Derating on Inductive Loads
Deliberately using a relay contact below the current rating given in the catalogue for a resistive load when the load is inductive. Common practice is a 40-60% reduction, or using only 20-40% of the resistive rating; a contact rated at 10A is thus planned for 2-4A on a general inductive load. If the load is a contactor coil (AC-15), the band narrows to as little as 1.5-2A.
Derating is not a safety margin but a calculation for a transition to a different physical condition. What wears the contact on an inductive load is not the continuous current but the roughly 10-fold current surge at start-up together with the arc at the moment of breaking. The catalogue ampere figure was measured under conditions where neither of these two events occurs.
In practice, follow this order:
- Determine the type of load you are driving: resistive, AC coil or DC coil.
- Calculate the pull-in current of the load (not the holding current), or derive it from the coil VA rating.
- Take 20-40% of the contact's resistive rating as your budget; if the load is an AC-15 contactor coil, reduce this share to 15-20% and confirm that the pull-in current does not exceed the budget.
- Add arc suppression: a snubber on an AC coil, a freewheeling diode on a DC coil.
The symptom of skipping derating is typical: the system runs without trouble for months, then the contact welds and the motion does not stop even though the command has been cut. On a crane this is a failure with a direct safety consequence. That is why the standard approach for large loads is to drive a contactor coil with the relay rather than the motor directly, and to size the contact with generous derating.
Context in which this term is used: Wireless Crane Remote Control Selection Guide