Shaft Current Improve Safety of Large Motor Bearing Systems

The motor is one of the most common machines. It is a device that converts electromagnetic energy into mechanical energy. During the energy conversion process, some simple and complex factors may cause the motor to generate shaft current to varying degrees, especially large motors, high-voltage motors and variable-frequency motors. There are many cases of motor bearings burning out and failing due to shaft current.

The necessary conditions for generating current are voltage and a closed circuit. To eliminate shaft current, from a theoretical point of view, one measure is to control or even eliminate the shaft voltage, and the other is to cut off the closed circuit. In practice, different manufacturers take different measures for different operating conditions. For operating conditions that are convenient to operate, guide carbon brushes are used. The principle is to create another circuit to separate the bearing from the circuit. In more cases, insulated bearing sleeves, insulating end covers, insulating bearings, or measures to insulate the bearing position are used in accordance with the method of cutting off the circuit.

In order to fundamentally reduce the hazards of shaft current, the rationality of the design scheme and the conformity of the manufacturing process to the design are very necessary. Lean control of the design scheme and process manufacturing is more economical and reliable than various subsequent measures.

AC millivoltmeter

Electronic voltmeter (also called AC millivoltmeter) generally refers to analog voltmeter. It is a commonly used measuring instrument in electronic circuits, using a magnetoelectric meter head as an indicator, and is a pointer-type instrument. Electronic voltmeter can not only measure AC voltage, but also be used as a wide-band, low-noise, high-gain amplifier.

Generally, electronic voltmeters are composed of two parts: amplification and detection. They are mainly composed of four parts: attenuator, AC voltage amplifier, detector and rectifier power supply.

Electronic voltmeter is mainly used to measure various high and low frequency signal voltages. It is one of the most widely used instruments in electronic measurement.

The voltage to be measured is first attenuated by an attenuator to a value suitable for the input of the AC amplifier, then amplified by an AC voltage amplifier, and finally detected by a detector to obtain a DC voltage, the value of which is indicated by the meter.

The deflection angle of the pointer on the electronic voltmeter is proportional to the average value of the measured voltage, but the panel is scaled according to the effective value of the sinusoidal AC voltage. Therefore, the electronic voltmeter can only be used to measure the effective value of the sinusoidal AC voltage. When measuring non-sinusoidal AC voltage, the reading of the electronic voltmeter has no direct meaning. Only by dividing the reading by the waveform factor of the sinusoidal AC voltage, 1.11, can the average value of the measured voltage be obtained.

Classification of voltmeters

1 Analog voltmeter

Analog voltmeters generally refer to pointer voltmeters, which add the voltage to be measured to the magnetoelectric ammeter and convert it into the size of the pointer deflection angle to measure. When measuring DC voltage, it can be directly or after amplification or attenuation converted into a certain amount of DC current to drive the pointer deflection indication of the DC meter. When measuring AC voltage, it must be converted into a DC voltage proportional to the measured AC voltage through an AC/DC converter, i.e. a detector, and then the DC voltage is measured. According to different classification methods, there are many types of analog voltmeters.

2. Digital voltmeter

The digital voltmeter converts the measured voltage value into a digital quantity through digital technology, and then displays the measured voltage value in decimal. The digital voltmeter uses an A/D converter as a measuring mechanism and displays the measurement results on a digital display. A digital voltmeter that measures AC voltage and other electrical parameters must convert the measured electrical parameters before the A/D converter to convert the measured electrical parameters into DC voltage.

Digital voltmeters can be divided into DC digital voltmeters and AC digital voltmeters according to the different measurement objects. DC digital voltmeters can be divided into three types according to the different A/D converter methods: comparison type, integration type and compound type. AC digital voltmeters can be divided into three types according to the different AC/DC conversion principles: peak type, average value type and effective value type.

Digital voltmeters use digital output to intuitively display measurement results. In addition to the advantages of high measurement accuracy, fast speed, large input impedance, strong overload capacity, strong anti-interference ability and high resolution, they are also easy to combine with computers and other equipment to form automatic test instruments and systems, and they are also occupying an increasingly important position in voltage measurement.