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meter question
By birdhouse, Section Newbies Posted on Sat Oct 10, 2009 at 06:01:12 PM MST
measuring dc amps?

hello all-
just bought a junky harbor freight meter. it is the type that clips onto a wire to tell you how many amps are flowing through it. i'm looking to determine cut in rpm and amps at various rpms on a three phase servo motor.
the real question- the meter only says it measured AC amps. i wanted to clip it onto a wire between the battery and rectifier to measure amps into the battery. will it work?
thanks for any help!

meter question | 22 comments (22 topical)

Re: meter question (3.00 / 0) (#1)
by wooferhound on Sat Oct 10, 2009 at 06:23:05 PM MST

Clamp-on meters will not measure DC voltage. Some meters can measure DC current by putting the meter in series with the wires going to the battery, but can't measure high amps over about 10 amps. You can do some fancy stuff with shunts where you use the meter in volt-mode to read DC currents.

W o o f -={(
Huntsville Alabama U.S.A.

Re: meter question (3.00 / 0) (#2)
by birdhouse on Sat Oct 10, 2009 at 06:35:54 PM MST

thanks woofer!

i may have described the meter wrong. when i say it clips on the wire, it doesn't actually touch the wire, just has a plastic clip that goes around the wire. mabe you understood what i meant. it's called a digital clamp on meter. will it work to measure dc amps?

thanks again!
"let it blow, let it blow"
[ Parent ]

Re: meter question (3.00 / 0) (#3)
by wooferhound on Sat Oct 10, 2009 at 09:01:20 PM MST

it is impossible for a clamp-on meter to measure DC Current. The wraparound clamp works a lot like a transformer, which also will not work on DC current.

W o o f -={(
Huntsville Alabama U.S.A.

[ Parent ]

Re: meter question (3.00 / 0) (#4)
by wooferhound on Sat Oct 10, 2009 at 09:03:17 PM MST

How much Current are you trying to measure ?

W o o f -={(
Huntsville Alabama U.S.A.

[ Parent ]

Re: meter question (3.00 / 0) (#5)
by rossw on Sat Oct 10, 2009 at 09:08:40 PM MST

it is impossible for a clamp-on meter to measure DC Current.

Hold on there guys, you're completely, totally wrong.

There are PLENTY of DC-measuring clamp-meters - Here's mine

DC clampmeters use a hall-effect device and measure the magnetic field around the wire. They are quite usefully accurate, very convenient and nowdays quite inexpensive.

[ Parent ]

Re: meter question (3.00 / 0) (#7)
by TomW on Sun Oct 11, 2009 at 05:38:20 AM MST

Ross;

Always with the negative waves...

I was gponna toss that out, too but do not own one so all hearsay.

Tom

[ Parent ]

Re: meter question (3.00 / 0) (#18)
by wooferhound on Sun Oct 11, 2009 at 07:17:35 PM MST

Ok , I'm proven wrong
but that is the first one that I have ever seen . . .

W o o f -={(
Huntsville Alabama U.S.A.

[ Parent ]

Re: meter question (3.00 / 0) (#6)
by birdhouse on Sat Oct 10, 2009 at 09:34:45 PM MST

thanks woofer and ross!

the meter is a pile. i was trying to use it to measure amps dc while benching my motor. then (just out of curiosity hooked it to the cord on my heat gun and plugged it into the mains. display was 0.0. it was $10.00 from harbor freight so my expectations were minimal. i did however find out the cut in of my motor for 12 volt is about 140 rpms! yay. the motor was VERY stiff to turn much over 150 rpms when hooked to my old truck battery (probably a terrible load) the battery measured 12.8 v (with my reliable meter) before the test. i was able to get it to over 13.8 volts. as soon as i stopped the motor, the battery volts jumped back down. then out of curiousity (i should be named george) i hooked a 12 volt 75 watt halogen bulb to the battery to pull some current out of it. to my supprise the volts kept slowly increasing with the bulb atached??? like i said it is my old truck battery, but i try to trickle charge it from time to time just to keep it in decent shape to play with. needless to say, i'm confused. i was hoping to measure the amps out of the motor at various rpm to figure out how large of charge controller ect to work with.

