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# Finding Motor amperage under load?

Posts: 2
edited 2005-02-02 - 06:29:21 in Robotics
How Do I find out the amps that a particular motor will pull under load? Ive got these motors rated 1 amp no load or 4 amps no load using different connectors on the same motor and I need to know what it will pull under use?
Thanks

• Posts: 200
edited 2005-02-01 - 05:31:29
Maybe a ammemeter? I'm also looking for the same thing.

Matthew
• Posts: 206
edited 2005-02-01 - 12:30:51
I have used an ammeter to measure motor current.·I found that the·numbers I measured were in the ball-park·with what had been published for the motors.

Russ
• Posts: 1,563
edited 2005-02-01 - 14:52:55
If you didn't want to break the circuit to measure the current everytime....then put in a large rated resistor.· Say 10Watts.· Ohmage....maybe 10ohms.
So, then you would just measure the voltage dropped across this resistor and that would give you an idea of the current going through.

So, ohm's law being V=I*R; I=V/R.·· So if you are measuring 40Volts across the resistor: 40volts/10ohms is 4Amps.
Now if the power equation is P=V*I then you have 40*4 = 160Watts.· Hmmm....you'd need a 160Watt resistor?· That's huge?!

If you use a 1ohm resistor: then you'd measure 1Volt for every amp.· Give that at 4amps, you'd have 4Volts across a 1ohm resistor.
So, power would be 16Watts.· Ok, so look for a 20Watter if you can....it'll make a nice heater for sure!

This is nice ifyou have to make common measurements....if not, then buy a clamp meter.· They're great....you don't need to break the circuit...but you do need to know how to use one.· It's easy...clamp one wire not both!· hehe·
You can get them for AC only or AC&DC (pricier).

Amprobe has some nice ones!

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Steve
"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."
• Posts: 73
edited 2005-02-01 - 15:55:56
Here's a chip that'll allow you to monitor current using the Stamp. \$5 from Digikey.

http://rocky.digikey.com/scripts/ProductInfo.dll?Site=US&V=175&M=MAX472CPA

Here is a tutorial on using these types of chips.

http://www.maxim-ic.com/appnotes.cfm/appnote_number/746

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Name: Bruce Clemens

Work:· [url=mailto:Clemensb@otc.edu]Clemensb@otc.edu[/url]
Good Stuff on my Bolg: http://theDeadBug.journalspace.com
• Posts: 262
edited 2005-02-01 - 16:57:37
Another approach is to use a lab power supply with AMP display. I use one for testing and it is great to check motors.
You can disable all the motors except the one you want to check or any set of them.
By the way, if you are using servos it is very interesting to look the AMP figures when the motor is doing different works. Under some efforts it takes much more power. Sometimes·a minimal bad adjust in the motor mount gives a huge difference in the current they take. When you listen some noise coming out from the motor it is taking probably a lot more current than expected.
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• Posts: 247
edited 2005-02-01 - 20:40:25
Motors draw current as a function of load. I assume you're using a DC motor?

Measure the resistance across the leads of your motor (if/when the contacts are touching, depending on what type of motor it is), and this is basically the non-moving max-load current draw. When the motor's moving, it generates its own back-EMF which reduces the current. Under max load (no movement), there's nothing to produce this back-EMF, so it's just a resistor. If you measure the resistance of your motor to be 4 ohms, not moving, then under 9V it's be pulling 2.25A. This is a worst-case scenario, and it's the instantaneous current your motor will draw on start up.

Look at the graph in the upper right hand corner of page two of this PDF document.
http://www.catalogue.crouzet.com/pdf/ENG/ndb12_eng.pdf

Hope that helps,
Dave
• Posts: 2
edited 2005-02-02 - 03:29:45
Thanks for the replies
• Posts: 200
edited 2005-02-02 - 06:29:21