The type of load and its inrush current characteristics, together with the switching frequency, are important factors which cause contact welding in relays, contactors, or switches. Particularly for loads with inrush currents, measure the steady state and inrush current. Then select a product which provides an ample margin of safety. Also, verify the actual polarity used since, depending on the relay, electrical life is affected by the polarity of COM and NO.
Fig 1. Relationship between typical loads and their inrush currents.
- Supercapacitors or ultracapacitors are not listed which can initially act as a short. Please refer to Initial Charging of a Supercapacitor.
Fig 2. Load Inrush Current Wave and Time
When using long wires (100 to 300m) are to be used in a relay contact circuit, inrush current may become a problem due to the stray capacitance existing between wires. Add a resistor (approx. 10 to 50Ω) in series with the contacts.
Fig 3. Equivalent long-wire circuit introducing stray capacitance and resistance.
If switching of the relay contacts is synchronized with the phase of the AC power, reduced electrical life, welded contacts, or a locking phenomenon (incomplete release) due to contact material transfer may occur. Therefore, check the relay while it is operating in the actual system. However, if problems develop, control the relay using an appropriate phase.
Fig 4. Potential Issues with Phase Synchronization in Switching AC Loads
Source - Panasonic General Application Guidelines
Related Content:
Initial Charging of a Supercapacitor
Relay Contact Protection Circuits
Relays, Switches, and Electrical Contacts in General
Common Contact Base Materials
Tips for Relay Operation
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