wave power supply, the axis voltage is generated by the alternating flux linkage of the motor shaft. These flux chains are made up of the rotor and stator slots, the connection between the core pieces, and the orientation characteristics of the magnetic material. The imbalance of power supply and other factors caused the flux imbalance [1]. In the 1990s; when the PWM inverter with IGBT as the power device is used as the motor drive power; the motor shaft current problem is more severe; and its mechanism and The sine wave power supply is completely different. The literature [1] points 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 is more specific. The relationship between bearing current generation and bearing current density and bearing damage is analyzed [2]; and the bearing current circuit model driven by PWM is established; however, the model does not reflect the bearing current and inverter switching frequency. The relationship between the motor shaft voltage and the shaft current when the high frequency PWM pulse voltage is driven; this paper is based on the shaft voltage and shaft current circuit model; The conditions and modes of the shaft current are generated; and the characteristics of the output voltage of the inverter are changed and the overvoltage of the motor end is observed; after the simulation analysis, the shaft voltage and the bearing current waveform are obtained under different conditions. /*250*250 was created on 2017/12/25*/ var cpro_id = 'u3171089';
In terms of pressing the bearing current; the method given in [1] converts the PWM voltage into a sine wave voltage with a sine wave filter; the motor operates under the sine wave power supply condition; but the method has a large inductance; the system dynamic response is slow. The voltage drop across the inductor and the power consumption increase. This article has a small inductance at the output of the inverter and is supplemented by the RC absorption network; it can be used to drive the shaft current driven by the PWM inverter.
2 Common mode voltage and shaft voltage are usually considered; magnetic circuit imbalance, unipolar effect and capacitor current are the main reasons for the shaft voltage in the motor [3]. In the normal motor of the grid supply; everyone usually pays attention to the magnetic circuit imbalance The effect of the shaft voltage in the inverter-powered motor is mainly due to voltage imbalance; that is, the zero-sequence weight of the power supply voltage occurs. Due to the imbalance of circuit, meta-device, connection and loop impedance; the power supply voltage will inevitably occur. Zero drift; this voltage will generate zero sequence current in the system; the bearing is a part of the motor zero sequence loop.
When the sine wave power supply is driven; after accounting, it can be known that the value of the inverter is driven by the PWM inverter; the value depends on the inverter switching condition; and the change period is common to the inverter carrier frequency. In fact; only one of the common mode voltages The way of expression; due to electrostatic coupling; there are large and small scattered capacitances between the motors; thus forming the zero-sequence loop of the motor. According to the transmission line theory; a scatter parameter circuit can use equivalent lumped parameters with the same input and output connections π network model replacement.
Therefore, the motor dispersion parameter circuit can be equivalent by the lumped parameter circuit; the windings constituting the shaft voltage--rotor coupling are shown in Figure 2a); Vbrg is the shaft voltage; Ibrg is the bearing current; Va; Vb and Vc are Motor input voltage. Although Iws does not flow through the bearing; but it has the same method as the bearing current on the stator winding; it must have an effect on the bearing current. For ease of analysis; the coupling of the intermediate point to the stator of the winding will not be considered. For the convenience of accounting; Figure 2 a) is simplified to the equivalent single-phase drive circuit model shown in Figure 2 b). Z1 is the midpoint impedance of the power supply; Z2 is the bypass impedance; characterizing the common mode reactance in the drive loop Coils, line reactors and long cables, etc., R0 and L0 are the zero sequence resistance and inductance of the stator, Csf, Csr and Crf are the stator-to-ground, stator-to-rotor and rotor-to-ground capacitance of the motor, Rb is the bearing loop resistance, Cb And R1 is the capacitance and nonlinear impedance of the bearing oil film, and Usg and Urg are separated from the neutral voltage of the stator winding and the rotor.
Regarding the motor powered by the inverter; when the bearing oil film is not broken down; because the carrier frequency is high; the capacitive reactance of the capacitor is greatly reduced. Xcb comparison; Rb is small and R1 is large; since the PWM driving voltage is non-sinusoidal voltage; Divide it first during accounting; then leave it alone; the useful values ​​for the axis voltage are:
3 bearing model and bearing current occur due to the presence of distributed capacitance and the excitation effect of high-frequency pulse input voltage; the coupled common-mode voltage is formed on the motor shaft. In fact; the presentation of the shaft voltage is not only related to the above two elements; The layout has a direct connection. The front and rear ends of the rotor are supported by a bearing; the layout is shown in Figure 3.
