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# What is the advantage of a three phase power supply to a single phase power supply?

Wiki User

2015-12-16 01:16:54

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Single phaseThe main advantage of 3 phase is that it is more efficient for running AC motors than one or two phase. Three phase was designed to be used for creating a rotating magnetic field in the motor, which starts rotation of the motor. Household service originally was intended for lighting and heating, while 3 phase was intended for industrial and commercial with large motors.

In US residential single phase service you have two hots, in phase with each other.

• Vhot1(t) = Vmax * sin(2 * pi * freq * t)
• Vhot2(t) = Vmax * sin(2 * pi * freq * t + pi)
Where:
• Vmax equals the maximum voltage to neutral. (Approx. 170V (120V * sqrt(2)) for US residential service)
• freq is the frequency of the line in Hz (60 for US residential service)
• t is the time in seconds
So:
• Between either hot and neutral you have 120Vrms to neutral.
• Between the two hots (Vhot1(t) - Vhot2(t)) you have 240Vrms
• The voltage goes to 0V 120 times a second, no matter what wiring you choose.
• You cannot get 3 phase from single phase without a special converter.
3 phase

In a 3 phase system you have 3 hots, out of phase with each other.

• Vhot1(t) = Vmax * sin(2 * pi * freq * t)
• Vhot2(t) = Vmax * sin(2 * pi * freq * t + 2 * pi / 3)
• Vhot3(t) = Vmax * sin(2 * pi * freq * t + 4 * pi / 3)
Where:
• Vmax equals the maximum voltage to neutral. (Approx 170V (120V * sqrt(2)) for 120/208V 3 phase)
• freq is the frequency of the line in Hz (60 for US service)
• t is the time in seconds
So:
• Between any hot and neutral you have 120Vrms to neutral. Since you are only using two conductors in this circuit, it is identical to 120Vrms single phase above. It can be used to power the same equipment that would be used on 120Vrms single phase power.
• Between any two hots you have 208Vrms. Again, this is the same as above, though the voltage is less than single phase due to the phase shift. This can be used like 240V single phase above, if the load will tolerate the slightly lower voltage.
• For heavy loads, all 3 hots can be used. Unlike single phase, one of the hots is always energized. The load will always have at least 140V (off a 208Vrms supply) between two hots. This makes it valuable for motors, as there is always a magnetic field present to pull the armature.
• You can easily get single phase power from 3 phase with no converters.
• 120/208V 3 phase service has 1.5 times the power capacity of equally rated 120/240V single phase.

From Wikipedia:

• A three-phase motor will be more compact and less costly than a single-phase motor of the same voltage class and rating
• Single-phase AC motors above 10 HP (7.5 kW) are uncommon
• Three phase motors will also vibrate less and hence last longer than single phase motor of the same power used under the same conditions.
To sum up
• Single phase is used in residential service. The split phase is used to power large loads that are too large to run off 120V. Otherwise, 120V single phase is used.
• US consumer appliances are single phase, so there is no incentive to run 3 phase to homes.
• 3 phase is usually only used in industrial settings with heavy equipment or motorized tools
• Unlike residential service which is always 120/240V, industrial 3 phase can be much higher, with 480V not uncommon. The designer of the building is expected to design the electrical system for the use of the building. It is not uncommon for these building to have their own step-down transformer to 120/208V for lower voltage loads.

Wiki User

2015-12-16 01:16:54
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