Watlow Electric Micro-Based Autotuning Control SERIES 980/985 - Manual (Page 8)

• Ground loops must be eliminated in the entire control system. There are
obvious loops which can be spotted by studying the"as-built" wiring diagram.
There are also the not-so-obvious ground loops that result from the technique
of connecting internal circuit commons in the manufacturer's equipment. An
example of this would be if a control circuit is designed to work with a
grounded sensor input.
• Do not daisy chain A.C. power (or return) lines, or output signal (or return)
lines to multiple control circuits. Use a direct line from the power source to
each input requiring A.C. power. Avoid paralleling L1 (power lead) and L2
(return lead) to load power solenoids, contactors, and control circuitts. If L1
(power lead) is used to switch a load, L2 (return lead) will have the same
switched signal and could couple unwanted noise into a control circuit.
• Grounding the chassis of each piece of equipment in the system is very
important. The simple practice of connecting each individuai chassis to the
overall equipment chassis immediately adjacent to that piece, and then tie all
the major chassis ground terminals together with one lead (usually green
wire) to ground at one single point will work best. Don’t connect ground to
the control case if the control is mounted in grounded enclosure (prevent
ground loops).
• Do not confuse chassis grounds (safety ground) with control circuit commons
or with AC. supply lines L2 (return or neutral line). Each return system
wiring must be kept separate. Be absolutely sure chassis ground (safety) is
never used as a conductor to return circuit current
How To Eliminate Noise
• Use “snubbers” ("QUENCHARC™") to filter out noise generated by devices
such as relays, relay contacts, solenoids, motors, etc. A snubber is a simple
filter device using a 0.1µf, 600 volt, non-polarized capacitor in series with a
100 ohm, 1/2 watt resistor. The device can be used on A.C. or D.C. circuits
to effectively dampen noise at its source.
• The general purpose Watlow snubber, described above, is 0804-0147-0000.
For other ‘QUENCHARC” sizes contact
PAKTRON
P.O. Box 5438
Lynchburg, VA 24502
Phone: 804-239-6941
• Metal Oxide Varistor (MOV) can be used to limit voltage “spikes” that
occur on the A.C. supply lines as a result of lightning strikes, switching large
motors, etc. The MOV is available in several varieties and for 115 or 230
volt lines. The device dissipates the voltage “spikes” to ground and in doing
so repeatedly, deteriorates its ability to function. MOVs have a limited life.
• Watlow stocks several MOVs. See Table 1.
8
WATLOW Series 980/985 User's
Manual Install and Wire, Chapter 2