Polyphase Circuits: Purpose & Benefits

[ranju]

Apart from single phase circuits , we use two phase , 3 phase mostly .. but I want to know what actually we mean by 2 phase , 3 phase circuits..!
I know in single phase supply , there is one voltage , one current .. 3 phase has 3 currents and voltages.! What is the purpose for these polyphase supply..?.

 

[sparkey]

Short answer, one "hot" wire per phase.

Long answer:
For single and two phase, think house wiring for simplicity sake. Your home power is typically called single phase because most of your stuff all runs off single phase 120Vac. Now, if you have any 240Vac appliances (washing machine, water heater, etc) you have two "hot" legs, one black and one red. The red and black are wired in the panel are wired to different buss' which are tied to separate phases coming from the power company. In a three phase service, add one more service feed conductor and buss to the panel. The practical application of this is that each phase supplied has a sine wave that is deliberately out of sync with the other. This becomes useful when applying motor loads, among other things.

 

[zoki85]

Some advantages of polyphase systems:

• Possibility of using one return conductor in m-phase system gives technical and economical benefit over using m return conductors in m single phase circuits
• If ∑E_k= 0 and ΣI_k=0, polyphase system is symmetric balanced, return conductor is not needed. For m≥3, the system has constant power at any instant of time: ∑p_k= const., which gives benefit in various respects like reducing costs of making AC induction machiones working on principle of rotating magnetic field, giving mechanical power/torque without pulsations etc

 

[ranju]

if you have any 240Vac appliances (washing machine, water heater, etc) you have two "hot" legs, one black and one red. The red and black are wired in the panel are wired to different buss'

But such appliances are having 3 wires..!

 

[sparkey]

Said appliances should actually have four wires running to them through the cable, but only two of them are the power supply conductors. Black and red would be the "hot" legs, white is the neutral, and green/bare would be the equipment grounding conductor. The phase doesn't refer to the amount of wires, only the amount of supply lines. See linked drawings.

Single Phase
Three Phase

 

[Averagesupernova]

In the USA 240 volt appliances will have 3 prongs. One ground and 2 hots. A neutral is required if any part of the appliance runs on 120 volts. Electric clothes dryers would fall under this category. Water heaters however do not use a neutral.