### Author Topic: Finding voltage and wattage of unknown DC motors  (Read 21729 times)

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#### Xyver

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• Posts: 125
##### Finding voltage and wattage of unknown DC motors
« on: July 09, 2010, 12:48:06 PM »
How do you find ou the specs for DC motors?  I have a few that I salvaged from old electronics, but i dont know how much power to put into them.  Trial and error?  But what means error?  Overheating? If I put a 9 volt to them, it kinda starts to smell like melting plastic after about 30 sec so i think thats too much...

#### random robots

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• he drives worse than my robot... thats sad
##### Re: Finding voltage and wattage of unknown DC motors
« Reply #1 on: July 09, 2010, 01:08:10 PM »
the best i can think of is to run the dc motor as a generator at 16000 rpm and connect that to a multimeter for results.

hope this helps,
patrick
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#### waltr

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##### Re: Finding voltage and wattage of unknown DC motors
« Reply #2 on: July 09, 2010, 03:03:42 PM »
Well it will take some measuring and guesses.

Start by measuring the DC resistance of the winding. This will give you an idea of the current draw (Stall current) at different voltages.
You can guess at the possible power rating (input power in Watts) by comparing size to other motors then calculate what voltage would give this power input.

Another guesstimate technique may be to apply a voltage and measure the temperature of the motor with no load. If the temperature rises more than a few degrees then the voltage (power input) in too high. By starting with a low voltage and measuring the temperature as the voltage is increased, waiting a minute between voltage increases, you can plot a curve of temperature verse voltage. Then a good guess of the max voltage could be determined.

#### Razor Concepts

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##### Re: Finding voltage and wattage of unknown DC motors
« Reply #3 on: July 09, 2010, 04:36:57 PM »
Post a picture of the motor. There are many standard packages of motors that have standard voltage ranges

#### Alfa_Zulu

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• Posts: 92
##### Re: Finding voltage and wattage of unknown DC motors
« Reply #4 on: July 09, 2010, 07:26:00 PM »
just start with AA batteries or something, slowly increase the voltage until it seems to be at a comfortable speed and not getting too hot, then grab a multimeter and put it in series with the motor and select DC amps (a conventional ammeter will work fine to) then record the current it draws at that voltage and multiply its voltage by its current

i.e. P=VxI

hope this helps

#### Soeren

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##### Re: Finding voltage and wattage of unknown DC motors
« Reply #5 on: July 10, 2010, 06:29:12 AM »
Hi,

And don't forget to get the loaded parameters too, as this is the most important. Making curves of speed vs. load will enable you to find the sweet spot where its max. efficiency lies.

If you increase voltage by eg. 5% each step and then wait until the temperature has stabilized, up to a temp of say 40..45°C outside casing with each amount of load (increase load at each attempt a similar amount).

Stall torque can be measured by using a wide "wheel", with an outside dimension of eg. 2" (precisely). Mount a piece of string on it and let the wheel outside a table edge. Add weight to the other end of the string until the motor can hold it, but not lift it further.
If the weight is 2.3 ounce and the wheel has a radius of one inch, the stall torque is 2.3 ozf-in and the stall current should be measured with the same setup, but do it quickly, don't have this magnitude of current going through the motor for long, to avoid burning the windings.

A Lab PSU with adjustable current limiting is the best supply for this.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Engineering is based on numbers - not adjectives

#### Xyver

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• Posts: 125
##### Re: Finding voltage and wattage of unknown DC motors
« Reply #6 on: July 11, 2010, 05:52:11 PM »
Ok I tried running it at 16000 rpms with my dremel, and it said about 3-3.5 volts.  So, I then attached it to 2 AA's, but it seemed to squeal when it was running, then when i stopped it and smelled it it smelled like melting plastic a bit, but it wasn't any hotter.  Ill try the other ways and get some pics as well

#### Cristi_Neagu

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