# Single phase to three phase

1. Oct 14, 2004

### greatEE

what are the major different other than the reason why you do three phase for motor and advantages. In detail in wiring why do you have 4 lines in three phase goig into three terminal and 4 lines goign into two termnial in single phases?

what is the best way to give a summary to a person who doesn't know the difference? Like people who are not familiar with electrical stuff at all?

2. Oct 14, 2004

### megashawn

well, by no means am I a pro, but from what I understand a 3 phase motor outputs more torque. By having the extra wires your talking about, this is similar to having more pistons in an engine.

But they also require more energy to operate.

3. Oct 18, 2004

### don rigby

*single phase motors require 3 conductors 1/ a ground or bond (as in all motors for safety) 2/ two power conductors, or a power conductor and a neutral conductor [from a source: motor starter/motor switch (internal connections may be confusing you)].
a single phase motor requires a start winding circuit that gives initial torque and direction until the motor speed is such that the the rotating field of the run winding is stable enough to maintain the rated horse power of the motor, at which time the start winding is released by way (usually) of a centrifugal switch.
*a three phase motor requires 4 conductors 1/ a ground or bond (as in all motors for safety) 2/three power conductors each of course providing ac voltage, and each 120 degrees out of phase with each other [consider a complete circle as 360 degrees divided by three (it is more complicated than this but do your reading)](a y configured three phase service has the three conductors described herin plus a neutral that is never required in the operation of a three phase motor as in your question)
due to the fact that we are utilizing three voltages 120 degrees out of phase from each other, we can see that a rotating field is much more easily satisfied (one might say, we already inherently have it), and therefore do not require a start winding of any sort.
so, we place this rotating field around a big chunk of ferrous metal (much more complicated than this ie: windings, eddie currents, losses... do your reading!) and we get a more reliable (no contacts), better electrically balenced (cost efficient, no a to gnd) ... do your reading!

Last edited: Oct 18, 2004
4. Oct 18, 2004

### GENIERE

I think that implies it would also be better mechanically balanced?

5. Oct 21, 2004

### don rigby

"I think that implies it would also be better mechanically balanced?"
that would be a question better put to a motor manufacturer(obviously easier).
however, from what i have seen...i could send a block of wood to a good electric motor repair shop for rotational balencing (if they were given solid points of reference).balencing equipment today, leaves little to be desired under most common rpm requirements.

Last edited: Oct 21, 2004
6. Oct 21, 2004

### Averagesupernova

I wouldn't say a three phase motor requires more energy to operate. They are more efficient than single phase motors. They are typically lighter in weight per horsepower, cheaper to manufacture and longer lived. You can buy three phase motors of the same horsepower cheaper than single phase. The current draw on three phase motors per conductor is less than a single phase. In some cases you can use smaller wires feeding the motor with 3 phase compared to single phase because of this difference.

7. Oct 22, 2004

### Ivan Seeking

Staff Emeritus
http://www.du.edu/~jcalvert/tech/threeph.htm

Note also that 3 phase allows for inherently reversible motors - reverse any two hot legs. Although there are single phase reversible motors as well now, they can get lost [out of phase] and go the wrong direction.

8. Oct 23, 2004

### don rigby

"In some cases you can use smaller wires feeding the motor with 3 phase compared to single phase because of this difference."
in all cases above 15 amps, by a factor of root 3. only because 15 amp wire is the smallest acceptable (except control wiring).

9. Oct 23, 2004

### don rigby

"Note also that 3 phase allows for inherently reversible motors - reverse any two hot legs. Although there are single phase reversible motors as well now, they can get lost [out of phase] and go the wrong direction."
i think what you meant to say is:
on a single phase motor, if the start circuit fails to a certian degree or completely, respectively, the motor will;
a/ either not come up to full rpm and then, thermal and/or over current protection opens the power circuit., or b/ just hum loudly until, thermal and/or over current protection opens the power circuit.
not to say that there wasn't ever a case in history where a single phase motor started in reverse rotation, but i doubt it. once wired, it is pretty much committed.

Last edited: Oct 23, 2004
10. Jun 14, 2009

### darkwood

Your electrical supply is made using a generator which supplies 3 phases out of phase with each other by 120 degrees, because a motor turns electrical energy into mechanical energy its just simply putting it a generator working in reverse thus its not too expensive to manafacture 3 phase motors and because the 3 phases are 120 degree out of step with each other the motor can run from power up using the existing magnetic fields already out of step.
Single phase motors dont have a natural out of step phase angle on the supplies so just connecting it to single phase supply would just melt the internal wiring without the motor turning to achieve the out of phase magnetic field single phase motors require a out of step field generated from the one phase this can be achieved in various ways e.g. capacitor start motors but ultimately it adds cost to design and manufacture.
The main advantage of a 3 phase motor is its balanced loading over the three phase which means their is no need for a neutral and is cost effective to run compared to a single phase motor and due to the higher voltage used in comparison to a single phase you can have a smaller motor doing the same work load.

11. Jun 14, 2009

### Staff: Mentor

This thread is 5 years old...

12. Jun 14, 2009

### darkwood

Thanks for the info', im new to this site just finding my feet.