Improving commutation in d.c. machines by the use of flux traps

It is suggested that sparking in a d.c. machine may be reduced by incorporating thin sheets of copper (`flux traps¿) to arrest the rapid changes of flux, at the end of the commutation period, that induce the high e.m.f.s that cause the sparking. The sheets need only be thin (typically 0.01 in) since they are intended to be effective only for the short periods (up to 10 ¿s) when sparking would otherwise occur. Skin effect makes thick sheets unnecessary. The thinness of the sheets eliminates the disadvantages of damper windings (substantially increased losses, and reduction of space available for active copper). Interpoles are retained. Bridge measurements on circular coils and armature coils show that the correct choice of flux traps can reduce the inductance at 20 kHz (corresponding to sparking periods) by a factor of up to 5, while leaving the inductance at 200 Hz (corresponding to commutating periods) substantially unchanged. Measurements on a test rig confirm that sparking is reduced. Measurements on a 300 kW traction motor fitted with flux traps show that, for a given width of black band, the running speed can be doubled. Losses appear to be increased by about 20%.