The purpose of measuring the motor shaft voltage is to understand the magnitude of the motor shaft current. When the motor shaft current reaches a certain level, it will cause the motor bearing to overheat, the bearing or bearing seat to have burn marks, or even damage the bearing. At the same time, fine metal particles may enter the bearing grease (oil), causing the bearing to wear or even heat up and fall apart.
This damage mainly occurs in motors powered by variable frequency power supplies of H315 and above and ordinary large and medium-sized motors. In comparison, the current variable frequency power supply phenomenon is relatively common, so bearing system problems caused by shaft current are also relatively common, especially in large-sized motors and high-voltage motors, where this problem is more concentrated.
Motor shaft voltage test method and requirements
● During the test, the motor under test should be operated at rated voltage and rated speed.
● Use a high internal resistance AC millivoltmeter (such as a transistor or thermoelectric potential millivoltmeter, digital millivoltmeter, etc.) to first measure the voltage U1 at both ends of the shaft, that is, the shaft voltage difference at both ends of the motor rotor. Under normal circumstances, this voltage is mainly caused by the asymmetric radial magnetic field of the rotor or the existence of an axial alternating magnetic field. The shaft voltage.
● Short-circuit one end of the shaft with its bearing seat and ground it, and measure the voltage U2 of the other end of the bearing seat to the ground.
● The surface of the measuring point should be in good contact with the lead of the voltmeter.
● During the test, the insulation resistance between the bearing seat and the metal gasket, and the metal gasket and the metal base should also be checked separately. Usually U1=U2. If the difference between U1 and U2 is more than 10%, it means that the insulation of the insulation pad is poor or there is a large measurement error. When U1=U2, it means that the insulation pad is well insulated. When U1>U2, it means that the insulation of the insulation pad is not good. When U1<U2, it means that the measurement is not accurate. The measurement method and instrument should be checked for accuracy and remeasured.

Causes and prevention of shaft voltage AC voltage is generated at both ends of the motor due to the asymmetry of magnetic flux. The asymmetry of magnetic flux is generally caused by the following reasons: 1) The local magnetic resistance of the stator core is large, such as the rust of the stator core, which leads to excessive local magnetic resistance; 2) The asymmetry of magnetic flux is caused by the uneven air gap between the stator and the rotor; 3) The uneven armature reaction of the fractional slot motor causes the asymmetry of the rotor magnetic flux.
Although the shaft voltage is not large, generally 1~30V (AC), it has a large amount of energy. If a loop is formed through the ground at both ends of the motor, the voltage source will generate a very large shaft current from one end of the rotor through the bearing to the housing or ground and then to the other end. In other words, the fundamental purpose of controlling the shaft voltage is to prevent large currents from damaging the bearing system. The shaft voltage itself actually has little effect on the motor.
There are two measures to prevent shaft current damage: one is to cut off the circuit, the other is to reduce the voltage, or the other is to discharge the current. The commonly used measures are: (1) adding grounding carbon brushes (both rigid and flexible); (2) using insulating bearing sleeves; (3) using insulating bearings.

Issues to be noted when measuring the shaft voltage of variable frequency motors
●The shaft voltage of variable frequency motors is generated by the PWM output of the inverter, and its signal spectrum is similar to the spectrum of the PWM wave output by the inverter, with rich harmonic content. Therefore, a wide-band voltmeter or measuring device is required
●The differential mode voltage of the shaft voltage is very low, generally within 10V, but the common mode voltage is higher, which is related to the bearing insulation and the distributed parameters in the motor, but it must be lower than the output PWM voltage, so you need to pay attention to the selection of the instrument.
●Since the shaft voltage is an inductive signal, the voltmeter or measuring device is required to have a very high input internal resistance, otherwise, the signal will leak through the measurement loop, and the measured signal will be greatly attenuated relative to the actual signal. The differential mode voltage is equal to the difference between the two input signal voltages, and the common mode voltage is equal to the average of the two input signal voltages; the differential mode voltage is generally a useful sensing signal, and the common mode voltage is generally caused by harmful temperature drift; give the highest possible amplification to the differential mode voltage, and give the highest possible suppression to the common mode signal.
low voltage electric motor, Ex motor, Motor manufacturers in China, three phase induction motor, Wolong Nanyang Explosion Protection