DC Generator Theory

Is there someone out there who is well versed in the theory of DC generators?
I have allways believed that the brushes of a two pole generator are always in line with the pole pieces but in the two Tiny Tim generators I have this is not the case, the pole pieces are ahead of the brushes in the direction of rotation by about 12 1/2 degrees. The brush carrier is rotateable but moving the brushes appears to have no effect. Could someone please explian.

Maurice

[Dave Hogan]

Right . . armature current moves the field off-axis somewhat, the phenomena is called armature reaction. If the generator were loaded and the bruch cage was moved, you should be able to find a point of best commutation, i.e. minimum sparking at the brush trailing edge. That position is only optimum for that position so it is set for some typical load current. I don't remember typical offset angles. Also have to point out that a machine may have a "compensation" windings in the field that carry armature current such that it cancels armature reaction at some speed.

Well, that's 110% of my knowledge on the subject, had to reach back 50 years to come up with this story!

Dave H.

[Simon Thomson]

Some dynamos have 2 sets of brushes and shunt windings so as the load increases and the neutral plane moves round the magnetic flux in one set of shunt windings decreases and the other increases keeping the output constant.

Other dynamos have interpoles which are smaller series windings in series with the main series windings so as load increases pulling the neutral plane round the interpoles magnetically pull the neutral plane back to where it is at no load.



I'll try and post some info later on.

simon.

Thanks very much for your prompt replies. It seems that Tiny Tim is quite a high tech design for the late nineteen thirties compared to motor car dynamos of up to twenty five years later. I look foreward to your additional information Simon.

Maurice.

[Harry]

I have a DeLaval generator that has the same adjustment. If you don't have a specification or have it marked, you almost to need to adjust it hot while looking sor sparks and maximum output.



http://www.old-engine.com/delaval.htm

[edisonlongwaist&oilpull]

The brush position with respect to field poles for neutral or no brush shift depends upon the way the armature wires are connected to the commutator i,e. you can build shift into the armature. Wherever this position is, the brushes can be shifted against rotation for a motor or with rotation for a generator. To determine neutral position, rotate as generator at no load and shift to maximum output voltage. Same can be done under load. Most DC machines books give a good explaination of armature reaction.