Motor Control with a PLC: DOL, Reversing, Star–Delta, I/O Lists, Wiring, Hard-Wired Interlocks and Contactor Feedback Monitoring

Moving the logic from a relay panel into a PLC gains flexibility, status display and fault detection — but the safety of the original circuit must not be lost. This page draws a clear line between what lives in software and what must stay in hardware, and gives working programs for all three motor circuits.

By Ajarn ModUpdated 14 Oct 2026

In 30 seconds

Keep in hardware: emergency stop via a safety relay · overload 95-96 · NC + mechanical interlocks
The PLC does: self-holding, sequencing, timers, feedback checks, alarms, run-hour counting
Feedback: $\text{Fault} = \text{TON}(\text{Cmd} \oplus \text{FB},\ 0.5\text{–}2\ \text{s})$
Star–delta: Y first → K1 → time elapsed → Y drops out → dead time → Δ

Dividing the jobs: hardware vs software Basic

Function	Where	Why
Emergency stop	Safety relay / safety PLC cuts coil power + PLC reads the status	Must work even if the standard PLC fails or its program is edited
Overload	95-96 in series with the coil + 97-98 into the PLC	Trips even if the PLC hangs, and the cause can be displayed
Short-circuit interlock	Auxiliary NC + mechanical + in the program	A shorted transistor output or a forced bit must not cause a short circuit
Self-holding, sequencing, timers	Program	Easy to change, settable from the HMI
Detecting a welded / non-pulling contactor	Program + feedback contacts	Only possible when there is a controller with logic

DOL with a PLC and feedback checking Basic

Address	Device	Type
I0.0	-S0 STOP	NC (1 = healthy)
I0.1	-S1 START	NO
I0.2	-F2 97-98 or auxiliary 95-96	NC (1 = healthy)
I0.3	-K1 13-14 feedback	NO
Q0.0	-K1 coil (via 95-96)	Relay or interposing relay
Q0.1 / Q0.2	Run lamp / fault lamp	—

M1 Run command

\text{Cmd} = I0.0\cdot I0.2\cdot\overline{\text{Fault}}\cdot(I0.1 + \text{Cmd})

M2 Feedback discrepancy

\text{Fault} \leftarrow \text{TON}(\text{Cmd} \oplus \text{FB},\ t_{fb}) \qquad t_{fb} \approx 0.5\text{–}2\ \text{s}

What M2 catches: commanded but not pulled in (blown coil fuse, open coil, 95-96 open) and not commanded but still pulled in (welded contacts, stuck output). The second case is dangerous: raise an alarm and disconnect with the upstream device.

Reversing with a PLC Intermediate

M3 Software interlock

Q_F = \overline{S0}\,\overline{OL}\,(S_F + Q_F)\,\overline{S_R}\,\overline{Q_R}\,\overline{FB_R} \qquad Q_R = \overline{S0}\,\overline{OL}\,(S_R + Q_R)\,\overline{S_F}\,\overline{Q_F}\,\overline{FB_F}

Adding $\overline{FB}$ of the other direction (feedback from the real contactor) stops the program commanding the new direction until the old contactor has actually dropped out. A reversal delay timer of 0.5–2 s is also commonly added to let the motor slow down.

Star–delta with a PLC Intermediate

Figure 1 · Outputs drive the coils through NC contacts of the opposing contactor (hard-wired) · inputs read the buttons, the overload and all three feedback contacts

M4 Star–delta sequence

K_Y = \text{Run}\,\overline{T_Y}\,\overline{K_\Delta} \quad K_1 = \text{Run}\,(K_Y + K_1) \quad K_\Delta = \text{Run}\,\text{TON}(T_Y,\ t_{dead})\,\overline{K_Y}\,\overline{FB_Y}

(* Structured Text: star–delta with feedback check *)
Run := (PB_Start OR Run) AND PB_Stop_NC AND OL_NC AND NOT Fault;
TY(IN := Run, PT := T#6S);                         // star period
KY    := Run AND NOT TY.Q AND NOT KD;              // K2 star
K1    := Run AND (FB_Y OR K1);                     // K1 after star has really pulled in
TDead(IN := TY.Q AND NOT FB_Y, PT := T#80MS);    // wait for K2 to really drop out, then time the dead band
KD    := Run AND TDead.Q;                            // K3 delta
TChk(IN := (K1 XOR FB_1) OR (KY XOR FB_Y) OR (KD XOR FB_D), PT := T#1S);
IF TChk.Q OR (FB_Y AND FB_D) THEN Fault := TRUE; END_IF

Note: the dead time starts when the feedback from K2 confirms it has dropped out, not when it is commanded off — this protects against a welded K2 contact. A plain relay circuit cannot do this. The simulator below is a simplified version without feedback.

Example M-1

Test the sequence in the simulator and answer the questions

Press START: record the order in which Q0.1 (Y) and Q0.0 (K1) turn on — why must Q0.1 come on first? (So the star point is made before the windings are energised.)
Watch the ET bars of T1 and T2: how long is Q0.1 off while Q0.2 is not yet on? (80 ms = the PT of T2.)
Edit the Q0.1 rung to remove the !Q0.2 condition and reload. Does the system still work, and why should this interlock stay anyway?

Frequently asked questions

With a PLC, do I still need auxiliary-contact interlocks in the coil circuit?

Yes. The interlocks that prevent short circuits (K1/K2 for reversing, K2/K3 for star–delta) must work even if the PLC fails, a transistor output shorts on, or someone forces a bit. An NC contact of the opposing contactor in series with each coil, plus a mechanical interlock, is still required. The program is an additional layer, not the only one.

Can I wire the overload relay only to the PLC?

Technically yes, but good practice is to put 95-96 in series with the contactor coil directly (so it trips even if the PLC hangs) and bring 97-98 or an auxiliary contact into the PLC for status display and event logging.

Ajarn Mod · Teerawut Savangboon
Lecturer in Electrical Engineering, Dhonburi Rajabhat University, Samut Prakan · Profile