Three-phase power

Electric Power Distribution

The backbone of every modern electric power distribution system is a three-phase alternating current (AC) transmission line. It consists of three primary current-carrying wires sometimes referred to as L1, L2, and L3 and in some cases a fourth wire called the neutral conductor. Single-phase distribution systems are also common because single-phase transmission lines costs significantly less than three-phase lines. They consist of one high-voltage line and a neutral. Most residential and rural areas are supplied with single-phase service. Three-phase power cannot be supplied from single-phase service unless a phase converter is used. Single-phase power is a single voltage that alternates between a positive voltage and a negative voltage for a specific number of times per second (in the U.S., 60 times per second or 60 Hz). Three-phase power is three distinct AC voltages, each shifted in time 120 degrees relative to one another as depicted in figure 1. The wave forms shown in figure 1 can be calculated using the sine function in trigonometry and are called sine waves. Notice that the voltage between L2 and neutral (L2-N) is delayed by 1/3 of a cycle from the L1-N voltage, and that the L3-N voltage is displaced 1/3 of a cycle from the L2-N voltage. A complete cycle of the one of the wave forms corresponds to one complete rotation around a circle or 360 degrees. The phase delay in the L2 and L3 voltages is often referred to as 1/3 times 360 degrees or 120 degrees for L2 and 240 degrees for the L3 voltage. Customers are supplied with electricity from the distribution system by placing transformers on the high voltage distribution system to reduce voltage to a level compatible with electric devices, for example, 240 volts. Three-phase service requires three transformers compared to one for single-phase service, and requires different metering equipment as well. Because of this, three-phase service costs more to install, so utilities usually prefer to install single-phase service unless there is a specific demand for three-phase power at the site.

Three-phase motors

Motors that convert electrical energy to mechanical energy comprise two-thirds of the industrial demand for electricity. Most of these motors are three-phase squirrel-cage induction motors which consist of an arrangement of coils wound in slots in a stack of iron laminations shown in cross section in figure 1 below. This part of the motor is stationary and is called the stator. The coils in the stator are connected in a manner to produce at least three separate windings which are at angles of 120 degrees with respect to each other. This is shown schematically in figure 1. If a set of three-phase voltages is applied to the windings shown, a magnetic field will be produced in the center portion of the stator. This magnetic field is constant in magnitude, and rotates at the frequency of the applied voltages (either 50 or 60 Hz depending on what country you're in). The second part of the motor (the rotor) is a set of round iron laminations that have been attached to a shaft with bearings. There are slots in this set of laminations as well. In this instance the slots are filled with very low resistance bars of aluminum that are shorted together at the outer edges of the laminated stack of iron. If the rotor is inserted into the center part of the stator, the magnetic field generated by the stator will cross through the shorting bars of the rotor causing a large current in the rotor. These rotor currents react with the magnetic field generated by the stator and cause the rotor to spin. The rotor will continue to accelerate until the shaft rotation speed is nearly equal to the velocity at which the magnetic field of the stator is spinning. The important point here is that if the stator had only a single coil driven by a single-phase voltage, then the magnetic field generated by the stator would not rotate — it could for example point either up or down, but not left or right. The motor could never start because there would be no rotational component of the magnetic field. Thus a three-phase system allows the mechanical energy being fed into the generator to be transferred to three-phase induction motors very efficiently. Three-phase motors also have the advantage of being very simple and reliable--there are no electrical switches contained in them. If they are not overheated, the only thing that wears out is the bearings, which are replaceable.