Page 121 - Offshore Electrical Engineering Manual

108 CHAPTER 10 Motor Control Gear

INTELLIGENT MOTOR CONTROL CENTRES
In a fully integrated intelligent MCC, all nonpower functions are digital and software
controlled.
Ideally, all the units should also have input points to monitor devices such as the
disconnect switch, contactor, overload relay and a hand-off-auto selector switch.
A network scanner module or network linking device must also be provided to
collect and distribute the device data in the MCC. An integrated intelligent MCC
should have at least the following components available.

1. Digital overload relays: Each motor starter should have
a. built-in network communication;

b. input points (for monitoring disconnect or selector switch);

c. output points (for controlling contactor);

d. light-emitting diodes for status indication;
e. protective functions – thermal overload, underload, jam, current imbalance,

stall, phase loss, zero sequence ground fault and positive temperature coef-
ficient thermistor input;

f. programmable parameters for the protective functions – trip level, warn-
ing level, reacceleration time delay and inhibit window, with the facility to
enable programming via a Laptop;

g. warning alarms to alert users of a potential trip and allow actions to avert
impending downtime. Time delays and inhibit windows allow recognition
of abnormal current loads (e.g., extended starting times with high currents),
without nuisance tripping;

h. current monitoring – phase, average, full load, ground fault, imbalance per-
cent and percent thermal capacity used are important monitoring features;

i. diagnostics – device, warning and trip status; time to overload trip; history of
last five trips and time to reset.
2. Small input/output (I/O) module for ‘nonintelligent’ units (e.g., isolators): The
preferable solution is an I/O module within the unit – small enough so that the
MCC unit size is not altered – to link the device and the network. The I/O mod-
ule should have an adequate number of inputs and outputs, according to the unit
functions (for a starter, four inputs and two outputs).
3. Network communication interface unit with input points: Intelligent devices often
require an external communication module. Ideally this module should contain
input points (again, to eliminate wiring to a distant I/O chassis) (five inputs each).

MOTOR CONTROL CENTRE SOFTWARE

Integrated intelligent MCCs require dedicated software that provides the operator
with essential data on MCC functions and is integrated into the operator’s displays.
MCC software should eliminate the need for creating customised MCC screens
within operator interface software.