DC motor question

[lumphammer]

The motor on my lavac pump died on me this morning, there is no problem with the supply. Taking the motor apart I could see there are three carbon brushes two bigger than the third.
The smaller brush is worn so much that I thing that is the problem, but I'm a bit confused as to why there are three brushes.
On the rotor opposite pairs on the commutator are connected, and these connect to the two thicker brushes. The smaller brush is at 90 deg to the other two
I've ordered a new carbon brush to replace the worn one, but I would like to understand why three brushes.

 

[Plum]

The motor on my lavac pump died on me this morning, there is no problem with the supply. Taking the motor apart I could see there are three carbon brushes two bigger than the third.
The smaller brush is worn so much that I thing that is the problem, but I'm a bit confused as to why there are three brushes.
On the rotor opposite pairs on the commutator are connected, and these connect to the two thicker brushes. The smaller brush is at 90 deg to the other two
I've ordered a new carbon brush to replace the worn one, but I would like to understand why three brushes.

Interesting, not come across this before but out of curiosity found this US3497788A - Variable speed d.c. motor with third brush - Google Patents may or may not be relevant

 

[rotrax]

People call Joe Lucas the 'Prince of Darkness' because of his motorcycle dynamo equipment.

The Miller dynamo af the pre and just post war period had three brushes.

Voltage regulation was, IIRC, the purpose of the third brush.

 

[Alex_Blackwood]

The motor on my lavac pump died on me this morning, there is no problem with the supply. Taking the motor apart I could see there are three carbon brushes two bigger than the third.
The smaller brush is worn so much that I thing that is the problem, but I'm a bit confused as to why there are three brushes.
On the rotor opposite pairs on the commutator are connected, and these connect to the two thicker brushes. The smaller brush is at 90 deg to the other two
I've ordered a new carbon brush to replace the worn one, but I would like to understand why three brushes.

The third brush is for the field winding. If that was worn and not making contact it would explain your dead motor There are on line references if you want to look up "DC motor third brush" Most refer to dynamos but same circuit for motor. They are basically the same thing.