Running-in electric motors

This article gives advice, hints and tips from an experienced club modeller. Please note that the SRFC accepts no liability or responsibility for any loss or damage resulting from actions taken by anyone acting on advice given in these articles. If you have any questions or would like to contact the authors please contact us

Just like internal combustion engines, electric motors need to be run-in before they can realise their full potential – similarly, if not run-in properly, parts of the motor can be damaged irreparably – in the case of an electric motor, the commutator & brushes. With brushless motors, a short, slow run to ensure the bearings are properly aligned is all that may be necessary, as they don’t have any other parts in contact with the output shaft. On the other hand, the majority of motors seen at our field contain the traditional contact brushes. These brushes are made by the gazillion to suit many different motors and so the contact faces are not curved to fit the commutator, but flat so that they can, over time, wear down to conform to the exact shape of the commutator. During this period, there is very little surface contact between the two, which results in electrical arcing & lack of efficiency, manifesting as the symptoms of high current & heat. If you simply run the motor flat out from the word go, the arcing & heat will score the thin metal of the commutator contacts (in extreme cases, cut through this metal) & form a resistive coating on the contact faces of the brushes – robbing the motor of what little go it had! The only recourse is to have the commutator machined flat & smooth and replace the brushes. With few exceptions, our motors cannot be disassembled so you will have to go and buy a new one…

There is, however, light at the end of this tunnel.
Following the instructions that come with most motors nowadays, you are advised to connect the motor to one or two cells and run it until the cells are discharged. One 480 sized motor I purchased recently stated that this running-in period should last for TEN hours! This is OK, but by running the cells down to nothing can do them harm, rendering them useless for anything else – perhaps you could keep them just for running-in purposes, I suppose.

running_in_electrics.
However, a quicker, safer (for the cells) method is to run the motor under water. That’s right; stick your electrical device into a cup of water! It runs quite happily this way and the water acts in a number of ways on the motor.

  • It’s a coolant, preventing the motor getting too hot & bothered.
  • The turbulence generated in the water carries away all of the particles ground off the brushes – in fact the water goes quite grey & yucky!
  • It’s also a cutting compound, speeding the process up.
I have found that 3 to 4 minutes running submerged makes a useful difference to the characteristics of the motor – more revs for less current drawn (Steeper climb for longer before the BEC gets in the way) Remove the motor and run for another few minutes to clear away surplus water and then oil the bearings – job done!

The speed 600 motor in my soarer started with figures, on a 9” x 5” CAM folding prop & 8.4 volts, of 8500 RPM @ 27 Amps. After running in, it returned figures of 9200 RPM @ 25 Amps – a useful increase in efficiency to better beat the opposition!

P.S.
Just to show there is an exception to every rule, I have recently bought a GRAUPNER SPEED 600 RACE motor and the replaceable brushes are machined to fit! No running in required!

Oh well, c’est la vie…

Tom Gaskin

Last updated on 13-March-07