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Abstract: The cause and harmfulness of the shaftcurrent of the frequency-control motors are described，the maximum value of the shaft-current is introduced in the practice and predicated inspecting and estimating method preventing the bearing from being damaged is presented as well．The means to protect from motors' damages taken place by the shaft-current is demonstrated at last in the paper．
Keywords: Frequency-control machine Shaft-current Bearing Insulation
With the rapid development of AC speed regulation technology, the application of AC variable frequency motors is becoming increasingly widespread. However, at the same time, the problem of abnormal bearing damage caused by the shaft current of variable frequency motors is also increasingly prominent. Baosteel Branch has also experienced a large number of abnormal damage issues to the bearings of variable frequency motors during the actual production process. The occurrence of these situations directly leads to equipment failures and causes huge losses.
1. Causes and hazards of shaft current generation in variable frequency motors
When an electric motor is running, the potential difference generated between the two ends of the shaft or between the shaft and the bearing is called shaft voltage. If the two ends of the shaft form a circuit through the motor base, etc., the shaft voltage forms shaft current. The shaft voltage exists with the generation of rotating motors. The main causes of shaft voltage generation in general power frequency motors are magnetic circuit imbalance, monopole effect, electrostatic induction, capacitive current, and other reasons, but these reasons are ultimately caused by magnetic flux pulsation. And in the case of sine wave (power frequency) power supply, if the design and operating conditions of the motor are normal, the potential difference between the two ends of the shaft is very small, and its harm is not serious yet.
At present, the widely used variable frequency motors are mostly powered by PWM variable frequency power supplies. At this time, the shaft voltage of the motor is mainly generated by the vector of the three-phase output voltage of the power supply and the non zero zero zero sequence component. The PWM pulse width modulation of the frequency converter leads to an increase in high-frequency harmonic components in the speed control drive system. These harmonic components generate electromagnetic induction in parts such as the shaft, stator winding, and cable. The voltage coupling effect of the distributed capacitors in the motor forms a common mode circuit of the system. This common mode voltage oscillates at high frequencies and is coupled with the rotor capacitance, generating a pulse voltage from the shaft to ground. This voltage will generate zero sequence current in the system, The motor bearing is a part of this zero sequence circuit. The zero sequence current schematic diagram of the variable frequency motor system is shown in Figure 1.
Figure 1 Schematic diagram of zero sequence current in variable frequency motor system
Shaft current is generated by a closed circuit composed of motor shafts, bearings, stator frames, or auxiliary devices to generate shaft voltage. Under normal circumstances, the shaft voltage of the motor is low, and the lubricating oil film inside the bearing can provide insulation without generating shaft current. But when the shaft voltage is high or the oil film does not form stably at the moment of motor starting, the shaft voltage will cause the lubricating oil film to discharge and break through, forming a circuit to generate shaft current. The high temperature generated by the partial discharge energy release of shaft current can melt many small areas on the inner ring, outer ring, or ball of the bearing and form grooves, resulting in noise and vibration. If not detected and handled in a timely manner, it will lead to bearing failure and have a great impact on production. The most significant feature of high-frequency shaft current on bearing corrosion in variable frequency speed control systems is the generation of "washboard" style dense groove stripes on the inner and outer rings and balls of motor bearings.
2. Limits for shaft voltage of variable frequency motors
As mentioned earlier, almost all motors generate shaft voltage to some extent during operation. The allowable shaft voltage or current of the motor is related to many factors such as bearing condition, oil film thickness, motor operation status, installation quality, on-site operating environment, and impedance of the shaft current flow path. Therefore, the limit value of shaft voltage is difficult to specify in detail. At present, there are no clear regulations on shaft voltage or shaft current limits in China or IEC. Only a few manufacturers or research institutions have put forward some suggestions and regulations on shaft voltage: for example, Siemens stipulates that the no-load shaft voltage of the motor at the factory should be limited to below 350mV, and if it exceeds this value, the bearing must be insulated.
We have tested the shaft voltage of a 1200kW variable frequency motor in a certain factory of Baosteel Branch. The motor bearings were insulated, but there were still two failures caused by bearing damage and failure due to shaft current. Later, we measured the shaft voltage of the motor according to the national standard GB1029 test method. Through testing, it was found that the motor shaft voltage was too high and the motor bearing insulation was poor.
During testing, it should be noted that the shaft voltage of the variable frequency motor is a high-frequency pulse voltage, and ordinary power frequency meters cannot accurately measure it. Therefore, a meter with a high response frequency needs to be used. Once the shaft current is detected, the development of bearing damage can be grasped through bearing failure analysis. We conducted regular vibration testing and failure analysis on the motor bearings mentioned above, and promptly identified early signs of bearing damage. According to the operating experience of Baosteel Branch's motors, in cases of motor bearing damage caused by shaft current, the motor bearing can generally maintain a duration of 1-6 months from the occurrence of early fault characteristics to the failure of the motor bearing. Generally, power frequency or DC motors usually take 1-2 months from the occurrence of large shaft currents to bearing failure. In the AC variable frequency speed control system using PWM modulation technology, due to the relatively small energy of the shaft current, it can last for six months or even longer from the early detection of bearing faults to the final damage.
