It’s a contactor and maybe a rectifier, that’s it. Look I realize you and the educated idiot that flaunts his college ego or I mean degree live in a world of power electronics. The transistor wasn’t even invented until the early 1950s (ignoring double cat whiskers on crystal sets). The SCR came even later in that decade, originally with a whopping 16 A output. A GE MD624, 1200 HP DC motor has a stall current of 6000 A, used on large mining equipment of the day. See the problem? Simply put there can’t be a drive because those didn’t exist for over a decade later! Even if it did, it was a toy suitable for engineers in school not the real world.
When I went to school to have my common sense removed (I hold two bachelors and a masters), in power class the professor first covered salient pole motors then told us he was skipping the DC chapter because we will never use or need it, and jumped right into synchronous machines, followed by induction machines. See? No common sense to be found when the Pentium CPU just came out. Computer technology was just on the cusp of field oriented AC control and IGBT based low voltage AC drives were just coming out albeit V/Hz only. Anything over about 100 HP was either a GTO (big explosions every few months) AC or DC. So as I said, the professor was just an educated idiot with a PhD, as in doctorate in dumb.
That was the early 1990s. Little did I know then that I would be working for one of the largest regional motor shops in the country and that not only would DC be a regular part of my job but that it’s making a comeback in high efficiency and very compact applications. Even the older wound rotor motors are prominent in car shredders and modern wind mills. How wrong that professor was.
And yes I am VERY familiar with Ward Leonard loops and MG sets. Those would not be used for this purpose. Military tech in this era is simple and reliable unlike say an M-16.
There are three kinds of DC motors. If you look at the wiring in a “modern” one you will find four wires and if they are labeled, A1, A2, F1, and F2. It may have more F wires and maybe S1 and S2 if those aren’t internally connected but that’s it. You drive the field and armature separate. Often rectified DC goes on the field. On the armature in years past you would run an AC motor mechanically coupled to a DC generator. The excitation.on the generator rotor comes from a rheostat fed with again rectified DC or in large generators an exciter which is just another stage of DC generators. This is Ward Leonard technology. Came out in the 1930s and revolutionized the older wound rotor AC tech that worked but was much more difficult to control. Later the MG set was replaced with solid state SCR drives in “smaller” DC applications but I’ve still personally worked on larger DC systems that still use MG set technology even in 2020. It’s still around.
That’s all shunt wound DC motors. Compound wound usually has S1 and S2 on the series or commutating coils or those are just internally connected and A2 is really S2 but I digress. That’s not the case here.
In this case what you have is a large “universal” motor. Today we get into brushless designs and even old WWII vintage toys used them. On small motors like the kind prominent now in battery powered tools the rotor is a permanent magnet (PMDC) but this has size and cost limitations. On larger DC motors of the time both armature and field are wound but on a siren they will be connected in series so just two wires.
To the OP your brushes could be arcing or by now it’s so dirty with carbon dust it’s probably flashing over occasionally and blowing fuses. DC motors all require periodic cleaning. Take it to any competent motor shop that still does DC. It’s expensive but your motor should hold up another decade or more. If you want to test first get an electrician with a digital Megger to disconnect the leads and test it for one minute at 500 V. If it’s under 2-3 megaohms time to wash it out and bake it, then retest and change the bearings. If you haven’t even been cleaning it, it will fail and what you are seeing is flash overs. Redip while you are at it for cheap life extension. Then every year take the brush cover off and clean all the dust out with a shop vac and check brush wear from now on. That’s it. Once a year.
If you went to school in the 1980s or earlier they would teach this. DC tech has all but been forgotten.
