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Author Topic: where do I connect the ammeter?  (Read 2589 times)

red181

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where do I connect the ammeter?
« on: April 14, 2008, 02:33:46 PM »

Hi all. I have a problem with short run times on nimh batteries, and want to measure the amp draw from the graupner 700bb turbo motor,(12.5amp  at max efficiency) and experiment with different props (currently using 35mm s blade plastic). I want to try a 3 blade, different sizes and pitches etc, to get the balance between speed and run time, before I invest in new nimh packs.

I was going to attach my (cheap!) digital voltmeter, which has a 20 amp limit across the motor terminals, but I have been told it needs to go between the motor and esc. Can someone please confirm?

Thanks
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boatmadman

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Re: where do I connect the ammeter?
« Reply #1 on: April 14, 2008, 03:22:52 PM »

Hi,

Essentially, yes. What you need to do is connect the negative terminal of the meter to the positive terminal on the motor, then coonect the meter positive to the esc positive outlet to motor, turn on meter, run motor and bingo!

Ian
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Bunkerbarge

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Re: where do I connect the ammeter?
« Reply #2 on: April 14, 2008, 03:40:52 PM »

Basically you measure voltage across the load, i.e. in parallel with it but you measure current in line with the load, i.e. in series with it. 

The best place as Boatmadman suggests is between the positive terminal of the motor and what ever type of speed control device is supplying it.

Don't forget that the actual current in the water under load is what you are after so putting the hull in the bath will ony give you a current under partial load.  The propeller could even be cavitating at full speed and your current could drop off considerably.  You need to try your best to emulate normal operating conditions somehow.  One possible way is to stick it in the bath and make a note of the current reading at different throttle settings.  If you notice it has dropped off at some point then it has probably started to cavitate but if you continue the line of the graph you are able to draw from your figures the real current should be somewhere along the line corresponding to the maximum throttle position.

The best way though (which very few people do so don't worry about it!) is to put an ammeter in line of the type that measures the maximum reading rather than a contunuous one.  Stick your boat on the pond and do a circuit and see what the meter reads as a maximum current.
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FullLeatherJacket

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Re: where do I connect the ammeter?
« Reply #3 on: April 14, 2008, 04:34:25 PM »

The best way though (which very few people do so don't worry about it!) is to put an ammeter in line of the type that measures the maximum reading rather than a continuous one.  Stick your boat on the pond and do a circuit and see what the meter reads as a maximum current.
Richard
Do you know where I might get my hands on such a beast without paying alimony-sized amounts (again..... :'()?
About 50A peak should do the trick.
FLJ
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nightowl2912

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FullLeatherJacket

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Re: where do I connect the ammeter?
« Reply #6 on: April 14, 2008, 10:26:32 PM »

Thanks, Andy. I shall make further enquiries.
FLJ
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wombat

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Re: where do I connect the ammeter?
« Reply #7 on: April 14, 2008, 10:34:12 PM »

Don't rely on the reading, especially if it is a cheap meter - the burden of the meter will affect the voltage across the load and hence the current - they are only really intended for mains voltage use

Wom
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Bunkerbarge

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Re: where do I connect the ammeter?
« Reply #8 on: April 14, 2008, 10:56:04 PM »

The one that Andy linked to looks like it should do the job and is obviously made for RC model use but it doesn't actually state it measures peak current.

This is a similar sort of thing but it actually states that it records peak current:

http://www.rc-electronics-usa.com/ammeters/amp-meter-specs.html
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malcolmfrary

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Re: where do I connect the ammeter?
« Reply #9 on: April 15, 2008, 02:03:43 PM »

A more entertaining (for the observers) method would be to have a range of fuses and a length of line fixed to the back end of the boat.  Start with a low value, set the boat off in an energetic manner, if the fuse blows, pull it back with the line.  Increase the fuse value, try again.  Repeat until the fuse survives.  You now know that the motor pulls somewhere between the last two values.
Is it a new build that is not coming up to expectations, or a developing situation? 
What run times are you actually getting, and what is the size of the battery?
Are the batteries charging fully?
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red181

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Re: where do I connect the ammeter?
« Reply #10 on: April 15, 2008, 04:17:12 PM »

thanks all for help, its not so much a new build, but a redevelopment really. I have almost finished a big refurb of a 42" Huntsman 28. On the plane it looks great, I am trying to optimse the speed/run time. The current set up is as listed at the start of the thread. After having a long conversation with a  rc good battery supplier, I will opt for running 2 x 9.6v (as 19.2v) 4600 mah packs. He has calculated that my run time, excluding the variables such as "is the drive line perfect", and "is the prop the most efficient", that I would actually achieve a better run time with 2 x 8.4v packs, as the motor will not be sucking as much current, however if I can resist the urge to run at max power, and settle for a bit less, then the run time will increase,as I will not be using max volts,  17 to 25 mins hopefully, which would be very acceptable.
So before I commit to the batteries, I want to experiment with the part that I do not understand, the propshaft, and try to minimise the drain, whilst retaining acceptable performance. Up to now, I have been swopping the props around, giving the boat a quick run, and if it looks faster keeping that prop!. Now its time to see what amps the prop is drawing, as the fastest may not be the best, if you know what I mean!
I suppose I am more interested in the difference between the readings with different props, not so much the actual reading, unless it was a lot different from that stated by graupner, at 12.5v max efficiency   .

Malc, batteries are fully charged, 2 x 3300mah 8.4v nimh as 16.8v, 10 to 14mins run, mostly at flat out, but I think I could do better. Tried 3 x 6v (as 18v) lead acids, died after 4 mins!
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malcolmfrary

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Re: where do I connect the ammeter?
« Reply #11 on: April 15, 2008, 09:45:22 PM »

As some have stated before, measuring in a tank or tethered at the poolside is not totally accurate, but should be good enough for comparing props if used with a gauge for measuring pull (normally something that tuggies do).  Testing like this cant show what happens when the boat actually gets on the plane.  A club member several years ago fitted a lighter, slightly lower capacity battery, increased both speed and run time.  Getting absolute best performance is a bit of a black art.
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red181

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Re: where do I connect the ammeter?
« Reply #12 on: April 15, 2008, 11:51:08 PM »

thanks malc, what I plan to do is attach the amp meter, facing one side, it has a large digital display, and run it past a volunteer to read the display, or follow it with a camcorder. Repeat the process with say 2 other props. I will have to keep the runs short, so the battery depletion does not affect the readings. If I do it at New Brighton, or at The Ellesmere Boat museum, I can run the boat very close to the side, particularly at New Brighton, its a raised lake, not sunken. Should be interesting,  O0
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