AC
MOTOR SPEED -The speed of an AC induction motor
depends upon two factors:
1.
The number of motor poles.
2.
The frequency of the applied power.
AC motor speed formula :RPM = 120 * Frequency / Number of poles.
Inverter Drive: An
inverter is an electronic power unit for generating AC power. By using inverter
type AC drive, the speed of a conventional AC motor can be varied through a
wide speed range from zero through the base (50Hz) speed and above (often to
90Hz or 120Hz).
PLC CONTROL
The
PLC mainly consists of a CPU, memory areas and appropriate circuits to receive
input/output data.
INPUT
RELAYS-(contacts) :These are connected to the outside world. They physically
exist and receive signals from switches, sensors, etc. Typically they are not
relays but rather they are transistors.
INTERNAL
UTILITY RELAYS - (contacts) : These do not receive signals from the outside
world nor do they physically exist. They are stimulated relays and are what
enables PLC to eliminate external relays. There are also some special relays
that are dedicated to performing only one task. Some are always on while some
are always off. Some are on only once during power-on and are typically used
for initializing data that was stored.
COUNTERS
:These again do not physically exist .they are stimulated counters and they can
be programmed to count pulses. Typically these in their counting speed. Some
manufacturers also include high speed counters that are hardware based. We can
think of these as physically existing. Most times these counters can count up,
down or up and down.
TIMERS
:These also do not physically exist. They come in many varieties and
increments. The most common types. Increments vary from 1 ms through 1s.
OUTPUT
RELAYS-(coils) :These are connected to the outside world. They physically exist
and send ON / OFF signals to solenoids, lights, etc. They can be transistors,
relays, or triacs depending upon the model chosen.
DATA
STORAGE : Typically there are registers assigned to simply store data. They are
usually used as temporary storage for math or data manipulation. They can also
typically be used to store data when power is removed from the PLC. Upon
power-up they will still have the same contents as before power was removed.Very
convenient and necessary.
PLC Operation
A
PLC works by continually scanning a program. We think of this scan cycle as
consisting of 3 important steps. There are typically more than 3 but we can
focus on the important parts and not worry about the others. Typically the
others are checking the system and updating the current internal counter and
timer values.
Step
1- CHECK INPUT STATUS - First the
PLC takes a look at each input to determine if it is on or off. In other words,
is the sensor connected to the first input on? How about the second input? How
about the third… It records into its memory to be used during the next step.
Step
2- EWXECUTIVE PROGRAM - Next the PLC
executes your program one instruction at a time. May be your program said that
if the first input was on then it should turn on the first output. Since it
already knows which inputs are on/off from the previous step it will be able to
decide whether the first output should be turned on based on the state of the
first input. It will store the execution results for use later during the next
step.
Step
– 3 UPDATED OUTPUT STATUS – Finally
the PC updates the status of the outputs. It updates the outputs based on which
inputs were on during the first step and the results of executing your program
during the second step. Based on the example in step 2 it would now turn on the
first output because the first input was on and your program said to turn on
the first output when this condition is true.
After the third step the PLC goes back to step
one and repeats the steps continuously. One scan time is defined as the time it
takes to execute the 3 steps listed above. 
