Craftsman Electrician study guide

Korean Electrician Public Problem 14 — Control Circuit (Automatic Forward/Reverse)

Last updated: 2026-06-18 · 3 min read

The 22 main-circuit conductors already built the path power takes. Yet applying power does nothing, because MC1·MC2's coils aren't energized yet. The circuit that decides *when* to energize them — the control circuit — is this guide's whole subject. Its backbone is a single self-hold latch, with 8-pin relays, timers, and limit switches layered on as conditions.

The operation sequence — what the circuit must do

Press PB1 → MC1 in — motor runs forward.
Reach end-of-travel → after a delay, MC2 in — reverse (a timer makes the delay).
Reach the opposite end → after a delay, MC1 in again — forward.
Press PB0 → all coils drop — stop.
EOCR trips → YL alarm lights + all coils drop — overload protection.

Problem 14 complete wiring diagram — main circuit (brown·black·gray·green) and control circuit (yellow) on one sheet. The 40 control wires fill in here.

Read it as three paths

The 40 control wires are clearest split into the schematic's top, bottom, and middle paths.

The top joins every coil's input (L1) into one node; the bottom gathers every coil's return (N) into another.

Top path — the L1 supply. The protected control supply runs through 9 terminals in turn, all joined into one node. Every coil's and timer's input side hangs off this single node. Wire it keeping each terminal within the 2-wire cap.
Bottom path — the N return. The fuse output, EOCR's N-side b-contact, every coil's A2 (pin 12), and the timer commons all gather into a second node. Routing order is free — all that matters is the 10 contacts land in one node.

Middle path — self-hold, timer start/timed outputs, interlock, indicator lamps. Every bit of logic between the two rails.

Middle path — the logic. Between the two rails sits the part that produces the actual behavior: self-hold a-contacts, timer start signals and timed outputs, MC coil drive, the interlock, and the indicator lamps — scattered across several small nodes. For each node, route however you like, as long as its contacts end up joined into one node.

This problem's new part — the limit switch (LS)

The heart of automatic forward/reverse is detecting when the motor reaches end-of-travel — the job of the limit switch. There are two, one at each end of travel (LS1·LS2).

A limit switch — pressing the lever/roller flips its SPDT pair (NC·NO) together.

Each LS is a single SPDT (COM·NO·NC). That one pair does two jobs at once in the forward/reverse swap:

NC (b-contact) — closed at rest, opens at end-of-travel. → Wired into the top path (L1 supply), so reaching the end cuts that side's coil supply.
NO (a-contact) — open at rest, closes at end-of-travel. → Wired into the middle path's self-hold input, so reaching the end triggers the opposite direction.

So one trip of an LS does both at once: kills this side's motor AND starts the other side's — that's the whole automatic forward/reverse mechanism.

The interlock — skip it and you get a 3-phase short (disqualification)

Having followed one forward/reverse problem from parts to layout to control, look at a complete problem of a different shape — Public Problem 1, fully analyzed.

Try it yourself

Wire Problem 14's control circuit and drive it with the PBs and LSs in the simulator →