The E-unit stays energized with AC voltage. However the horn relay, requires a DC pulse. Aren’t both units just a coil of wire that uses a magnetic field to either lift the pawl on the e-unit or the latch on the horn relay? It doesn’t matter whether the magnetic field uses the ‘S’ or ‘N’ pole to lift the pawl or the latch. So why does the e-unit stay energized with AC voltage and the horn relay requires a DC pulse?
Earl
Dang fine question! I don’t know either, tho…
The E-Unit will work on ac or dc,but if the horn relay did that the horn would blast all the time. so the dc activates the relay and blows the horn. The D battery is used only as power for the horn itself.
Oh and before someone corrects me, some engines did use a C battery in smaller diesels. The same relay is also used to blow the whistle but that motor is powered by the ac from the rails.
If you look closely at the whistle relay, you can see that there are two thick copper washers at the bottom end of the coil, closest to the armature. These act like a big shorted turn on a transformer secondary winding. The current induced in them by the relay winding cancels the alternating magnetic field that would otherwise exist in the relay’s magnetic circuit, especially at the end where the copper washers are, which is the business end of the magnetic circuit where the armature is.
So that keeps the relay from operating on AC. But the DC component of the whistle signal, being steady and unchanging, cannot induce a current in the copper washers, any more than a transformer can work on DC. So the washers have no effect on the magnetic field produced by that DC current; and the relay operates when DC is applied to its coil.
Bob, Thanks for the explanation. Electromagnets often have an iron core center. Do those type of electromagnets only work with DC? Actually, the e-unit has an iron core center, the plunger. Why does the plunger not cancel the AC magnetic field?
Earl
Good question. I think that the answer has to be that iron is not nearly as good a conductor of electricity as copper is, only about 1/6. Also, the iron does not completely surround the magnetic circuit the way the copper washers do, but is actually part of the circuit. So any current induced in the iron encircles only part of the magnetic path.
You may have noticed that most magnetic circuits intended for AC are laminated, like the iron parts of transformers and motors. This is done deliberately to interrupt any current paths in the iron. It would be appropriate for the e-unit solenoid’s slug as well, except that the thing is small enough to make lamination difficult. Likewise for turnout motors.
Ah yes, but, the E-Unit is a “Solenoid”, a coil of wire wound around an insulator and having a “moving plunger” in the center. When energized, the “plunger” is pulled into the center of the coil. When power is turned off, gravity or a spring pulls the “plunger” back out (down with “most” E-Units). If AC power is applied, the Solenoid Plunger snaps in at a high velocity as the sign wave goes positive, then tries to drop back as the sign wave goes negitive, but residual magnetism delays it enough receive the next positive pulse a 60th of a second later. If all is tight and new, all is well. As the E-Unit wares, the "plunger is free to move in the coil when energized and the famous E-Unit “Buzz” at 60 cycles develops.
Don,
The magnetic field is generated at both the positive and negative cycles of the AC voltage curve. The only difference is the magnetic poles are reversed, but the plunger is pulled up regardless of which magnetic pole is active. The buzz is generated when the AC voltage nears the zero volts crossing and the magnetic field is too weak to hold the plunger in place. The magnetic field then changes poles and the magnetic field builds again to retract the plunger into the upright position. The 60 cycles is fast enough that the plunger stays near the top of the solenoid and the e-unit does not cycle. So at the peak of either the positive or negative AC voltage cycles, the peak magnetic field occurs and the plunger is up. There is a phase angle due to some induced current when the magnetic field collapses but I am ignoring it here for illustrative purposes.
Earl
Have you noticed that, when you operate an e-unit on DC, the slug is attracted to the center of the solenoid without regard to the polarity of the DC current? The same thing happens with AC: The slug is attracted on each half-cycle of the coil current. It is not attracted on positive half-cycles only, dropping back on negative half cycles. The buzz is therefore at 120 hertz, not 60 hertz.
I don’t see what this has to do with current’s being induced in the slug however.