Shaft voltage

A voltage on a motor shaft is a symptom of insulation breakdown.

Why didn't the installation earth trip operate (rhetorical question)?

"A voltage on a motor shaft is a symptom of insulation breakdown."

Not necessarily. I remember reading about this a couple of years ago.

A quick Google... appears to a fairly common problem with variable-frequency drives (VFD). The basic fix is a "shaft grounding brush/ring". Low-impedance shielded cables are also recommended.

I remember having read some good papers on the subject in the ABB Website. Pl. search in the ABB Website for Shaft Voltages/Shaft Currents.

What will be nature of shaft voltage if there is no VFD used.?

"What will be nature of shaft voltage if there is no VFD used.?"

Answered in posts #5 and #6, above.

It can also occur with large wound rotor motors. Normally one bearing is mounted on insulators to stop circulating currents.

SHAFT VOLTAGE

The shaft of a rotating machine ideally should have no voltage end-to-end or shaft to ground. However, such voltages do exist on almost all shafts due either to electrostatic charge build up or residual magnetism and, in electrical machines, to asymmetry in magnetic fields of the stator and/or rotor. Electrostatic charge build-up can come from charge separation in connection with steam or oil flow. Residual magnetism is often left in the unit from such causes as welding, magnetic particle inspection, plus permanent magnets or electromagnets possibly employed for holding, transport, etc. Magnetic asymmetries in the stator and/or rotor of electrical machines come from causes such as air gap eccentricity, shorted core laminations, broken rotor bars, winding shorted turns and/or ground faults, plus transients and harmonics from variable frequency or PWM power supplies or from blown diodes or fuses in excitation systems. So long as the shaft peak voltage to ground does not exceed 1 Volt, there should be no consequential effects to the bearings. When the peak shaft voltage exceeds 1 volt, there is a possible risk that current discharge will occur across that bearing, depending upon the magnitude and frequency of the voltage, the bearing loading, its alignment, shaft vibration and the condition and cleanliness of the oil film. The effect of the shaft voltage on bearings is electrical spark erosion, spark tracking or welding at shaft journals and bearing babbitted surfaces

IEEE Std 11 5-1 995

PART I-ACCEPTANCE AND PERFORMANCE TESTING

3.6 Shaft current and bearing insulation

3.6.1 General

Irregularities in the magnetic circuit may cause a small amount of flux to link the shaft, with the result that an electromotive force is generated between the shaft ends. This electromotive force may cause a current to flow through the shaft, bearings, bearing supports, and machine framework, and back to the other end of the shaft, unless the circuit is interrupted by insulation.

NOTE-

While other causes may produce a shaft voltage not involving a difference in potential from one end of the shaft to the other, special tests are not provided for the resulting effects because each of these sources requires specially adapted methods of test, essentially of an investigative research nature.

3.6.2 Method 1. Across end shafts

The presence of shaft voltage may be determined by measuring the voltage from end to end of the shaft with a high-impedance voltmeter.

3.6.3 Method 2. Across bearing oil film, uninsulated bearings

This method requires that the insulating properties of the bearing oil film be adequate to withstand the shaft voltage without breaking down. The presence of shaft voltage or current may be determined by running the machine at rated speed and voltage, and connecting a low-resistance conductor from the shaft to the frame of the machine at one bearing, and a low-range ac voltmeter (or a high-range ac ammeter) with low-resistance leads from the shaft to the frame at another bearing. Deflection of the instrument indicates the presence of a voltage that may produce shaft currents. If the instrument does not deflect, there is either insufficient voltage present or the bearing oil film is not acting as an adequate insulator.

3.6.4 Method 3. Across bearing insulation

Many machines have one or more bearings insulated to eliminate shaft currents. For these methodls as described in this subclause as well as 3.6.5 through 3.6.7, it is assumed that insulation is located between the bearing and the frame of the machine. To determine the presence of a voltage that will produce shaf,t currents in such a machine, a low-resistance conductor is connected from the shaft to the uninsulated bearing in order to short-circuit the oil film, and a low-range ac voltmeter (or a high-range ac ammeter) is connected between the shaft and the frame successively at each insulated bearing. Deflection of the instrument indicates the presence of a voltage that will produce shaft currents if the bearing insulaticln is not present.

3.6.5 Method 4. Bearing insulation

The insulation can be tested by connecting a low range altern,ating-current voltmeter (or a high range alternating-current ammeter) across the insulation. A low-resistance conductor may be applied from the shaft to each bearing to short-circuit the oil film. Deflection of the instrument, in this case, is evidence that the insulation is at least partially effective. If there is no deflection of the ,instrument, either the insulation is defective or there is no shaft voltage present.

3.6.6 Method 5. Bearing insulation

A layer of heavy paper is placed around the shaft to insulate the journal of the uninsulated bearings. The coupling of the driving or driven unit should be disengaged if it is not insulated. Then from a 110V - 125V scale with a resistance in the range of 100 Ω/V - 300Ω/V placedin series with the voltage source, two leads should be run, one of the insulated bearing and the other to the frame (across the insulation), if the lamp filament does not glow (or if the reading of the voltmeter does not exceed 60V) the insulation may be considered satisfactory.

A 500 V megger may also be used. This is much more sensitive than the above method and may tend to reject insulation which is adequate to prevent the small shaft voltage from causing injurious current.

Fig Ref, Bearing Damaged and Insulation