Questions about DC MG Set

[rockman7892]

I'm looking at the attached drawing for a 750V DC MG set for rollers in an aluminum mill and was hoping that some of our gen/motor experts here could help shed some light on a few items for me. These gen and motor units have separate exciter control enclosures and the main 750VDC distribution is fed from generators to motors through a 750V Switchboard lineup which contains breakers and switches shown on one-lines.

1) For both the DC generators and motors there are both separately excited field windings as well as armature windings. From my little experience I'm used to seeing one or the other but not both....either a separately excited field winding or a shunt/series winding but not both. Is there an operational advantage to having both?

2) What are the C&C windings on both gen and motors (purple cloud)? Are these typically located on the units itself or the control/distribution board?

3) What are the resistors and OV relay on the generators for (green cloud)? These appear to be for some sort of overvoltage relaying/detection?

4) the DB resistor in the on the motors (blue cloud) appear to be used for dynamic braking, to slow the motor and prevent it from producing back EMF?

5) What are the resistors (red cloud) used for on the motor circuit. I'm assuming these are used for speed control of the motor with the (3) different contactors being used as means to change resistor value for speed control? What about the resistors on the generators (red cloud) Are these resistors typically on the unit itself or on the control distribution board?

 

[rockman7892]

Gr8blu

Thanks for follow-up.

I'm trying to envision a scenario where a GF or other fault would occur that would not be seen by the positive side breaker and thus require a breaker on the negative side as well?

I agree that on AC 3-phase you need to open all three phases, but with a DC circuit being only 2-wire and current flow being from positive to negative i'm missing how current could flow that would not be seen and opened by the positive OCPD?

For example if the negative side of the system became grounded with an initial GF and then a 2nd GF occured on the positive side wouldn't the current still flow from positive to negative and thus trip the OCPD on positive?