oh- by the way- it's a fanuc 3 phase ac servo motor. yea there's magnet already in there. no re-wire.
2000 rpm 140 volts 20 amps stall. 50 lbs.

any suggestions to get multiple rpm/amp readings out of this thing?
"let it blow, let it blow"

Re: meter question (3.00 / 0) (#22)
by wingman1776 on Wed Oct 21, 2009 at 07:43:43 AM MST

I checcked my faunc servo motor with a dc amp meter that you add to your car. I got it at auto zone. placed it inline after the rectifer. It may not be the perfic way to do it but it seemd to work for me. I could only spin the motor at 400 rpm used my millwalke hole hog I use at work but I got about 17 amps 13.9 volts.

[ Parent ]

Re: meter question (3.00 / 0) (#8)
by ghurd on Sun Oct 11, 2009 at 05:58:06 AM MST

OK.
The HF Clamp meter 'probably' works... BUT...
It measures total combined current through ALL the conductors in the wires in the clamp.
You clamped the 2 conductor wire of the heat gun, which one conductor has 10A going one direction, and the other conductor has 10A going the other direction.
The Combined Total = 0.0A

My solution (I am Not saying it is a good idea, I am saying it is what I did) was to take a 99-cent polarized extension cord, cut the 'hump' off with a utility knife so a 3-prong jambs in, separate the extension cord's conductors for a foot or 2 in the middle so the clamp has a place to go around 1 conductor.
Extension cord middle looks like ---<>---
It works.

And ALL the HF clamp meters only work with AC.
G-
Ghurd.info

Re: meter question (3.00 / 0) (#9)
by kurt on Sun Oct 11, 2009 at 06:01:39 AM MST

basically in a nutshell unless the meter specifically says dc amps it will not accurately measure dc amps they will usually read something but not an accurate reading.

most likely the reason you got a 0 reading trying to measure the amps of your heat gun is you clamped it around both wires of the cord. to get a reading you would have to split the cord to individual wires and clamp around just one wire. unless the meter is DOA that is never know about harbor freight.

IRC inst.
just a personal rant carry on.

Re: meter question (3.00 / 0) (#10)
by birdhouse on Sun Oct 11, 2009 at 07:50:28 AM MST

wow-

thanks for the detailed replies!

i thought it might be the case that since the heat gun had all wires in the clamp that it may net zero. i also thought it may not work for dc amps. i guess i'll walk down to radio shack once they open and see what kinda ammeters they've got.

so another question- why is my motor seem to be excessivley electrically stiff even just a few rpm above cut in? is it the old truckk battery not wanting to accept amps? would it react differently to say a fresh 200 ah 12 volt bank that was in need of charging? it is so stiff with the truck battery it seems like it would instantly stall even a ten or twelve foot set of blades at 150 rpm.

thanks agin to all helping me through this! i'd be lost without this board!
"let it blow, let it blow"

Re: meter question (3.00 / 0) (#11)
by ghurd on Sun Oct 11, 2009 at 08:27:04 AM MST

"i'll walk down to radio shack..."
Or you could drive some place decent!

RS supposedly has a deal with Fluke, but the end result is not on the shelf around here. And if it was, I expect the RS retail would be "a lot".

The $3 HFs do fine. Within a percent or 3% of my Sperry on every comparable setting, which is as good as or better than the rest of my tests.

A few RPM above cut in is when power is flowing.
Power comes from somewhere, and the stiffness is where the power is coming from.
Completely normal.

