Why does a large-capacity motor have a shaft current?
At an academic exchange meeting in S City, several motor technology Dajia conducted in-depth and detailed discussions on the theory and suppression practice of large motor shaft current. There are many foreigners with high noses and blue eyes in the big gamma.
“Yes” and “No”, the academic atmosphere of the true and the rate is full of discussion. Ms. participated in the "Music Without Borders" relatively recognized, this time also appreciates the charm of academic borderless, the awareness of the axis current problem is undoubtedly a sublimation.
Axis current problem
During the operation of the motor, a potential difference, that is, the shaft voltage, appears at both ends of the shaft. In small motors, the shaft current is slightly unaffected, generally not noticeable; large motors are not, the shaft voltage is often high enough to break down the oil film with insulation in the bearing, generating shaft current, flowing through the shaft, bearing and bottom plate. The circuit produces electrical corrosion pitting or slab-like burn marks on the journals and bushings or overheats the rolling bearings.
Shaft current cause
The shaft current is caused by two factors: (1) non-uniform magnetic circuit, unipolar effect caused by asymmetry; (2) capacitive current.
●About the unipolar effect
The monopole effect is a characteristic description of a single pole motor. The so-called unipolar motor refers to a single polarity without a commutator when viewed from one end of the armature. The armature is a metal disc (as a radial conductor combination) or a cylinder (as an axial conductor combination). When the armature conductor rotates, it induces a constant radial or axial electromotive force in the armature conductor. It can be used as a low voltage, high current power supply device. In order to reduce the contact voltage drop and brush friction loss, a liquid metal collector is used. Figure 1 is a model diagram of a unipolar motor.
1 cylindrical copper rotor; 2 disc-shaped bracket, the armature is fixed on the rotor;
3 rotating shaft; 4 hollow cylindrical stator yoke; 5 two annular exciting coils;
6 ring carbon brushes listed at the ends of the armature.
