A case history of a Motor Control Center (MCC) revision.
Copyright © 2010 Francis J. Martino
The customer purchased components to revise the MCC modules so as to operate
new production equipment. This note allowed the purchasing personnel to understand
the composition of an MCC Module.
The Motor Circuit Protectors (MCP) had to be sized properly. Sizing as per NEC will
be added to this note shortly.
Other considerations were the choice of the overload relay types and the addition of
GE Multilin TM ground fault relays.
Below are photographs of a starter mounted in the General Electric Spectra Series TM / 8000
motor control center:
The lower photograph on Page 14 shows a size 0 or size 1 starter. It is mounted
to the white panel by screws. If the holes in the panel do not match the mountng
holes of the starter then holes are drilled into the panel and self-threading
screws are then placed through the mounting holes in the starter and into the
holes that were drilled into the panel. When the screws are tightened the
starter will then be held firmly against the panel.
To the left is the disconnect switch. There is no MCP in the module. Motor protection
is given by the three fuses to the lower right.
Notice that the Module is not in the MCC.
The upper photograph on Page 14 shows a Hand-Off-Auto switch in the door,
a disconnect switch and a reset pushbutton. There are no indicating lights in the door.
The upper photograph on page 15 shows the rear of the module. The three stabs on
the bottom are connectors that will be pushed into the vertical bus when the module
is placed in the MCC. The stabs make the mechanical and electrical connection to
the bus. It is the vertical bus which carries the current to the modules. Notice
the holes in the panel. It is those holes that are used for mounting the
components in the module.
The lower photograph shows indicating lights in the door. Pages 14 and 15 show
Modules that will be placed in the MCC.
A size 5 starter is shown on page 16. The starter is attached to the white panel by
screws. The MCP is to the lower left. A control panel (Item 11) for special equipment
is on the lower right. The control panel will mount in the large hole that is
in the door. The special equipment to be controlled is not in the MCC.
Notice the size of the module with the size 5 starter and MCP. It has a height that is
greater than the module with the size 1 starter. Page 1 shows a MCC with several sizes
MOTOR STARTER OVERLOAD RELAYS
One of two types of overload relays need to be chosen, either bimetallic or solid-state:
General Electric bimetallic overload relay with Class 20 protection. The bimetallic unit
requires heater elements that are sized for the motor full load amp rating.
General Electric solid-state overload relay with selectable class 10, 20 or 30 protection.
The solid-state units require no heater elements. The current sensing range is adjustable.
For most applications, Americans use Class 20 and the Europeans use Class 10.
Class 10 has a faster trip time and thererfore affords greater protection than Class 20.
Class 30 has a long trip time and is used for high inertia loads and long acceleration
times during start.
To select the overload amperage rating, yoy may use either the motor horsepower and
voltage or the motor full load amp rating.