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Old 09-21-2011, 06:59 AM   #31
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Amp / Watt analysis


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Originally Posted by jbfan View Post
It does, but watts is watts.

100w/120v=.833amps
100w/240v=.416amps

Watts is the measure of power.
Thanks you, that was my point
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Old 09-21-2011, 08:37 AM   #32
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Amp / Watt analysis


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Originally Posted by dmxtothemax View Post
No matter what voltage you run a motor on,
The amount of power used is still the same. !
Some of the economy from choosing the higher(est) voltage the motor is rated for is due to less power loss in the wires feeding the motor.

The number of volts (not the percentage of volts) dropped in a given piece of wire depends on the amperes flowing. Losing 10 out of 240 volts is less significant compared with losing 10 out of 120 volts.

For the typical multi-voltage motor the amperes drawn (and the voltage drop in the lines) with the motor set for 240 volts is about half that compared with 120 volts. So we are now talking losing 5 out of 240 volts compared with losing 10 out of 120 volts in the wires. (example only)

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Originally Posted by LivingLight View Post
I strenuously disagree...
Amps = Watts DIVIDED by VOLTS
How can volts going up not change amps in a three way formula?
Increasing the voltage and leaving everything else the same (not reconfiguring the motor for the higher voltage) generally does increase the amperes drawn.

The load (motor, etc.) has a given resistance. Since amperes equals volts divided by resistance at all times, doubling the volts will also double the amps if the resistance stays the same. In practice the resistance usually increases in this situation because the load heats up and the resistance increases so it is not a proportional increase in amperes for an increase in volts.

When the motor is reconfigured for the higher voltage, its resistance is greater so the motor delivers about the same output in terms of work performed, the watts consumed is about the same as before under the lower voltage, and the amperes drawn is less.

Undervoltage can also burn out motors. The physical load imposed causes the resistance to go down (due to such factors as lowered RPM), the amperes drawn goes up, and the motor can oveheat.
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Last edited by AllanJ; 09-21-2011 at 09:09 AM.
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Old 09-21-2011, 09:50 AM   #33
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Amp / Watt analysis


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Originally Posted by LivingLight View Post
Ok so here is my question. HairDryer = 1875 watt

Amps = Watts divided by volts

We use 220volt here, so

Amps = 1875 / 220
= 8.2

Or


Amps = 1875 / 110
= 17



So unlike Ohms law, in this equation, more volts = less amps.

I Still can't see how a hello kitty hair dryer is gonna such 15 amps. Then a stove on grill would take like 80 amps, more than a average house hold breaker.
I see your problem
BTW 1875/220=8.5 amps - needed to make numbers work
Ohms Law is a three part equation. Voltage, Current, and resistance (or impedance)
The flaw in your above example is that power is constant regardless of the applied voltage to the hair dryer. The only thing constant is the resistance of the hair dryer so the resistance of your hair dryer is 220/8.5=25.9 ohms

If you were to plug this hair dryer into a 110 power source:
I=E/R so I=110/25.9=4.25 amps as compared to 220/25.9=8.5 amps.
If the hair dryer would even operate at 110 volts the power used would be 110*4.25=468 watts

Power is also this: P=IČR and therefore R=P/IČ
P=IČR or 4.25Č*25.9=468 watts

In regards to the 80 amps for the stove, are you adding the value of two 40 amp breakers?

Last edited by a7ecorsair; 09-21-2011 at 10:02 AM.
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