Why is 3ph better?

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Forum topic by Timberwerkz posted 03-12-2009 08:00 PM 8267 views 0 times favorited 5 replies Add to Favorites Watch
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20 posts in 3489 days

03-12-2009 08:00 PM

What makes 3ph better than regular singular phase power?

5 replies so far

View PurpLev's profile


8536 posts in 3671 days

#1 posted 03-12-2009 08:25 PM

the extra 2 phases…

in all seriousness though 3 phase circuits are more efficient then single phase circuits and have a more balanced load on them (less ups and downs). Also single phase motors need to have a separate starter-motor to get the main motor up to speed, while 3 phase motors don’t – thats why on single phase motors you’ll have a major electrical overload when you start up a machine (lights may dim…etc) while 3ph machines will not have the same behavior, this also means that 3 phase motors require less parts, and are less expensive to make

less expensive parts and motors, and a more efficient load balance on the circuits are the main advantages of 3 phase power over single phase.

-- ㊍ When in doubt - There is no doubt - Go the safer route.

View 8iowa's profile


1580 posts in 3784 days

#2 posted 03-12-2009 08:35 PM

Actually, for fractional and low HP motors there isn’t a great deal of practical difference. A three phase motor will have a somewhat higher efficiency, and is easily reversible by interchanging two of the three leads. Also, as far as I know, there aren’t any three phase motors wound for 120V. The vast majority are 240V or 480V. The advantage of higher voltage being lower amperage.

Single phase motors have the additional complexity of having a starting winding, since there is no torque available at the initial energizing. Once the motor comes up to speed, a centrifugal switch cuts out the start winding. When you turn a single phase motor off, if you listened carefully, you can usually hear the centrifugal switch engage again when the rpms drop to a low level. Many single phase motors must have a capacitor, connected to the starting winding, to give extra “juice” for starting the motor against a load. Single phase motors are almost always unidirectional, so when you order one, you must know whether you need cw or ccw rotation.

-- "Heaven is North of the Bridge"

View marcb's profile


768 posts in 3696 days

#3 posted 03-12-2009 08:49 PM

Mechanically 3 Phase has less inherent vibration. Single phase motors have a 120HZ vibration inherent in the system because of the zero crossings on the sine wave.

120V AC sine wave

3 phase has 3 sine waves working in unison.
3 phase image

Notice that when 1 wave crosses 0 you have 1 other on positive and 1 wave on negative. You never have no power.

Because of this 3 phase motors also have higher torque, a single phase motor does not have any power at the 0 crossing. Its like turning the motor off 120 times a second.

View CharlieM1958's profile


16275 posts in 4241 days

#4 posted 03-12-2009 08:53 PM

Pretty cool explanation and graphics. Even a liberal arts major like me can understand it. :-)

-- Charlie M. "Woodworking - patience = firewood"

View Timberwerkz's profile


20 posts in 3489 days

#5 posted 03-12-2009 10:25 PM

Thank you for your help. All explanations are understandable, which I appreciate

Thanks again Chris

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