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| '''Three phase''' is a common method of ] ] in industrialised countries. It is a type of ]. | '''Three phase''' is a common method of ] ] in industrialised countries. It is a type of ]. | ||
| At the power station an ] converts mechanical power into a set of ] ]s, one from each electromagnetic coil or winding of the generator. The currents are ] functions of time, all at the same ] but with different ]s. In a three-phase system the phases are spaced equally, separated from each other by 120° (which is the maximum phase separation possible). The frequency is typically 50 ] in ] and 60 Hz in the ] (see ]). The three phases are typically labelled by colors, traditionally ''red'', ''yellow'' and ''blue''. | At the power station an ] converts mechanical power into a set of ] ]s, one from each electromagnetic coil or winding of the generator. The currents are ] functions of time, all at the same ] but with different ]s. In a three-phase system the phases are spaced equally, separated from each other by 120° (which is the maximum phase separation possible). The frequency is typically 50 ] in ] and 60 Hz in the ] (see ]). The three phases are typically labelled by colors, traditionally ''red'', ''yellow'' and ''blue''. Generators output volatge range from hundreds of volts up to over 20,000 volts. This voltage is usually "stepped-up" to a higher voltage with a transformer. The reason the voltage is increased is to reduce losses. Power essentially is equal to the product of voltage and current - so as you increase the voltage, you will reduce the current for a given value of power. Heating losses in a transmission line are proportional to the sqaure of the current so if you can halve the current in a line, you will reduce the losses by four. For this reason you will see transmission lines operating at level in excess of 500,000 Volts. | ||
| At the destination, a substation or transformer supplies the power stepped down from the high-voltage transmission line to three sinusoidally varying electric currents of 120 ] (in the US) or 230 V (in Europe) alternating current (VAC). This is then delivered to the customer's circuits at a master breaker panel through four conductors. One conductor is the neutral or ground at the power source, the other three lines or phases carrying electrical power to point destinations or supply transformers. Connecting an electrical circuit from ] to the neutral supplies 120 VAC (or 230 VAC) to the circuit. | At the destination, a substation or transformer supplies the power stepped down from the high-voltage transmission line to three sinusoidally varying electric currents of 120 ] (in the US) or 230 V (in Europe) alternating current (VAC). This is then delivered to the customer's circuits at a master breaker panel through four conductors. One conductor is the neutral or ground at the power source, the other three lines or phases carrying electrical power to point destinations or supply transformers. Connecting an electrical circuit from ] to the neutral supplies 120 VAC (or 230 VAC) to the circuit. | ||
Revision as of 22:51, 10 September 2004
Three phase is a common method of electric power transmission in industrialised countries. It is a type of polyphase system.
At the power station an electrical generator converts mechanical power into a set of alternating electric currents, one from each electromagnetic coil or winding of the generator. The currents are sinusoidal functions of time, all at the same frequency but with different phases. In a three-phase system the phases are spaced equally, separated from each other by 120° (which is the maximum phase separation possible). The frequency is typically 50 Hz in Europe and 60 Hz in the US (see List of countries with mains power plugs, voltages & frequencies). The three phases are typically labelled by colors, traditionally red, yellow and blue. Generators output volatge range from hundreds of volts up to over 20,000 volts. This voltage is usually "stepped-up" to a higher voltage with a transformer. The reason the voltage is increased is to reduce losses. Power essentially is equal to the product of voltage and current - so as you increase the voltage, you will reduce the current for a given value of power. Heating losses in a transmission line are proportional to the sqaure of the current so if you can halve the current in a line, you will reduce the losses by four. For this reason you will see transmission lines operating at level in excess of 500,000 Volts.
At the destination, a substation or transformer supplies the power stepped down from the high-voltage transmission line to three sinusoidally varying electric currents of 120 V (in the US) or 230 V (in Europe) alternating current (VAC). This is then delivered to the customer's circuits at a master breaker panel through four conductors. One conductor is the neutral or ground at the power source, the other three lines or phases carrying electrical power to point destinations or supply transformers. Connecting an electrical circuit from one phase to the neutral supplies 120 VAC (or 230 VAC) to the circuit.
