Hi

Being a complete novice I see lots of stuff about current and to be honest often have very little understanding how it is calculated, bar measuring in series.

I have some small motors and I wondered what is the best way to work out the current they draw.

I read somewhere that you can measure the resistance of the motor then use ohms law to work out the current.

Is this how its done?

Or should it be done by creating a circuit and using a multimeter?

Thanks

 

There are many different types of motors.

What kind are you talking about? AC, DC...?

 

Apologies

DC motor. Lego types and other small ones

 

Ok I get 1280mA for one of my little unbranded motors.

The resistance was only 2.8 Ohms.

My batteries measured at 3.6V.

For a 4v Lego motor I got 34mA, using the same batteries. Its resistance was 105 Ohms.

 

You cannot use the motor resistance for current draw, this depends on the motor load and the degree of BEMF generated related to motor rpm.
Max.

 

Ok, so back to my question how would current be ascertained? Would this have to be from a datasheet?

When posting questions regarding motors people often ask for that value so they can help answer my questions.

 

Actually the best way is to add a very low resistance (around .1 Ω) in series with the motor and measure the voltage on the resistor with an oscilloscope and apply ohms law to get the current. You might be surprised to see that it’s not a flat value.

The easiest way to get temporary .1 Ω resistor is just to use a piece of wire. Measure it with a good meter and don’t forget to add in the resistance of your meter wires.

 

if you use your multimeter or dvm to measure current, be sure you put it in series with the motor, and start at a high range, switching down till you get a good reading. NEVER PLACE AN AMPMETER ACROSS THE LOAD!!!! all that will tell you is wither how long it takes to burn out your meter or how much your supply will put out.

 

Either from the manuf data sheet from the rated continuous torque curve, or simply by measuring current with a meter, as mentioned, this will obviously vary with load.
If the load is not controlled and the motor operated at maximum voltage, the current rating can be exceeded.
An example, if you have a motor rated at 24vdc and you apply this to a stationary motor, the initial current will only be limited by the resistance of the motor (very high current).
The load has to allow the motor to increase RPM rapidly in order for the BEMF to be generated and oppose the supply voltage, therefore limiting the current.
Max.

 

There's an oscilloscope picture of the type of current you'll be measuring at the bottom of this page. I doubt that a meter will show you these values.

http://www.solarbotics.net/starting/200111_dcmotor/200111_dcmotor3.html

 

I haven't got an oscilloscope at present but thanks for clarifying

 

Motor of the miniature variety are usually classed in watts or by torque (Nm = current).
So you either have to know this type of info from the manufacturer or come by it empirically by a series of load test while monitoring the temperature.
Taking an arbitrary reading does not really mean anything.
Look at some of the precision motor web sites for more ideas, Maxon etc.
Max.

 

I haven't got an oscilloscope at present but thanks for clarifying

I wasn’t trying to be arrogant. The current is complex wave form and a meter will not measure it. You will likely get an average of what it see’s. If you select ‘Peek Hold’ it may or may not be influenced by the inrush current, depending of the window of measurement.

A meter will likely only give you an average of see’s which probably slightly better than guessing.

 

The second page gives more information on extracting the motor size empirically.
http://www.solarbotics.net/starting/200111_dcmotor/200111_dcmotor4.html
There is a little bit more to it than this but gives an idea of what is involved.
The data sheet method is a lot easier.
Max.

 

Simpler than an oscilloscope and integral calculus, connect the power source to a small resistance, then add a large capacitor (thousands of microfarads) in parallel with the motor. The capacitor will smooth out the wave form and you can get an average voltage reading across the resistor that is pretty close to the truth.

 

Simpler than an oscilloscope and integral calculus, connect the power source to a small resistance, then add a large capacitor (thousands of microfarads) in parallel with the motor. The capacitor will smooth out the wave form and you can get an average voltage reading across the resistor that is pretty close to the truth.

Like you said, it will give you an average. It won't help so much if you want to control it with a silicone part.

 

These appear to be micro sized motors, so I don't think there is a great deal to worry about.
But this doesn't really address the original question WRT what answer would he give?
I have a T.M. motor that is rated for 2.5HP and 20amps, I don't think I would get much life from it if I were to run this in a continuous fashion at this plate value!
This is where data sheets are important when running/controlling motors and ensuring that they operate withing safe continuous limits.
Max.

Ok, so back to my question how would current be ascertained? Would this have to be from a datasheet?

When posting questions regarding motors people often ask for that value so they can help answer my questions.

 

Like you said, it will give you an average. It won't help so much if you want to control it with a silicone part.

How hard is it to find a transistor that can deliver 2 amps, just in case your measurement of about 1 amp isn't very accurate?

 

Sorry, I work in an environment where, if you can save a penny, you do. In the hobby world where you only need o 1 amp, a 10 amp transistor work just as well as a 2 amp one.

Looking at the measurements posted; the 1.2 amp is probably equivalent to a locked rotor, which should really be blowing a fuse. Just a guess without actual running measurements but this motor sounds like a typical toy type motor that could be controlled equally well with a cheap 2N3904 transistor.

 

Do this neat experiment.

Put a motor, a battery and an ammeter in series. Watch what happens to the ammeter when you apply friction to the shaft. In place of the ammeter, you can also use a small incandescent lamp.