I could use some information from someone who really knows DC motors.

I'm fooling with a treadmill motor I found. This is a cheap permanent magnet motor. It's my second. I used the first one in a belt grinder. I took the armature out so I could machine a keyseat in it, and when I put the motor back together, it ran fine.

Now someone is telling me treadmill motor magnets lose magnetism when the armatures are removed, even for an instant. He says you have to put an iron "keeper" between the magnets to protect them.

I looked this up and found some references. What I gathered was this: modern cheap DC motors use barium-ferrite magnets which do not require keepers. I read that old motors used iron or Alnico, and that these substances were very poor choices because they required keepers and could also lose magnetism from things like vibration. Barium-ferrite is supposed to be much better, although it's inferior to rare earth magnets.

The person who says treadmill motors need keepers claims he ruined three motors in a row by taking the armatures out briefly. His motors were the same brand as the motor in my belt grinder, which works perfectly.

What's the real story? Also, if I do need a keeper, I'll have to make it myself, so what exactly is it?

Thanks.

 

Another imaginary problem.
I had a DC motor apart for a week in 1984 and it didn't suffer a bit.

 

Over the years I have removed armatures for overhaul with never any apparent problems, I believe the rare earth magnets used now are not that susceptible to this.
If you are concerned, you just need to place a steel pipe in the rough dia of the armature when you slide the armature out.
I am using some old field magnets I have removed over the years and they still appear to be as strong as ever.!

On one site, one manuf of stepper drives claims it will happen to any stepper motor that is dismantled this way.
I would have thought that steppers would be the least susceptible.
Max.

 

Your replies are completely consistent with what I've read on my own, and with my own experience. I don't understand how this guy ruined his motors.

Max, does your experience include treadmill motors? I believe they use ferrite-barium magnets, which are better than Alnico but inferior to rare earth.

He says the armatures were hard to pull out, and then when he put them back in, the pull was much weaker. He says the motors don't pull the way they used to, and they suck "huge current."

I read that remagnetizing these things is very quick, but you have to have a lot of wire and some suitable chunks of iron.

Doesn't sound like a big concern, since, as you say, I can find a piece of steel and jam it in there.

 

Well the symptom would be a larger current when ran under the same load, although another symptom would be a much higher off load rpm for the same voltage in.
Most of the T.M. motors are Chinese origin, and China has the market on rare earth magnetic materials, AFAIK!
Max.

 

I don't know what to think. My grinder is wonderful. I just fired it up and cleaned up the end of a huge wrench in about 45 seconds. I had to saw the wrench in half in order to make a tool.

I guess it doesn't matter. I just hate believing old wives' tales and other forms of BS.

 

Late to the conversation, but, doesn't the steel shell of the motor also count as a "keeper"? At least when no armature is present.