What is the purpose of a primary resistor ahead of the thyristor controller? The resistor is rated at 100 amps and has a very small ohmic value of 0.089 ohms.
It is probably not "ahead" of the thyristor controller, it is probably connected to the rotor windings to keep the rotor current from being too high when starting. The thyristor controller (assuming you mean a soft start) most likely replaced the original wound rotor control system, but they would have needed to leave one set of rotor resistors in place to make it work properly.
"Venditori de oleum-vipera non vigere excordis populi"
Another advantage to adding a soft start (voltage controller) on a wound-rotor motor, in conjunction with the resistors, is that you can get a smooth, controlled acceleration without torque pulses due to switching of the rotor circuit resistor(s). Makes for a nice, smooth ride.
Thank you for reply. A little more history is probably needed. The primary resistor is located after the feeder breaker and before the thyristors. Since the resistors are rather larger and give off a lot of heat they are located external to the drive. The Thyristor Drive was manufactured by Westinghouse and dates back 30 plus years. Recently one of the primary resistors burned up and an electrical controls firm was brought in to replace it. They did replace it, but in the process of doing so they asked why it was even necessary. Several days ago they ran into me and asked if I knew the purpose behind the resistor. I thought initially they were the rotor resistors, but then they showed me the schematic verifying the resistor's location. It peaked my interest and after researching several books, I have yet to find the answer.
Sounds like a mistake from the early years of thyristor controls. I work for a company that was at one time Vectrol, who was bought by Westinghouse to get into the Soft Starter market (and subsequently split off again). There was never a design in our products that was to be used down stream from resistors. In older bridge crane travel starters, there were "ballast resistors" used in a two-step starting process. Step 1 would have been to start through the ballast resistor, lowering torque so the bridge didn't jerk. Step 2 was a bypass contactor that allowed the motor to run at full voltage. Soft starts were sold to REPLACE the ballast resistors. Sounds like someone didn't understand the concept, and put them in-line! check out the 2nd diagram on this page for ballast resistor connections.
The only other possibility is that the motor nameplate voltage is lower than your line voltage, and they did this in an attempt to make it work. Given the short duty cycle of bridge crane motors, it is possible that it would work for a long time, although the motor would have permanently reduced torque. If on the other hand it were oversized for the application, that may have not been a problem either. It bears checking it out.
"Venditori de oleum-vipera non vigere excordis populi"
I sketched the diagram and created a PDF file, but I am unable or incapable of attaching the file to this email message. Any way, there's no mistaking the primary resistor.
The resistors could also be there to limit repetitive inrush current when the thyristors switch on.
If this thyristor drive is of the 3-phase bridge type that drives a DC motor there has to be a deliberate resistance, inductance, or universal harmonic filter in the line to limit what happens during commutation. A 3-phase bridge rectifier (or inverter when regenerating) creates 6 short circuits every cycle because the thyristors do not turn off instantaneously. If you do not have this or the equivalent in a long branch circuit your thyristors will not live long.
It is now a well known phenomenon that when 2 bridge rectifiers are connected to the same bus without imdividual impedances voltage uncertainty when one rectifier is commutating will tend to burn up the other rectifier.
