Using Clamp-On Multimeter to Check a/c Surge Current

HiTech wrote:
Excellent. Did it work for a warm restart as well?

Jim

I bought it and used it over the weekend. There was one time when I had turned the ac off and tried to restart it about 15 mins later and it overloaded. I just waited 5 more minutes and tried it again and it was fine. I'm happy with that, and it was running the fridge at the same time as the ac because the lp setting wasn't working. So now I get to figure that out.

Typical starting current of an induction motor (compressor) with a start and running winding is 3-5 times it's running current. Yes--its only for a split second and a locked rotor figure would state that I believe.

An analog meter, an AC voltmeter large enough to easily see the difference between 100 and 110 volts can work. An analog meter has a NEEDLE and a back scale.

If I start a roof air ON HIGH HEAD and the needle drops like a stone to or near 100, that's the end of the game for me.

With a larger correct generator, the stutter load to start a roof air on high head is barely noticeable. If the generator bogs, even for a second (1000 milliseconds) that is too much sag. Time to get better measuring equipment.

If the "penalty" was losing a nickel and dime item it's a lot different than what a camper moaned to me on a remote Mexican beach. "The air conditioner started slower and slower, and now my generator is dead. I borrowed the big worksite generator from Fred, and when I try the AC, it clicks the breaker on the generator".

How much do you think that ended up costing him? I haven't a clue. I do not want to find out. I'll let somebody else do that.

otrfun wrote:
Wow, obviously got some forum members here with some very in-depth electrical knowledge! Appreciate everyone taking time to offer their advice and help. After reading everybody's responses I decided it's just too risky depending on a clamp-on multi-meter to make a go, no-go decision on purchasing the Yamaha EF2400iSHC.

So, I attempted what I previously thought was a very long-shot. I called a few Yamaha dealers to see if any of them would allow me to demo a unit with my RV’s a/c unit. Although they're over a 100 miles away, I did find two dealers who would do a demo. They only asked that I agree to purchase the unit from them if it worked satisfactorily. IMO, that's more than fair. But, I have to admit the real surprise was---their price (with tax) was within $50 of the lowest price I found for the EF2400iSHC online!

Problem with this solution is that it may very well start at the dealer and NOT start when you want it to. As an example, my coleman AC will easily start from my honda 2000 in eco mode in my driveway at sea level and 80F. It won't start it even in non eco mode much above 3000 ft, or above 90F. so it doesn't start it when I need it most.

With a pair of honda's, in eco mode at 5,000ft, and 95F generators hardly strain. Voltage drops from 125 down to about 110 on an analog meter for less than a second and it's cooling away. Even in non eco mode I'll see a drop down to about 115 then back up to 120ish due to the inrush current till the motor develops some back emf.

Wayne Dohnal wrote:
Even if somebody has real numbers I don't think you can put a lot of faith in them. AFIAK there's no standard definition for a surge measurement. When a motor starts, the first AC cycle generally draws the most current and it tapers down from there over a second or two. Does a surge measuring meter capture the peak at an individual cycle level? Does it sample over a certain time window and report the average? Is it reporting peak, average, or RMS values? I don't think there's much if any standardization between meters. Just as trivia I've measured the eu2000i peaking out at around 43 amps which it can sustain for only a couple of AC cycles.

very true. Also, it's hard to find an affordable clamp on AC current probe that will capture single cycle or even multiple cycle peaks. yes, they exists, I've been involved in the design of some, but the only one's I'd rely on for quantitive data would be a AC current probe for an oscilloscope so I could look at peak current each cycle. And then your looking at a few hundred $$$ for the probe plus an oscilloscope.

Wayne,

Like getting an immunization against ebola. It's wonderful when you have it. Soft start is fabulous, but I have yet to source something affordable for retrofitting.

But apart from that, thank you once again. The RV OEM soft-start system was unknown to me and I cannot endorse such a system with high enough praise. It can be a quantum leap in longevity for borderline power situations.

How does the fear of damaging the motor with impaired startup power stack up against the growing number of active soft start controllers that limit the startup power? This one http://telinklff.en.ec21.com/Air_conditioning_Soft_Starter--4946810.html claims to lengthen the life of the air conditioner, and the Dometic offering http://www.dometic.com/enie/International/Site/Marine/Marine-Leisure/Dometic---Air-Conditioning/Air-Conditioning-Accessories/products/?productdataid=95453 says "May eliminate the need to upgrade your generator."

Thanks Wayne!

I have seen motor windings overheat and burn in just a few seconds of locked-rotor simulations - identical to what happens to an AC compressor motor when it tries to hi-head restart and encounters low voltage. Motor winding insulation can not tolerate repeated "almost to the point of charring" over amperage even on a momentary basis. The bright copper with translucent yellow varnish starts to change color, first to orange, then brown then (you don't want to know). One single burn spot (short) in all those windings starts the process.

Just because something "seems to work 10-times in a row" does not cut it in my book. Only accurate voltage and amperage readings taken at time of high head restart are valid. Many of you are financially comfortable enough to not have a crisis when an A/C compressor burns out after a year or so. I do not have that luxury. So boasting "my tiny generator starts MY air conditioner" is misleading and fraudulent - unless the boast includes "The inrush does not slump the voltage to 105 volts or less". I am not being picky here. I am attempting to warn other folks that there is more to choosing a successful power source than chortling "See! I told you so! It Starts!"

The specs on my 25+ year old 15k AC's says starting current is 19 amps.I would think that more modern units would require fewer starting amps.

My 4 year old 15k RVP Coleman heat pump isn't any better. For the first ~100 ms. of startup with a strong power source it pulls 27 RMS and 73 peak amps. This tracks well with its LRA spec of 71. For the next 200 ms. the peaks go down to 40 amps. The whole startup sequence takes about 750 ms. With a 14 ga. 50 foot extension cord introduced, the startup peaks go down to 60 amps, and the startup time goes up to a full second. This is a "big picture" capture of the startup.



And the first ~200 ms.:

Got my Fluke 187 owners manual out. It has a Min and Max mode. Will capture peaks to 250 microseconds.

The specs on my 25+ year old 15k AC's says starting current is 19 amps.
I would think that more modern units would require fewer starting amps.