Programming Star-Delta Motor Start with Siemens LOGO! Logic

Overview: Why Star-Delta on a LOGO! Logic Module

Three-phase induction motors draw 4× to 10× their nominal full-load current (FLC) during direct-on-line (DOL) start, depending on the motor's voltage class and design. The reduction of inrush is a recurring engineering problem in plants where:

• The utility supply is soft or the transformer is small relative to the motor.
• Mechanical couplings (belts, gearboxes, pumps) cannot absorb the DOL starting torque step.
• Cable cross-sections have already been sized to DOL service but must be reused.
• Regulations (e.g., local utility limits) cap the locked-rotor current contribution.

A star-delta (Y-Δ) starter addresses the problem by first connecting the stator windings in star, then transferring them to delta once the rotor has reached ~75–80 % of synchronous speed. The transfer is implemented with three contactors and an interlock timer, all driven by a logic relay such as the Siemens LOGO! module.

Electrical Principle and Key Ratios

The relationship between DOL, star, and delta values is fixed by transformer vector math and is the same whether the controller is a hard-wired timer block, a LOGO!, or a S7-1200:

Single-phase apparent power is kVA = V · I / 1000; three-phase apparent power is kVA = √3 · V_LL · I_line / 1000. The 1/3 reduction in starting kVA is what makes star-delta attractive for 7.5 kW and larger motors where utility contribution rules apply.

Prerequisites

• LOGO! base module with relay outputs (e.g., LOGO! 8 12/24 RCE, 230 RCE, or 230 RCo) — three discrete outputs are required to drive the three contactors (K1 main, K2 star, K3 delta). Transistor-output variants need interposing relays because of the inrush on the contactor coils.
• LOGO! Soft Comfort (V8.x or later for LOGO! 8 generation) installed on a PC; LOGO! Soft Comfort V8.4 download entry is the current engineering tool.
• Three contactors in the same utilization category (AC-3) and size. The K2 (star) contactor only carries motor current during the start phase, so it can be one frame size smaller than K1 and K3 if your local code permits; the conservative choice is identical frame sizes.
• Mechanical and electrical interlocks between K2 (star) and K3 (delta). The LOGO! program provides a software interlock, but a hard interlock (DIN-rail mounted mechanical interlock or back-of-contactor mechanical interlock) is mandatory because contactors can chatter before the LOGO! de-energises the opposite coil.
• Thermal overload relay (F1) sized at 0.58 × motor FLC, because the motor windings in delta carry only the phase current. Alternatively, place the overload in the line and use the standard FLC setting; many designers prefer this for simplicity, accepting that the protection curve is conservative.

Power Topology and Contactor Sequence

Three contactors act on the motor terminals U1, V1, W1 and the shorting links U2, V2, W2. A typical arrangement:

1. K1 (main contactor) closes to connect supply L1/L2/L3 to motor U1/V1/W1. K1 must be closed for the entire start and run period.
2. K2 (star contactor) closes simultaneously with K1 to short the U2/V2/W2 terminals together, forming the star point.
3. After the preset transition time t_Δ, K2 opens first, then K3 (delta contactor) closes to apply line voltage across the U2-V2-W2 terminals, putting the windings in delta.

LOGO! I/O Assignment

Function-Block Diagram Program (LOGO! Soft Comfort)

The logic has four required blocks and one optional diagnostic block. Build in the order shown; each block reference is the LOGO! Soft Comfort name.

Step 1 — Latching Run Bit (B001, RS flip-flop)

Set input (S) wired to I1 AND NOT I2 AND NOT I3. Reset (R) wired to NOT I2 OR NOT I3. Output B001 is the "Run" flag, latched until stop or overload.

Step 2 — Star Pulse Generator (B002, on-delay / edge-triggered)

On the rising edge of B001, generate a single fixed-length pulse of 50–100 ms to drive K2. This guarantees K2 opens before K3 closes.

Step 3 — Transition Timer (B003, on-delay, parameter T = t_Δ)

Set T = 5.000 s as a starting value for a typical pump or fan. After B001 has been true for 5 s, the timer's output goes high and enables the K3 path. Adjust empirically:

• Pumps with water column: 2–4 s
• Centrifugal fans: 4–7 s
• Compressors (unloaded): 5–10 s
• Light conveyors: 6–12 s

The transition should be just long enough for the motor to reach at least 75 % of synchronous speed. Too short and the motor stalls on the transfer; too long and K1 plus K2 carry unnecessary current and the motor may already be at full speed in star (excess heating).

