I'm looking for data on how the ripple current of a DC brushed motor changes over the lifetime of the motor.

Intuitively, it would decrease as the magnets lose some of their field strength, but I can not find any data or specs that confirm this. Looking at a paper called, "Brush wear detection by continuous wavelet transform," I found some graphs that seem to indicate the ripple increases, but local minima/maxima were not the central point of the paper so I can't draw any definitive conclusions.

Can anyone help?

 

We tried this one time. There seemed to be multiple effects if i remember correctly. The age (thickness) of the brushes is fighting the effect of the weakened magnetic field of the permanent magnets. Thinner brushes means less resistance to the power supply (?? don't remember for sure - but it wasn't straight forward).

 

That is mostly what I've found as well. There are a lot of papers available on how the brushes wear but nothing on how the wear is going to change the magnitude of the ripple (just how it changes the skew on the rising edge). None of the ones I've seen have included magnet degeneration and focused on the ripple.

I have a gauss meter so I can remove magnets from motors and see how they deteriorate but it is hard to get a statistically significant sample without spending $$$$.

 

I think you will find the applied voltage to obtain a given RPM will decrease as the magnets loose strength. Weaker magnets produce weaker back EMF at a given speed, so less voltage is required to obtain a given speed. That's the main reason for having "field loss relays" for wound shunt motors. Prevents over-speed run-away. Industrial brushed PM motors have a Kv rating that is used to check field strength.