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Author Topic: How to measure Amps about 10 amp limit  (Read 1798 times)

Joystick

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How to measure Amps about 10 amp limit
« on: April 14, 2008, 02:16:45 PM »

How do I measure Amps drawn by my motor as my DMM says only upto 10 Amps.. My motor may draw more when I stall it.
 willl I cook the meter above this...
Is there a simple way or do I need another meter ? :(
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HS93 (RIP)

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Re: How to measure Amps about 10 amp limit
« Reply #1 on: April 14, 2008, 02:34:27 PM »

Get a 0.1ohm resistor about 10 plus watts and put it in line as you would your meter then put your meter across the resistor on the 20v range and move the decimal point one place to the left ,you cannot blow your meter this way .

HS93
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wombat

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Re: How to measure Amps about 10 amp limit
« Reply #2 on: April 14, 2008, 10:48:37 PM »

Problem with this is that you will be dropping 1V across the resistor at 10A. If you are on a 12V supply you are only going to be getting 11V across the load - if it is a motor, this could have all sorts of effects no the current, especially if you are testing under load.

Wom



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malcolmfrary

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Re: How to measure Amps about 10 amp limit
« Reply #3 on: April 15, 2008, 01:48:41 PM »

Just add about 10% to the reading to compensate for the resistor loss. 
Most "10A" dmms will actually read higher without instant damage, but another cheap way is to pay a visit to a car scrap yard and get an ex-dashboard ammeter.  Not clinically accurate, but near enough to let you know what thickness of wire is good and value of fuse to fit.
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andrewh

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Re: How to measure Amps about 10 amp limit
« Reply #4 on: April 15, 2008, 03:25:49 PM »

Joystick - you ask the question about pulling higher current when stalling the motor

Why would you want to do this?  or is this something you worry about?

As an electric flyer;  stalling the motor is the thing I always Don't want to do - most especially with my finger, as this is when they start getting dangerous to health.

A stalled motor will draw a huge current, and can do so for as long as it is stalled (it is effectively the same current as the starting current, but the start current reduces as soon as the motor rotates).

This is why fuses are a good idea - they will blow and protect your hand, wiring, boat or settee depending on what is stalling the motor. 
Experience suggests that if you are using car fuses, you can protect a 10A (full power) motor with a 10 Amp fuse (cos they take time to heat) - it should not blow on starting but it will (quickly) if the motor stalls. 
Ideally you do not want the second prop blade to do again what the first has just done!

Are you getting the help you requested, or are we pontificating about matters you don''t need to know?
andrew


 
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Joystick

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Re: How to measure Amps about 10 amp limit
« Reply #5 on: April 15, 2008, 03:38:57 PM »

The reason I wonder about the stall current is that I am building my new craft for use in the sea.. :D. I live 30m from it so seems a waste to miss out. I have a large 56" scratch built hull I'm trying to kit out. Pilot boat in other side of the pond scale....
As we get a fair bit of weed close in I dont want to cook a speed controller  :(

Its all a bit hit and miss with the sea, recovery is'nt a problem as we have a coble so the project our amusment..

I let you know when I've lost it (the boat that is) :P
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FullLeatherJacket

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Re: How to measure Amps about 10 amp limit
« Reply #6 on: April 15, 2008, 03:59:16 PM »

Joystick

I used to muse about stall currents too, as some of my customers get paranoid about it. My attitude changed when I tried to stall a geared MFA Torpedo 500 with my fingers (because I'm like that..........mad). At the point where it was about to do some serious damage to my fingers it was pulling 7.3A. I then got to wonder what sort of load it might encounter in the water to create such a current and which wouldn't instantly get minced by a decent brass prop............and I stopped worrying.
 
If you do manage to stall a motor you'll probably blow up the ESC - whatever it says on the tin about current handling -  but, if you fuse the circuit properly,  then your model won't catch fire as well. Ref the ammeter, Dave Mainprize does a 0-20A panel-mounted jobby for less than a tenner. Go Google Electronize, huh?

FLJ

 
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Joystick

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Re: How to measure Amps about 10 amp limit
« Reply #7 on: April 15, 2008, 04:39:33 PM »

Yep I did consider the force thats needed to stall the motor, I was just trying to figure out what would be a safe figure for a ESC. After a bit of thought  :o I have decided to get a Electroize 30 amp controller.. if I pop that I stand a good chance of getting it fixed.. I can live with that.

As for lopping of fingers trying to stall it I play safe.. I just mount it solid and then throw a damp rag at it :D

 I did'nt  think seaweed would have much chance against  fast/torque  brass prop much
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Stavros

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Re: How to measure Amps about 10 amp limit
« Reply #8 on: April 15, 2008, 05:32:50 PM »

What a Joke so you think you can get it repaired eh well think again I have had 2 of mine at Electronise for over 4 months DON'T WASTE your money but Action BUY with confidence that is what I would do


Stavros
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malcolmfrary

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Re: How to measure Amps about 10 amp limit
« Reply #9 on: April 15, 2008, 09:56:03 PM »

The big problem with weed is not stopping the motor but gathering a huge clump of itself around the prop like candyfloss, stopping the prop actually shoving water.  Monofilament left by anglers, however, can work its way between prop and shaft tube and create an amazing amount of drag when it melts and re-sets. 
Best cure for weed is a sharp prop, and the best fuse arrangement is next value up from the max running value, but lower than the max that the ESC can deliver.  That way, weed will mostly be minced, the motor will not be over protected, and the boat can be protected against any internal problems that the ESC might develop.
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