Have you heard of stray voltages? These take place on revolving shafts in magnitude starting from micro-volts to numerous other volts. These voltages are generated due to rotation of shaft in the magnetic field of the earth or born out of electromagnetic communication signal induction. Grounding currents can be provoked by shaft rotation that is linked to asymmetric magnetism of any electrical machinery. This can take place due to the residual effect of magnetism in a shaft and by induction that is caused by switching on electronics, brushes that carry electric current and exciters.
Shaft voltages can either be your friend or foe. If they are your friend, they will warn about the development of problem quite long before the issue becomes apparent on instruments. When they become a foe, they can generate circulating currents, reduce the efficiency of the unit, the current that is generated can damage the seals, bearings, couplings and gears. Controlling shaft voltages can bring down the potential for sheer damage. This can be passively controlled by just keeping the grounding brushes. If you wish to do it actively, you can inject countering current signals in the rotor.
Let us talk about ESP in brief. Electrostatic precipitators bring down particulate emissions from sintering operation, blast furnaces and are also used for fly ash control. All ESPs are designed in such a way so that they can collect particles of diameters of 0.1?m to 10?m. ESPs can handle huge volumes of exhaust gas between 175 and 700C.
Shaft grounding is a very important concern. This term was highly used few years ago from OEM of motor. But now there are companies that supply carbon bushes and also brush holders that are used in shaft grounding. Here are some uses:
? In turbo generators
? In AC and DC motors and also used as special application
? In ships
Take a look at the factors that cause shaft voltage:
? When there is an asymmetry in the magnetic circuit of any rotating electrical machine.
? Static charges build-up within the shaft.
? Due capacitive coupling of voltages in static systems.
If the current passes through the bearings, it might cause a rise in current densities on tiny contact points in the bearing. This can result in a melting of metals and this boosts the formation of craters and serrations. The ideal shaft voltage of 500mV is best suitable for rolling bearing.
Due to high frequency ground currents there can be sparks, electrical shocks, arcing and all these damage the bearings. So, some of the techniques to resolve this issue includes using shaft insulators, Faraday shield, alternate discharge paths, ceramic bearings, insulated bearings, shaft grounding ring and grounding bush.
