Re: MOT current draw question

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "davep by way of Terry Fritz <twftesla-at-qwest-dot-net>" <davep-at-quik-dot-com>


> << I'm just not sure why my meter says it's pulling 40 amps.  If that
>>was true, my breaker should trip *FAST*, shouldn't it? >>

	IF it was true.  IF, brother, if., then maybe the 
	breaker would trip.  (the comments previous comments
	below, at >, are apropos...)

	There are a number of ways meters can, uhhhh, lie.  (1)
		At 40A (or thereabouts) the sine wave may be so
		distorted that the meter is just guessing.
	(detail specs of the meter will tell which waveform(s)
	each will indicate accurately.)
		If measuring short time flows, the needle (if
		a needle meter) may over shoot.

	(and analagous effects to both of these can fool
	digital meters, if used...).

> Not necessarily. While a dead short across the line (hundreds of
> amps) will cause an almost instant trip, a breaker can easily carry
> two or three times it's rated load for quite a while. My coil
> draws 40 to 50 amps with the Variac wide open, and it will run for
> 30~45 seconds from a 20 amp breaker.

> << I also had a voltmeter hooked up so I could do true wattage
>>measurements.

	How does one measure true power with a voltmeter?
	(Only way i can think of is with a wattmeter...)

>>  According to my math, at 39 amps and 95 volts one MOT was pulling
>>almost 4000 watts.

	That's VA, rather than watts.

>>This can't be right, so can somebody tell me what I'm doing wrong? >>

> That doesn't sound too outrageous for having a shorted secondary.
> Of course, your primary will fry in short order, but I suspect your
> meter is working correctly.

	best
	dwp

[1]
Some YEars Ago (in tech school) we were doing a motors lab.  We ended
up with 3 AC Ammeters in parallel to get them on scale.  (and all
the lab teams got the same result....).  Mr Instructor was monitoring
the lab switchboard.  (He and we suspected summat was RONG, but no one
could see what, so he said Press On.).  He got notably different
(tho still high) readings on the master meter...

Next Day:
	Discard all the labs.  We were supposed to do that at
	10% normal line voltage.  THAT line got left out of the
	lab sheets...

(The difference in current indications was because we were distorting
the sine wave, with the overload, so HIS meter read one way, OURS
read another...)