well i have about the 2 main locomotive types now (i mean aside from EMD and GE). AC and DC. but what’s the difference between an Alternating Current unit and a Direct Current unit? and why were the prime movers not able to handle Direct current on 6000 HP locomotives?
With AC the direction of current flow reverses at the frequency. So AC at 60 Hz. (60 cycles per second) completes 120 reversals of current flow every second. With DC the power flows in 1 direction only.
On the 6000 hp. motors it wasn’t the prime mover that could not handle the power it was the traction motors. What made a traction motor able to be powerful enough to handle the 1000hp. or 746 Kw of power is not the fact that they were AC motors. Rather they were 3 phase AC motors. I suspect if I go too deep into electric theory I will lose you . So I will simply say that a 3 phase motor powered by a variable voltage variable frequency power source (the inverter) does not need copper windings in its core and no power flows into the core so it does not need commutators, a high maintenance and power limiting item. This allows a lot more power to be provided to the motor than a comparable sized and weight DC motor. More simply put is that a DC motor able to handle 1000 hp. in slowspeed service wouldn’t fit inside the locomotives trucks. Also with a asynchronous 3 phase drive wheelslip is almost impossible as the motor tries to rotate at a speed determined by the frequency of the power supplied.
Well said!!!
indeed. but how do AC and DC affect the performance of the locomotive itself? is alternating current better for grades?
IIRC this was discussed in Trains a while back - let’s see what I can remember…
And it isn’t much, aside from the max current ratings already mentioned. DC works best at a certain minimum track speed (which varies by locomotive, etc). Seems like speeds around 10 mph figure in there. Below that you can run into problems.
AC can run down to some pretty impressive speeds - fractions of a mile per hour, in run 8. There is more danger of losing a coupler somewhere than burning up a traction motor (consider the load necessary to drag a pair of 4400 HP locomotives down to 1/2 mph).
My recall - This is a question for Randy to weigh in on.
Just remember that on A/C traction motor that VOLTAGE leads current , Low motor RPM= low voltages= low current , instead of 2 opposing fields you have three running syncronized (in phase)with the alternator. That’s why the traction motor leads on an A/C engine are about a third the size of a D/C engine.
There was a good explanation by the second poster, not alot to add.
The weak point of a DC motor (Single phase AC motor too) is the commutator and its brushes. If you have ever taken apart a small electric motor you would have seen these brushes. A small square chunk of hard carbon to which a copper wire is connected. A coil spring presses this piece of carbon against a copper segmented wheel mounted on the motor shaft (commutator). Each segment is wired to a portion of the rotor windings. As the motor spins the commutator (copper wheel) rotates under the brush which is pressed against it. Over time the brush is gradually worn away and must be replaced. Now scale that small brush up to the size needed for a large motor on a locomotive. On a locomotive the motor is not sprung from the axle so when the locomotive hits a low spot on the track the impact is transmitted to the motor, imagine how long the drill motor would last if you were constantly dropping it. Another problem is that during the impact the force may lift the brush off of the commutator surface. With the brush off the surface the current will continue to flow but the current must jump the gap as an arc. This causes pitting of the commutator surface which increases the speed which the brush is worn down. This is the reason why Engineers must throttle down locomotives when they are about to cross a diamond. Asynchronous 3-phase AC motors have none of these limitations.
So why would any railroad still buy DC motored locomotives? Cost. DC motored locomotives are quite a bit cheaper than those built with AC motors. Each railroad will calculate the purchase price then add the expected lifetime repairs for both types, plus the value of the savings on the cheaper one that can be used elsewhere. So AC wins when you use the locomotive hard like on coal, grain and other heavy trains on heavy grades. Otherwise DC wins, at least until the manufacturers can reduce the price differential. Note that CP is the only railroad to purchase exclusively AC motored locomotives. CN and NS are purchasing DC only, while BNSF, UP, a