One thing to remember in the formula of E Ã· R = I is that "R" is Resistance, not Reactance.
"R" could be the component of Pure Resistance - dispersing True Power (Wattage), or may be part of an Impedance (Z) figure - which would make the figure more dependant on the total Reactance and Resistance.
For simple calcs across pure resistance loads (like Resistance Heating elements or Incandescent lamps when they are at operating temperature), the true power (P) will be a direct representation of E Ã— I, and the level of current flowing (I) will be a result of the impressed Voltage (E) across the fixed Resistance (R).
Changes in either one will result in a Linear type change overall. The total consumed true power (Wattage) changes in a linear fashion - like how an Incandescent lamp dims when a motor starts.
The linear effect of true power works both ways: If Voltage is decreased across a fixed resistance, the load current is also decreased - which results in a decreased consumption of true power.
If the Voltage is increased across a fixed resistance, the load current also increases, which results in an increased true power consumption on the circuit/element.
That's what makes an Incandescent lamp blow out from higher than rated Voltage being impressed across the filament.
On Reactive elements, only the Resistive element (if any) recieves / dissipates the true power - hence this is the only part with a linear characteristic. Any Resistance within the Reactive elements also becomes affected (for instance - winding resistance of reactor coils).
An Induction Motor is effected by changes in voltage because the TOTAL OUTPUT HORSEPOWER is linear (true power). The Motor works similar to a transformer as viewed by the input KVA (Windings are the primary, the squirrel cage rotor is the secondary).
Only the Reactive component of these circuits is non linear.
Raise the input voltage too far and the motor lets smoke out!
For a simple VA formula (E Ã— I = VA for an AC circuit that's not pure resistance), the equation reflects Apparent Power, not True Power only.
Even though, generally speaking, a higher voltage results in a lower current draw, this only applies in the way the load device has been designed, not increasing or reducing nominal voltage to a given device without compenasting / connecting it accordingly.
Raising the nominal voltage on XL / XC or Z components anymore than 10% will not lower the current draw, but will swing things into a new power level and result in a higher current draw!
Same applies in the opposite direction!
The device being driven must be able to have connection alternatives available, in order to drive it at a higher voltage - otherwise the true power which is drawn from the supply will be greatly affected.
That old idiom of "The Higher The Voltage, The Less the Amps" is another one in the line of "Voodoo Electrical Statements", and has been repeatedly miss quoted over and over again.
The complete statement is more like:
[*] The KVA draw of an AC circuit is a component of the Voltage times the Line Amperes. To reduce a high load current and still maintain a given KVA value, devices may be designed to utilize a higher voltage via connections of the internal components.
[*] To reduce a large input current flow on pure Resistance loads connected to AC or DC, the device can be driven with a higher voltage as long as the device can be connected to allow a fixed wattage to be consumed. This would be from increasing the fixed Resistance in proportion to the impressed system voltage.
Get the idea?
p.s. if I sound harsh, please take no offense! I am only trying to make a few points about the whole true power formula, since the entire point of all this stuff is to transfer the correct level of true power (wattage) from the generating source, to the load device... then transpose it into heat energy (either instantly or eventually).
Funny thing about all the stuff we connect to power supplies - we help make the surrounding air get heated, and we pay money to heat the air / enviroment around a given load!
I could burn leaves and do the same thing! Only need to buy a book of matches!!!
(trying to make humorous joke references to energy transfers, but now it sounds stupid!)
p.s. had to edit a typing boo-boo!
[This message has been edited by Scott35 (edited 06-01-2003).]