Installation
4.4 Wiring guidelines
S7-1200 Programmable controller
80 System Manual, V4.2, 09/2016, A5E02486680-AK
Guidelines for inductive loads
Use suppressor circuits with inductive loads to limit the voltage rise when a control output
turns off. Suppressor circuits protect your outputs from premature failure caused by the high
voltage transient that occurs when current flow through an inductive load is interrupted.
In addition, suppressor circuits limit the electrical noise generated when switching inductive
loads. High frequency noise from poorly suppressed inductive loads can disrupt the
operation of the PLC. Placing an external suppressor circuit so that it is electrically across
the load and physically located near the load is the most effective way to reduce electrical
noise.
S7-1200 DC outputs include internal suppressor circuits that are adequate for inductive
loads in most applications. Since S7-1200 relay output contacts can be used to switch either
a DC or an AC load, internal protection is not provided.
A good suppressor solution is to use contactors and other inductive loads for which the
manufacturer provides suppressor circuits integrated in the load device, or as an optional
accessory. However, some manufacturer provided suppressor circuits may be inadequate
for your application. An additional suppressor circuit may be necessary for optimal noise
reduction and contact life.
For AC loads, a metal oxide varistor (MOV) or other voltage clamping device may be used
with a parallel RC circuit, but is not as effective when used alone. An MOV suppressor with
no parallel RC circuit often results in significant high frequency noise up to the clamp
voltage.
A well-controlled turn-off transient will have a ring frequency of no more than 10 kHz, with
less than 1 kHz preferred. Peak voltage for AC lines should be within +/- 1200 V of ground.
Negative peak voltage for DC loads using the PLC internal suppression will be ~40 V below
the 24 V DC supply voltage. External suppression should limit the transient to within 36 V of
the supply to unload the internal suppression.
Note
The effectiveness of a suppressor circuit depends on the application and must be verified for
your particular usage. Ensure that all components are correctly rated
and use an
oscilloscope to observe the turn
-off transient.
To Next Page
Loading...
