Discussion relating to the past and present operations of the NYC Subway, PATH, and Staten Island Railway (SIRT).

Moderator: GirlOnTheTrain

  by Fan Railer
Have you ever wondered what the term Field Shunting is I did, but for a long time i didn't bother to find out. Finally, when i did, i came up with this :
Field Weakening:
The DC motor can be made to run faster than the basic "balancing speed" achieved whilst in the full parallel configuration without any resistance in circuit. This is done by "field shunting". An additional circuit is provided in the motor field to weaken the current flowing through the field. The weakening is achieved by placing a resistance in parallel with the field. This has the effect of forcing the armature to speed up to restore the balance between its magnetic filed and that being produced in the field coils. It makes the train go faster.

Various stages of field weakening can be employed, according to the design of the motor and the intended purpose. Some locomotives used as many as six steps of field weakening.
Thought it was pretty interesting.
if the MTA ever does rebuild the R62/A and the R68/A, they should re-include Field Shunting cause those trains are mad slow.
Back in the late 70's through late nineties, Field shunting was gradually phased out from the fleet to improve the safety margin starting with a test train of R42's.

Link to article:
this website has really detailed information of almost every aspect of what makes a train run and work.