The whole "notch"' notion of fixed-RPM-for-HEP locomotives is a bit of a misnomer. It is not as if the engineer on an F40 literally has to leave the throttle handle in 8, only opening the Generator Field switch when he wants to move. Rather, these fixed HEP RPM locomotives are fitted with appropriate governors or computers to keep the diesel engine itself constantly running at the appropriate speed. (This model includes the GE Genesis series.) Tractive power is still controlled by the engineer's throttle handle, but it only controls electrical input to the traction motors, in such a case. Such power is referred to as "excitation" in the operating and troubleshooting manuals.
"Constant RPM HEP"' and '''non constant HEP" locos are compatible in a M.U. configuration for just this reason. The engine supplying the HEP - whether by separate alternator or separate engine PLUS alternator - will adjust its tractive effort automatically, according to the throttle position commanded thru the trainline by the lead unit. Similarly, non-HEP-supplying units in consist will simply act as if HEP doesn't even exist, and will respond directly to trainline throttle calls. Non HEP supplying units will naturally develop higher horsepower and traction amps than the HEP supplying unit, but the difference is small enough to avoid undesirable wheel slide, slack action, etc.
There are two arguments for keeping the HEP on the head end of a multiple unit consist. One - the HEP startup/shutdown controls are readily accessible to the engineer. Two, the engine producing HEP will naturally produce slightly less H.P. than the others, and thus is at less risk for wheel slip. Wheel slide only really matters on the lead unit, since when the first wheelset traverses wet rail (whether with sand on or off), it tends to "clean" the rails off, to a certain extent, for the trailing vehicles.
The disadvantage, as mentioned earlier, is the notable increase in cab noise. And as far as the P42s go - while a solid P42 consist should theoretically perform about the same no matter where the HEP is, there does seem to be a noticeable difference in acceleration capability when HEP is on a trailing unit. Don't ask me how that works.
The only real notation to be made here is that, in single unit operation, a locomotive with prime-mover-powered HEP cannot be moved while HEP is in "standby" mode. This is because the HEP electrical load is thus shifted to the Traction Alternator, which is normally reserved for tractive movement. In this mode, the diesel engine is also reduced to a lower RPM (somewhere in the 500-600 range), to provide for lower fuel consumption and emissions. In some terminals, complete shutdown of engine HEP, in favor of ground power, is standard. It is only with ground power that single-unit prime-mover-powered-HEP trains can truly shift into "low idle" mode, apart from HEP being shut down completely.
edit: for reference: P42 with Normal HEP is 900 rpm engine, while engine without HEP load goes almost 1100 rpm. A well-maintained P42 can pull a seven car set with HEP with about 1200 traction amps max, while with no HEP it would be somewhere in the neighborhood of 1500 amps.
To directly answer the OP's question, only one source can supply 480v power, no matter what. Even if a train has 8 locomotives, only one can supply HEP. If ground power is hooked up to the train, then no locomotive will be able to supply HEP, until the ground power is unplugged. This begs the question - how long can passenger trains be? Normally, at Amtrak, they do not exceed 15 cars, to avoid exceeding the limits of the single-source-HEP model.
hey there guy