Title: Reading and Audio Visual Assignment
Course: WGF 107 DEVELOPMENT > Section: Content

Sub-Title | Understanding Three-phase Voltage System Fundamentals
Content
In a three-phase system, three circuit conductors carry three alternating currents (of the same frequency) which reach their instantaneous peak values at one third of a cycle from each other. Taking one current as the reference, the other two currents are delayed in time by one third and two thirds of one cycle of the electric current. This delay between phases has the effect of giving constant power transfer over each cycle of the current and also makes it possible to produce a rotating magnetic field in an electric motor.

The sum of the currents is always zero and each line returns the current from the other two. Thus a three-phase system can operate with only three wires. ^[3] Three-phase systems may also have a fourth wire, particularly in low-voltage distribution, which is the neutral wire. The neutral allows three separate single-phase supplies to be provided at a constant voltage and is commonly used for supplying groups of domestic properties which are each single-phase loads. The connections are arranged so that as far as possible in each group equal power is drawn from each phase. Further up the supply chain in high-voltage distribution the currents are usually well balanced and it is therefore normal to omit the neutral wire.

Three-phase has properties that make it very desirable in electric power systems:

- The phase currents tend to cancel out one another, summing to zero in the case of a linear balanced load. This makes it possible to reduce the size of the neutral conductor because it carries little to no current; all the phase conductors carry the same current and so can be the same size, for a balanced load.
- Power transfer into a linear balanced load is constant, which helps to reduce generator and motor vibrations.
- Three-phase systems can produce a rotating magnetic field with a specified direction and constant magnitude, which simplifies the design of electric motors.

Most household loads are single-phase. In North American residences, three-phase power might feed a multiple-unit apartment block, but the household loads are connected only as single phase. In lower-density areas, only a single phase might be used for distribution. Some large European appliances may be powered by three-phase power, such as electric stoves and clothes dryers.

Wiring for the three phases is typically identified by color codes which vary by country. Connection of the phases in the right order is required to ensure the intended direction of rotation of three-phase motors. For example, pumps and fans may not work in reverse. Maintaining the identity of phases is required if there is any possibility two sources can be connected at the same time; a direct interconnection between two different phases is a short-circuit.
Image | A transformer for a high-leg delta system; 200-V/3-phase motors would be connected to L1, L2 and L3. 200 V Single-phase load would be connected L1 and L2. Single phase 100 V load between either L1 or L2 and neutral (N). L3 (wild or high leg) will be 173.2 V to neutral.
Content | Three-phase Induction Motor An important class of three-phase load is the electric motor. A three-phase induction motor has a simple design, inherently high starting torque and high efficiency. Such motors are applied in industry for many applications. A three-phase motor is more compact and less costly than a single-phase motor of the same voltage class and rating and single-phase AC motors above 10 HP (7.5 kW) are uncommon.^[^citation needed^] Three-phase motors also vibrate less and hence last longer than single-phase motors of the same power used under the same conditions.^[^citation needed^] Line frequency flicker in light can be reduced by evenly spreading three phases across line frequency operated light sources so that illuminated area is provided light from all three phases. The effect of line frequency flicker is detrimental to super slow motion cameras used in sports event broadcasting. Three phase lighting has been applied successfully at the 2008 Beijing Olympics to provide consistent light level for each frame for SSM cameras.^[16] Resistance heating loads such as electric boilers or space heating may be connected to three-phase systems. Electric lighting may also be similarly connected.
Links/Uploads | Three-phase Electric Power – Wikipedia – The Free Internet Encyclopedia                              http://en.wikipedia.org/wiki/Three-phase_electric_power#Single-phase_loads
Attributions | Images and content courtesy of Wikipedia – The Free Internet Encyclopdia