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Steve Aiken
04-07-2005, 11:00 AM
I've never really understood 240V single phase, especially in the multi-wire circuit. The current in the 2 legs don't really 'cancel each other out' does it? The current from each leg still has to get back to the panel then to the transformer, right? In a multi-wire circuit with different current in each of the 2 legs, the neutral carries the difference because not all the current can make it's way back through the hot legs.

Am I out-to-lunch on this?Steve

Ken Fitzgerald
04-07-2005, 11:25 AM
Steve.....I was at a loss on this too but....

With 240vac...imagine a transformer that is centertapped. If you measure from one end of the transformer to the other end of the transformer you have 240vac. If you measure from either end to the center tap you have 120 vac.

When you wire your t/s for instance for 220/240, the current flows from one leg through the motor to the other leg. No current flows through the ground or 3rd wire. It's there for safety.

When you run your t/s on 120 current flows from one leg (one end of the transformer) to the neutral. The 3rd wire is ground and is there to ground your t/s and for safety.

Normally in you breaker panel depending on local electrical code, the ground bus and the neutral bus are "bonded" or screwed together and thus are at the same potential.

There is more confusion injected because in the "old days" it was 220/110 and now it's referred to as 120/240. The transmission lines have been improved and we are getting more voltage today.

Only real advantage IMHO of 240/220 operation is more starting potential at 220 versus 110. Therefore, you saw should achieve maximum speed faster and if it incurs a heavier load ( tight spot or tougher wood) it should recover or achieve it's normal speed quicker. There's been arguments about it being cooler but the same amount of current flows through each of the motor windings regardless of how the motor is wired

220/240 is referred to as "single phase" because from one end of the transformer to the other is a single phase.

That's my understanding ...could be wrong.

Ken Fitzgerald
04-07-2005, 11:32 AM
Additionally...as Chris Padilla did in wiring his shop's 120 circuits ......there is the Edison method. You run one cable say....10/3 which had (4) wires in it. 2 hot legs, neutral, and ground. Each hot leg is wired to alternate outlets as you go down a wall and go to separate breakers and therefore you have 2 circuits in one cable. The neutral and ground are common to both circuits. The problem he encountered was a GFCI measures the current difference between a hot leg and neutral and if it sees a enough difference it trips because it thinks the additional current is going from the hot leg to the ground wire and that can kill you too! He ended up pulling an additional wire to provide separate neutrals for each of the 2 120 circuits......I considered the same in my shop but I thought the GFCI's might not work so I wired 2 separate circuits and cables down each wall and each adjacent outlet is on the "other" circuit and an individual GFCI protects each circuit.....Clear as Mud? :confused: :D

Chris Padilla
04-07-2005, 11:34 AM
achieve it's normal speed quicker. There's been arguments about it being cooler but the same amount of current flows through each of the motor windings regardless of how the motor is wired

Ken,

It isn't cooler in the motor but in the wire feeding the motor since the current is halved. That's it.

Ken Fitzgerald
04-07-2005, 11:37 AM
I'll go along with that!