DC crane

[SparkyTris]

perhaps not completely relevant to this forum but...
with reference to the attached vintage american schematic for a vintage american electric crane;
what is the likely purpose of the arrangement highlighted in the upper rhs of the diagram?
the symbology is a little alien:
the zigzag line is the contactor's coil
the capacitor-esque symbols are contactor contacts, NO appear as capacitors, NC appear as capacitors with a diagonal stroke through them.
the resistor symbols are resistors but most of them in this diag are huge low-ohms shunts for controlling current and speed of the large 175hp motor.

the mystery of the moment is the purpose of contactor contained within the red box at top rhs of the diag.
the contactor fires when the volts across R2-R3 are high enough to drive it (this voltage would be a function of current in the motor).
the NC contacts associated with this coil will immediately open and add the small 500 ohm resistor into series with the coil.
we are at the moment unable to guess what TPR stands for.

this function may be associated with releasing the brake from the motor which is necessary in order to lower or hoist efficiently. (however, it is of course essential that the brake is applied the moment any lowering or hoisting action is stopped)

comments welcome.

it's a 250V DC arrangement with currents in the order of 0-300A in normal working conditions.

 

[magicmushroom666]

I wonder if they are just switching in that 500R resistor to limit the holding current through the coil once its been activated, as I presume the voltage R2-R3 will keep rising and could eventually damage the contactor coil. But if they put the 500R in all the time, it would trigger too late?
This assumes there's some more contacts not shown here for TPR that actually makes it useful?

 

[bernardgreen]

It looks like the names for the contactor coils are not used for the contacts operated by the coil so somewhere there should be a list that links coils to the contacts they operate.