Shaft current in variable frequency motor

Ordinary cage asynchronous motors suitable for general drive systems are also suitable for use in variable frequency speed regulation systems. Therefore, the cage type variable frequency asynchronous motor powered by the frequency converter, Its structural design parameters (the frame number and size can refer to the general cage asynchronous motor, but it should be noted that since the variable-frequency motor operates at various frequencies, it should be noted that this factor has various adverse effects on the motor operation when designing and manufacturing the cage variable-frequency asynchronous motor. One of the most important effects is the influence of shaft current on the motor bearing when the variable-frequency motor operates. Now the nine star editor expounds the variable-frequency motor from the following aspects The influence of shaft current on motor bearings and preventive measures.

1. Types and causes of bearing current.

Low frequency bearing current

The asymmetry of motor magnetic circuit will produce low-frequency shaft current, which is common in motors with a capacity greater than 400KW. This is because the asymmetric magnetic circuit will produce annular AC flux (annular flux) in the yoke, resulting in AC induced voltage in the conductive circuit composed of motor shaft, bearing, end cover and motor stator frame shown in Figure 1. When this induced voltage destroys the insulation ability of bearing lubricant, The current will flow through this circuit, including the front and rear bearings of the motor.

2. High frequency shaft current

Cause of high-frequency shaft current: the power frequency three-phase sinusoidal power supply voltage is balanced and symmetrical, so the neutral point voltage is zero, but the output voltage of the frequency converter is generated by PWM pulse width modulation, which converts the DC voltage into three-phase sinusoidal AC voltage through the inverter (see the following figure for the principle)

Although its fundamental frequency component is symmetrical and balanced, because the disconnection of diodes in the inverter unit cannot be absolutely synchronized, asymmetric high-order harmonics can be generated, resulting in the increase of zero sequence voltage component, that is, the neutral point voltage is not zero. This zero sequence voltage is defined as the common mode voltage in the standard. This voltage can be measured at the neutral point in the winding of the load motor. Its frequency is the same as the breaking frequency of the diode in the inverter unit, and its amplitude is proportional to the DC bus voltage. The standard stipulates that the shaft current generated by the common mode voltage is called high-frequency shaft current.

Types of high frequency shaft current:

1. Under the action of common mode voltage, the high-frequency induced voltage is generated by the high-frequency magnetic flux circulating along the stator yoke. When the induced voltage is high enough to destroy the insulation of bearing lubricant, the generated circulating current flows along the circuit formed by the bearing, shaft and stator base.

2. Under the action of common mode voltage, there will be a voltage drop of more than 100V between the motor base and the frequency converter frame, and the current leaked into the stator base will return to the frequency converter through the metal coupling and the driven mechanical equipment, resulting in the shaft grounding current.

3. Under the action of high-frequency common mode voltage, the impedance formed by various stray capacitors in the motor becomes smaller, thus providing a low impedance path for current flow. Therefore, when the capacitance inside the motor is discharged, high-frequency bearing current will be generated. The current returns to the power supply through the grounding conductor and capacitor of the frequency converter.

Harm of shaft current

The current flowing into the bearing changes rapidly, and its change rate depends on the process of the bearing. When the ball of the bearing is completely immersed in the lubricant and is not conductive, the existing bearing capacitance is in the state of electrostatic charging. If the voltage of electrostatic charging exceeds the insulation performance of the bearing lubricant, the oil film formed by the bearing lubricant will be destroyed, In addition, the induced voltage generated by the asymmetric magnetic circuit of the motor can also damage the insulation performance of the bearing lubricant and then form a large bearing current. When the density of the bearing current exceeds 1.5a/mm, the high temperature generated by the partial discharge energy release of the shaft current can melt many small areas on the inner ring, outer ring or ball, and form grooves, resulting in noise and vibration. If it is not found and handled in time, it will lead to bearing failure, Great impact on production

Prevention methods of bearing current damage

Insulated bearings are used.

Electrically insulated bearings can fundamentally eliminate electric erosion by integrating the insulation performance into the bearings, so as to improve the reliability and increase the normal operation time of the machine.

Electrically insulated bearings usually have outer diameter surface and end face of outer ring with aluminum oxide coating. For insulated bearings with inner hole and end face of coated inner ring, since the coating surface area of inner ring is smaller than that of outer ring, these bearings can provide enhanced protection against high-frequency current.

The insulated bearing adopts a special spraying process. The outer surface of the bearing is sprayed with high-quality film. The film has strong adhesion with the substrate and good insulation performance, which can avoid the electric corrosion of the bearing caused by the induced current, prevent the damage of the current to the lubricating grease, rolling elements and raceways, and improve the service life of the bearing. It is applicable to motors and generators, especially variable-frequency motors.