The technical field of this invention is electric motors, and particularly DC traction motors for diesel-electric locomotives. Such motors are necessarily powerful, typically generating up to 750 horsepower, and must be ruggedly built to withstand the forces generated. The commutators of such motors may comprise as many as 210 copper commutator bars, weighing close to one pound (2.2 kg) each, all of which must be held in a stable structure on a rotor against centrifugal force at rotational speeds up to 2,900 RPM and at temperatures up to 150.degree. C. The commutator bars must further be electrically insulated from the supporting steel members of the rotor at voltages up to 1,500 volts and in an environment exposed to water and contaminants which can create electrical creepage paths at the high voltages across exposed surfaces. Locomotive traction motors are operated at varying speeds and temperatures and are subject to severe shock loads and vibration. The resulting widely varying loads and thermal expansion/contraction cause relative movement between the commutator bars and their supporting members during motor operation.
The need to retain structural stability while allowing some internal movement due to the high and greatly varying loads has resulted in a prior art commutator structure in which the commutator bars are provided at opposing axial ends with V-shaped grooves and are retained against centrifugal force by a pair of retaining rings each having a V-shaped projection to engage the V-shaped grooves of the bars. Force exerting means such as threaded bolts pull the retaining rings together axially to firmly retain the bars while allowing a small amount of relative motion. Differences in the distribution of the total tensile load among the various surfaces have produced a number of different commutator types, of which this invention is concerned with a modified arch-bound construction.
Since the retaining tings are generally made of steel, which is electrically conducting, they must be electrically insulated from the copper commutator bars. A favored method has been to provide a V-cross-sectioned insulating ring of a high density, calendared, polyamide paper such as NOMEX .RTM. over the V-shaped projection of each retaining ting to physically and electrically insulate the ring from each of the commutator bars while helping to provide mechanical stability of the parts. However, the commutator bars and retaining rings are physically very close together--separated by only the thickness of the insulating ring. Thus, at each end of the commutator bars, the respective retaining ring and its accompanying insulting ring are extended axially significantly beyond the end of the commutator bars. This provides an insulated surface distance between the adjacent exposed surfaces of the retaining ring and commutator bars which is sufficient to resist the formation of creepage paths thereacross by water and contaminants. To prevent radial expansion, due to centrifugal force, of the exposed axial ends of the insulating rings, which would allow contaminants to creep under the insulating ring, each is radially restrained by a circularly wrapped string or similar substance, and various sealing parts and materials are provided to attempt to seal the joints between the commutator bars, retaining ring and insulating ring. However, the arrangement is not as rugged as is desired and further takes up axial rotor length which thus cannot be used for rotor windings.