Need to get some solid RPM/Voltage/output/etc numbers for someone to make an estimate for the blades.
G-
Ghurd.info
[ Parent ]

Re: meter question (3.00 / 0) (#12)
by birdhouse on Sun Oct 11, 2009 at 08:47:28 AM MST

ghurd-
well i kinda went about this all backward. :) i am almost finished with a three blade 8.5 foot dia. tsr of 6.5 set of blades. they turned out very nice. used oztules chainsaw method. you can see it here

http://www.youtube.com/watch?v=aNQ_SW1Njf4

so the three dollar ammeter from HF... guessing it's a analog type? does it use a shunt or direct? what is the range of this meter?

the reason i'm so intent of the stiffness is cause i'm using a hand help drill taped to the motor and i really have to hold the drill hard to try and increase rpm at all above cut in.

i guess this is why you guys use lathes direct chucked to get good bench tests.

i'm really just trying to figure out the potential power this unit may produce.

i brought the motor to jerry to check it out seeing how he is more knolwedable than i and he thought it may be good for upwards of 1000 watts. but then again, he doesn't furl any of his mills, so he may be quoteing max power in a storm ect. not sure. he was pretty impressed with it.

looks like i may take a trip to HF.
"let it blow, let it blow"
[ Parent ]

Re: meter question (3.00 / 0) (#13)
by ghurd on Sun Oct 11, 2009 at 09:40:13 AM MST

The $3 HFs are digital and direct. 10A max, which melts the lead wires if done very long.
Could use a shunt with it.

There are about 50 HF part numbers that change on a regular basis, but this is typical of what they are,
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=90899

Can you get an arbor for the shaft, file it down to fit a drill chuck?
I doubt the drill (or drill press) will turn it with enough gusto to get 1000W 12V output.
G-

Ghurd.info
[ Parent ]

Re: meter question (3.00 / 0) (#14)
by birdhouse on Sun Oct 11, 2009 at 10:38:38 AM MST

ghurd-
just got back from HF got the meter you posted a link of. was pretty easily able to get it up to 4.5 amps. then the tape job spun loose from my little hand held 3/8" drill. so here are the limited numbers i got-

158 rpm = 1 amp (12.8 watts)
166 rpm = 2 amps (25.6 watts)
181 rpm = 4.5 amps (57.6 watts)

battery voltage at 12.8 at beginning of test.

any conclusions on blade matching? i know it's a little late for this. aka you think my 8.5 foot tsr 6 blades will be a decent match?
do these numbers sound good?

i know i have no torque info, so it may not be possible for recomendations.

oh- the shaft- it's a stainless taper about 1.1/16 to 1.1/4 over a few inches with a threaded end. going to have my machinist buddy make me a blade mounting adapter of sorts.

any thoughts are much appreciated!

thanks agin for all of the help!
"let it blow, let it blow"
[ Parent ]

Re: meter question (3.00 / 0) (#16)
by ghurd on Sun Oct 11, 2009 at 02:47:11 PM MST

LOL
(coffee just shot out of my nose. A lot. Twice.)

That "someone" knowing about blades would not be me.
I ask for help with questions about good blades.

If you made some small, very poorly suited, PVC blades, I can help make them better,
But 'good' blades are past anything I should be giving advice about.

People who know what they are talking about will need to know the resistance per phase.
And how the thing is wired for those output numbers (Y/star?).
And everything else you can think of. Battery voltage at x amps at the same time, how the RPM was measured, open voltage at several known RPMs, etc.

Jerry, as in the non-furling Jerry I am thinking of, can probably make a decent guess if he hangs it in the lathe for a while.
G-

Ghurd.info
[ Parent ]

Re: meter question (3.00 / 0) (#17)
by birdhouse on Sun Oct 11, 2009 at 03:53:12 PM MST

did i miss something? i wasn't trying to be funny, or disrespectful, or put anyone down.

resistance is .6 ohm between phases. (just measured with a decent meter, i understand this may not be true resistance) i donno know if it is star or delta ect. haven't gotten open voltage results at given rpms yet. just the loaded numbers above. the open volts are easier with my tape drill combo due to lack of torque.

ghurd- you still selling charge controllers? i think i could use atleast one.

thanks!
"let it blow, let it blow"
[ Parent ]

Re: meter question (3.00 / 0) (#20)
by ghurd on Mon Oct 12, 2009 at 07:35:45 AM MST

I kind of figured I was famous for being bad with blades, and blade questions.
If it wasn't for RichH making blades, I wouldn't have a decent set at all.