The power transmission grid is organised so that each phase carries the same magnitude of current out of the power station; the currents returning from the customers' premises to the power station all share the neutral wire, but the three-phase system ensures that the sum of the returning currents is approximately zero.
Connecting between two phases provides √3 or 173% of the single-phase voltage (208 VAC in US; 400 VAC in Europe) because the out-of-phase waveforms add to provide a higher peak voltage in the resulting waveform. Such connection is referred to as a line to line connection and is usually done with a two pole circuit breaker. This kind of connection is typical of heaters, such as, for example, a 2kW, 208 volt baseboard heater.
All three phases are typically used in large industrial motors, or efficient air conditioners (e.g. most York units above 2.5 tons are available in 3 phase) as this is the most efficient way to transmit large amounts of electrical power. The greatest power demand is when starting the motor.
Some devices are made which create an imitation three-phase from single phase center-tapped power (240 volts AC in the United States; with phase separation of 180°). This is done by creating a third "subphase" between the two polarities, resulting in a phase separation of 180° - 90° = 90°. Many three-phase devices will run on this configuration, but at lower efficiency.
Sometimes single phase center-tapped 240 VAC is incorrectly referred to as "two-phase". It should be noted that a two-phase system is a system in which the two voltages are 90° out of phase. For example, if one is and the other is , where t is time, then you have a two phase system, also known as a quadrature system (one being referred to as the real part and the other being referred to as the imaginary part). A two phase system for 120 VAC line to neutral will measure approximately 169.7 VAC line to line. Two phase systems are seldom used because they require the same number of hookup wires as three phase (i.e. one for sine, one for cosine, and a common wire) delta connection, and the two phase system also does not balance the same amount of electricity in each of the three wires (although the cosine and sine are balanced, the neutral is not the same as the other two). A two-phase system is said to provide complex power and such systems are used at lower voltages (e.g. to run stepper motors, and the like) but not commonly distributed at high power levels.
and the other is 
, where t is time, then you have a two phase system, also known as a quadrature system (one being referred to as the real part and the other being referred to as the imaginary part). A two phase system for 120 VAC line to neutral will measure approximately 169.7 VAC line to line. Two phase systems are seldom used because they require the same number of hookup wires as three phase (i.e. one for sine, one for cosine, and a common wire) delta connection, and the two phase system also does not balance the same amount of electricity in each of the three wires (although the cosine and sine are balanced, the neutral is not the same as the other two). A two-phase system is said to provide complex power and such systems are used at lower voltages (e.g. to run stepper motors, and the like) but not commonly distributed at high power levels.
In particular, if we plot phasors of a two phase or three phase system around the unit circle in the complex plane, we have a form of complex power.
A single phase 240 VAC split phase (center-tapped) power system, when plotted as phasors on the complex plane, can exist entirely along the real axis. It is this lack of complex power capability that impairs its ability to create a rotating magnetic field, and it is the rotating magnetic field that makes motors run very efficiently. For heating, such power is fine, but for example, running an air conditioner, it is far better to use complex power.
How to test three-phase electrical supply
A three-phase electrical supply consists of three active conductors and an earth ground.
A three-phase induction motor cannot function correctly if its electrical supply is not within certain parameters.
Typical parameters are 208 or 415 volts between phases, 120 or 240 volts from any phase to earth or ground, voltage within 12% of nominal values, and each phase within 5% of each other.
In a typical three-phase induction motor circuit, an appropriate place to test is at the line side of the direct-on-line motor starter.
Figure 1:
A B C earth/
O O O ground
/ / /
/ / /
O O O
Tests should be made between A and B, A and C, B and C, A and earth, B and earth, and C and earth.
Note that listed voltages are for countries which use 120 or 240 volts only!
How to test three-phase pumps
Electricians may not often encounter three phase induction motors used in domestic watering systems.
Procedures to follow to field test these motors and their controls are listed:
Topics including testing motor coil resistance and testing earth fault resistance are covered.
For further information on three phase circuits see: