The Issue of Shaft Currents in Variable Frequency Motors

Ordinary cage asynchronous motors, which are suitable for general-purpose drive systems, are also suitable for use in inverter speed control systems. Therefore, by the frequency converter powered cage asynchronous motor, its structural design parameters (seat number and size can be referred to the general cage asynchronous motor, but it should be noted that due to the frequency converter motor in a variety of different frequencies, so in the design and manufacture of cage inverter asynchronous motor to pay attention to this factor on the operation of the motor to produce a variety of undesirable effects, which one of the most important impact is the frequency converter motor operation, the shaft current on the motor bearing. Shaft current on the motor bearing impact, now nine star editor from the following aspects of the frequency motor shaft current on the motor bearing impact and preventive measures.

Types of bearing currents and reasons for their generation.

Low-frequency bearing currents

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

High-frequency shaft currents

The reason for the high frequency shaft current: the power frequency three-phase sinusoidal power supply voltage is balanced and symmetrical, so the voltage at the characteristic point is zero, but the output voltage of the inverter is generated through PWM, that is, the inverter converts the DC voltage into three-phase sinusoidal AC voltage.

Although its fundamental frequency component is symmetrical and balanced, but because the diode in the inverter unit can not be absolutely synchronized, so it can produce asymmetrical high harmonics, resulting in an increase in the zero-sequence voltage component, that is, the neutral point voltage is not zero. The standard will be the zero sequence voltage is defined as the common mode voltage, this voltage can be measured in the load motor winding in the neutral point, its frequency and inverter diode switching frequency is the same, and its amplitude is proportional to the dc bus voltage. The standard specifies that the shaft current generated by the common mode voltage is called high-frequency shaft current.

Types of high-frequency shaft currents.

Under the action of common-mode voltage, high-frequency induced voltage is generated by the high-frequency magnetic flux circulating along the stator yoke, and when this induced voltage is high enough to destroy the insulation of the bearing lubricant, the resulting circulating current flows along the bearings, shafts, and stator seats connected to the circuit.

Under the action of common mode voltage, there will be a voltage drop of more than 100V between the motor frame and the inverter frame. The current leaked into the stator frame returns to the inverter through the metal coupling and the driven mechanical equipment, forming the shaft grounding current.

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

The hazards of axial currents

The current flowing into the bearing changes rapidly, and its change rate depends on the bearing process. When the bearing ball is completely immersed in the lubricant and does not conduct electricity, the existing bearing capacitance is in the state of electrostatic charge. If the voltage of electrostatic charge exceeds the insulation performance of the bearing lubricant, the oil film formed by the bearing lubricant will be damaged, 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 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 of the bearing, and form grooves, thus generating noise and vibration, If it is not found and handled in time, it will lead to bearing failure, which will have a great impact on production.

Methods of preventing current damage to bearings

With insulated bearings

Electrically insulated bearings can radically eliminate galvanic corrosion by integrating insulating properties into the bearing, thereby improving reliability and increasing machine uptime.

Electrically insulated bearings usually have an outer ring bore and end faces coated with aluminum oxide. Insulated bearings with coated inner ring bores and end faces provide enhanced protection against high-frequency currents due to the smaller surface area of the coating on the inner ring compared to the outer ring.

Insulated bearing adopts special spraying technology, spraying and plating high-quality coating on the outer surface of the bearing, the coating and the substrate combination is strong, good insulation performance, can avoid the induction current on the bearing of the galvanic corrosion effect, to prevent the current on the lubricating grease and the rolling body, the raceway caused by the damage to improve the service life of the bearings. Applicable to motors, generators, especially frequency conversion motors are more widely used.