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. Motor switching also requires an appropriate utilization category and rating; contact size alone does not establish suitability. 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. The homepage relay example uses 8-6 for holding. In this MC model, place 4-10 across the same two nodes as START, keeping STOP in the common supply path.
- 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
Open the practical project list →
Check your understanding
Question: What happens to 4–10 and 5–11 when this MC energizes?
Answer: 4–10 closes and 5–11 opens in this model. Auxiliary contacts are not substitutes for motor power contacts.
Practice path and references
The practical project list may require sign-in. Create a project, choose a task and place the relevant components, then compare the contact/wiring paths explained here. The link does not preload a specific completed circuit.
Reference: Q-Net public-problem notice, 2025-08-04 edition, problem 14 pages 11-1 to 11-4 (work/wiring instructions) and 11-9 (component diagrams). Read the original for each task and any amendments. Site examples are distinct from general installation design requirements.