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Old 04-18-2010, 01:02 PM   #1
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Electric dryer

If i am connected to 15A or 30A shore power the dryer outlet shows 120VAC on the hot legs (ie +120VAC on red and +120VAC on black w.r.t neutral and w.r.t ground). The voltage difference between the red and black is zero. Will a 240VAC electric dryer work in this situation if accidentally turned on ( i don't want it to)?

On 50A shore power or generator power, i am fine as the voltage difference between red and black is 240VAC (ie (120VAC) - (-120VAC))

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Old 04-19-2010, 02:40 AM   #2
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The motor of the dryer will run (it is 120vac) but the heating element will not come on. Your generator must also be something like a 12 k; not the 7.5 or 8 k quiet diesels as they do not put out 240 vac.

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Old 04-19-2010, 05:59 AM   #3
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That is good to know that heating element will not come on, since it sees 0V instead of 240VAC , right?
In the generator mode (10kW onan, outputs +120VAC and -120VAC in each hot leg), i am not worried. Neither is it a problem in 50A, 240VAC shore power mode.
Only when shore power is 30A, 120VAC and 15A,120VAC i don't want the heating elements to energize or damage anything in the dryer. In these modes i see 120VAC in each leg of the dryer power outlet.
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Old 04-19-2010, 01:13 PM   #4
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correct me if I'm wrong (probably am ) but isn't your "minus 120vac actually a PLUS 120vac, but 180 out of phase with the other leg?
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Old 04-19-2010, 02:38 PM   #5
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yes, you are right. it is a vector.
-120VAC = 120VAC (magnitude) and 180deg (phase)
+120VAC = 120VAC (magnitude) and 0/360 deg (phase)
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Old 04-20-2010, 07:01 AM   #6
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The plus and minus actually indicates the phase difference in a 240v alternating current circuit. One leg is in the "negative" phase whenever the other in in the "positive" phase. Mathematically, the plus and minus signs make the calculation work, i.e. the voltage difference between them is 240v ( +120 and -120). Clear as mud, right?

It is merely convenient to think about the legs as plus and minus, as if it were two DC circuits. We use a similar mathematical convenience in DC circuits when we talk about plus and minus. The plus and minus actually represent direction of current flow rather than the physical charge present, but the arithmetic works and it makes it easier to comprehend the effects. And it all gives electrical engineers and physicists something to argue about over cocktails!

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