3. Measures and Countermeasures
Shaft voltage is generated along with the design, manufacturing, installation, and operation of equipment, and is generally unavoidable for users. However, two conditions must be met for damage caused by shaft voltage: firstly, the existence of shaft voltage; The second is that the insulation (lubricating oil film) of the bearing is damaged, providing a path for shaft current, and both are indispensable. Since shaft voltage cannot be avoided, the focus should be on the prevention and control of shaft current. For rolling bearing motors, due to the thin oil film or lubricating grease film, which is sensitive to the effect of shaft voltage, it is generally necessary to take measures to prevent shaft current for motors with a capacity of around 100kW (or frame number 355).
When using a frequency converter for power supply, measures such as correct grounding of the system, reasonable selection of flat wave reactors, dV/d t filters, and consideration of the standing wave effect of the cables between the frequency converter and the motor can reduce the shaft current as much as possible. The cable length between the frequency converter and the motor is generally recommended to be 50m~150m, depending on the motor capacity, voltage, and system design. Another most effective and simplest way is to insulate the motor bearings. Taking a general variable frequency speed control motor as an example, shaft current is the current generated by the closed circuit composed of the motor shaft, bearings, stator frame, or auxiliary devices through the shaft voltage, as shown in Figure 2.
Figure 2
The motor system may include four grounding points, one of which is an auxiliary device installed at the non load side shaft end of the motor, such as a tachometer, encoder, etc; Secondly, the bearings at both ends of the motor; Furthermore, it involves the coupling of the motor to the mechanical equipment. Under the operation of the motor, if the bearing insulation is damaged after the formation of shaft voltage, a path is formed between the rotating shaft and the base, that is, between the two bearings or between the front bearing and the auxiliary device, the shaft current circuit is formed through the base and the ground, and the shaft current is also generated. Therefore, when the shaft voltage is high, not only should one end of the motor bearing be insulated, but special attention should also be paid to the insulation of the motor auxiliary devices.
Motor bearing insulation is usually achieved through non load side motor bearing insulation. There are various forms of motor bearing insulation, including insulated bearings, motor "shaft" insulation, bearing chamber insulation, motor end cover insulation, and motor bearing seat insulation. Using insulated bearings is the simplest method.
At present, insulated bearings are only "special" products from various bearing manufacturers, with significant limitations in their models. A deep groove ball bearing with an inner diameter of about 150mm costs 5000-6000 yuan and a supply cycle of about 10 months, which is expensive. The insulation of the motor's "shaft" actually refers to the coating of insulation at the joint between the motor rotor shoulder and the bearing inner race, and the insulation paper is placed on both sides of the bearing inner race to insulate the bearing from the shaft. This is a relatively new bearing insulation design, but the process is complex and difficult to promote. And bearing seat insulation is only applicable to large individual bearing seat motors. Therefore, the most commonly used bearing insulation methods for variable frequency motors are bearing chamber insulation and end cover insulation. These two methods utilize insulation at the junction of the bearing chamber and the end cover or the junction of the end cover and the base, ultimately resulting in bearing insulation. This type of insulation design should pay special attention to the impact of dirt and short circuits on the insulation at the junction, as well as the insulation of sleeves and gaskets for fixing screws and locating pins.
4. Conclusion
With the development of variable frequency speed regulation technology, the issue of shaft current in variable frequency motors cannot be ignored. Research in this area is not yet mature at home and abroad, and there is no clear standard to regulate it. But in engineering, we must take corresponding measures to prevent shaft current. On the premise that it is fundamentally impossible to eliminate shaft current, adopting insulation measures for motors is currently the simplest and most effective method to prevent shaft current. In addition, regular vibration testing of motor bearings, combined with bearing failure analysis, can help identify early bearing damage caused by shaft current.
More about KYOCM Slewing Bearing ：
Rotary bearings consist of an inner ring and an outer ring, one of which usually contains a gear. Together with the connecting holes in the two rings, they enable optimized power transmission through simple and fast connections between adjacent machine parts. Bearing raceways are designed with rolling elements, cages or gaskets to accommodate loads acting individually or in combination in any direction.
Features and advantages:
High carrying capacity
High stiffness for rigid bearing applications
Low friction
Long service life
Surface protection and corrosion resistance
Integrate other features including:
Driving mechanism
Control device
Lubrication system
Monitoring system
Sealed cassette tape
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2023-08-14