Sharing information - question I received recently, what benefits does the HEP "Standby" mode provide on the conventional (main engine driven HEP) F40PH?

The "Standby" HEP function is available when the locomotive will not need to be operating the main generator (AR10 traction alternator) for traction. It drops engine speed from 893 (nominal) RPM (Notch 8 speed used for traction while generating HEP with the gearbox driven alternator) to 720 (nominal) RPM (Standby speed) for generating HEP with the AR10 main generator. Most notably this:

Reduces Noise Level
Reduces Fuel Consumption at comparable HEP outputs
Reduces wear on the engine, particularly the turbocharger gear train
Reduces somewhat the soot accumulation in the engine, exhaust manifold, turbo screen, and turbo exhaust nozzle ring and wheel, stack, and silencer.

The "standby" position was a standard feature in the conventional F40PH locomotives.
This discussion is not applicable to the "stretch" F40PH-2C (MBTA) and subsequent clone units where HEP is provided by a separate engine.

PC

F40PH's are MUCH quieter in standby mode.

Phil Mulligan

Did the output of the AR10 presumably bypass the rectifiers to directly feed the 480 VAC trainlines?

The HEP output from the AR10 in Standby Mode is tapped off between the stator coils and the diode banks (rectifiers). Because the traction circuit "P" power contactors are open in HEP Standby Mode there is no output path for DC traction power through the rectifiers. The 3-phase AC power generated by the AR10 is fed to the HEP distribution system by a motor operated transfer switch that concurrently disconnects the HEP circuit from the gearbox driven alternator.

PC

What's the formal gear ratio for the 'gear-driven alternator' to produce the 60Hz power?

It is 117:58, close to a 1:2 speed increase. The 117 tooth gear is on the drive shaft powered by the engine. The 58 tooth gear is on the alternator rotor shaft.

PC

To make 60hz power (North American standard frequency), you have to pick a paticular RPM for your alternator to spin. Absolute top speed is 3600, then 1800, 1200, 900, 720, 600, 514, 450. RPM needs to be held to within a tight tolerance.

117/58 gear radio drives the alternator at close to 1800 RPM

And the choice of 117:58 rather than 116:58 (which would numerically be exactly a 2X speed multiplication) helps to equally distribute wear on the gears. If the ratio had been exactly a 2x multiplication you would have the same teeth on both gears in contact repeatedly on each revolution. The 117:58 (sometimes referred to as a "walking" ratio) puts different teeth in contact each successive revolution.

PC

Aren't Amtrak locomotive still using engine driven HEP? I've heard conflicting reports that the engine can turn at varied speeds even while applying head end power but nobody can quite figure out how they do it.

This is routinely done on modern passenger locomotives by rectifying AC output of an alternator to direct current and then inverting it to 60Hz 480V AC for HEP use. Exact details vary by manufacturer and locomotive type and are beyond what I am able to type out for this discussion. The MBTA Wabtec Motivepower HSP46 manuals give a very complete technical description of how this is done, if anyone wants more detail. A nice opportunity to learn about modern GE inverter systems at the same time. Railroads and the manufacturers are paying good wages for locomotive and car technical support jobs.

PC

Short version: the AC is converted to DC and then run through chopper(s) which create square waves which are separated by 60 degrees. This output is then fed through smoothing reactors which rounds these off to something which reasonably approximates sine waves. Not sure how voltage regulation is accomplished, but its done using today's magical solid-state electronics. I have one of these kinds of things on my lathe, and it converts 240 single phase AC house current to 240 three phase AC, with the output frequency being variable frequency for speed control.