1 Causes and hazards of motor shaft current formation
The shaft voltage in rotating electrical machines is generally generated by the shaft alternating magnetic flux. The stretching line variable frequency motor is powered by PWM variable frequency power supply. At this time, the shaft voltage of the motor is mainly generated by the vector sum of the three-phase output voltage of the power supply and the non-zero zero-sequence component. The frequency converter PWM pulse width modulation leads to the increase of high-frequency harmonic components in the speed control drive system. These harmonic components generate electromagnetic induction in parts such as the rotating shaft, stator windings and cables. The voltage coupling effect of the distributed capacitance in the motor forms the system common mode loop. This common-mode voltage oscillates at high frequency and is capacitively coupled to the rotor to generate a pulsed voltage from the shaft to ground, which will generate a zero-sequence current in the system, and the motor bearing is part of this zero-sequence loop. See Figure 1
The shaft voltage forms a closed loop through the motor shaft, bearing, stator frame or auxiliary device to generate shaft current, as shown in Figure 2. The post-processing AC motor adopts rolling bearings. During the operation, under the action of the shaft voltage, the motor rotor forms a metallic contact with the motor stator through the bearing, which generates a considerable shaft current. When the running shaft comes into contact with any part other than the rotating body, a discharge occurs through that part. If the dielectric strength of the bearing oil film is exceeded, the shaft voltage will cause the discharge breakdown of the lubricating oil film to form a loop to generate shaft current. The high temperature generated by the partial discharge energy release of the shaft current can melt many tiny areas on the inner ring, outer ring or ball of the bearing to form grooves, resulting in noise and vibration. If it is not detected and handled in time, the bearing will fail.
In addition, when the frequency conversion motor is running, in the stator coil, the DC-AC voltage inverter applies a square wave voltage whose frequency and amplitude can be changed. The 3-fold high-frequency harmonics contained in it generate high voltage peaks, and the peaks have a fast rise time. The generated magnetic field generates capacitive electromagnetic resonance in the grounding circuit of the rotor and bearing, stator and motor casing, and the super-high voltage charge is accumulated and released. It will break down the grease film formed between the bearing balls and the inner casing, causing an instant impact on the bearing.
2 Measures and countermeasures
Since the shaft voltage is generated along with the design, manufacture, installation and operation of the equipment, it is generally unavoidable for users. However, the damage caused by the shaft voltage must meet two conditions: one is the existence of the shaft voltage; the other is the destruction of the insulation (lubricating oil film) of the bearing, which provides a path for the shaft current, and the two are indispensable. Since the shaft voltage cannot be avoided, the emphasis should be placed on the prevention and control of the shaft current, that is, the shaft current should be reduced to a harmless level. Generally, the surface density of the shaft current should be guaranteed to be 0.4A/mm2 or the shaft voltage should be below 0.35 mV.
2.1 Suppress power harmonics
When the inverter is used for power supply, the system should be properly grounded, the smoothing reactor and d V/d t filter should be selected reasonably, and the standing wave effect of the cable between the inverter and the motor should be considered, and the length of the cable between the inverter and the motor should be shortened as much as possible. Reduce harmonics, reduce shaft current, noise, vibration and motor temperature rise. To reduce or suppress the amplitude of overvoltage spikes, two methods can be used: one is to add a reactor (choke coil) or filter on both sides of the cable, which can effectively slow down the rising speed of the output voltage pulse at the power supply end; the other is to add a reactor (choke coil) or filter on both sides of the cable In the design, the cable length between the inverter and the motor should be minimized, and the oscillation period of the transient wave process between the two should be reduced by shortening the cable length, thereby reducing the overvoltage at the motor end.
2.2 Grounding of the motor load side bearing and insulation of the non-load side bearing
For the shaft voltage caused by static charge, a carbon brush grounding mechanism is installed on the drive shaft of the motor drive system near the motor side, so that the grounding brush can be reliably grounded and contacted with the rotating shaft, and the motor shaft current is introduced into the ground to ensure the rotating shaft. The zero potential effectively avoids the occurrence of harmonic electromagnetic resonance and the accumulation of charges on the inner and outer walls of the bearing, and avoids the instantaneous shaft current formed by the charge discharge. Its schematic is shown in Figure 3.
Since the shaft current is the current generated by the shaft voltage through the motor shaft, bearing, stator frame or auxiliary device to form a closed loop, the motor system may include 4 grounding points, one of which is the auxiliary device such as a tachometer installed at the shaft end of the motor on the non-load side. , encoder, etc.; the second is the motor 2-end bearing; the third is the motor coupling to the mechanical equipment. Therefore, at present, when the shaft voltage is high, the frequency conversion motor with rolling bearing structure should use insulating bearings, or add a sleeve in the bearing chamber of the end cover, add an insulating layer between the sleeve and the end cover, and fasten the bolts of the inner and outer covers with insulating sleeves and sleeves. Insulating pads and other means to ensure bearing insulation.
3 Conclusion
With the development of frequency conversion speed regulation technology, the shaft current problem of frequency conversion motor cannot be ignored. In order to effectively maintain the running state of the variable frequency motor and prolong the service life, corresponding measures must be taken to prevent the shaft current. Under the premise that it is impossible to eliminate the shaft current fundamentally, taking insulation measures for the motor is the simplest and most effective way to prevent the shaft current. In addition, regular vibration inspection of motor bearings, combined with bearing failure analysis, can help detect early bearing damage caused by shaft currents.
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2022-09-09