# 220v conversion question

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• posted on September 2, 2009, 6:07 pm
On 09/02/2009 11:29 AM, krw wrote:

Why? It's already power, not voltage or current.

Fair enough. I've got a 1.5HP dust collector on a 20A 120V circuit. I've been meaning to switch it over to 240V but haven't gotten around to it yet.
Chris
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• posted on September 2, 2009, 7:11 pm

Volts is volts. Watts is watts. Watts is volts squared. ;-)

I just bought a 2HP DC (came in a crushed box yesterday :(). I went with 2HP because as long as I was going 240V anyway, might just as well do it right. ;-) Still gotta run the line, so replacing the crushed parts isn't an emergency.
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• posted on September 2, 2009, 6:22 pm

During motor startup the current could be more like 40-60 amps. Breakers are designed to allow for momentary inrush currents of that magnitude, maybe even higher. What does that 6.8% figure change to when you do your calculations on 40 amps instead of 10?
--
There are no stupid questions, but there are lots of stupid answers.

Larry Wasserman - Baltimore Maryland - lwasserm(a)sdf. lonestar. org
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• posted on September 2, 2009, 7:04 pm
On 09/02/2009 12:22 PM, Larry W wrote:

Okay, but let's use a more realistic wiring length of 30 feet. (20 feet from the panel, 10-foot cord.)
Power loss in the wiring is:
Pc = I^2 * R
For 30' of #12 copper, R=0.048 ohm
At 120V, assuming 60A inrush: Pc = 173W
At 240V, assuming 30A inrush: Pc = 43W
So for a total inrush power draw of 7200W (60*120 or 30*240), at 120V we lose 2.4% of the power to supply losses, while at 240V we lose 0.6%.
We get 1.8% more power delivered to the motor by switching to 240V.
Chris
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<%-name%>
• posted on September 3, 2009, 4:23 am
On Wed, 2 Sep 2009 08:34:57 -0700 (PDT), " snipped-for-privacy@gmail.com"

My old Beaver table saw wouldstart like a bomb when plugged directly into the wall, or on a short 14 ga cord. It often popped a fuse on startup.
Putting it on a 25 foot 16ga cord it never popped a fuse, and took about 3 times as long to come up to speed (this was a repulsion start induntion motor - you could hear when the brushes came out)
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• posted on September 2, 2009, 3:41 pm

I understand what your are saying. When on 120v the saw was on a circuit with 4 outlets. The only other item plugged in was a cordless phone charger.
There is a clear difference in start up. I am definitely not an electrical professional, so I can't explain it.
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• posted on September 2, 2009, 2:38 pm
DLB wrote:

OP: You have gotten a little good advice and a whole lot of very bad advice. You problem is figuring out which is which. If I someday have a death wish, I'll have to remember to ask such a question on the wreck.
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• posted on September 2, 2009, 3:46 pm

I will probably just wire a separate cord for the 120v lamp. I do like reading all the comments and thank everyone for posting. It all helps me understand more than I did. It makes sense that once running the motor has the same power (one hot leg at 15amps vs. two hot legs at 7.5amps each).
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• posted on September 2, 2009, 9:05 pm
On Wed, 2 Sep 2009 08:46:16 -0700 (PDT), DLB

Just remember those "7.5amps each" are not additive. (Robatoy, don't look ;) ) It's the same amps in each leg - one's coming to the motor, the other's returning it from whence it came. Just like 120v, one line brings the amps in, the other line takes it back.
Tom Veatch Wichita, KS USA
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• posted on September 3, 2009, 10:47 am

Not additive? Now I am back to the beginning of understanding. Isn't the motor running at 15amps on either 120v or 240v?
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• posted on September 3, 2009, 11:36 am

No. It runs at 15A @ 120V, or 7.5A @ 240V.
The amperages on the two legs of the 240V circuit don't add, because the two legs are in series with each other. Current is the same at all points in a series circuit.
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• posted on September 3, 2009, 11:57 am
DLB wrote:

No. If it draws 15 amps on 120 it should draw 7.5 amps on 240. On 120 15 amps come in the hot and go out the neutral, on 240 7.5 amps go in the hot and go out the other hot.
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• posted on September 3, 2009, 9:13 pm

OK, so if I could run the motor on 60volts it would draw 30amps?
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• posted on September 3, 2009, 9:29 pm

Assuming the same horsepower motor, yes. If the motor had four windings (instead of two, like the typical dual-voltage motor) wired in parallel instead of series, yes. Remember 1HP ~= 750W (add for motor inefficiency).
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• posted on September 3, 2009, 9:32 pm
DLB wrote:

According to Ohms's Law, yes.
Current = Volatge / Resistance, resistance is a constant given the same motor.
--
Froz...

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• posted on September 3, 2009, 9:46 pm
DLB wrote:

If it's designed to run on 60 volts with the same power output as it gives on 120, yes.
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• posted on September 4, 2009, 2:45 am
On Thu, 3 Sep 2009 14:13:09 -0700 (PDT), DLB

If the motor was designed to run on 60 volts, yes. If you tried to run the 120 volt motor on 60, no.
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<%-name%>
• posted on September 3, 2009, 11:27 pm
On Thu, 3 Sep 2009 03:47:21 -0700 (PDT), DLB

No.
With 120, 15 amps are running through the motor. 15 amps are coming in one wire and 15 amps are going out through the other.
With 240, 7.5 amps are running through the motor. 7.5 amps coming in through one wire and 7.5 amps going out through the other.
Two wires in each case. You could almost think of it like a waterwheel, one wire pouring in the juice, one wire draining it out, and the juice spinning the motor as it goes through.
OK, OK, I said "almost". It's just an analogy. (;-)
Tom Veatch Wichita, KS USA
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• posted on September 4, 2009, 12:50 am
[snipped for brevity]

But, but, but... THEN where does it go?

Oh... okay then... *smirk*
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• posted on September 4, 2009, 2:35 am
On Thu, 3 Sep 2009 17:50:54 -0700 (PDT), Robatoy
Well, from industrial customers, it goes back to the electric company and into their settling ponds for waste treatment and sanitizing before being recycled out to the residential customers.
Tom Veatch Wichita, KS USA