OK, it sounds like you have a current source dual converter there. It is very similar to the drives I did back in the 1980's.
The large inductor is called the DC Link inductor or DC Link reactor. Often there are two of them (one on the positive bus, and one on the negative bus). The capacitors are probably there on the DC link for smoothing, but more importantly to provide the energy to commutate (switch the current flow from one SCR to another) the inverter.
The reason you have a synchronous motor is so you can more easily commutate the inverter. With a synchronous motor, you get more counter EMF (here we go again) from the motor than you would with an induction motor (same thing as an asynchronous motor, guest). In other words, the motor is being used to help you turn off the inverter SCR's. It actually has to do with the beta angle of the load, which gets into more detail than we need for this discussion. Suffice it to say that when the load appears capacitive to the inverter, the inverter SCR's will tend to turn off. An induction motor will pretty much never appear capacitive, except during rapid deceleration.
The inverter SCR's/thyristors are on a constant polarity DC bus. SCR's won't turn off in this case. The motor is generating CEMF (sometimes called back EMF) which is what causes the SCR's to turn off.
Of course the above is just speculation.
As far as being typical, I worked on more asynchronous/induction motors than the synchronous type. We made both types of drives, and the only time I thought a synchronous motor was required was for very high inertia loads, which a pump is NOT. Induction motor drives needed an additional circuit to commutate the inverter SCR's. This circuit was called, you guessed it, the "commutator."
You probably have an older design than we made, though, so that design needed the CEMF from the synchronous motor to help it commutate. Our design did not, but it was "revolutionary" 20+ years ago.
I reiterate my original advice: Get an induction motor and an IGBT voltage source drive when you replace this ancient beast.
I'm up to 10 cents now, I think....
Guest, there are about ten books that you'll need to really answer everything you're asking. It depends on your present level of knowledge which books I'd recommend. The power converter handbook is the final say on most matters concerning motor drives. That's about $120, the last time I checked. Probably $150 by now.
If you want a non-mathematical introduction to motors and drives, there are many good ones out there. The training manager for ABB wrote one that's fairly complete. The name of the book is "Motors and Drives: A Practical Technology Guide" As of this writing, it costs $85 and there are only 3 left at Amazon.
A good "beginner's" book on electricity is "Electricity 1 - 7" It used to be seven little booklets, but is now a single book. It's been updated recently so now it has pretty color pictures. Completely non-mathematical approach to things electrical. I'd say any fairly intelligent 9th grader could probably read it and understand it. Harry Mileaf is the author.
Regards,
Don