Shaft Currents in Motors and Generators

The magnetic field in a motor or a generator is ideally conducted along paths that are symmetrical. However, sometimes the magnetic fields within an electric machine such as a motor or a generator is asymmetrical. This asymmetry can be caused usually by variation in the quality of the steel used in the motor construction or in some cases to shaft eccentricity.

These asymmetrical magnetic fields which are varying over time can induce currents in the shaft of the motor or generator. These currents which are induced in the shaft tend to flow from one end of the shaft to the other through the bearings and then through the earth.

When these currents flow through the bearings, the tend to cause arcing and consequent pitting in the bearings. This can lead to failure of the bearings.

Shaft currents can be prevented by insulating one of the bearings. A Teflon layer is usually placed between the shell of the bearing and the bearing housing. This ensures that the shaft voltage induced does not have a return path. This prevents shaft currents from flowing.

Flame Proof Motors

Flame Proof motors are used in hazardous locations usually in chemical, petroleum and gas industries, where the environment may contain gases or inflammable vapours. Electric equipment are notorious for being the source of sparks during switching. This could be catastrophic in areas where inflammable gases are present.

Hazardous areas classified into zones depending on the possibility of the presence of gases and combustible dust-air mixtures.

The following are the classification for hazardous areas

Zone 0, An Explosive atmosphere is continuously

Zone 1, An Explosive atmosphere is present for 1000 hours in a year.

Zone 2, An Explosive atmosphere is present for 10 hours in a year.

Flameproof motors are specially designed so that any explosion which occurs within the motor is contained within the motor frame itself. This prevents any secondary explosion in the surrounding area. The flame-proof motor is specially enclosed within a protective shield which covers the entire motor including the bearings.

Flame proof motors are heavier that motors of similar rating due to the increased weight of the reinforced design of the end shields so as to withstand the impact of an explosion.

Flame proof motors have special arrangements for cooling. Since conventional air circulation is not possible with these motors, special arrangements such as using the internal air to carry heat to the surface of the motor where the heat is transferred to the external air by means of special tubes have been devised.

Another method of cooling is the use of a coolant such as water which circulate in the stator and rotor of the motor. This water is brought to the near the inside of surface of the motor where it is cooled by means of the atmospheric air.

The cooling system ensures the isolation of the internal cooling medium from the external atmospheric air.

Can Single Phase motors be reversed by changing the polarity?

Single phase motors have two windings, the main winding and the auxilliary winding. The auxiliary winding is used to start the motor and may be disconnected once the motor picks up sufficient speed.

Reversing a single phase motors cannot be done by reversing the polarity of the supply to the entire motor. To reverse the single phase motor, the polarity of the supply to only one of the windings needs to be changed.

This can be done by reconfiguring special links which may be provided in the terminal box of the motor.

Why are Power Cables Shielded?

Shielding in Power Cables involves covering the insulation of a single cable or a group of cables by a conducting material usually a sheath which is grounded.

The objective of a shield is to ensure that the cable insulation is subjected to a uniform electric stress. The shielding also prevents transient overvoltages which are induced along the cable by ensuring a eliminating surge potentials and ensuring a uniform surge impedance.

Shielding also protects personnel from dangerous shocks which may be caused by intense electric fields.

The Energy Star Classification

The Energy Star Classification is a system of classification introduced by the United States Environmental Protection Agency in 1992. The Program aims at reducing energy consumption by developing more efficient electric gadgets and devices.

The program has been adopted by other countries such as New Zealand, Japan, Taiwan, the European Union and Australia.

The labelling program is voluntary. Devices which bear the Energy Star Logo consume about 20-30% less energy that normal ones.

The Energy Star labelling System is now used in numerous electric appliances such as heaters, televisions, computer monitors, lighting and even LED traffic lights.

Energy Star certified refrigerators use about 20% less power than the minimum standard. Fluorescent lighting which is standardized by Energy Star use about 75% less energy than conventional incandescent lighting.

The Energy Star standard is also being extended to commercial building which use less energy and to industrial facilities.

What are Zero Watt Bulbs ?

A Zero Watts Bulb is a bulb with low light output.  It is frequently used as a night lamp and other applications requiring low intensity lights such as.  It is called a zero watts bulb as it consumes relatively low power, around 15 W.

When all the appliances were switched off at night and only the zero watts bulb was switched on, the power consumed in the house was too small to be measured by the older electro-magnetic meters.  Hence, the moniker "zero watts".

Zero watts bulbs are increasingly being replaced by more efficient CFL bulbs which are also last longer.

Triplen Harmonics - An Overview

Triplen Harmonics are harmonics whose orders are multiples of three.  Thus the 3rd, 6th and 9th Harmonics are known as Triplen Harmonics.  These harmonics flow through the neutral conductor of Star connected transformers, causing heating.  This heating also affects cables and causes nuisance trippings of relays.

These are usually caused by power supply units for electronic appliances.  The power supply units are usually connected to only one phase.

Triplen Harmonics can be avoided by use of the delta connection for transformers and three phase loads.  Increasing the sizing of the neutral conductor is also a means of attenuating the effects of these harmonics.