Taking a bearing in between as an example; the raceway of the bearing consists of an inner raceway and an outer raceway; when the motor changes; the balls in the bearing are surrounded by a smooth oil layer; due to the insulating effect of the smooth oil; between the bearing raceway and the ball Forming the capacitor; as shown in Figure 3b). These two capacitors exist in series in the rotor stator loop (for ease of analysis; do not consider the impedance of the ball); can be equivalent to a capacitor Cbi; i represents the i-th in the bearing Balls. Regarding the entire bearing; the capacitance between each ball and the raceway exists in parallel. Therefore, the entire bearing can be equivalent to a capacitor Cb. According to the analysis of the bearing; the bearing can be used with an internal inductance and resistance The switch is equivalent. When the ball is not touched by the raceway; the switch is disconnected; the rotor voltage is set up, when the rotor voltage exceeds the oil film threshold voltage; the oil film breakdown switch is turned on; the rotor voltage is agilely discharged; Large discharge current.
Va, Vb and Vc are the three-phase input voltage of the motor; L', R' and C' are the equivalent convergence parameters of the input voltage coupled to the rotor shaft; Cg is the equivalent capacitance after the parallel connection of Crf and Cb. When bearing the ball and When the raceway touches or the oil layer in the bearing is broken down; Cb does not exist; at this moment, Cg only represents the coupling capacitance of the rotor shaft to the casing.
The capacitance Cb is a function of a plurality of variables: Cb(Q, v, T, η, λ, Λ, εr) [2]. During which Q represents power; v represents oil film velocity; T represents temperature; η represents smooth agent viscosity λ represents the smoothing agent additive; Λ represents the oil layer thickness; εr represents the smoothing agent dielectric constant. Bearing capacitance Cb and stator-to-rotor coupling capacitance Csr; much smaller than stator-to-case coupling capacitance Csf and rotor-to-case coupling capacitance Crf .
In this way, the voltage coupled to the motor bearing is not too large; this is because the capacitance of Crf in parallel with Cb is much larger than the Csr in series with the coupling loop; in series capacitor loops, the larger the capacitance is accepted The voltage is smaller. In fact, according to the characteristics of the distributed capacitance; a large part of the common mode current is transmitted to the earth through the coupling capacitor Csf between the stator winding and the iron core; thus the bearing current is only one of the common mode currents. Some. As can be seen from Figure 4; there are two fundamental methods for forming bearing currents.
First, due to the existence of the distributed capacitance; the stator winding and the bearing form a voltage coupling loop; when the input voltage of the winding is a high-frequency PWM pulse voltage; the dv/dt current must occur in this coupling loop; this current is transmitted to the earth by Crf. The other part is transmitted to the earth through the bearing capacitor Cb; that is, it constitutes the so-called dv/dt bearing current; its size is related to the input voltage and the scattering parameters in the motor. Second, due to the existence of the bearing capacitance; the shaft voltage occurs on the motor shaft; When the shaft voltage exceeds the breakdown voltage of the bearing oil layer; the raceway in the bearing table is equivalent to a short circuit; thus forming a large discharge current on the bearing; so-called electric discharge machining (EMM) current. Other; when the motor At the time of the transition; if there is a touch between the ball and the raceway; the same will form a large EDM current on the bearing.
In order to quantify the influence of EDM and dv/dt current on the bearing; the current density in the bearing is very important. To establish the current density, it is necessary to estimate the point touch area of ​​the inner surface of the ball and the raceway. According to Hertzian point contact theory; bearing electrical The number of lives can be obtained by the following formula [2]:
Elec Life(hrs)= (7)
In the formula; represents the bearing current density. Generally speaking, the dv/dt current has a great influence on the bearing life. The bearing current density of the EDM is very large; the bearing life is greatly reduced. Others; the bearing damage degree at no load is instead The load time is much larger; this is due to the increased bearing contact area during heavy loads; the bearing current density is reduced invisibly.
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