Generation of Motor shaft current

The causes of shaft voltage and shaft current

1. Magnetic imbalance generates shaft voltage

When the ac asynchronous motor operates at sinusoidal alternating voltage, its rotor is in the sinusoidal alternating magnetic field.The unbalanced magnetic field in the magnetic circuit is caused by the fan-shaped impingement of the stator rotor of the motor, the lamination of the silicon steel sheet, the existence of the iron core slot and the vents, etc.When the magnetic field in the peripheral direction of the stator core of the motor is unbalanced, the alternating flux of the intersecting chain with the shaft will be generated, thus generating the alternating potential.When the motor rotates, namely the magnetic pole rotates, the magnetic flux of each magnetic pole changes, and the shaft voltage is induced at both ends of the shaft, resulting in the magnetic flux intersecting with the shaft.As the magnetic pole rotates, the flux of the intersecting chain with the shaft changes alternately. This voltage is generated along the axial direction. If a closed loop is formed with the bearings on both sides of the shaft, an axial current is generated.Generally, this kind of axial voltage is about 1~2V.

2. The inverter power supply generates the shaft voltage

When the motor is operated with inverter power supply, the power supply voltage contains high-order harmonic components, which generates electromagnetic induction between the end of the stator winding coil, the wiring part and the rotating shaft, thus generating the shaft voltage.

Asynchronous motor stator winding is embedded in slot stator core, the distributed capacitance existed both  in stator winding turn-to-turn and between stator winding and the motor base, when run under general inverter in high frequency, common-mode voltage of the inverter will produce sharp change, will form leakage current by the motor winding capacitance between the motor shell to the ground terminal. The leakage current may form radioactive and conductive electromagnetic interference.Because of the unbalance of the motor magnetic circuit, electrostatic induction and common mode voltage are the causes of the shaft voltage and shaft current.When the input end of the stator winding is suddenly added with a steep voltage change, due to the influence of distributed capacitance, the voltage distribution at each point of the winding is uneven, which makes the voltage close to the end of the input end winding highly concentrated and causes insulation damage or aging.This phenomenon generally destroys the part of the stator winding, the voltage is often concentrated in the endpoint of the invasion.

In addition, due to large reactance of windings, the input voltage high frequency component on nearby capacitance will focus on the input terminal, through the wire, winding, chassis distributed capacitance between the earth wire to the flow of electric current, form a LC series resonance circuit, when which produces high frequency harmonic current, can produce all kinds of faults.General purpose inverter drive capacity small asynchronous motor, the problem of shaft voltage can not be considered, but the use of more than 200kW motor, especially the existing fans, compressors, such as the occasion of frequency conversion speed adjustment, it is best to confirm the size of the shaft voltage in advance, in order to take preventive measures as early as possible.

3. Electrostatic induction generates shaft voltage

At the running site of the motor, due to the strong electric field of the high-voltage equipment, the shaft voltage is induced at both ends of the shaft.

4. Static charge

In the running process of the motor, the fluid in the aspect of load runs friction with the rotating body and generates static charge on the rotating body.The axial voltage produced by this condition is different in principle from that produced by magnetic alternating current.The axial voltage generated by static charge is intermittent and aperiodic.Such as static charge accumulation, temperature measuring element insulation damage and other factors may lead to the generation of shaft voltage.Once the axial voltage is established, the axial current will be generated once a path is formed between the rotating shaft, the housing and the housing.

5. External reasons

The intervention of an external power source generates the shaft voltage.Due to the complex field wiring, especially large motor protection, measurement components wiring more, which live wire lap on the shaft, will produce the shaft voltage.

Harm of motor shaft current

Normally, the lubricating oil film between the shaft and the bearing ACTS as an insulation.For lower shaft voltage, no shaft current is generated.When the shaft voltage increases to a certain value, especially when the motor starts up, the lubricating oil film has not been formed stably, and the shaft voltage will break through the oil film to form a circuit, generating a considerable shaft current, which can reach hundreds of amperes or even thousands of amperes.Because the metal contact surface is very small, the current density is large, so that part of the bearing melt, melting bearing alloy will be spattered by the rolling pressure , so small pits will burn in the bearing internal surface.The common symptom is a striated arc scar on the inner surface of the bearing, severe enough to burn out the shaft neck and bearing bush.