OT... All you electrical experts - "Single phase"?

I've read alot of the other posts... and now I'd like to get some confirmation that what I'm thinking is correct...

Power company makes three phase power. I get one of those three phases running to my house. Transformer on pole steps down to "single phase" at 230V (w/ center tapped ground).

Here's where it gets a little fuzzy for my not-so-electrical mind...
If "L1" to ground voltage was displayed simultaneously on an oscilloscope with "L2" to ground - would they in fact be 180 degrees out of sequence? If so - would that really make it two phase?

Yup and nope.

Yup the two voltage traces you describe would be 180 degrees out.

Nope it's still single phase. The line trasformer up on the pole has a center tapped secondary. The centertap is the neutral and the two legs form the 230 volt line connections.

Two phase (used to be a common distribution standard before WW II) has the phases 90 degrees out. Mix and match the four leads coming from a two phase motor to the line and the motor shaft runs fwd or reverse accordingly.

Three phase has the power 120 degrees out of phase. Interchanging any two leads from a three phase motor reverses the shaft rotation. Except for a few museum pieces all electrical power distribution in the US and Canada is three phase. The single phase is split off for residential services at the neighborhood level.

Single phase induction motors are not intrinsically self starting. They'll just sit there and hum - and soon smoke. There has to be a gimmick to make them start; like shading poles, capacitors and start switches, a wound rotor and a centrifugal shorting ring, or something. Once started a single phase induction motor provides its own phase shift (sorta) and develops useful power.

If you have three 11 KV line up on the pole or have a local three phase opendelta feed to a nearby sanitary or potable water pumping station pumping station, you might be abel to bring in three phase to your shop but I wouldn't reccommend it unless you have 20 HP equipment you wish to run to ratings.

[This message has been edited by Forrest Addy (edited 01-24-2003).]

Thanks - It's all starting to make a little more sense now.

The 90 degree phase shift on two phase would explain the need for four wires in that setup (I think).

The 180 degree voltage shift on single phase explains how 1+1 magically becomes 3 phases with a phase converter.

Joe,

You're right on. In the days when two phase (also called quarter phase) was in use some electricians attempted to make the four wire system into a three wire system; it sort of worked -- very badly.

FYI, there's a quite wierd transformer called a Scott connection which converts two phase to three phase and vice versa. These were used alot in the 1950s to serve older two phase industrial applications.

bnelson

The Scott-connected transformer is alive and well.

Many three-phase distribution transformers presently being installed are T-connected.

You will see these indicated as 120T208 or 277T480 or 346T600, among other choices.

Historically, T-connected distribution transformers were used by existing two-phase-based utilities (primarily those in the Buffalo and Philadelphia areas) to supply their new three-phase customers, and by new (or recently converted) three-phase-based utilities to supply their existing two-phase customers.

However, a T-connection transformer bank can also be used to transform two-phase to two-phase and three-phase to three-phase.

It is the second case which is quite popular today.

The benefits of a T-connected three-phase to three-phase transformer, over an Open Delta-connected transformer (which uses the same number of transformers), include better regulation *and* lower "per unit" impedance, which are significant advantages for both the utility and the customer.

A T-connected distribution transformer, intended for three-phase service, can provide a neutral in the secondary (customer side) but cannot provide a neutral in the primary side.

Therefore, these T-connected transformers (with a neutral) are usually employed in cases where the primary is a Delta and the secondary is simulating a Wye.

T-connected distribution transformers may not be a good choice where the load includes certain types of technical equipment, including CNC machinery and solid-state broadcast transmitters, both of which can be sensitive to line-to-line variations.

Peterh,

Thanks for the info. I've never had the opportunity to be involved with a Scott transformer. Hope to some day.

My second post on this forum, "Large converter questions??", outlines the problems of the open delta connection for machine shops. Bottom line is you don't want to go this route if running CNC.

bnelson

If your converter can maintain +/- 10 percent on phase B ( the"manufactured" phase) under all load conditions, you should have no problem with CNC equipment.

+/- 5 percent is better, however.