Elec467 Power Machines &
Transformers
Electric Machines by Hubert, Chapter 13
Topics: protection circuits, ladder logic,
and PLC (programmable logic controllers)
Contactor
A contactor makes and breaks power circuits. Because sparks are associated with live
circuits and motors draw large currents, sparking is an issue which is dealt with by use
of a blow-out coil. Other constructions minimize the time the stays in contact with the
actual contacts.
How a blow-out chute works
When the circuit breaks a spark is created. This spark is actually a current
flow of electrons jumping through the air because of the potential difference.
The current is changed into a magnetic field by the blow-out coil which
affects the path of the current which is not restrained by a wire. As the
length of the spark grows the resistance increases eventually causing the flow
of electrons to stop.
Shading coil
The shading coil changes the flux across the contacts so that the armature
doesn’t chatter when the operating coil closes. Chatter is created when the
armature closes so quickly it bounces off the stationary contact. The shade coil is
wrapped around ½ of the flux path so the current flow created in the shade coil
creates a counter emf flux. This delays a portion of the flux created by the
operating coil when it is turned on. Once the flux stabilizes the shade coil field
collapses. The initial flux which caused the armature to change states (closing) is
not strong enough to hold the armature closed without bouncing. This is when
the flux delayed by the shade coil shows up in time to hold the armature closed
and not allow it to bounce.
Thermal overload relay
The Thermal overload relay operates somewhat like a thermocouple
but uses different coefficients of expansion properties of two dissimilar
metal to pull contacts apart when there is too much current causing
the heater element to overheat. .
Schematic showing operation of
thermal overload relay
There is a range of currents that cause the relay to open (trip) and a delay time to
allow the motor to cool before it can be reset.
Over-current relay
An over-current relay is
designed so a high current
flow will cause the contacts
to open. This is a normally
closed relay under normal
current flow. Its placement
in the motor branch circuit
breaks the circuit and the
relay holding the M
contacts closed opens
when they lose power. The
over-current relay loses
power also but by then its
job is done.
A motor
circuit
diagram
shows
connections
labels, wiring
and physical
placement.
This is a ladder
network. The
rungs of the
ladder are
branches of
circuits wired in
parallel. It reads
top to bottom,
left to right.
Normal
arrangement has
the functional
circuit on the first
rung and
subsequent
branches control
relays above
their rung. PLC
software
facilitates the
design of a
control circuit.
Automatic circuit breakers
This slide illustrates two types of button designs. (a) is a forever Start and you have to
press Stop to break the contact. (b) is a Start that stays connected until the voltage
drops or the Stop button interrupts the voltage. Both stop the motor when M’s operation
coil looses power causing the flux to collapse that was holding M’s contacts closed
Programmable Logic Controllers
PLCs are used to control machines or processes that are sequential in
nature, using “discrete” inputs and outputs that have defined states. For
example, if a limit switch detects the presence of an object, it provides
an “ON” signal to the PLC; if no object is detected, it provides an “OFF”
signal. The machine or device typically performs actions based on time or
events in a pre-defined order.
PLC basics
A PLC consists of three basic sections: power supply, the central
processing unit (CPU) and the input/output interface system. The
CPU, which controls all PLC activity, can further be broken down
into the processor and memory system. The input/output system is
physically connected to field devices (e.g., switches, sensors, etc.)
and provides the interface between the CPU and the information
providers (inputs) and controllable devices (outputs).
One of
many
optional
I/O units
Power
supply
CPU with I/O