4-Stop Lift — Relay & VFD Control

The 24 V transformer feeds a relay ladder. Any station's UP or DOWN button latches the U or D relay, which closes the VFD's FWD or REV input — the car runs at full speed. Pressing B or C (before or during the run) latches that floor's call relay.

Each floor has a slow zone: a roller switch held closed by a cam strip on the car. When the car enters a zone it is targeting — the B zone with the B relay latched, the C zone with C latched, or a terminal zone (A / D) in the direction of travel — the SL (slow) relay picks up. SL closes the VFD's preset-slow input, so the drive ramps down to creep speed, and simultaneously starts the on-delay timer TR. Exactly 5.0 seconds later, TR's timed contact breaks the HOLD bus that feeds every seal-in contact: U/D, B/C, and SL all drop out, the VFD ramps to zero, its "running" relay opens, and the brake relay lets the spring-set brake close.

Any STOP button (NC contacts, all four in series) breaks the whole control bus — everything drops and the car stops immediately. Final limit switches block travel beyond A and D, and hard overtravel switches cut the control feed entirely as a backstop.

Relay contact allocation

Point-to-point

1. Supply L → Q1 pole 1 → VFD L1. Supply N → Q1 pole 2 → VFD L2/N.
2. VFD U, V, W → motor terminals, direct run, no devices in between.
3. Tap L and N on the load side of Q1 → TX primary.
4. TX secondary terminal 1 → FU2 → this is 24 V H (ladder hot). TX secondary terminal 2 → 24 V C (ladder common, right rail).
5. Bond PE throughout (supply → panel → VFD PE → motor frame).

The left rail carries 24 V H through the protective chain (fuse, overtravels, all four STOP buttons) — everything below that point is the CTRL bus. Rung R0 derives the HOLD bus (dashed) through the timer's timed-NC contact; every seal-in contact feeds from HOLD, while the pushbuttons feed from CTRL. When TR times out, HOLD dies for an instant and every latch releases at once.

Rung-by-rung

The VFD's logic inputs are switched with dry relay contacts only — jumper them to the VFD's own COM terminal, never to the 24 V control transformer. The brake relay coil is powered from the 24 V ladder through the VFD's relay output, so the spring brake can only lift while the drive is producing torque.

VFD parameter setup (names vary by brand — find the equivalents)

Mount the roller switches on fixed brackets in the shaft, in vertical columns. Cam strips (angle stock or UHMW strips) bolt to the car; a switch is held actuated for exactly the length of car travel over which its cam overlaps the roller. Different zone lengths therefore need cams in different columns.

Switch schedule

Car parked at A; goods loaded; operator sends it to C:

1. Press C — C relay latches via C-1 on the HOLD bus.
2. Press UP — U latches (D-2 and LS-TF are closed), U-4 closes the VFD FWD input, the drive ramps up, its R1 "running" output pulls in BR, and the brake lifts. The car accelerates to full speed.
3. LS-A is held at the start (car in the bottom zone) but D-3 is open, so SL stays out. The car passes the B zone; B relay is out, so nothing happens.
4. The cam edge reaches LS-C: C-2 and LS-C are both closed → SL picks up. SL-1 closes the VFD slow input (drive ramps to creep), SL-2 starts TR.
5. 5.0 s later TR-1 opens: HOLD dies, U / C / SL all drop, FWD and SLOW open, the VFD ramps to zero, R1 opens, BR drops, and the spring brake sets with the car level at C.
6. TR loses its coil (SL dropped), TR-1 recloses, and the ladder is idle, ready for the next call.

Running to a terminal is the same, except the zone is armed by the direction relay itself (U-3 · LS-D going up, D-3 · LS-A going down), so the car always stops at D or A even with no floor button pressed — exactly the behavior you specified.

1. Bench-test the ladder first. Power only the 24 V circuit (leave the VFD inputs disconnected). Verify each latch, the interlocks, the STOP chain, and that shorting SL's coil terminals for 5 s drops everything.
2. Set VFD parameters per the table in Section 5, then check motor rotation unloaded (swap two motor leads if UP runs down).
3. Set creep speed around 10–15 Hz and measure the actual creep velocity over a marked metre.
4. Position the cams using d from Sheet 4. For each slow-zone switch, place the cam's leading edge (the edge the car hits first on approach) d before the floor. A short cam (~75 mm) is sufficient — SL-3 holds the relay in after the cam passes. For B/C floors, centre a short cam on the floor and verify its leading edge is ≥ d from centre in both approach directions.
5. Trial runs: if the car stops short of a floor, move that cam edge closer to the floor (shorten d); if it overshoots, move it away. B/C cams are centered, so trim each end to tune each approach direction independently. A/D landing can also be trimmed by nudging LS-TF / LS-BF, which stop the car directly if the timer runs long.
6. Verify the safeties last: press each STOP mid-travel; block LS-TF/LS-BF and confirm OT-U/OT-D kill the control feed; confirm the brake holds the rated load with power off.

• One call at a time. The timer clears every latch at each stop, so if B and C are both pressed the car stops at the first zone it reaches and forgets the other call — press it again. True call memory needs per-floor reset logic (about two more relays).
• Don't press the button for the floor you're on. If the car sits at B with B latched and you press UP, SL picks immediately and the car creeps for 5 s before stopping slightly above B. Harmless, but avoidable by habit.
• Leveling is time-based at B and C: accuracy depends on consistent creep speed and cam edges. With a VFD holding a fixed creep frequency it's quite repeatable; load-dependent slip is the main variable.
• Worthwhile additions: gate/door interlock switches in series with the CTRL bus (car can't move with a gate open — do this one), a slack-rope or overload switch in the same chain, and a pilot lamp per station wired across the U/D coils so operators can see the car is in motion.