Overmod 'll let Erik explain the induction of back EMF in wound-armature motors, but note that a good locomotive induction motor has essentially none, meaning that the maximum synchronized rotational speed of the motor is determined only by characteristics of the modulated field current, and for a synthesized AC drive this can be relatively fast without counter-losses related to speed.
'll let Erik explain the induction of back EMF in wound-armature motors, but note that a good locomotive induction motor has essentially none, meaning that the maximum synchronized rotational speed of the motor is determined only by characteristics of the modulated field current, and for a synthesized AC drive this can be relatively fast without counter-losses related to speed.
Back EMF in a motor can be thought of as the equivalent of the back pressure provided by a piston during the power stroke of a piston engine (energy equals force times distance moved). In essence, the back EMF is an inherent part of the motor converting electrical energy to mechanical energy. This simplest in case of a DC motor as there is no phase shift between the voltage and current.
Back EMF for AC motors is a bit more complicated as it provided by the field (from field windings or permanent magnet) in the case of a synchronous motor or the slip between. The proportion of electrical power that is converted to mechanical power is the proportion of the current that is in phase with the applied voltage - hence power engineers reference to real power versus reactive power. An unloaded induction motor will draw current from the lines, but the current is almost 90 desgrees out of phase with the line voltage, hence little real power is transferred to the motor.
FWIW, inverters do have to be designed to handle the reactive power needs of the motors, mostly in a higher current rating than what would be expected from a resistive load of the same power.
I was under the impression the C4's were considered versatile machines, at least in comparison to modern DC motored C-C power like SD70M-2's or ES44DC's. Equivalent or better performance to a ES44DC, yet no concerns about short time ratings and such on a heavy drag freight when mixed in a consist of more capable six motored AC power.
Obviously a less capable machine with less tractive effort, but one that's still better able to blend in on something like a heavy coal train up a grade in a pinch than an ES44DC or Dash 9 could.
BNSF is apparently pleased with their performance as they have taken delivery on Tier 4 locomotives from GE with the same arrangement. They also have a small batch of SD70ACe-P4's with a similar setup.
Leo_AmesObviously a less capable machine with less tractive effort, but one that's still better able to blend in on something like a heavy coal train up a grade in a pinch than an ES44DC or Dash 9 could.
YoHo1975UP (at least up till now) will regularly throw in SD70Ms on trains going over Donner.
BNSF is also rebuiling its older Dash 9s into AC44C4Ms.
BNSF only did a small batch of AC44CM's back around the 2014-2016 time frame (There's no C4 in the BNSF model designation even though they're rebuilt along the lines of factory built C4's).
20 of the earliest Santa Fe Dash 9's from the 600 class went through a pilot program back then. But since then, BNSF Dash 9's have only been going off lease or been sold, including more of the early 600's and newer examples of the model.
Almost 600 of their one time fleet of almost 1800 Dash 9's are gone if my tally off the Diesel Shop roster just now was accurate. And over 500 of the survivors are laid up per the latest Loconotes BNSF update.
No sign of further rebuildings that I'm aware of for these or the SD70MAC's, despite trialing rebuild programs for both. In fact they don't seem to be doing much of anything in this area, unlike the current trend for most Class 1's.
The last two Locomotive special issues from Kalmbach show just two GP23ECO's in 2019 and nothing for 2020 for major locomotive capital rebuilds for BNSF. And while they've been a fairly regular visitor for new C4's (Including an order for some T4 credit units in 2020), one has to go all the way back to 2009 for the newest GE C-C's on the roster and 2014 for the newest SD70ACe's.
So not only do they not do rebuilds, but they're also not buying any new six motor road power it would seem.
Actually the rebuilt Dash 9s are in fact designated as AC44C4M's on the BNSF.
They're marked that way on the frame below the nose as well. Not sure why I thought what I did.
Thanks
http://www.rrpicturearchives.net/showPicture.aspx?id=4411284
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