AutomationDirect DL105, User Manual

Page: 54 / 350

Installation,
Wiring,
and
Specifications
2--23
Installation, Wiring, and Specifications
DL105 PLC User Manual, 3rd Edition
Output Point Wiring
Ground
Neutral
Line
+
--
Fuse or
Circuit
Breaker
Inductive load devices (devices with a coil) generate transient voltages when
de-energized with a relay contact. When a relay contact is closed it “bounces”, which
energizes and de-energizes the coil until the “bouncing” stops. The transient
voltages generated are much larger in amplitude than the supply voltage, especially
with a DC supply voltage.
When switching a DC-supplied inductive load the full supply voltage is always
present when the relay contact opens (or “bounces”). When switching an
AC-supplied inductive load there is one chance in 60 (60 Hz) or 50 (50 Hz) that the
relay contact will open (or “bounce”) when the AC sine wave is zero crossing. If the
voltage is not zero when the relay contact opens there is energy stored in the
inductor that is released when the voltage to the inductor is suddenly removed. This
release of energy is the cause of the transient voltages.
When inductive load devices (motors, motor starters, interposing relays, solenoids,
valves, etc.) are controlled with relay contacts, it is recommended that a surge
suppression device be connected directly across the coil of the field device. If the
inductive device has plug-type connectors, the suppression device can be installed
on the terminal block of the relay output.
Transient Voltage Suppressors (TVS or transorb) provide the best surge and
transient suppression of AC and DC powered coils, providing the fastest response
with the smallest overshoot.
Metal Oxide Varistors (MOV) provide the next best surge and transient suppression
of AC and DC powered coils.
+24 VDC --24 VDC
Module Relay Contact
--324 VDC
+24 VDC
For example, the waveform in the figure below shows the energy released when
opening a contact switching a 24 VDC solenoid. Notice the large voltage spike.
Surge Suppresion
For Inductive
Loads