Rockwell Automation MinPak Plus DC Drive User Manual - Manual (Page 69)

C:1
Appendix C: TheoryOf Operation
This is a generalized theory of operation for the MinPak Plus controller. It covers internal components and their
functions. It also explains therelationship of thedrivemotor and thecontroller.
Motor - A Reliance Super RPMt wound field DĆC motor is compatiblefor usewith theMinPak Plus controller. The
Super RPM allows adjustable speed service as a straight shunt machine. It gives constant shaft torque capability
from 50% of base speed to base speed when provided with fixedĆshunt field excitation current and a source of
adjustable DĆC voltage - such as the MinPak Plus - for armature power. (The permanent magnet Super RMP DĆC
motors do not require a separate field excitation.)
The fixedĆshunt field provides a constantĆstrength magnetic field against which the armatureĆinduced field can react.
Armature (and shaft) speed is then controlled by varying the terminal voltage to the armature. Motor speed is nearly
proportional to armature terminal voltage.
Armature current is drawn, as needed, to provide motor torque. It is approximately proportional to load torque at the
motor shaft plus a small amount required to support motor losses.
With the motor's operational needs so described, the motor controller, here the MinPak Plus, must consist of an
adjustableĆvoltage armature supply, a fixed potential shunt field exciter (for woundĆfield motors only), and a regulator
to control and adjust armature voltage under varying torque and speed requirements.
Armature Rectifier - The armature rectifier, here the internal Power Cube in the MinPak Plus controller, is a
conventional singleĆphase, fullĆcontrolled, fullĆwave bridge. (Refer to Figure CĆ1 where it is shown in schematic form.)
Using 4 thyristors, thePower Cubeprovides adjustableDĆC voltagefor themotor armaturecircuit by
phaseĆcontrolled rectification of singleĆphase AĆC plant power. Each of the thyristors is turned on" by pulsing its gate
at a point in the AĆC input power cycle when its anode potential is positive with respect to its cathode.
Under these conditions, the application of lowĆpower gate pulse approximately 350 microseconds in duration will
cause the thyristor to conduct for as long as its anode remains positive with respect to its cathode. AnodeĆtoĆcathode
polarity reversal, brought about by the sinusoidal nature of the incoming plant line, will commutate, or turn off," the
thyristor.
Figure CĆ1. Full Wave Bridge Configuration