I have a general question about early haul trucks with AC wheel motors. When the motors are being used to retard the truck, is the amount of retard variable or does it come in steps? Siemens talks about eliminating mechanical contactors in the chopper circuit but it's not clear if the they were eliminated only recently or in all of their AC designs. Curious about other manufacturers as well. Thanks, ISZ
I know little about Siemens systems, but GE 240 ton A/C systems use only 2 mechanical RP contactors. RP1 picks up then as more dynamics are needed chopper 1 picks up it's on/off time. If still more dynamics are needed RP 2 picks up and chopper 2 will raise it's on/off time, up to full dynamics. A portion of the dynamics are also when phase modules fire, as in dynamics the phase modules fire to motor in a way that is lagging north/south poles of armature thus helping slow down motors with magnetic fields. hope this helps.
So RP1 basically kicks in half of the resistor grid while the chopper varies the load, then when more braking is needed RP2 kicks in the other half of the grid? Thanks for the help. ISZ
Pretty close. A GE A/C drive resistor grid box is basically set-up into 4 quadrants, RP1, RP2, chopper1, and chopper resistors each fill one of the quadrants. After RP1 kicks in then chopper 1 steps up on-time, then RP2 kicks in and chopper2 steps up on time unitl full retarding. Think of RP1 and RP2 as steps, there would be a big jump in dynamic effort between after RP1 kicks in until RP2 kicks in. the choppers smooth out the stair pattern and make it a smooth transition. The choppers feed less resistors grids than are controlled by RP1 and RP2.