Understand motor behavior

Hi, I really want to understand the motor behavior. After reading many posts, I decided to experiment with it.

I use a run of the mill motor taken out of an old RC car, I have no idea what the spec is, the only thing I know is that it has a 40ohm resistance.

On my bench power supply I set the voltage to 6V and I gave the current 3A.

Now when I power up the motor, initially the ammeter indicates that it draws 0.5A, after 5 minutes or so, the ammeter starts to drop and it fluctuates quite a bit. Then after 20 minutes or so the current continues to drop then it goes down to 0.17A and becomes very steady.

The motor is under zero load condition all this time.

My question is is about the current drop. Is it because because the back EFI causes the fluctuated current until it gets stabilized?

Thanks.

It can't have a 40 ohm resistance if 0.5A flows at 6V. When you measure the resistance of a brushed DC motor you sometimes have to turn it a bit and find the position with the lowest resistance through the brushes.

It sounds like the motor was old, dry bearings, dusty. As it and its bearings warmed up the mechanical friction changed and decreased. You may find a drop of oil on each bearing improves things even more.

The winding resistance will also increase as the copper heats up, which will be part of the effect too.

Brushed motors have significant friction from the brushes, usually dominating unless the bearings are dry.

You are wrong about back EMF, such effects are instant (from a human perspective).

Mark's explanation is right on. Basically, all you are going to understand using an old motor is the behavior of old motors. I don't think that will be very useful. If you truly want to understand motor behavior, buy a new motor.