elect motor question

[look 171]

My elec motor knowledge is limited. The Skill Saw thread got me thinking. I have an old Makita job site saw that bog down with load. It basically slows when a piece of simple 3/4 lumber is fed through to the point it just stops. Brushes or?

[908/930]

Sounds like brushes, but could be the switch or just bad electrical connection not letting enough current through. I have also had the springs for the brushes go soft.

[Zeke]

Or, again, the armature could have a local short.

[Steve Carlton]

Sounds like aliens hovering above and dampening the local energy field.

[look 171]

Ok, time to open that puppy up and have a look. Can the armature be solder back?

[zakthor]

Low current? Take the switch apart and see if its blackened.

[look 171]

Originally Posted by zakthor

Low current? Take the switch apart and see if its blackened.

Really, never thought of that? I should bypass the switch and see how it works before I start tearing into it. Thanks

[Zeke]

Originally Posted by look 171

Ok, time to open that puppy up and have a look. Can the armature be solder back?

The copper wires in an armature are wound over each other in layers. Care is taken so that none touch and then the whole thing is dipped in resin. When they get hot often you will not see where the insulation melted and allowed a short. It's likely buried in the windings. A meter can show if the armature is good from the commutator bars on a brushed motor.

See YT for instructions. Incidentally, induction motors can be checked as well using only the leads.

[look 171]

I looked for a new armature this morning but found nothing or its no longer available. The question is, is it worth fixing if its the armature at a motor rebuilding house? I like that little saw. Light, and easy to use.

[Zeke]

I found a motor service in NLB or Bellflower (Or both, IDR) but they were 50 bucks just to set it on the counter.

[look 171]

Armature do not wear out normally? They copper wired just come loose due to usage or age? I see parts offered on ebay from that saw. 80 bucks but who know how good they are?

[Zeke]

I think some wear is to be expected. Brushes were more or less designed to be replaced although I took apart a newer brushed tool that I picked from the trash and disassembled it just for grins. I found that the brushes were not really accessible in a way that it made sense to repair the tool. I tossed in back into the bin after cutting the power cord.

AFA wear on armatures, sure the commutator can get messy and needs a light cut on the lathe. You can do somewhat of a cleanup while it's in the tool The armature itself is a lifetime part of the tool. It all depends on how hard to tool was used and how hot it got. Long skinny extension cords will drop the voltage enough to make them get hotter faster. If a tool feels quite warm, you're pushing it and it needs to completely cool. Not just 5 minutes either because the armature has soaked up all that heat to the core.

High speed tools wear out bearings. I've rebuilt my Makita planer 2wice. It's the 100 model and NLA. The new ones like it approach 500. When parts become NLA for the 1100 I won't have a planer. Once you have the big boy you won't use the little ones.

[zakthor]

I know a place that rewinds armatures (romaine electric) but its special work done only when required and its ‘spensive.

They said they get starters for armored trucks because theyre nla and insurance requires that everything on the vehicle is oem. I think they said rebuilds start at $500.

[908/930]

If it spins up to normal high RPM with no load I do not think it is the armature, from what I have seen burnt armature runs like a 6cyl engine running on 4cyl, and wont hit full speed, quite noticeable that something is wrong Did your brushes and springs look good? I don't think anybody rewinds those, just too much time involved. The last motor I saw that needed to be sent in for a rewind was about 300hp, it released a large amount of magic smoke about 20' away from me.

[look 171]

Originally Posted by 908/930

If it spins up to normal high RPM with no load I do not think it is the armature, from what I have seen burnt armature runs like a 6cyl engine running on 4cyl, and wont hit full speed, quite noticeable that something is wrong Did your brushes and springs look good? I don't think anybody rewinds those, just too much time involved. The last motor I saw that needed to be sent in for a rewind was about 300hp, it released a large amount of magic smoke about 20' away from me.

No smoke and it does go up to speed without load. Maybe I have the incorrect brushes in there? That was the first thing I changed out

[Zeke]

Originally Posted by 908/930

If it spins up to normal high RPM with no load I do not think it is the armature, from what I have seen burnt armature runs like a 6cyl engine running on 4cyl, and wont hit full speed, quite noticeable that something is wrong Did your brushes and springs look good? I don't think anybody rewinds those, just too much time involved. The last motor I saw that needed to be sent in for a rewind was about 300hp, it released a large amount of magic smoke about 20' away from me.

And it wouldn't be a voltage issue either. But, and I'm just postulating here, a couple of dead bars on the commutator would slow the tool under load. I think most of the time you get sparks with that condition and things go south quickly. So what is wrong with this saw is a bit of a mystery.

However, there are shorts and then there are opens.