6 pole, 8 pole DC machines

[biggeorgetechnical]

Briefly: How common are 6 and 8 pole DC machines? The rail transit propulsion motors I have worked with are 4 poles, and I have heard of 6 pole DC (never actually seen), but before a conversation with a client today - I had never even heard of 8 pole.
?

 

[aolalde]

Small and medium size DC motors are normally associated to high rotor speeds. The most convenient construction is in two and four poles and small diameters. The speed does not depends on the poles but the armature frequency (HZ) is linked to the speed (rpm) and number of poles, ( f = rpm*P/120 ).
High frequency implies high eddy and hysteresis losses in the armature or rotor laminations and high grade materials to handle them.
Large DC motors for heavy industries like Mining, Paper, Marine, Metal, etc have custom made motors not only with 6 and 8 poles but 24 or more and large diameters. Certainly I have not seen all these custom made applications.

 

[UKpete]

In the UK the rail traction motors are virtually all 4-pole. I can think of only one locomotive type which has 6-pole, everything else that is dc (locos, London Underground, all the EMUs) is 4-pole and has been for decades.

With an increased pole count the armature frequency does go up, so more expensive laminations are required but conversely the thickness of the frame iron can be reduced assuming the mechanical strength is still sufficient. Also the total amount of copper on both the armature and the field coils can be decreased because of the shorter end windings. However, the manufacturing cost will go up (all those extra pole bricks and armature coils) and also because the number of brush arms = no of poles on a lap wound machine, the brush arm accessibility for changing brushes under the vehicle becomes a problem. The commutator can be made shorter though.

So for typical traction motor ratings I think it comes down to the lower pole count being cheaper. Assuming you are talking about a traction motor, I can only imagine that an 8-pole was used for some marginal weight saving and reduction in size.

I don't know about industrial dc machine practice.