Brushed DC motor draws more current in one direction than the other?

Thread Starter

Rahulk70

Joined Dec 16, 2016
521
HI,

I was recently trying to test the capacity of a battery pack I'm building and was trying to measure the current draw using a 35 Turn 540 brushed DC motor (apparently on the website I had purchased from said something about it being sensored, but I couldn't find any sort of sensor inside). But I did read somewhere that the cuts/notched are related to some sensor.

I connected the DC 540 motor to a 1.2V 7000mAh NiCd battery and measured it current. In one direction it spins a bit faster and reads 1.58A and in other direction it spins a bit slower and draws 1.89A. Both the carbon brushes seem so have even wear on them too. Any idea what could be causing this?

Thanks.
 

Attachments

Thread Starter

Rahulk70

Joined Dec 16, 2016
521
It could be mechanical resistance. Can you feel a difference when you rotate the shaft by hand?
It felt the same I would say when spun by hand. But shouldn't the armature, magnet and bearing behave the same in both directions?
 

Yaakov

Joined Jan 27, 2019
3,483
It felt the same I would say when spun by hand. But shouldn't the armature, magnet and bearing behave the same in both directions?
Not necessarily. If there is something eccentric or asymmetrical it could cause differential friction. If you have light out, you could try to put a drop on the bushings to see if it makes any difference.
 

Tonyr1084

Joined Sep 24, 2015
6,253
Back in the 80's I had a friend (yes, I had A friend) who raced slot cars. He would take his electric motors apart and defeat the alignment of the brushes so he could advance or retard the timing when the brushes contacted a particular winding. HE SAID he can tweak a motor for torque or for RPM. I'm wondering if maybe the brushes on YOUR motor might not be 90˚ to the magnetic field. Or maybe the magnet may have its flux off kilter. Just guessing here. And I don't know if what my friend said was accurate. He seemed pretty smart, so - - - .
 

MrSoftware

Joined Oct 29, 2013
2,015
My guess is the timing is not symmetrical. In the remote control toy world, many of the motors have adjustable timing to change the performance. One way gives you more torque and the other way gives you more RPM. I imagine advancing the timing in one direction will retard it in the other. So that motor, whether by design or by loose manufacturing tolerances, probably has the timing not perfectly symmetrical in both directions.
 

MaxHeadRoom

Joined Jul 18, 2013
23,612
Back in the 80's I had a friend (yes, I had A friend) who raced slot cars. He would take his electric motors apart and defeat the alignment of the brushes so he could advance or retard the timing when the brushes contacted a particular winding. HE SAID he can tweak a motor for torque or for RPM. I'm wondering if maybe the brushes on YOUR motor might not be 90˚ to the magnetic field. Or maybe the magnet may have its flux off kilter. Just guessing here. And I don't know if what my friend said was accurate. He seemed pretty smart, so - - - .
That would be my guess, some uni-directional motors did offset the brushes for optimum performance in one direction.
 
Top