On 09/02/2009 01:08 PM, firstname.lastname@example.org wrote:
Consider a motor that draws 15A at 120V. Let's call it a 90A inrush
current, with a wiring length of 30 feet (20 feet from the panel,
10-foot cord) and #12 wiring.
Power loss in the wiring is:
Pc = I^2 * R
For 30' of #12 copper, R=0.048 ohm
At 120V, assuming 90 A inrush:
Pc = 389W
At 240V the inrush should be half, or 45A:
Pc = 97W
This makes sense, we double the voltage and cut power loss by a factor
So for a total inrush power draw of 10800W (90*120 or 45*240), at 120V
we lose 3.6% of the power to supply losses, while at 240V we lose 0.9%.
We get 2.7% more power delivered to the motor by switching to 240V. I
wouldn't be able to notice the difference, so I call that "small".
The line losses of pushing 1000 watts through a given conductor at a
higher voltage creates less resistance than at a lower voltage. Hence
500KV power lines.
Current is your enemy, voltage is your friend.
Speaking of Watts... Charlie did NOT, I repeat, DID NOT quit he
At 240v there's slightly more power available to the motor because of
less power loss in the circuit wiring due to the higher current at
120v. This is especially evident under high current conditions like
startup when the motor is temporarily drawing several times it's
normal current draw.
If the 120v circuit had half the resistance of the 240v circuit, there
would likely be little or no difference in the behavior of the motor.
But that's typically not the case. A 20 amp 120v circuit would
typically be wired with 12ga wire. A 20 amp 240v circuit would also
typically be wired with 12ga wire. Therefore, all other things being
the same, the 120v and the 240v circuits would have roughly the same
resistance. I'd expect the motor running on 240 under those conditions
to have faster startup characteristics as well as being slightly
harder to stall.
Technically true. But on a properly designed circuit the NEC requires
at most 5% voltage loss. Even assuming that we got perfect efficiency
at 240V I defy you to notice a tablesaw coming up to speed 5% faster.
This is not a "wow, start up is almost instant" sort of difference.
As I mentioned in my first email, if it makes that much difference then
it's likely the motor itself was not operating optimally at 120V.
Aside from supply losses which have been discussed extensively :) in
this thread, there's no inherent reason why a motor would operate more
efficiently at higher voltage.
On 09/02/2009 09:34 AM, email@example.com wrote:
As Dan Coby pointed out, with a 314ft run of 12awg you'd have 6.8% more
power available due to reduced supply losses if drawing 10A at 120V.
With a typical shop wiring configuration the difference would be less.
There's no way that the reduction in supply losses results in a "wow,
start up is almost instant" sort of difference when switching to 240V
unless the 120V circuit was way undersized.
Try it and you'll change your tune. Your numbers are only good for
steady state current draw. There is a large inrush to start the
motor, which will be limited by the supply impedance. At 240V that
impedance is the same, though the potential is doubled.
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