110305-1005 EST
Bigrick31:
I going to simplify your question and work back to your question as stated.
Consider a 240 ohm power resistor center tapped in the middle at the 120 ohm point.
For a 120-0-120 single phase source the transformer center tap will be grounded and also connected to a wire called neutral. This provides two output voltages relative to ground or neutral. Each is 120 V, but they are 180 degrees out of phase with respect to each other.
Connect the 240 ohm resistor across the two hot leads. This puts 240 volts across the 240 ohm resistor and 1 A flows thru the resistor. What is the voltage difference between the resistor center tap and the transformer center tap?
Next disconnect the transformer center tap from ground, and connect a 100 V battery between the transformer center tap and ground. What is the voltage between the resistor center tap and ground? What is the voltage difference between the resistor center tap and the transformer center tap?
Disconnect the battery, leave nothing connected to either the transformer center tap or the resistor center tap. Now connect one end of the 240 V supply to the neutral wire and ground. What is the voltage from ground or neutral to the transformer center tap, or to the resistor center tap?
During all of these gyrations has the current thru the resistor changed?
Now consider your airconditioner.
Suppose it has a 4 wire plug with EGC, neutral, and two hot wires for use in a single phase application with 120-0-120 as the supply. If internally the neutral wire is not used, meaning no current flows thru this wire under any normal operating conditions, and that the EGC only serves its intended purpose, then this airconditioner could be connected to a 240-0 power source.
On the other hand if the airconditioner were designed with some 120 V circuitry connected between one hot wire and neutral, then you could not run this from a 240-0 supply without the addition of a 120 source.
An isolated lighting ballast would only be a two terminal device and could be operated across either a 120-0-120 or 240-0 supply.
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