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Author Topic: Measuring Current draw?  (Read 1039 times)

aamcle

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Measuring Current draw?
« on: July 26, 2018, 09:07:05 AM »

This seems like a good place to ask.


What do I need to measure the current drawn by the motor, I fairly sure my multimeter although it measures Amps would be fried.




Thanks All.       Aamcle
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john44

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Re: Measuring Current draw?
« Reply #1 on: July 26, 2018, 09:26:29 AM »

Hi, there is a product from hobbyking called a Watts-up meter, component shop also do
A similar unit, they measure watts amps, volts including peaks. They connect in between
Battery and load and scroll through the readings until the supply battery is disconnected
They will read 100 amps. If I am wrong I will be corrected by the experts.
The meters are small enough and light enough to be left on the craft on your voyage
So you get actual performance readings so you know what size fuses to fit.


Hope that helps, (most multi-meters only read upto 10 amps)


John
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malcolmfrary

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Re: Measuring Current draw?
« Reply #2 on: July 26, 2018, 09:35:00 AM »

Many digital meters have a 10A range.  Being digital, they can display up to 19.99, but the test leads might not live through that.
Apart from meters that have external "range resistors", which are neither common nor cheap, an old world solution was to use an ammeter from a car scrapyard, which could be had in 30 and 60 amp versions.  And things like the watts up meter.  Some might record the figures for posterity.
Then there are clip-on or clamp meters that you clip over the wire without interrupting it and take a reading from the magnetic field generated by the current.
The down and dirty method is to get a bag of assorted fuses and experiment, noting the two values where failure happens regularly, and the higher value where it doesn't.
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aamcle

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Re: Measuring Current draw?
« Reply #3 on: July 26, 2018, 10:00:08 AM »

Thanks Gents.




Aamcle
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plastic

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Re: Measuring Current draw?
« Reply #4 on: July 26, 2018, 10:34:43 AM »

When measuring current out of the water, you are just measuring the friction and inefficiency of the drive train.

You can do some interesting experiments seeing what is going on mechanically.

With no losses and no bearing/grease friction, the current should be almost nothing.

With multiple motors, any differences between them might mean you are looking at slight misalignment of propshafts and tight thrust washers.
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Bob K

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Re: Measuring Current draw?
« Reply #5 on: July 26, 2018, 11:27:14 AM »

I had had a similar problem.  I purchased a cheap 30VDC ammeter from Ebay, around 4.

I connected it in series in the positive DC line from battery to propulsion system.
The wired-in ammeter was temporarily stowed in the hull.
This gave me reasonable measurements, both in air, and in the water.

It is of course the readings whilst the ship is afloat that you are looking for.

Dependent on the current drawn a regular multimeter may or may not be suitable.

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Colin Bishop

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Re: Measuring Current draw?
« Reply #6 on: July 26, 2018, 11:47:09 AM »

Also worth bearing in mind that the current draw when the model is stationary is likely to be slightly higher than when it is in motion as there is more resistance to the prop(s).

Colin
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aamcle

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Re: Measuring Current draw?
« Reply #7 on: July 26, 2018, 02:29:02 PM »

Now I know what to look for I've found some items on "the bay " that seem to offer the functions I want.


Next I need to modify a 12v power supply to run a hot wire cutter and I can make a start, I picked up a lifetimes supply of XPS foam today.




Atb.     AAmcle







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NickelBelter

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Re: Measuring Current draw?
« Reply #8 on: July 27, 2018, 04:38:49 AM »

You can make a shunt resistor for your multimeter to allow it to measure amperage over the internal fuse limitation.  You can also buy them pre-made if you feel like burning up some cash. 
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aamcle

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Re: Measuring Current draw?
« Reply #9 on: July 27, 2018, 08:10:43 AM »

I can pick up a device on the bay for so little that making something isn't a great option.  :-))


Aamcle




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david48

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Re: Measuring Current draw?
« Reply #10 on: July 27, 2018, 08:20:29 AM »


Could Ohms law be used  measure current ? .
David
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aamcle

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Re: Measuring Current draw?
« Reply #11 on: July 27, 2018, 08:29:55 AM »

I don't believe so or at least not in a useful way as the amount of current drawn by the motor will change with load.




Aamcle
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malcolmfrary

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Re: Measuring Current draw?
« Reply #12 on: July 27, 2018, 08:59:46 AM »

Could Ohms law be used  measure current ? .
David
You do indeed use Ohms Law to figure out the value of shunt resistor needed to drop a suitable voltage for the voltmeter to read as current.  To do that you need to know the characteristics of the meter being used so that the numbers that it shows you actually mean something.
Motors rely on being inductive devices to work, rather than plain resistance, they also have impedance.  This varies with the switching rate and the speed of rotation of the motor and is also affected by the actual construction of the motor.  Because of that, a simple resistance measurement of the motor and knowing the supply voltage doesn't give any meaningful clues as to what current it will take under load.  Thats why an ammeter is used to get the actual number in one go.
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john44

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Re: Measuring Current draw?
« Reply #13 on: July 27, 2018, 11:27:19 AM »

With Ohms law, it states that current is proportional to the voltage which is inversely
Proportional to the resistance the resistance being constant and in Ohms.
Great for working out a set loads but not variable loads like motor acceleration
Because the resistance is not constant.


John
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bfgstew

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Re: Measuring Current draw?
« Reply #14 on: July 27, 2018, 12:25:51 PM »

You can make one very cheaply and it works. Tested it against high end Fluke clamp meter costing 300+, within 2 to 4%, I didn't have correct diameter copper rod, mine was a fraction under size, so compensated by adding a few mm on the overall length, hence the 2 to 4% out of tolerance. Make it as instructed and you will have an accurate shunt for pennies.


https://www.rcgroups.com/forums/showthread.php?393591-Home-Made-Current-Shunts-for-Measuring-Motor-Current
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malcolmfrary

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Re: Measuring Current draw?
« Reply #15 on: July 28, 2018, 10:06:59 AM »

You can make one very cheaply and it works. Tested it against high end Fluke clamp meter costing 300+, within 2 to 4%, I didn't have correct diameter copper rod, mine was a fraction under size, so compensated by adding a few mm on the overall length, hence the 2 to 4% out of tolerance. Make it as instructed and you will have an accurate shunt for pennies.


https://www.rcgroups.com/forums/showthread.php?393591-Home-Made-Current-Shunts-for-Measuring-Motor-Current
If it is smaller diameter, it needs to be shorter.  Larger diameter, lower resistance for a set length, so more length for the same resistance.  Smaller diameter, the same length has more resistance, so it needs to be shorter.  But either way, if it is a case of needing to ask, a pre-built item is needed.  For ease of use, the clamp-on type of meter is probably the easiest.
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