General information on suppression modules
24 V DC supply voltage
Suppression modules for inductive products simplify the
fulfilment of the EMC law!
1024 V
Old laws have not lost their validity now. The Ohm´s law is still valid everywhere,
as well as the Lenz´s rule. This states that when an inductive load is switched off,
current has the desire to continue flowing in the same direction and strength as
before. A voltage source is created from the inductive load, which can be many
times higher than the nominal voltage. This voltage spike is the cause of many
EMC problems.
What does the EMC law achieve?
From the 1st January 1996 only products which fulfil the requirements of § 4 of
the EMC guidelines, can be brought in to the European market.
– 1000 V
M
0V
By using semi-conductor elements such as a transistor the usual values of
UCEmax = 100 V must be taken into consideration. A short term voltage spike of
1024 V, as shown in the example, can destroy a transistors and in many cases,
mean a new PLC or bus card must be bought.
That means:
Murrelektronik solves your interference problems:
Products cannot send out interference nor be influenced by it.
Suppression components do not fall under the EMC guidelines. These products do,
however, come under control of the low voltage guidelines, so long as they have
a voltage between 50…1000 V AC/75…1500 V DC.
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Our service for you:
• high availability
• complete assortment
• world-wide sales network
• we offer complete systems from
PLCs to control peripherals
• EMC service center
Our partners are:
We understand the problem…
faults in the control process
defective control system
defective coil on the motor, contactor, valve or transformer
interference emissions causing a reduction in functionality
EMC problems
Suppression modules for control systems
Murrelektronik supply suppressors of all types and for all control voltages
(24 V…690 V).
Research into the suppression of various inductive coils (contactors, motors,
solenoid valves) has enabled Murrelektronik to produce made-to-measure
quality products with the following characteristics:
• AC voltage suppression is normally done with an RC-network, which places
special demands on the capacitors and resistors.
1. High inductive voltage spikes at inductive loads:
The high voltage spikes when switching off exceed the normal supply voltage many
times and overload the electronics. The voltage can in some cases be so high, that
the windings are shorted and the inductive product is destroyed.
• The type of capacitor foil, the impulse capacity, the short time overload
voltage rating and the small housing are important factors.
2. High inductive voltage spikes in the automated system:
The voltage spikes climb at a very high speed. These very high climb rates contain
high frequencies. Every cable and wire in the control system acts as a transmitter
or receiver for these high frequencies.
These high frequency interferences can therefore move unhindered from cable to
neighboring cable.
• The diodes for DC suppression must satisfy strict specifications. Special
switching characteristics, high blocking voltages and a special crystal structure
are required.
In order to suppress voltages from 24 V…240 V DC with just one suppressor
Murrelektronik have developed a special diode-network.
3. High inductive voltage spikes in the controller:
The present voltage spike at the moment of switching off adds itself in opposite
polarity to the supply voltage and affects the controller i.e. PLC. These high
voltages in the controller cause energy rich arcs, due to the current flowing on in
the same strength and direction at the moment that the contact is opened. The arcs
cause material deformation and reduce the life expectancy of the switching elements.
3.3.1
• The resistors must be suitable for a range of voltages.
• Zener diode circuits are available for interference suppression on DC
inductances. This circuit combination gives very little switch-off delay time,
especially suitable for fast acting processes.
• The newly available small contactors demand varistor suppressors in view of
the size limitations. Varistors give very good over-voltage absorption and long
life.
General information on suppression modules
• A newly developed suppressor combines a varistor with an RC-network. With
compact dimensions this arrangement not only cuts the voltage peak but also
dampens the steepness of the voltage rise.
The mechanical mounting of the modules is therefore exact and simple as well as
guaranteeing the optimum suppression characteristics. Connection is via flexible
wires or direct contacts.
• The mechanical strength of the housing, consisting of accurately fitting
moulded parts, is achieved in Murrelektronik's own tool shop and injection
molding departments.
Contactors, motors and solenoid valves
Why must coils be suppressed?
The table shows the theoretical values, and those obtained in practice, of the
back - e.m.f. caused when a coil is switched off.
Murrelektronik…leaders against the voltage spike
(L S =25 H; C S =100 pF; I 0 =10 mA) Theoretical In practice
To combat high voltage switch off overloads, which lead to contact burn out and
destruction of the neighboring electronic control systems, contactors should be
suppressed.
Murrelektronik has for years developed and produced suppression modules, which
are carefully designed together with the contactor manufacturers, to work with
specific contactors.
Switch-off peak (back - e.m.f.)
5000 V
100 - 4000 V
Speed of voltage climb
V
0,1 ns
V
0,1 - 20 ns
Frequency
3,8 KHz
1KHz - 10 MHz
Extra power required
due to suppressor
Components
–
Very large 1 V
–
–
Advantages:
• matches wide range of loads
• best possible damping
• simple construction
Disadvantages: • long delay time
Yes
Small
UVDR
Yes
Yes
Advantages:
Yes
Small
UZD
–
–
Advantages:
• limits positive and negative voltages
• suitable for AC and DC
• matches wide range of loads
Disadvantages: • no damping below UZD
Yes
Small
UVDR
–
–
Advantages:
Yes
Small
1,5xUNom Yes
Yes
Advantages:
Back e. m. f.
limitation
Damping also occurs
below ULIMIT
Incorrect polarity
protection
and also suitable for AC
Additional switch-off
delay
Characteristics of load
current and voltage
Circuit
An overview of different types of suppressors
• HF-damping due to RC-network
• high energy absorption
• short delay time
Disadvantages: • must be matched to the load
• limited lifespan
• matches wide range of loads
• high energy absorption
• very simple construction
Disadvantages: • no damping below UVDR
• limited lifespan
• HF-damping due to RC-network
• immediate de-energization
• excellent results with AC
Disadvantages: • must be matched to the load
• limited lifespan
3.3.2