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Electricity as Fuel

To get started, think of electricity as fuel, which may be burned to produce light or heat. For our purposes, electricity is "burned" to produce a magnetic field that turns a motor.

Electricity is measured in terms of Voltage and Current. Think of voltage as the pressure of the electrical fuel, and current as a measure of the fuel's flow, or volume demanded. This is commonly measured in AMPS. Electricity is distributed and metered as Alternating Current (AC). Where a battery has 2 terminals, one that is always positive (+), and one that is alwaysnegative (-), AC voltage changes, or alternates, from positive (+) to negative (-) at a set frequency, usually 60 times a second (60 cycles).

An electric motor operates on the principle of one magnetic field chasing another. As the electrical polarity on the AC line changes (from + to -), the magnetic poles in the motor change from north to south in relation to the rotor poles, causing the motor to turn.

With each change in polarity the voltage rises and falls as a wave, with a brief period of no voltage, called a zero crossing. Each time the voltage rises--either above or below zero crossing--the motor receives power, much as a car is propelled by the engine firing (Fig. 1).

Why 3-Phase Is Better

Think of the spark plug on a running engine. If you took hold of that spark plug, you'd swear the electricity was a continuous flow rather than an intermittent spark. Single-phase AC power is a lot like that, and the flow of power--in mechanical terms--is more like a pulsating shower head than a garden hose running freely. This zero crossing shows up as a subtle but persistent power interruption in single-phase and is the reason that single-phase motors above 5 Hp are rare and expensive.

Power every 1/60th second sounds fairly often. But consider that a motor turning 1800 revolutions per minute gets only 2 power strokes per turn. This is the same as the crank on a bicycle or a 4-cylinder car engine--power every 1/2 revolution. To appreciate the difference between single and 3-phase, think of this flow of electrical current as a song. Each line, or stanza, has a pause at the end that represents the zero crossing (no music = no power). You hear single-phase as:

  Three blind mice (pause) Three blind mice (pause) See ...

By contrast, 3-phase is like the same song sung in rounds. Each voltage of 3-phase is displaced in time from the others, so now you hear:

  Three blind mice (pause) Three blind mice (pause) See ...

       Three blind mice (pause) Three-phase mice (pause ...

            Three-phase mice (pause) Three-phase mice ...

Notice how the power flow of each phase overlaps the dead space in the others. This overlap in power is the key to the smooth, continuous and universally adaptable power of three-phase.

Figure 1 - In an AC single-phase wave form the power passes through a positive and negative cycle 60 times a second. 3-phase has 3 such waves.