That one MG set provide DC for ALL the arcs! In reality, normally no more than two of the seven arcs would be burning at one time.
The five switches on each ballast resistor allow the operator to adjust the current for different carbons, etc.
Here's more data:
#17: The Westinghouse motor generator set. The motor is three phase, with an automatic starter (we think it's wye/delta) as seen in the next pic. It still runs smoothly and with a contented gentle whine even under full load. Below the cast base at the bottom of the pic is a felt and wood pad, all resting on a huge solid concrete base! (About 2'x6'x3') My pics of the data plates didn't turn out, so I can't remember the particulars.
#18: The starter. This evil little box almost caused me a change of shorts during my first visit to the theatre. The union rep hit the start button just as I was walking by that thing...it starts with a loud thunk, joined by very loud buzzing from those open coils, then a few seconds later another loud thunk as it switches to run mode! The armature for the swich is between those coils and from the force it operates with, I promise anyone dumb enough to get in its way will lose fingers or hands! We have never opened the cover, but think the contacts are oil-immersed. The #1 referres to the fact that there used to be another MG set (we don't know what for) in the anteroom behind this room. This and the MG above are in the same room as the ballast bank and back of the switchboard.
#19: A close-up of the DC brushes. This was taken while the generator was running and under load of one arc. Despite the dust build-up, there are no overheating issues and virtually no sparking of the brushes. The commutator is also very smooth and shows minute wear. (After about 70+ years, no less.) I have been advised by an old-school motor repairman to leave it alone, any attempt to clean it out could damage the insulation on the windings. Only if the dust gets thick enough to cause heating issuse should it be very gently cleaned with very low pressure air.
#20: The ballast resistors used to limit current to, and balance out intensity of, the arcs. The five knives allow the operator to trim each arc for best performance individually. The top left set, "A.C. Emer." sets the overall current from the A.C. backup to match the generator characteristics as close as possible. I haven't located the "A.C. Emer." supply in this case, some theatres used a step-down transformer, some had an autotransformer connected to mains, and some (I think really small theatres) connected directly
to the mains!
#21: The absolutely beautiful D.C. system busbars. They supply D.C. (and A.C. emergency) power to each arc through enclosed DPDT with center off switches on the face of the switchboard. The wirenuts and THHN are for the START/STOP buttons for the generator. Too bad the installer didn't feel compelled to match the quality of the original workmanship. The round object peeking into the pic is the field rheostat for the generator.
#22: The other side of the D.C. control panel. You can see the meters for the Generator, amps on the left, volts on the right. The pushbuttons for the generator are in the AB hack job (Sorry, this still bugs me.) Above the rheostat are the holes where the booth exhaust fan buttons used to be, they were identical to the intake fan buttons at top right. Two of the arc control switches are visible, in the D.C. position. Center is off, left is A.C. emergency.
The exhaust fan is started by using the main fuse disconnect switch in the cabinets behind the switchboard. Thankfully the contactor I installed seems to be taking the arc on startup, not the switch.
About the booth fans: They're both located in rooms some distance from the booth, and are driven by 5hp 3phase motors, with very long flat leather belts! Until a few years ago, the exhaust fan was driven by a 1940's vintage Westinghouse motor. It unfortunatly went down in flames (literally) due to a bad connection in the terminal block of the motor (and a bypassed overload relay.) I replaced the Disconnect at the fan location, overload relay, contactor and motor. Quite a job made even more difficult by the stench from the old motor. I had to fabricate a plate to mount the motor on to the fan bed, what a chore. The intake fan was already converted some years ago, but they left the original overload in place. I'm waiting for a call to replace that motor sometime soon.
#23: The inside of one of the Brenkert arc lamps. You can see the trim of carbons in use , and a spare trim at the bottom keeping dry. The cast pieces shield the reflector from metal splatter when striking the arc.