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Contacts
Relay manufacturers assume you will be using resistive loads when they rate their relays.
The load is a simple resistive element, and it is assumed that the current flow through the contacts will be fairly constant, although
some increase may occur due to arcing during "make" or "break." Ideally, a relay with a purely resistive load can be operated at its
stated voltage and current ratings and attain its full lifetime.
Precautions for the Contacts
Contacts are the most important elements of relay construction. Contact life is influenced by contact material, voltage and current
value applied to the contacts, the type of load, switching frequency, ambient atmosphere, form of contact and the contact bouncing
etc.
The material transfer, welding, abnormal usage and the increase in contact resistance bring about the failure of the contacts. Please
pay attention to them in application.
In order to better apply the relay, please refer to the following precautions of the contacts.
Inductive loads
Switching inductive loads is difficult, primarily because current tends to continue to flow in inductors, even as contacts are being
broken.
The stored energy in inductors induces arcing; arc-suppression schemes are frequently used.
When you are switching inductive loads, you typically will want to derate relay contacts to 40 percent of the resistive load rating.
Capacitive loads
Capacitors resemble short circuits when they are charging, so the in-rush current from a capacitive load can be very high. Series
resistors are often used to limit in-rush current; without a limiting resistor, contact welding may occur.
When you are switching capacitive loads, you typically will want to derate relay contacts to 75 percent of the resistive load rating.
Motor loads
When an electric motor starts up, it has very low impedance and requires a large in-rush current to begin building a magnetic field
and begin rotating. Once it is running, it generates a back electromagnetic force (emf), which can cause a large inductive spike when
the switch is opened. The result is a large in-rush current at "turn-on" and arcing at "turn-off."
When you are switching a motor load, typical industry practice is to derate to 20 percent of the resistive rating.
Type of load and inrush current
The type of load and its inrush current characteristics, together with the switching frequency, are important factors which cause
contact welding.
The table shows the relationship between typical loads and their inrush currents.
Type of load
Resistive load
Solenoid load
Motor load
Incandescent lamp load
Mercury lamp load
Sodium vapor lamp load
Capacitive load
Transformer load
80618_MAN_IO-LINK IMPACT_12-2019_ENG
10. RELAY APPLICATION NOTES
Inrush current
Steady state current
10 to 20 times the steady state current
5 to 10 times the steady state current
10 to 15 times the steady state current
Approx. 3 times the steady state current
1 to 3 times the steady state current
20 to 40 times the steady state current
5 to 15 times the steady state current
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