Prevention of Bearing Current in Variable-Frequency Motor

Abstract: The paper discusses that for the variablei-frequency motor fed fromthe inverterinducted voltage appears at the winding and shaft because the power supply has many high-voltage pulses the high-frequency har monic increasesand the magnetic circuit of motor core is not balance. To decrease the harm caused by bearing current some measures such as mounting the power filter and insulating the front and rear bearings are presented in addition that the magnetic circuit is required to be balance in manufacturing the motor.

Key words: Variable-frequency motor, bearing current, prevention

1. Introduction

Usually in large motors, especially those that use fan-shaped laminated iron cores, if the magnetic field is asymmetric, it is easy to generate shaft currents. In recent years, small motors using variable frequency drive devices have also experienced significant shaft currents, leading to premature bearing damage and directly affecting and reducing the reliable lifespan of motor operation. This has attracted the attention of motor users and manufacturers.

2. Hazards of shaft current

The auxiliary devices installed at the shaft end of the variable frequency motor, such as tachometers, encoders, etc., can easily form a shaft current circuit with two bearings or one of the two bearings (see Figure 1 for the mathematical model of shaft current). The shaft current causes damage to the motor bearings and poses a threat to the safety of auxiliary devices such as tachometers and encoders.

Figure 1 Mathematical Model of Axis Current

Under normal operation of the motor, a lubricating oil film is formed inside the bearing, which can provide a certain insulation effect. Even if the motor operates at a lower shaft voltage, it will not generate shaft current. But when the shaft voltage reaches a certain value, it will break through the oil film and discharge. Especially when the motor is started, the lubricating oil film inside the bearing has not yet been stably established, and the instantly generated shaft voltage will break through the weak parts of the oil film and discharge, forming shaft current. When the metal contact points in the discharge area are very small (especially in ball bearings), the current density passing through these points will be high, instantly generating high temperature, causing local burns on the bearing and small pits on the ball or raceway. If there are small pits and pits in the shaft current for a long time, it will increase, and grooves may even appear on the raceway of the rolling bearing, or the journal and bearing shells of the sliding bearing may be burned, or the bearing may be completely damaged, resulting in noise, vibration, or shutdown. Electric motors operating in explosive hazardous environments may cause serious accidents such as explosions due to overheating or sparks generated by excessive shaft currents.

The allowable shaft voltage and shaft current of an electric motor are related to many factors such as bearing type, operating status, lubricating oil quality, speed, installation quality, on-site operating environment, and impedance of the shaft current flow path.

Almost all motors generate shaft voltage to some extent during operation. Generally speaking, if the shaft voltage generated at both ends of the shaft reaches 500m, V is prone to generating harmful shaft current. According to the American NEMAMGI standard, testing is carried out according to the IEEE112 standard. If the peak shaft voltage exceeds 300mV, insulation measures need to be taken.

The influence of shaft current on sliding bearings and rolling bearings is slightly different. For sliding bearings, if the shaft current is less than 10A, there is basically no ablation. If the shaft current value reaches 10-40A, it can only maintain operation for 3000-12000 hours. If the shaft current is above 100A, it is very dangerous and may burn out within a few hours.

For rolling bearings, due to the small contact area between the ball (roller) and the inner and outer raceways of the bearing, the sensitivity to shaft current is greater than that of sliding bearings. The damage and destruction caused by shaft current to rolling bearings are more severe. Damage may occur within a few hours when the shaft current is greater than 2A; If the shaft current reaches 1-1.4A, the bearing can only continue to operate for 200-700h; When the shaft current is below 1A, general rolling bearings can operate without significant damage.

3. Generation of shaft voltage and shaft current for variable frequency motors

(1) Variable frequency motors are powered by inverters, which often use pulse width modulation (PWM) technology. The voltage waveform contains many high-voltage pulses, resulting in an increase in high-frequency harmonic components in the speed control drive system. These harmonic components generate electromagnetic induction in the shaft, stator winding, and wiring parts, and the midpoint potential of the winding may be far from the grounding point, This common mode voltage oscillates at high frequencies and couples with the rotor capacitance to generate a pulse voltage from the shaft to ground, with a peak value of 10-40V.

(2) The magnetic circuit of the motor's iron core is unbalanced, such as uneven thickness of the entire round stator lamination, which causes the accumulated thickness of the entire stator iron core to appear on one side thick, one side thin, or one side loose, and one side tight after positioning and lamination, resulting in asymmetric magnetic circuit of the stator iron core circumference. The use of fan-shaped lamination and inappropriate iron core slots and ventilation slots can also cause magnetic circuit asymmetry. When the asymmetric magnetic field cuts the motor shaft, shaft voltage is induced at both ends of the shaft.

(3) If there are many high-voltage equipment around the motor during operation, the shaft voltage will also be induced at both ends of the motor shaft under these strong magnetic effects.

(4) Intervention of external power supply. Due to the complexity of on-site wiring for motor operation, such as the protection and monitoring detection components of the motor, auxiliary devices such as tachometers, encoders, cooling fans, etc., any live wire tower connected to the shaft may cause the generation of shaft voltage.

