What Is an MC (Magnetic Contactor)? 12-Pin Main and Aux Contacts
An MC (magnetic contactor) is, in one line, a relay sized to carry motor current. The principle from the 8-pin relay — "current through the coil flips every contact at once" — carries over unchanged. The only difference is that its main contacts are big and rugged enough to break a motor's heavy current. It lets a small control signal switch the motor's heavy power on and off, which makes it the panel's workhorse.
The coil terminals carry the industry names A1 · A2 — pins 6 · 12 on the 12-pin layout. Land control voltage there and all five of the 12-pin contacts snap to their energized position at once.
Main contacts (1-7 · 2-8 · 3-9) — the heavy 3-phase switch to the motor
- At rest (coil off) — all three pairs open → motor stopped.
- Coil energized — all three close together → L1=7 · L2=8 · L3=9, three phases hit the motor at once → it spins.
- Where the 8-pin relay had COM-NO-NC sets, the MC's main contacts are three plain on/off pairs. The terminals are chunky precisely to carry that heavy current.
Aux contacts (4-10 a · 5-11 b) — self-hold and interlock
The aux contacts are the small switches that report the MC's state back to the control circuit. They can't carry motor-grade current, but they're plenty for relay and lamp signals.
- 4 ↔ 10 (a/NO) — closes when the coil energizes. The go-to pins for self-hold and "running" indicator lamps. Just as L3 tied the 8-pin's 1-3 in parallel with the start button, the MC ties 4-10 in parallel with the start button for the same self-hold.
- 5 ↔ 11 (b/NC) — opens when the coil energizes. Used for the interlock in forward/reverse circuits, to keep the opposite MC from ever closing at the same time.
See it work
- Press the button and the coil becomes a magnet, flipping all five contacts at once.
- 4-10 closes, lighting the green lamp via L1 → 4 → 10 → GL. 5-11 does the opposite and opens.
- Let go and the spring returns everything to rest; the lamp goes dark.
Next
Now see how this MC connects to the motor and the outside world — Motor + terminal blocks (TB) covers the boundary between inside and outside the panel.
Try it yourself
Watch the MC pull in, in the Korean Electrician practical simulator →