The Influence of Variable-Frequency Operation on Motor Shaft Current

Why Does Shaft Current Occur in Motors Operating at Power Frequency?

The basic conditions for current formation are voltage and a closed loop. The prerequisite for generating shaft current is the presence of shaft voltage and a closed loop. So why does shaft voltage occur in motors operating at power frequency? During the operation of rotating motors, there are two causes of shaft voltage: one is alternating magnetic flux, and the other is static charge accumulation.

The shaft voltage generated by the former is continuous and periodic. Under normal circumstances, the motor's rotor operates in a symmetrical, sinusoidally alternating magnetic field. The alternating electromotive force induced by the rotor cutting the magnetic field and the resulting alternating current are also symmetrical. Therefore, there is normally no asymmetric voltage between the two ends of the rotor. However, when the magnetic reluctance of the motor's stator core is unbalanced in the circumferential direction, an asymmetric alternating electromotive force will be generated, which in turn produces shaft voltage—this voltage is generated along the axial direction. In contrast, the shaft voltage generated by static charges is intermittent and aperiodic. During motor operation, fluids on the load side rub against the rotating parts, generating static charges on the rotating body. The gradual accumulation of these charges gives rise to shaft voltage.

Hazards of Shaft Current to Motors

During the operation of large and medium-sized Ac Motors, once a loop is formed for the rotor shaft voltage, shaft current will be generated, which is a typical low-voltage and high-current mode. Oil lubrication is used between the shaft and the bearing bush, and the motor bearing rests on the oil film. Due to the low amplitude of the shaft voltage, the insulation of the oil film will generally not be broken down.

During high-speed rotor operation, if the lubricating oil quality is substandard or there is oil shortage, the oil film will break and be punctured, resulting in metallic contact between the shaft and the bearing bush. At the moment of contact, the shaft voltage will form a closed loop, leading to low-voltage breakdown. At this time, the generated shaft current is quite large, reaching several hundred amperes or even thousands of amperes in an instant, which is sufficient to burn out the journal and bearing bush.

The gradual accumulation of static charges generated on the shaft by operational friction continuously increases the potential of the shaft due to charging. When the rotating shaft comes into contact with any component outside the rotating body, it discharges through that component. If the rotating shaft does not come into contact with components outside the rotating body, charges will continue to accumulate, eventually generating excessive voltage. If this voltage exceeds the insulation strength of the bearing oil film, the charges will discharge in an extremely short time, forming shaft current.

Shaft current flows through the loop composed of the rotating shaft, bearing inner ring, bearing outer ring, and bearing housing. The most prominent phenomenon is small and deep circular erosion points on the shaft bearing position and the surface of the bearing inner ring caused by arc discharge. Shaft current not only destroys the stability of the oil film and the conditions for oil film formation but also generates many erosion points on the surfaces of the rotating shaft and bearing inner ring due to discharge, damaging the good fit between the rotating shaft and the bearing and thus rendering the bearing inoperable. In special cases, strong shaft current will generate intense electric arcs on the contact surface between the journal and the bearing bush, leading to damage to the journal and bearing bush, causing motor vibration and noise, and ultimately making the motor unable to operate normally.

The oil film of rolling bearings is generally relatively thin and sensitive to shaft voltage. When the shaft current flows through the fine contact points between the rolling elements and the inner and outer rings, if the voltage is relatively high, breakdown may occur on the contact surfaces of the inner and outer rings. Generally, the shaft voltage of rolling bearings is required to be less than 300mV.

Modern motor design and manufacturing processes have added many measures to eliminate low-frequency shaft current. However, due to the adoption of variable-frequency power supply and the use of fast-switching components in modern AC drive systems, high-frequency current pulses will be generated on the bearings. If the energy of these pulses is sufficiently high, it will also cause bearing damage.

1. Fault Manifestations of Bearing Ablation Caused by Shaft Current

The main basis for judging whether bearing burnout is caused by shaft current is to check the traces on the bearing surface. For any ablation caused by shaft current, striped ablation marks like a washboard will appear on the raceways of the inner and outer rings of the bearing, which is a typical feature of rolling bearing damage caused by shaft current. At the same time, there are also pitting, scars, and even cracks on its surface; the temperature of the motor bearing rises rapidly, accompanied by the outflow of lubricating grease.

The washboard-like ablation marks are formed because when the rollers or balls roll on the raceways of the bearing rings and roll the raceways, the contact resistance at the rolling contact point is very small, and the lubricating grease is squeezed to both sides. When the rolling elements are about to leave their original positions, a small gap is generated. The shaft current discharges on the surfaces of the rotating shaft and the inner ring of the bearing, producing many erosion points and burning the raceway surface into striped marks.

The number of stripes is related to the shaft current frequency, motor speed, and internal conditions of the bearing. When the subsequent rolling elements continue to rotate forward, the ablation marks are flattened and polished due to rolling, so the raceway surface will appear bright.

2. Empirical Values for Bearing Burnout Caused by Shaft Current

The allowable magnitudes of shaft voltage and shaft current for motors are related to many factors such as bearing type, operating status, lubricating oil quality, speed, installation quality, on-site operating environment, and the impedance of the shaft current flow path. Generally speaking, if the shaft voltage generated at both ends of the shaft reaches 500mV, harmful shaft current is likely to be generated. If the peak value of the shaft voltage is greater than 300mV, insulation measures need to be taken.

The influence of shaft current magnitude on sliding bearings and rolling bearings is slightly different. For sliding bearings, if the shaft current is less than 10A, ablation will occur in most cases; if the shaft current reaches 10~40A, the normal operation time of the motor will be shortened, and some can only last 2000~10000 hours.

For rolling bearings, due to the small contact area between the balls or rollers and the raceways of the inner and outer rings of the bearing, they are more sensitive to shaft current than sliding bearings, and the damage caused by shaft current to rolling bearings is more serious. When the shaft current exceeds 2A, damage may occur within a few hours; if the shaft current reaches 1~1.5A, the bearing can only operate for 200~600 hours; when the shaft current is less than 1A, the rolling bearing can generally operate stably.