Once the shaft of a motor with shaft voltage is broken down with the motor casing, such as the lubricating oil film of the bearing, and forms a circuit with auxiliary devices such as tachometers, encoders, or coupling devices (as shown in the mathematical model diagram of shaft current), destructive shaft current is generated.

4. Prevention of shaft current in variable frequency motors

4.1 Suppression of power harmonics

The speed control system powered by inverters is equipped with filters or matched frequency conversion speed control devices are equipped with common mode filtering circuits to provide high-quality sine wave power supply, reducing harmonics, shaft current, noise, vibration, and motor temperature rise, extending the life of bearings, windings, and motors, and ensuring the safety of motor bearings, tachometers, encoders, and other auxiliary devices.

4.2 Insulation measures shall be taken for both ends of the bearings

If auxiliary devices such as tachometers and encoders are installed on the shaft head of the variable frequency motor, insulation measures should be taken not only for the non transmission end bearings of the motor, but also for the transmission end to cut off the shaft current passing through the transmission end bearings to form a circuit (see Figure 1).

Insulated bearings should be selected for bearings with a rolling bearing structure, and plasma spraying method can be used to evenly spray 50-100 on the inner surface and end face of the bearing inner ring μ High performance insulation layer can also be sprayed on the outer surface and two end faces of the bearing outer ring, but the insulation layer of the outer ring is prone to scratches and loss of insulation ability when disassembling the end cover. It is also possible to add a sleeve in the end cover bearing chamber, with an insulation layer sandwiched between the sleeve and the end cover, and to tighten the bolts of the inner and outer covers with insulation sleeves and pads. This structure and process are relatively complex.

Adopting a sliding bearing structure, the traditional method is to pad the fixed bearing area with epoxy glass cloth plates, etc. Insulated pipe joints are added to the oil inlet and outlet pipelines.

4.3 Measures taken for motor design and manufacturing

(1) Insulation measures shall be taken for the front and rear bearing platforms and shoulder of the motor shaft using a rolling bearing structure, such as plasma uniform spraying of 50-100 μ M-thick high-performance heat-resistant ceramic insulation layer.

(2) In terms of motor design and manufacturing, efforts should be made to achieve basic balance in the magnetic circuit of the motor. For example, the iron core lamination of the entire round punching plate can rotate the uneven thickness of the punching plate to 120 ° lamination. When using fan-shaped punching, according to the number of poles p of the motor, the number of sheets per circle t, and the number of cycle layers n of the cyclic staggered stacking, it is necessary to meet the highest single fraction of q=nt/p=even or even numerator.

4.4 Insulation measures taken for connecting shafts

In order to improve the safety of auxiliary devices, it is necessary to cut off the path of shaft current between the motor shaft and the load machinery, as well as the shaft connecting auxiliary devices such as tachometers and encoders (see Figure 1). Special measures must also be taken, such as insulation for the shaft directly connected to the motor shaft.

4.5 Strengthening insulation of monitoring lines

In order to avoid shaft current caused by insulation damage to the wires of various monitoring devices and auxiliary devices of variable frequency motors, on-site operation and maintenance personnel need to regularly inspect and strengthen their insulation to eliminate unnecessary shaft current hazards.

4.6 Improving the operating environment of motors

The operation site of the motor should maintain an appropriate distance from high-voltage and strong electric field equipment, avoid inappropriate grounding arc welding next to the motor, eliminate static electricity generated during the production process, and reduce interference with the magnetic balance of the motor.

4.7 Operational auxiliary measures

If there is significant destructive shaft current in the on-site operation of the motor or if the surrounding environment is humid for medium and large motors, and the above insulation measures are inconvenient to implement or cannot meet the requirements due to reasons, grounding brushes can be installed on the inner side of the bearings at both ends of the motor to guide the shaft current into the ground, avoiding the burning of the motor bearings and protecting the safety of auxiliary and monitoring devices.

5. Conclusion

Generally, based on the quality of power supply, motor size, bearing type, coupling device, monitoring device, motor operating environment, and shaft voltage of the variable frequency motor, the above measures can be adopted separately to reduce the harm of shaft current, extend the operating life of bearings, and improve the reliability of variable frequency motor operation.

2023 December 1st Week KYOCM Product Recommendation:

Linear motion rolling bearing:

Linear motion bearing or linear slide rail is a bearing designed to provide free motion in one direction. There are many different types of linear motion bearings. Mobile linear slideways such as mechanical slideways, XY worktables, roller worktables and some dovetail slideways are bearings moved by the drive mechanism. Not all linear guideways are motorized, and non-motorized dovetail slideways, ball bearing slideways and roller slideways provide low-friction linear motion for inertia or manually driven equipment. All linear guideways provide bearing-based linear motion, whether it is ball bearing, dovetail bearing, linear roller bearing, magnetic bearing or fluid bearing. XY table, linear platform, machine slide and other advanced slide rails use linear motion bearings to provide multi-axis motion along X and Y.