How to Solve Shaft Current Problems and Protect Motor Bearings

The motor is one of the most common mechanical devices, converting electromagnetic energy into mechanical energy. During this energy conversion process, both simple and complex factors may cause the motor to generate shaft currents to varying degrees. This issue is particularly common in large motors, high-voltage motors, and variable frequency motors. Numerous cases have shown that shaft currents often lead to bearing failures or even severe bearing burnouts.

The necessary conditions for current generation are voltage and a closed circuit. From a theoretical perspective, eliminating shaft currents can be approached in two ways: by controlling or eliminating shaft voltage, or by breaking the closed circuit. In practice, manufacturers adopt different methods depending on operating conditions. For applications with relatively simple operating environments, conductive carbon brushes are often used. Their principle is to create an alternative circuit, isolating the bearings from the current loop. In many other cases, the focus is on breaking the circuit path through measures such as insulated bearing sleeves, insulated end shields, insulated bearings, or insulating the bearing seat itself.

To fundamentally reduce the hazards of shaft currents, the rationality of design and the consistency of the manufacturing process with the design are crucial. Lean control in design and production is both more economical and more reliable than remedial measures applied afterward.

AC Millivoltmeter:
An electronic voltmeter (also known as an AC millivoltmeter) typically refers to an analog voltmeter. It is a commonly used measurement instrument in electronic circuits, employing a moving-coil meter as the indicator, which makes it a pointer-type instrument. The electronic voltmeter can measure AC voltage, but it can also serve as a wideband, low-noise, high-gain amplifier. Generally, electronic voltmeters consist of two major parts: amplification and detection, mainly composed of an attenuator, AC voltage amplifier, detector, and rectifier power supply. They are primarily used to measure voltages of various high- and low-frequency signals and are among the most widely used instruments in electronic measurement.

The measured voltage is first attenuated to a suitable level for the AC amplifier, then amplified, and finally detected to obtain a DC voltage, which is indicated on the meter. The deflection angle of the pointer is proportional to the average value of the measured voltage, while the scale is calibrated in terms of the effective value (RMS) of a sinusoidal voltage. Therefore, electronic voltmeters can only directly measure the RMS value of sinusoidal AC voltages. When measuring non-sinusoidal AC voltages, the readings have no direct meaning. To obtain the average value, the reading must be divided by the waveform factor of a sine wave, which is 1.11.

Classification of Voltmeters:

1)Analog Voltmeters
Analog voltmeters generally refer to pointer-type instruments. The measured voltage is applied to a moving-coil meter, where it is converted into a deflection angle of the pointer. For DC voltage measurement, the voltage may be directly applied, amplified, or attenuated to drive the DC meter head. For AC voltage measurement, an AC/DC converter (detector) is required to first convert the AC voltage into a proportional DC voltage for measurement. Depending on classification methods, analog voltmeters can be further divided into several types.

2)Digital Voltmeters
Digital voltmeters convert the measured voltage into digital values using digital technology and display the result in decimal numbers. An A/D converter serves as the measurement unit, with the result shown on a digital display. For AC voltage or other electrical parameters, digital voltmeters require signal conversion before the A/D stage, transforming the measured parameter into DC voltage. Based on measurement applications, they can be classified into DC and AC digital voltmeters. DC digital voltmeters can be further divided into comparison-type, integration-type, and composite-type according to the A/D method. AC digital voltmeters can be divided into peak, average, and RMS types depending on the AC/DC conversion principle.

Digital voltmeters provide direct digital output, offering high accuracy, fast response, high input impedance, strong overload capacity, strong anti-interference ability, and high resolution. They are also easy to integrate with computers and automated test systems. As a result, digital voltmeters play an increasingly important role in voltage measurement.