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Author Topic: multimeter question  (Read 885 times)

Trucker

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multimeter question
« on: October 17, 2019, 06:08:44 PM »

can some one explain why i havent got the numbers i was expecting on my digital multimeter, in preperation for the install of the running gear i thought i would follow a few youtube vids and work out the load / stall / and no load amps on the two M500 brushed motors, with every thing hooked up i get 1.95amps with no load, but the stall reading seems to race through the numbers on the meter and settle on 1, on left of the screen, i have the setting of the meter on 10amps and the red lead in the 10amp jack. i also have a data sheet for the motors which say stall 21 amps, is it as simple as my meter isnt capable of reading the stall on these motors...
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Taranis

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Re: multimeter question
« Reply #1 on: October 17, 2019, 06:23:23 PM »

Yes you need a higher current range
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ANDY
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DaveM

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Re: multimeter question
« Reply #2 on: October 17, 2019, 06:32:53 PM »

is it as simple as my meter isnt capable of reading the stall on these motors
That's the answer. Few multi-meters will offer a scale of more than 10A DC and those that do will cost a lot of money. You'd be better off buying something like this http://www.4-max.co.uk/wattmeter-budget.htm
No doubt there are other very similar units - and probably cheaper - but this is the one I bought a while ago.
DaveM
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roycv

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Re: multimeter question
« Reply #3 on: October 17, 2019, 06:43:02 PM »

Hi Trucker you are right the current reading has gone off the scale.  Not a good idea to stall a brushed motor for longer than a second or two as it will affect the magnetic field in the magnet.

Magnets are created by zapping them inside a coil of wire for a very short time with a high current.  I looked into how to do this and all I can say is don't go there!

It is much more useful to measure the current when the prop is in the water and running at full power.  The current read then can be multiplied by the actual voltage and if you want accurate numbers you need to measure the voltage when on load as it will be lower than the nominal voltage.

This will give the power used which is measured in Watts.  However when the boat is travelling the current will be less.
 
There is a rule of thumb that maximum efficiency is reached when the current is 20% of the stall current, So about 4 amps with your motor.  The power consumed is determined by the size and pitch of the propeller.

regards
 Roy
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RST

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Re: multimeter question
« Reply #5 on: October 17, 2019, 07:15:53 PM »

If you're lucky the internal fuse blew and you'll always see a reading off the scale now. If not it didn't blow and knacked the meter. Both have happened to me before but a half decent meter should be ok. Suggest you unscrew the back and check fuse. It could all be ok. It used to be a case of nipping quickly to maplins for a new one but not now. Local electrical outlet will probably have one.
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Trucker

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Re: multimeter question
« Reply #6 on: October 17, 2019, 09:02:53 PM »

guys thanks for your prompt replies, i guess im going to need to make friends with a sparkie in order to loan his gear, good job my neighbour is one :-)) , my own meter has survived the extra power going through it so will fight another day..
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microgyros

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Re: multimeter question
« Reply #7 on: October 17, 2019, 11:52:23 PM »

Open up the meter and take a look inside or look at the youtube vids to see why it survived, and with no fuse in 10 amp range. The meter measures via the voltage drop across a wire shunt that internally connects the 10 amp jack to -ve.

The data sheet's 21 amps is the useful number you want. It is virtual, determined graphically by extrapolation and referred up to 75 Celcius. By direct measurement you get a ball park figure. The complicating factors in the motor are magnetisation and copper temperature coefficient.
Another meter may be the easiest way for you. However you could:-
1) take a measurement at lower voltage and then refer it upwards  (sensible and perfectly valid)
or
2) measure volt drop across a 0.01 Ω resistor, in a similar way to the internals of the meter connected in the 10 range.

Copper resistance/temperature coefficient is high. Multiplied by 100 C temperature rise shows resistance increases by the order of 40%. That is why:-
1) the meter shunt and resistor material is an alloy.
2) as the motor winding heats up the current drops before your eyes.
3) a rough home made copper shunt is only within calibration in a particular current/ temperature point.
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Taranis

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Re: multimeter question
« Reply #8 on: October 18, 2019, 09:04:15 AM »

Far more simple and easy to comprehend is to fit an auto fuse holder with a 25amp blade fuse. If you can't get it to blow reduce to a 20 amp and so on.
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ANDY
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