Cutler-Hammer D320 PLC, User Manual

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190
D320 PLC User's Manual
Application Characteristics Selection of Parts Countermeasures
AC
Load
DC
Load
Attach a surge
suppressor:
Inductive load
Relay output
module
COM
OUT
If the load is a relay or a solenoid,
the load is slow to return to the
normal status. When using a DC
power source, place the surge
suppressor across the inductive
load.
When using an AC power source,
place the surge suppressor across
the switching relay contacts.
The example shows how to connect
the surge suppressor for a DC
power source
For a contact voltage of 1 V and a
contact current of 1 A, use the
following C and R values:
C: 0.5-1.0
µ
F
R: 0.5-1.0 Ω
Another example; for a contact
current of 0.5 A and a contact
voltage of 200 VAC use the
following C and R values:
C: 0.25-0.5
µ F
R: 100-200
Ω
For DC circuits use a minimum of a
250 V rated capacitor. For AC
circuits use a minimum of a 1000 V
rated capacitor.
Attach a flyback
diode:
Inductive load
Relay output
module
COM
OUT
×
The diode connected in parallel
allows the energy accumulated in
the inductive load to flow back into
the inductive load in the form of an
electrical current. The energy is
then dissipated as heat based on
the resistance of the inductive load.
The time required to return to the
normal status is longer than the
surge suppressor method. Use diodes with low reverse
leakage current and with a reverse
voltage value that is at least three
times greater than the nominal
applied voltage. Verify the diode
has the proper power rating.
The steady state current that flows
when the inductive load is turned on
should be greater than the current
produced when the inductive load is
turned off.
Attach a varistor:
Inductive load
Relay output
module
COM
OUT
A varistor functions as a voltage
clamping device. When the applied
voltage exceeds the rated voltage
value of the varistor, the varistor
turns on, creating a short circuit
connection across the inductive
load.
This method has a slow recovery
time.
When using a DC power source,
place the varistor across the
inductive load.
When using an AC power source,
place the varistor across the
switching relay contacts. To specify the varistor do the
following:
Chose a maximum continuous
voltage rating just above the
expected applied voltage.
Chose a varistor that can handle
the energy level that will be
generated by the inductive load
BUT avoid overspecifying. As the
varistors energy level capability
goes up so does the capacitance
which will slow down the response
time of the system.