As with all diesel electric locomotives the horsepower rating is engine input to the main generator or traction alternator for traction with the engine at full throttle and conditions stabilized. There is an efficiency loss through the main generator or traction alternator, through electrical switchgear, in the traction motor, pinion and wheel gearing. Only the efficiency loss of the main generator or traction altenator (including the rectifier assembly) is considered when verifying the engine horsepower rating because the power is measured as output voltage and amperage while the main generator or traction alternator is connected to a fixed resistance, either a load box or the locomotive dynamic brake grids. The efficiency loss of the main generator or traction alternator has to be considered in the horsepower calculation; Volts x Amps / 746 x %efficiency. But in order to state the engine horsepower input using factors measured at the main generator or traction alternator output, the formula commonally used is V x A / 700. 746 watts = 1 HP at 100% efficiency, 700 represents 1 HP at 94% generator efficiency.
The service manual for the DDA40X recommends formula V x A / 715 which represents 96% traction alternator efficiency. The DDA40X employed an AR12 traction alternator instead of the sandard AR10 type and EMD considered it more efficient during load test conditions.
The air compressor is shaft driven off the front of the engine crankshaft, and like all EMD locomotives starting with the GP30, the traction motor blower and traction alternator blower are shaft driven off the engine grar train at the rear, the same shaft drives the auxiliar generator. All of these loads are mechanical and are a steady load, the compressor will vary between unloaded and loaded (pumping) but the difference in load on the engin