what is the difference between single phase voltage and three phase?

Answer 1

"Eddie", is this your name? I take it as your name.
Now eddie to answer this quest I need to know what electronic background do you have.If you are a child or
a layman I will say "single phase power is what we get at our residence & three phase power is supplied to industries." why is it so? Common sense will tell that three phase system is more efficient in power delivering. Now if you are a student studying electronics and have come across this doubt ,your doubt is solved as follows.
To say in simple language for a begginer like you I will not be using certain technical terms but a lucid language.
To begin with ,Phase has something to do with time.
A single phase line has a voltage which varies sinusoidally with time.At time t=o say the voltage is zero.
now say the max (peak) voltage is 240 volts. so as time advances voltage also does so but in a different way .
Example : when time is 2 sec, the voltage is 240sin(f*2) volts. Here f is the frequency of the voltage.
Now three phase voltage is consisting of three such above mentioned voltages which differ in phase by 120degrees. That is say if a voltage starts rising at time t=0,then the second voltage will start after the first has undergone a phase change of 120degree.same does the third voltage after the second has undergone a phase change of 120degree.This kind of voltage line requires
4 cables. Three of which supply voltages in three phases as written above and the fourth is a neutral cable. All the three phases are measured with respect to this neutral cable.These voltages are used in industries for running machineries which have differential induction coils.Meaning each coil requires a voltage which should be lagging behind the other.
Now eddie if you are not satisfied with this answer ,then
you should refer to a website "How stuff works", or certain books on "Basic Electronics"

Answer 2

Electricity in our home is single phase which is one of the phases of three phase system.
In single phase,current in live wire is always changing its polarity with respect to neutral and is called cycle.If your country use 50Hz, changing will take place 50 times in every second.So the time for one cycle is 1/50 = 0.02 sec or 20 mSec.During this time live wire current will cycle once from positive to negative .We can say that it has 10 mSec in positive and another 10 mSec in negative and so on.
Now in three phase system we have to share this 20 mSec for all three phase.If the first phase start its postive at 0 sec then second phase will start its positive at 20/3 mSec or 6.67 mSec and third phase will start at 13.33 mSec and first phase will start again at 20 mSec and so on.
Three phase system is more efficient.
Starting winding is not required for three phase induction motors, due to phase difference in windings, rotating magnetic field is already produced for starting.Thus reducing the size of motors.
In single phase, induction motors require starting winding and capacitor to produce two phase current which then produce starting torque to run the motor in required direction.
Hope this will help.