capacitor start on power tools

The tools you mention would all have Series Universal motors. A tool like a mounted masonry saw (a biggish motor and possibly a belt driven blade) would likely be a capacitor start/ capacitor run induction motor.
Series Universal motors are used where a small motor with a a high power output is required. However, as pointed out by the Sarge they have brushes and a commutator (wearing parts) that makes them less reliable than a regular induction motor.
Hope this helps.


Joined Jan 31, 2009
I thought the same regarding cap start, IE not needed on Univ mtrs.
However I have two vacuum cleaner mtrs I intend using & each has a cap fitted.
One a 1600 watt rated mtr has the cap fitted on a speed conrol board. When this board is bypassed the mtr starts & runs just fine. Long term maybe this will induce premature failure?

The other (larger) mtr about 1000 watts, is also fitted with an encapsulated assembly of caps across the 240 VAC/50Hz input. The diagramatic illustration on the cap assembly indicates the use of 3 caps total but the other markings are:
2 @ 0.1mF, 2 @ 4700 pF. It looks like the larger cap is directly across the supply & each of the smaller caps is from the supply terminal to earth.

Can someone please explain what's going on here?

Rgds to all from newcomer,
Peter O


Joined Nov 20, 2008
Those caps are likely for commutator brush RF noise suppression and certainly not for starting or running the motor, which is most likely the universal type in a vacuum cleaner.


Joined Jan 31, 2009
Yes I am now pretty sure you're correct about the function of the caps I described.
But each motor seemed to be "suppressed" differently.
I guess I would now like to know:
1) Does RF noise suppression have any benefit other than the obvious "green & clean" effect?
2) Can the commutation be improved, perhaps by spark suppression, so that the brushes & comm life is extended.
3) Is any "design" involved here or do manufacturers just test alternatives until they meet an arbitrary standard?

A final question on different subject:
My two motors were significantly different in size yet the smaller was rated at 1600 watts which seems high to me for the vacuum cleaner application & at odds with the larger unit rated at 900 watts.
Are there accepted international standards for rating motors of this type in such applications?

Peter O