Trying to determine a motors voltage

[RAF96]

I have a few motors without any markings and wondered if there is a way of finding out their service voltage without using specialised kit.

 

One way is to ramp up the voltage until the magic smoke appears, but I would like a less destructive way.

 

Starting with a pair of micro motors 6mm x 12 mm which spin up on 50mA from a Maplins led test box.

 

Rob

[kevinlms]

I have a few motors without any markings and wondered if there is a way of finding out their service voltage without using specialised kit.

 

One way is to ramp up the voltage until the magic smoke appears, but I would like a less destructive way.

 

Starting with a pair of micro motors 6mm x 12 mm which spin up on 50mA from a Maplins led test box.

 

Rob

Try some AA batteries and gradually add more in series and see what happens. Could be that they are 3, 6, 9 or 12 volt. It is a subjective test, but without any information to go on, not much else you can do.

[DCB]

Get a Morley or OnTrack power unit. They will deliver a very nearly constant voltage almost irrespective of the load, I use one to test LEDs on one of the lower settings and yet it runs heavy Hornby Dublo locos at a scale 100 MPH.  Just stick a Multitester across the motor and read the voltage as it increases from zero.  In fact zero is not exactly where the knob says and the voltage changes seamlessly from negative to positive at zero volts, there is no isolated off position. At some point a low voltage motor will be screaming its head off before throwing its windings and chucking in the towel.  H&M Variable Transformers are similar but minimum voltage is around 5 volts off load

To use motors on cheap and nasty resistance controllers you need a motor which can cope with at least 15 volts as plenty of controllers deliver 20 volts. 

[AndyID]

Hi Rob,

 

You can get some idea of the voltage rating by measuring the motor's resistance with a meter. Small 12V motors (eg a 1420) typically have a resistance of 30 to 45 ohms. Large 12 volt can motors like a 2236 will have a resistance of around 12 ohms. You should be able to scale the numbers from there. So, if you have a small motor with a resistance of maybe 10 ohms it's probably only good for 3 to 4 volts.

 

Obviously this isn't very precise, but it should get you into the right range.

 

Andy

[RAF96]

Thanks all.

I may trawl for data sheets to find representative resistance figures for various size motors and their voltage ranges to see if there is any correlation.

Rob

[AndyID]

Thanks all.

I may trawl for data sheets to find representative resistance figures for various size motors and their voltage ranges to see if there is any correlation.

Rob

 

Rob,

 

If you can find a manufacturer that makes versions of the same motor with different maximum voltage ratings you should see that the resistance is approximately proportional to the voltage rating. If they don't specify the resistance you can determine it if they specify the stall current at a particular voltage. Just apply Ohm's Law (R = voltage over current).

 

The torque produced by a DC motor is a function of the ampere-turns (current times the number of turns) wound on the armature. Lower voltage motors use fewer turns and larger cross-section wire that can handle greater currents. Higher voltage motors use more turns of smaller gauge wire (more resistance), but in both cases the ampere-turns are roughly equal for different voltage versions of a motor.

 

Andy