Testing DC current output

[TheEquineFencer]

OK, stupid question time again. I do not own a DC Amp probe or amp meter that will do over 10A, how can I test the output current without one? I'm not about to try to put 10A or less through my Fluke 87 and hope I don't blow the fuse either. What I do have laying around are a couple of dead cheap battery chargers with amp gauges on them. Can I just put a load from the DC output on a brush type motor I have in my lathe through the gauge and get a reading? What would make a good controlable load I might have laying around? My plan is to turn the servo motor I have in the lathe and check Volts/RPM/Current at differant speeds. If I can remember how to adjust the seimins micromaster VFD I'm using to drive the lathe I might hit the target 13.6 VDC I'm looking for and record the RPM.

[madkane]

it might need others to say yes to this idea but if you spin up the motor and get open volts reading  then with volts per rev reading  and resistance of the windings you could infer  what the amp readings will be  ie  1 volt over cut in /r gives you the amps at that speed and just  guestamate from there

thats why dead short amp readings dont mean anything for gen output

[TheEquineFencer]

I think you lost me somewhere.

[madkane]

if you put the motor in the lathe and note the volts at x rpm

then with the meter measure the ohms of the generating coils you can infer the amps at any speed

ie  if you get 14 volts at 300 rpm for cut you have 14/300=0.0466 volts per turn

so if you go  and say 0.0466x400=18.64 volts open circuit

18.64-14=4.64 volts/by ohms = amps to use  

ie 4.64/2 ohms =2.32 amps

hope this helps

[Flux]

The most useful thing is the speed to reach cut in volts and open circuit volts are linear you can get that from one spot reading. If you plot a curve it will just be a straight line.

If you know the generator internal resistance and you know what you are doing you can calculate the current at different speeds from that. If you can't do that then the best thing is to use a battery as a load and not mess with any other form of load and cause confusion.( adding a diode as in the final wind turbine helps simplify the tests)

Unless you have or make a shunt you can't measure over 10A with the Fluke. A shunt can be made near enough from a length of wire. Tom W has the figures here somewhere.

If the battery charger meters are of suitable range you can use them, they are not generally very accurate, it seems perfectly acceptable to sell battery chargers that do far less than the nameplate claims and far less than the meter shows. Near enough for your tests no doubt.

Flux

[TheEquineFencer]

The OEM data on the Servo motors I have are as follows, Cont. stall DC Volts 8.1 @ 16.1 amps, 3 ft/lbs torque, Peak Stall DC Volts 35.7 VDC @ 90.0 Amps, 15 ft/lbs torque, for the 4000 RPM one. It is rated for 90VDC at 4000 RPM. That's 44.44444444 Volts/RPM which would require 604 RPM to be at 13.6VDC if I'm right. Now to measure the resistance, I have 4 brushes at 90 degrees to each other, they have the wires separtate from each other. Let's say they are A,B,C,D. A and C (green leads)are opposite each other and B and D (red leads) are the others. When I measure the resistance it reads like a dead short. I took the brushes out and measured at 180 degrees across the comutator and it reads like a dead short, the fluke is reading .3 ohms, that's the amount for the resistance for the Fluke leads. So how am I suppose to measure the resistance? On the small commutator I took out of the end, it had a set of 4 brushes, opposite brushes from each other tied togather. When I check the voltage on this main rotor/commutator still in the servo, from A-C and B-D I get almost nothing, when I go from A-D I get 6.7VDC on low speed. So if my math is right, I'm running at 300 RPM on my lathe. Rated RPM/Rated Votage = Volts/RPM = 44.4444. So 6.7VDC x 44.44444 = 297 RPM aprox. I checked the voltage with the green leads tied together and the red ones also, checking across the two from Green to red. I'm trying to figure out how much "generator" I have so I'll know what size mill I need to build.

http://www.otherpower.com/images/scimages/8380/4000RPM_90VDC_SERVO_MOTOR_repaired_1.jpg

[SparWeb]

Hi Fencer,

Somebody mentioned it, but you might want to make a simple shunt to measure current.  It's not that hard to make something "close enough" for what you need.  The next step is a careful re-learning of Ohm's law.  I had some big mis-conceptions myself, when I was starting this hobby, so it's smart to get your brain smoothed out on how Ohm's law works.

Use a piece of #6 or #8 threaded steel rod.  About 12 inches will do fine.  Using enough nuts and washers to hold everything together, connect the ends of the rod in series with one of the power leads.  Make good tight connections, use ring terminals where appropriate.  See if you can find some furniture angles or something that will serve as "legs" so that you can attach the whole thing to a wooden board.

Current will flow through the threaded rod.  Steel conducts, but isn't great.  Take advantage of this fact to make a shunt.  There's not much resistance in a threaded rod, but it's enough to make a multimeter show a voltage drop across the ends.  The resistance will be too small to measure directly.  Expect about 0.01 Ohm or so.  You might find that you can adjust the position of the connections on the rod, in or out, to get an exact resistance so that the math is easier, but even if it's something 0.008132 ohms you can use a pocket calculator anyway.

For example, if 10 Amps flows through your threaded rod, Ohms law dictates that you will measure 0.08132 volt across the ends of the shunt. That is:

V=IR

(10A) * (0.008132 ohm) = 0.08132 volt

To calibrate a shunt, you have to know exactly how much current and voltage is present simultaneously in a test circuit.  Use Ohms law to work that out.  Once calibrated, the shunt will work for a variety of test situations.  When you use it after calibrating it, you know its resistance, and whatever voltage potential you measure across the ends is proportional to the current in the circuit.

Eg

if you measure 0.054 volt, then   0.054V / 0.008132 = 6.64 Amps

Good luck.

[TheEquineFencer]

Where do you get a meter that will measure the Ohms down that low? I think it's a novel idea and method.

[rossw]

You can do this to get "close enough" if you like.

If your fluke can measure (lets say) 5A safely, get your 12V battery and a 50W headlamp or similar load. Something you can rig up that will draw a current you CAN measure safely.

Hook a wire from the battery to your shunt (above), through the shunt to your load, from the load through your ampmeter and from there back to the battery.

Measure the current and note it down.

Now set your fluke up to read low volts (and check you have the leads in the right place) and put them across the shunt. Reconnect the load. The current will be the same as before (lets say it was 4A)

Rather than measuring the resistance of the shunt, simply adjust the length of your shunt until you get some "convenient" value. 0.4V for example, or 0.04V.

Once you've tweeked the length of threaded rod and it now shows you (say) 0.04V (40mV), you know that you can safely read a VOLTAGE across your shunt, and convert it directly to AMPS.

If you unhook things and then run your generator through it and measure 0.2V then thats 20 amps. 0.35V = 35 amps. Etc.

The exact values don't really matter, as someone else said, you can always calculate it, but I think if you've built it with allthread so you can adjust it - why not make it read something useful/convenient?!

[TheEquineFencer]

I'll give this a try this weekend.