Why brushes on a power tool?

[lcm1947]

I've noticed that some tools like my 12 volt Makita drill/driver have brushes and some like my recently purchased 12 volt ID by Hitechi doesn't. Anybody know anything about brushes and what they do or don't do? Also when do you know to replace them? Does the tool lose power or just won't start or what? Any info would be helpful.

[steve-u]

I bet your Hitachi ID has brushes also [run it in a dark room and look for the sparks] but they can not be easily replaced such as those that have external brush holders. You still may be able to replace them but you would have to take the cover off to access them. Some really cheap tools do not have replaceable brushes because the life of the brushes is expected to outlast the tool or the service cost would exceed purchasing a new tool kind of like no one repairs consumer VCRs that generally go for $50 or so.

My Bosch CS10 circular saw has replaceable brushes but you need to remove the motor cover. The brushes suppy electrical power to the motor via the armature. The links below would be worthwhile to check out for more details. Since brushes ride on the armature via spring pressure and they are made of carbon or such they wear and can eventually need replacing depending on how much the tool is used. Us casual power tool users may never need to replace brushes on any of our tools. Other types of motors such as the induction motor used in the BT3100 do not use brushes. Hope that helps a bit. --- Steve

http://electronics.howstuffworks.com/motor5.htm

http://en.wikipedia.org/wiki/Induction_motor

[gmack5]

"Slight" correction.

Other types of motors such as the induction motor used in the BT3100 do not use brushes. Hope that helps a bit. --- Steve
http://electronics.howstuffworks.com/motor5.htm
http://en.wikipedia.org/wiki/Induction_motor

Steve, I hate to bust your bubble, but the Motors in the BT3000 and the BT3100 are "Universal" motors and they DO have Brushes.

[steve-u]

Sorry I did not answer your question about knowing when to replace brushes. I have never had a tool wear them out so I can't speak from experience but if the brushes are easily accessible you can remove them and check them. The ones I have seen are probably about .75 inches when new. My guess is when they get down to .2 inch or so you want to replace them and the motor can be damaged if they get too bad and it would probably be making quite a racket by then. I noticed that the current Porter Cable circular saw does not have easily replaceable brushes and PC recommends service after 100 hours of motor time use so that might give you some idea on what to expect for life of brushes. I don't know how they work but some qualilty tools such as some Boschs have designed the tool to protect itself from brushes wearing too far. -- Steve

[steve-u]

Thank you. I stand corrected! My Craftsman radial arm saw has the induction motor. --- Steve

[RodKirby]

Interesting.

Even though my Ryobi R150 1/4" router has had VERY regular use over 12 years (mostly weekends), it still has around 5/8" of "brush" protruding from the clip/s. Which probably proves the comments above - the motor will probably die before the brushes wear out.

[LCHIEN]

Lee,
the general answer to the question is, there are two types of motors used in tools, those with brushes and those without .


Ok, well that's actually the truth, but they are officially called universal motors and induction motors.

Universal motors have brushes and are generally used in portable, hand held machines; they are lighter and smaller and cheaper for the power but do have wearing parts (the brushes) so they are not suitable for continuous or real heavy duty operations; better for intermittent use. Since the brushes and the commutator make the current alternate, the speed is independent of line frequency and depends on average voltage and load (which is why you can use the solid state speed controllers on them). Universal motors are found on most routers, portable drills and hand held tools (due to size and weight). Most are custom made to fit the tools.

Induction motors are bigger and heavier for the same power; they come in standard sizes and frames. They can run 24/7 for years. They can have split windings and be easily rewired for 110/220V operations (an advantage over universal motors) Their no-load speed is fixed by the number of poles and the line frequency - most have 1 or 2 poles so their speed is 60Hz or 30 Hz which turns out to be 3600 or 1800 RPM. As you load them they tend to slip in speed, hence most are listed as 3400 and 1700 no load speeds since there's a little friction. Most stationary tools with fixed motors, e.g. larger band saws, Drill presses, jointers, and table saws (the BT3x being a notable exception) have induction motors. They can't be run on a sold state speed control, since the frequency stays the same but the voltage is dropped, this just burns up the motor when loaded. Speed control is possible with expensive variable frequency AC power drives. Single phase motors also need starter capacitors and associated centrifugal switches to make them start correctly (and in the right direction). really big industrial motors as well as those that need to operate refrigerators and compressors are usually induction motors.

There's a FAQ on woodworking motors (just search for same on Google) that explains some of this in more technical detail.

[lcm1947]

Wow! good answers and hey I understand now. I really appreciate all the replies. You guys are great and I am so glad I discovered this forum ( and you guys ) back when. You guys are really neat - and not to forget who made it all happen - the people that run this site so well. My hat is taken off at this point to all of you. Thank you.

[sneezer17301]

replace the brushes before the copper shunt makes contact with the commutator. or you will have to tear it apart and and retrue the copper and possibly reundercut the mica insulation. and don't ask what happens if the spring hits the copper

[lcm1947]

Thanks sneezer I was wondering just how far down you let them get. Appreciate it.

[pirinst]

Cheaper, throw away tools and brushes........

I've had experience with brushes when younger which was way before
cordless tools were even thought of.

I've noticed the cheap throw away tools in my experience also have very cheap bearings. Cordless drill and also a cheapo belt sander I've torn apart looking for brushes to replace both had very cheap brass like friction bearings which were basically toast at the same time the tools were going bad.

I've never put out the cash for the expensive cordless like DW or others in that category, but I'd bet the bearings are probably bigger and better than
the craftsman and skill ones I've been using.

[LCHIEN]

I've had experience with brushes when younger which was way before
cordless tools were even thought of.

I've noticed the cheap throw away tools in my experience also have very cheap bearings. Cordless drill and also a cheapo belt sander I've torn apart looking for brushes to replace both had very cheap brass like friction bearings which were basically toast at the same time the tools were going bad.

I've never put out the cash for the expensive cordless like DW or others in that category, but I'd bet the bearings are probably bigger and better than
the craftsman and skill ones I've been using.

cheap tools probably use oil-impregnated brass or bronze bushings as bearings.
Better tools use real ball roller bearings.

[Tom Slick]

you won't find a brushless induction motor in cordless tools. one of the reasons is there is no phase change in DC like there is with AC which is one of the ways a "brushless" motor works. it is also easier to make a DC "brushed" motor variable speed. I have vary expensive, heavy duty, 5hp, variable speed motors at work that have brushes. older CNC milling machines used high precision DC "brushed" motors to control their motion within .0001".
moral of my story is brushes aren't used in motors just because it is a cheap way to go.