Motor Axial Voltage Testing Procedure & Wiring Setup - MPT New-tech (Shanghai) Co., Ltd. - 常规

Motor Axial Voltage Testing Procedure

and MPT New-tech (Shanghai) Co., Ltd.

Motor Wiring Setup

Firstly, ensure that the test motor/test bench has a good ground connection. It is recommended to have a single-point ground system to minimize potential differences between different ground points that may cause testing interference.

Use a flexible, low-friction, conductive probe to make contact with the motor shaft (contact point can be at the shaft end or the circumference of the rotor) and extract the axial voltage signal. Connect the extracted axial voltage signal to the signal terminal of the oscilloscope test probe. The ground clip of the oscilloscope test probe should be connected to the motor casing.

Note: When extracting the axial voltage wire, ensure that the probe or its lead is insulated from the casing. If using the standard test probe from MPT New-tech (Shanghai) Co., Ltd. (Product Model: VGS-M10-1; Patent Number: ZL202221574986X) - as shown in Figure 1 - an M10x1.5 threaded hole can be opened at a suitable position on the motor housing for the installation of the fixed test probe. If the casing at the shaft end is open, it can be clamped and fixed using a magnetic base or other auxiliary fixtures.

Figure 1 - MPT New-tech VGS-M10-1 test probe

If the motor is an oil-cooled motor and there is a possibility of oil leakage at the opening, it is recommended to wrap the conductive fiber with polytetrafluoroethylene tape at the appropriate position of the test probe to ensure good contact between the conductive fiber and the motor shaft. Refer to Figure 2 for the completed wiring connection.

After completing the wiring connection as shown in Figure 2, measure the resistance between the casing connection wire and the rotor connection wire using a multimeter when the motor is in a static state. The resistance value should be less than 5 ohms. (This requirement does not apply if it is a single motor and both front and rear bearings are insulated bearings.)

Oscilloscope:

Recommended bandwidth ≥ 100M, preferably with statistical functions.

Measurement mode: Sampling

Measurement parameters: Voltage waveform, peak-to-peak voltage

When reading the values, pay attention to the multiplier setting of the signal probe.

Figure 2 - Wiring setup illustration

It is recommended to set up the wiring for both the front and rear ends of the motor simultaneously, using a dual-channel or multi-channel oscilloscope to measure the axial voltages at the front and rear ends of the motor simultaneously, improving testing efficiency.

Axial Voltage Detection

It is recommended to measure the front-end axial-to-ground voltage, rear-end axial-to-ground voltage, and the voltage difference between the front and rear ends of the shaft based on a matrix of speed and torque settings. Record the axial voltage waveform and peak-to-peak value (Vpp). It is also recommended to measure the axial voltages under acceleration and deceleration conditions.

Testing Conditions

500RPM 2000RPM 5000RPM 8000RPM 12000RPM 15000RPM Without any measures No-load 20% load 50% load 80% load 100% load With measures No-load 20% load 50% load 80% load 100% load

Table 1 - Example matrix of conditions

Note: Table 1 is for reference only. Customers can adjust it according to their own motor technical parameters and operating conditions.

During the measurement of axial voltages, adjust the vertical knob of the oscilloscope to ensure that the axial voltage waveform falls within the oscilloscope screen range. Adjust the horizontal knob to clearly display the axial voltage waveform shape (usually between 50-500μs).

Typical Axial Voltage Waveforms

Axial voltages typically exhibit one of the following three typical waveforms, or a combination of two or even all three:

Resistive discharge waveform
Non-discharge waveform
Capacitive breakdown discharge waveform

Typical waveforms and waveform interpretation:

Waveform Type Interpretation Remark

Resistive discharge waveform Typically occurs when the bearing continuity oil film has not been established under low speed, high load conditions, or when the bearings have obvious wear after long-term use.

Non-discharge waveform Typically occurs when the bearing oil film is good under high speed, low load conditions. This waveform shows that the oil film can withstand the axial voltage without significant discharge. In this waveform, all axial voltages are displayed. The lower, the better.

Capacitive breakdown discharge waveform A certain oil film has formed inside the bearing, but the oil film thickness is insufficient to withstand the axial voltage, resulting in breakdown. In this waveform, the higher the voltage, the better the quality of the oil film.

Data Analysis, Judgment, and Recommendations

After completing the axial voltage testing without taking any measures to prevent bearing corrosion, analyze the recorded axial voltage waveform and axial voltage Vpp based on the recorded matrix of conditions.

If the axial voltage waveforms mostly exhibit non-discharge patterns under most operating conditions, it indicates that the bearing oil film is in good condition and can withstand the axial voltage without breakdown. In this case, limited measures or even no measures to prevent axial voltage are required.

If the axial voltage waveforms mostly exhibit capacitive breakdown patterns and the peak-to-peak voltage is generally greater than 15V under most operating conditions, measures should be taken to dissipate the axial voltage and prevent corrosion.

If the axial voltage waveforms mostly exhibit resistive discharge or capacitive breakdown discharge patterns but the peak-to-peak voltage is generally less than 10V, it is recommended to combine measures such as conductive ring dissipation with insulated bearings to prevent corrosion.

After implementing measures to prevent bearing corrosion, repeat the axial voltage testing under the set matrix of conditions using the above axial voltage testing method. Record the axial voltage waveform and axial voltage Vpp and compare them with the axial voltage without measures. The greater the reduction in the axial voltage Vpp, the better. It is recommended that the reduction be no less than 60%.