Author Topic: Motors...?  (Read 1849 times)

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Offline zabuza™Topic starter

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Motors...?
« on: March 07, 2010, 10:52:59 AM »
Motors...?

Any motors have their data on a data sheet, they all contain basic data like stall torque, stall current, free running rpm, free running current, gear ratio and the voltage it is at. (By free running i mean it doesn't have any load)

from above data i want to know how i would calculate the
1.power of the motor at a load
2.current of the motor at a load

i googled this, but there are so many different equations iam getting so confused

thank you in advanced for the help ^^ :-*
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Offline waltr

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Re: Motors...?
« Reply #1 on: March 07, 2010, 11:55:31 AM »
Quote
i googled this, but there are so many different equations I am getting so confused

Yes, this is not easy to calculate at all. Most of the inexpensive motor we use do not have the full motor data required. Take a look at more expensive motors like these:
http://www.micromo.com/n112053/n.html
and look at their data sheets. There is much more information.

Another piece of information is the Speed-Torque curves (http://www.micromo.com/n390432/n.html). This is what is really needed to calculate the power and current at 'load'. It is possible to measure your motor and create a speed-torque curve. Not the easiest but doable. The efficiency varies greatly from motor to motor but the other values are fairly linear so this curves can be created from some basic data.
MicroMo also has a number of fairly good motor tutorials that should help.

Another way is to take some guesses that fall between no load and stalled. Then calculate the estimated min and max power/current expected.
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Offline zabuza™Topic starter

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Re: Motors...?
« Reply #2 on: March 07, 2010, 02:46:54 PM »
So the best way to actually find if a motor is good for moving some load is by testing it? or having more data.
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Offline waltr

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Re: Motors...?
« Reply #3 on: March 07, 2010, 03:55:21 PM »
More or less that is correct. Since some of the curves are linear you can do these with minimum data.
Look at the Speed-torque curve in the second link I posted (scroll to bottom) and read how to produce this curve.

If you have the no load motor current (easy to measure), the stall torque (most motors have at least this info), the no load RPM (not hard to measure and most motors also have this info) and the Stall current then a speed-torque curve can be generated. The output power and the efficiency can be calculated from these curve as described in the link with the addition of the motor's terminal resistance.

The only value that is hard to measure is the Torque but using the Stall torque from the motor specs will work.
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