AC-15 Load Category
This is the category defined for switching inductive alternating current control loads such as contactor coils and solenoids. It is the load type the relay output actually faces in a crane panel; the AC-15 capacity of the same contact stays at roughly 15–20% of the AC-1 rating.
Two physical phenomena make AC-15 difficult. The first is pull-in: before the coil has pulled in the air gap is large, so the inductance is low and the current rises to roughly 10 times. The second is interruption: when the current is cut, the magnetic energy stored in the coil is released as an arc across the contact gap.
In the field this corresponds to something clear:
- A contact rated 10A AC-1 is typically planned for the order of 1.5–2A under AC-15 conditions. Unlike the looser 20–40% budget (2–4A) given for general inductive loads, AC-15 is a tighter condition specific to contactor coils.
- On cranes that start and stop frequently, contact life is used up far faster than in a continuously running application.
- An arc suppression measure (a snubber on an AC coil, a freewheel diode on a DC coil) noticeably extends life.
When making a selection, look at the coil data of the contactor you are driving: the pull-in power (VA) and the holding power are given separately; size the contact according to the pull-in value. If you need to drive a motor directly instead of a contactor coil, do not forget that it is the motor's current characteristic and not the relay output that is decisive.
The context in which this term is used: Wireless Crane Remote Control Selection Guide