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Author Topic: How to test what a motor is drawing..  (Read 3410 times)

The long Build

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How to test what a motor is drawing..
« on: November 16, 2007, 09:09:40 pm »

How do you test what ampage an actual motor is drawing underload.. I have a multi meter but other than checking a voltage Have no Idea how to check for this..

TLB
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boatmadman

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Re: How to test what a motor is drawing..
« Reply #1 on: November 16, 2007, 09:23:51 pm »

Hi,

You need to set the multimeter to amps dc with a range greater than you think the motor will draw.

Disconnect the  positive forward lead between the motor and esc, connect the positive lead of the meter to the esc output lead, the negative lead of the meter to the motor terminal, turn on the handset and boat system and, with the prop in the water, push the motor to full speed and the meter should tell you how many amps its pulling.

By positive forward lead I mean the one that carries the highest voltage when the motor is driving ahead. find this out by running the motor ahead and with the multimeter set to dc volts, check the voltage difference across the motor terminals. you will want the one with the high voltage.

Hope this helps

Ian
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Shipmate60

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Re: How to test what a motor is drawing..
« Reply #2 on: November 16, 2007, 09:30:26 pm »

I use a voltmeter/ammeter unit I made with Tamiya plugs on each end.
I plug it into the battery and into the ESC lead.
This gives running voltage, so checks the battery condition and amps drawn by the motor(s).
I can load the motors by gently pinching the coupling to give a more realistic running amperage.
You can see the unit on the deck of the model.
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The long Build

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Re: How to test what a motor is drawing..
« Reply #3 on: November 16, 2007, 10:22:18 pm »

Thanks for the explanation there , new I should have listened at school O0
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cos918

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Re: How to test what a motor is drawing..
« Reply #4 on: November 16, 2007, 11:14:26 pm »

hi there if you still have the box your motor came in it should have the free running and stall current on it. The current you are looking is some were between them. its sort of a  guide of what current to expect. john
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The long Build

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Re: How to test what a motor is drawing..
« Reply #5 on: November 16, 2007, 11:29:19 pm »

Hi John
No the motor is one stripped out of I think a old hand vac possibly , rated at 4.8 volts being powered of a 9vlt battery high I know but it keeps blowing 10 amp fuses which I would not thought it should do , so after speaking to some friends , they advised to see what the motor was drawing in-case of bent prop etc. , then realised I had no Idea how to test this..
Cheers anyhow..
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Colin Bishop

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Re: How to test what a motor is drawing..
« Reply #6 on: November 16, 2007, 11:33:57 pm »

If it's drawing over 10 amps then you may need a different motor unless you have a huge model.
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cos918

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Re: How to test what a motor is drawing..
« Reply #7 on: November 16, 2007, 11:38:14 pm »

just out of intrest what boat you got it lenth prop size etc john
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The long Build

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Re: How to test what a motor is drawing..
« Reply #8 on: November 17, 2007, 09:12:28 am »

Hi John
Its only a 20 inches long , built years ago from a model boats plan "Bobby" as a mess about boat for a bit of fun when I was  a lot younger, I use it now just to experiment with electrics, wiring etc..Oh and to chase off the ducks on the pond when they get a bit close although they tend to ignore it noway days. Prop is 35mm with a 1850+ model power battery.(Prob over powered)
I am going to test it in the bath later as is and then change all the wiring still using  the current motor and then swap it for a electronize one and see if there is any change..
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FullLeatherJacket

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Re: How to test what a motor is drawing..
« Reply #9 on: November 17, 2007, 09:33:44 am »

If you're popping 10A fuses with a set-up like that I would advise you to redirect the motor to the bin......... A 35mm prop should be fine on an Electronize 540 or 545 as long it's a scale type i.e. not a surface-piercing thingy, but I'd not use 9.6 volts if I were you! I reckon a cheapo Sub-C sized pack with six cells  (7.2v) would do the trick, and you'll probably get one for next to nothing if you don't insist on huge capacity. Try Component Shop or Model Power.....but bin that motor anyway!!
Suit yourself.
FLJ  8)
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cos918

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Re: How to test what a motor is drawing..
« Reply #10 on: November 17, 2007, 09:39:24 am »

hi have to agree with FLJ 10 amps is very wrong for that set up.If you take the motor out of the boat check the free running of the prop shaft. Because if the nuts at each end were over tightened this would make the propshaft hard to turn and cause high current draw. john
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boatmadman

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Re: How to test what a motor is drawing..
« Reply #11 on: November 17, 2007, 09:59:05 am »

I recently measured a basic 540 type motor, it pulled 6a under no load! So you may have something similar.

I binned it and replaced with an electronize 545, think it was less that 2a no load.
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John W E

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Re: How to test what a motor is drawing..
« Reply #12 on: November 17, 2007, 11:10:55 am »

Hi all

I have scanned a couple of graphs and bits & pieces from books that I have.  They give various outputs of different motors.  I know the articles are 'old' but they are still relevant to today's motors and it should give you some ideas of what amperage/voltage the motors will use.  Do not take these as gospel though.   I use them to give me a rough ballpark figure so, when I actually do an amperage test, using an amp meter with the motor and the model, I know what I am aiming for.  If you have any problems reading the graphs, I suggest you download them onto your PC and then enlarge them.   

I hope this is of some help.   As I have said they have helped me in the past.

aye
john e
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John W E

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Re: How to test what a motor is drawing..
« Reply #13 on: November 17, 2007, 11:12:34 am »

and more......

john
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John W E

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Re: How to test what a motor is drawing..
« Reply #14 on: November 17, 2007, 11:14:13 am »

the last chart comes from the Company Electronize and its from their findings on different motors.

aye
john e
bluebird
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malcolmfrary

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Re: How to test what a motor is drawing..
« Reply #15 on: November 17, 2007, 11:17:31 am »

In this day and age, I would reccomend using a digital meter.  At the bottom of the range they are not much more expensive, if any, than analogue ones, and you dont have to be as careful about connecting them up the right way round.  There is no needle to wrap round the stop - you just get a reading of "-" whatever it was.
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Shipmate60

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Re: How to test what a motor is drawing..
« Reply #16 on: November 17, 2007, 11:30:32 am »

TLB,
If you go back to my pic. Those motors (MFA 800's) with 50 odd mm props only pull 2 amps each in air even in this cold weather (propshaft grease thicker).
As suggested bin that motor!!

Bob
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HS93 (RIP)

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Re: How to test what a motor is drawing..
« Reply #17 on: November 17, 2007, 11:44:29 am »

I use this method only because I've been using it for a few years to monitor charging of nicads etc. I use a 0.1ohm resistor of "xxx" high a wattage as I can get but a 5 or 7 w is ample and connect it up as picture. it has The advantage of it it exceeds the max on your dvm it does not blow the fuse on amps as can happen with some. I use it when aligning motors to shafts . you can make a set of leads with a switch so instead of moving one wire the switch does it. I find it works for me  the only other thing is that when you read amps you do not have to alter the dvm just move the decimal point one to the right.

Peter
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Re: How to test what a motor is drawing..
« Reply #18 on: November 17, 2007, 03:29:13 pm »

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The long Build

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Re: How to test what a motor is drawing..
« Reply #19 on: November 17, 2007, 03:55:01 pm »

To all
Thanks for all the help, will prob bin (storage) the motor as FLJ says , but now I have been heducated in working out the amps drawn I will test it any way..and then test with the new motor , should be intersting, well to me anyway. :D

Cheers Bluebird for the graphs will download them for future reference..

Always forget about Martins Useful information Guide to modelling on the front page,

Thanks all ..
Larry
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