Step 4 — Output Coils (B004, B005, B006 OR-gated to Q1–Q3)

Q1 (K1 main)  = B001
Q2 (K2 star)  = B001 AND NOT (B003 output)        // drop star when timer expires
Q3 (K3 delta) = B003 output AND NOT (B002 pulse)  // close delta only after star pulse ends

Step 5 — Optional Diagnostic (B007, edge detector + memory)

Count the number of starts per hour. If the count exceeds the motor manufacturer's allowed number (e.g., 3 starts/h for a 22 kW TEFC motor), latch a flag to Q4 that drives a "wait" lamp and inhibits a new start until the hour window elapses.

Ladder Text Equivalent (FBD ↔ LAD)

For users preferring ladder view in LOGO! Soft Comfort:

       I1   I2   I3        B001   I2   I3
RUNG 1 |--|/|--|/|--|(S)  R:  B001--|/|--|/|--(R)  Q1

RUNG 2  B001  T_done            Q2
        --| |------|/|---------( )--

RUNG 3  T_done  T_pulse_off     Q3
        --| |------|/|---------( )--

       where  T_done   = B003 output (T = t_Δ)
              T_pulse_off = NOT B002 pulse

Timing Diagram (ASCII)

       0s          t_Δ        t_Δ+0.1s   run
B001  __|‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾|__
Q1    __|‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾|__
Q2    __|‾‾‾‾‾‾‾|___________________________
Q3    ____________________|‾‾‾‾‾‾‾‾‾‾‾‾‾|__
       |<-star pulse 50 ms→|
                |<-- t_Δ = 5 s typical  -->|

Wiring Diagram (Inline SVG)

Commissioning Procedure

1. Disconnect the motor from K1, K2, K3. Lock out and tag out per site procedure. Verify zero energy with a CAT IV meter on every output terminal.
2. Power the LOGO! alone. From LOGO! Soft Comfort, run an online simulation; force Q1, Q2, Q3 one at a time and verify the contactor coils buzz. Replace any contactor that does not pull in within 50 ms.
3. Connect the contactor control wiring. Force Q1+Q2 simultaneously (star mode) and measure the contactor auxiliary contacts: K1 NO closed, K2 NO closed, K3 NC closed. Any deviation indicates wiring error.
4. Reconnect the motor. Perform the first start with the shaft unloaded (decoupled). Use a clamp-on ammeter on L1 and capture the inrush: expect a peak of 1/3 ± 10 % of the DOL locked-rotor current.
5. Verify the transfer: at t_Δ, the line current should rise briefly to ~1.3× the running current and settle within 200–400 ms. A sustained higher current means the motor is not up to speed; increase t_Δ in 1 s steps.
6. Re-couple the load and repeat the start. The total start time including acceleration should be ≤ 15 s for a healthy motor; if the star phase exceeds 20 s, the motor or load is too large for star-delta and a soft starter should be used instead.
7. Trip the overload F1 manually. The LOGO! should drop Q1, Q2, Q3 and the start must not be re-latchable until the overload is reset and the stop pushbutton is cycled.

Verification Checklist

Troubleshooting Matrix

Design Constraints and Field Notes

• The closed-transition variant (with a fourth resistor contactor) avoids the open-circuit transfer transient and is mandatory for motors above ~30 kW in many industrial sites. The LOGO! program gains one extra timing stage and one more output.
• For motors above 75 kW, the contactor inrush on K1 can disturb the LOGO! supply; fit a line filter or use a separate 24 VDC power supply sized to LOGO! specifications (see the LOGO! manual system manual, article 109744812).
• For hazardous areas, the stop function must be hard-wired to the contactors (Category 1 stop) — the LOGO! must not be the sole means of removing power (EN 60204-1). Wire a separate NC from the E-stop relay in series with the K1 coil.
• LOGO! 8 modules retain the program in non-volatile flash. Battery is not required for the program, only for the real-time clock. Cold start below 0 °C requires the LOGO! 8 12/24 RCE variant with extended temperature rating.

FAQ