It is probably Star. I never saw a servo wired in Delta, but have not seen all that many servo motors either.
If you get 0.6 ohms, for 3 coil sets, and have 6 pairs of power wire ends, it may be not internally wired at all.

It is not generally a good idea to take a servo motor apart. Pretty high probability it will lose a lot of output, or can even go down to near 0 output.

Yes. Controllers still available. http://ghurd.info/
Never gets updated, but still there.
G-
Ghurd.info
[ Parent ]

Re: meter question (3.00 / 0) (#15)
by tanner0441 on Sun Oct 11, 2009 at 11:26:55 AM MST

Some cheap battery chargers have an ameter that the wire just passes through a metal clip across the back and it works by detecting the magnetic field round the wire.

Brian.

[ Parent ]

Re: meter question (3.00 / 0) (#19)
by Flux on Mon Oct 12, 2009 at 01:12:46 AM MST

This seems to have raised a lot of issues and most of it seems sorted out but meters do seem to cause a lot of confusion here so a bit more information may not be out of place.

Normal cheap clamp meters use a current transformer principle and only work on AC. As someone else already said, it it doesn't claim to measure dc it won't. the dc ones work on a very different principle and are far more expensive and they make a big point of telling you it will measure dc.

The dc measuring types either have a moving iron movement or a moving magnet that senses the magnetic field as in the battery charger thing that Tanner mentions and as used in some car ammeters. These are crude and are basically dc indicators. in the past there were a few moving iron clamp meters with jaws and these were capable of measuring ac or dc but neither to a high degree of accuracy. The better ones such as Crompton managed about 3% on dc if you took the average of readings in both directions to eliminate hysteresis. These are old technology and you won't buy a new one.

The modern dc clamp meters mainly use hall sensors and they can be accurate enough for most normal jobs but they are not cheap and don't confuse them with the transformer type ac ones.

All clamp meters measure the vector current in the jaws. You must only clamp a single wire to get a sensible answer. Clamping the power cord of an appliance will give you zero for single phase, 3 phase or a pair of dc conductors. The resultant fields cancel.

The 3 phase case is not quite so obvious but the instantaneous sum of 3 phase currents is again zero.

The ac type will measure the current in the 3 phase leads feeding a rectifier but the waveform they are dealing with is way beyond what they are calibrated for so it will not be very accurate so don't expect much correlation between what they measure on the ac side and what you measure as true dc on the rectifier output. You probably won't be able to do the maths anyway to do this comparison so the meter inaccuracy may not be a big issue in this case.

Flux

[ Parent ]

Re: meter question (3.00 / 0) (#21)
by Jerry on Tue Oct 20, 2009 at 09:50:25 PM MST

Hi birdhouse.

I have the HF clamp on digi meter. It measures AC amps only. As the guys said just clamp around one wire at a time. It works prety fair and will get you in the ball park.

Its true I've never built a furling wind generator. One reason is I've never flown a machine larger then 7 ft.

I'll be building some 8 to 12 ft machines in the next coming year and they will have furling.

When you and I opened wiring acces area on your Fanuc as you recall the star point was there and very accesable.

If you wire this unit JR you'll get more rpm and power from your drill bench test.

This should remove some of the stall on your drill and gain you abit more amperage.

I should recive the 2 Fanuc motors from my son next week. I'll do many tests and report my findings.

I hope mine comes with the shaft adaptor shown in the net pictures. If not I'll have my son machine the nessesary adaptor.

I have a 100 Lb. 180 volt dc motor that should drive the Fanuc into some decent power. If not I'll add a second 100 Lb. 180 volt motor. We'll find out what this baby can do.

I'll keep an eye on its thermal condition with an inferred thermometer.

I will need some guidence for blades as well. I think a good assortment of test info should be suficiant for the blade GURUs here to make a sugjestion.

Keep us posted on your progress.

Jerry

Airheads Page

[ Parent ]

meter question | 22 comments (22 topical)

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