Why 240?

[Kelly C. Hanna]

I thought the difference in amps was only because the power was spread over two legs when 240 was used...hence instead of 15 amps, it would be 7.5 PER LEG. Correct me if I am wrong, but the difference can be felt at my house. Less bogging down on the old contractor saw I had and quicker start time.

[Ken Fitzgerald]

Kelly you are correct.

As stated by someone on this thread earlier...this is a subject that gets beat to death regularly! It's complex and it's easy to overlook a point in trying to make it work in theory and practically. And I'm sure Chis P., Rob and Ken G. will tell you that theory and practice don't alway agree.

Given that all things are equal (resistance in each leg) the current is halved when going from 120 to 240 with equal currents in both legs.

I just spent some time going to several manufacturers websites looking at schematics for table saws that will run 120/240. I wanted to verify that the motors were, in fact, wired for 240 and using 240 and that 240 was, in deed, being placed across the motor windings ....DUH or DOH! It is.

With that in mind and the points Ken Garlock brought up about less power loss due to IIR (heat)losses being less and less power loss due to reduced current flow through inherent resistances, the same motor wired for 240 should spin up faster because of a higher starting potential applied to it and should recover faster in the event it bogs down during cutting.


Maybe...

[Ken Garlock]

But in reality, the current is still less than half at 240 of what it is at 120. If you use your situation of a 2 Ohm resitance in the wire and a 1200 Watt load.
Then the current draw at 120V is roughly 12.68A and at 240V it is about 5.23A.

Hey Wm. we are close enough to be acceptable, particularly in government work Mine is a classroom solution, yours is a real life solution.

[Ken Garlock]

I thought the difference in amps was only because the power was spread over two legs when 240 was used...hence instead of 15 amps, it would be 7.5 PER LEG. Correct me if I am wrong, but the difference can be felt at my house. Less bogging down on the old contractor saw I had and quicker start time.

I can see what you mean. The motors I know about have two windings that are either in parallel for 120V, or in series for 240V. If each winding draws 5 amps, the motor will draw 10 amps on 120V with the parallel configuration. When configured for 240V, the windings are in series, and the each winding will draw 5 amps. Why you might ask? If we ignore the startup situation, and just look at the up-and- running condition, you can say that at any point in time each winding will draw the same current, and hence has the same impedance/resistance. Now for the fancy math: E=I*R

Case 1: 120V = 10 amp*R. Or R=12 ohms. But remember that the windings are in parallel, hence with a little thought we can say that each winding has a resistance of 24 ohms. Each winding draws half the current, 5 amps. Hence E=I*R, 120V/5amps = 24 ohms.

Case 2: let us reconfigure the motor for 240V. We know from above that the resistance of each winding is 24 ohms, thus the new resistance of the motor is 48ohm. Back to E=I*R. Now plug in the knowns E and R. 240V = Iamps*48, solving for I we get 240/48 = 5 amps.

It really isn't comon usage to say that each leg of the 240V carries 5 amps. Yes they do, but we must have two wires to form a circuit just as with the 120V case, the neutral was carrying 10 amps and the hot was carrying 10 amps. So, a more common statement is that the circuit is carrying 5 amps at 240V or the circuit is carrying 10 amps at 120v.

Hope this helps more than it confuses

[Ray Moser]

I was preparing to come up with examples but Ken beat me to it. The higher voltage and lower amperage to move the same amount of power results in lower I^2R loss to resistive heating in the wire carrying the power. The resistance in the wire results not only in heating the wire but there is a voltage drop across that resistance. That is why Edison's DC electrical distribution lost out to the AC system we have today. DC can't be stepped up to higher voltage thru a transformer and with Edison's system long range transmission was not possible. He needed a DC generating station every few miles because losses in the wire made it impossible to get much power more than a few miles from the source. Tesla's AC system and its support by George Westinghouse won the day. When they built the AC generating station at Niagra Falls they were able to sent the power all the way to New York City with much less loss to heating and voltage drop in the transmission wire. Your saw or whatever runs better mainly because there is less voltage drop in the house wiring because you're drawing half the current when you provide twice the voltage. The higher wiring voltage drop when you draw high current at 110V starves the motor as compared to the same motor being supplied with 220V at half the current draw. It is most noticeable during the high current draw on startup.

[Ted Shrader]

Stefan -

On thing about having 240V available, is when you get some bigger tools, you are all set.

Power consumed (and what you are charged for) is the same no matter which way the machines are wired.

With 240V you can do the Tim Allen, "More power! AAARRGH, AAARRGH, AAARRGHHH!"

Ted