Siemens LOGO! Automatic Generator Start: Relay Output Wiring

Overview

Auto-starting a backup generator from a Siemens LOGO! is one of the most common entry-level applications for the platform, but it is also one of the most common places where first-time users stall. The recurring failure point is conceptual: the user has a logical condition (battery low, mains failed, pressure low) and intuitively wants to "close two contacts" to call the generator, exactly the way a manual start panel is wired. The LOGO! does not have separate "open contacts"; it has digital inputs, internal flags, and physical outputs, and only the physical outputs can drive an external load.

This article consolidates the engineering answer into a complete reference: which LOGO! hardware to select, which terminal is the relay output, why marker (M) flags cannot be wired to a generator, how to scale a battery-voltage threshold into a reliable auto-start decision, how to add hysteresis and run-on cooling time, how to wire the start input of a typical genset controller, and how to verify and commission the finished logic. The objective is a deterministic, fail-safe interface between a 24 VDC logic module and an industrial/large-engine generator controller.

Prerequisites

• Siemens LOGO! 8 base module (6ED1052-1xxx08-0BAx) or LOGO! 8.3 base module (6ED1052-1xx08-0BA1), with at least one relay output.
• LOGO! Soft Comfort V8.3 or newer programming environment (6ED1058-0BA08-0YA1, free download with a 14-day trial licence, perpetual licence sold separately).
• DC supply matching the LOGO! variant (12/24 VDC or 24 VDC), or 115/230 VAC variant for the mains-fed plant.
• Battery voltage to be supervised: 12 VDC, 24 VDC, or 48 VDC nominal system.
• Voltage divider or signal conditioner (e.g. 0-10 V transducer) sized for the LOGO! analog input range.
• Genset controller with a remote-start input rated for a dry contact closure (most do; verify the controller manual).
• Appropriate DIN-rail space, ferrule crimper, and Cat5/6 patch lead for LOGO! 8 Ethernet programming.

LOGO! Hardware Selection and Output Identification

The LOGO! 8 platform uses the 6ED1052 family. Outputs are not identical across variants. The base module ordering suffix determines whether outputs are relay or transistor, and how many of each you get.

For generator control, use a relay-output variant unless you are driving a low-voltage DC interface. The 10 A relay contacts (Q1-Q4) on 6ED1052-1MD08-0BA1 are rated:

• 10 A resistive at 250 VAC (AC-1)
• 3 A inductive at 250 VAC (AC-3, cosφ = 0.4)
• 10 A resistive at 30 VDC
• Mechanical endurance 10 million operations, electrical 100,000 cycles at full load.

Source: LOGO! 8 System Manual, entry ID 109741041. Always check the manual revision shipped with your hardware; the 6ED1052-1xx08-0BA1 generation uses manual A5E33039758-AC.

Why M Flags Cannot Drive a Generator

The recurring confusion in beginner posts is the attempt to "wire" a flag (M8 in the source thread) directly to the external device. Flags are internal memory bits inside the LOGO! scan cycle. They are written to by the user program and read by the user program; they do not appear on any terminal block. M8 has no physical existence outside the program.

Special flags (M8, M25, M26, M27, M28) carry pre-defined semantics and writing to them in some firmware versions is rejected. In LOGO! 8 firmware releases from FS04 onward, the reserved flags are:

Use a normal flag (M1-M7, M9-M24) for general purpose, or drive a digital output Q1-Q4 directly. The M-flag block in the FBD program is therefore a programming error if you intend to energise external hardware.

Relay Output Wiring to the Generator

The only correct way to close a contact in front of the genset is to energise a LOGO! relay output (typically Q1) and wire one pole of that dry contact to the genset remote-start input. The genset controller does not see M8; it sees two screw terminals on the LOGO! marked Q1 (common) and Q1 (NO).

Two wires. One from a Q1-COM terminal (or Q1 if labelled NO/COM as a single pair on the LOGO!) to one side of the genset remote-start input. The other from Q1-NO to the other side of that input. The two pins of the genset terminal are not powered by the LOGO!; they are an isolated contact pair inside the genset controller, often pulled to a small internal DC voltage (typically 12/24 VDC) through a sensing resistor.

Battery Voltage Sensing for the Auto-Start Condition

The most common reason to start a generator from a LOGO! is "the battery of the monitored system is low". The cleanest implementation is a LOGO! analog input (AI1-AI4 on 12/24 VDC variants) reading a scaled voltage. The procedure below assumes a 24 VDC system with a 0-10 V analog input range on the LOGO!.

1. Choose AI1. On the 6ED1052-1MD08-0BA1 the analog channels share terminals with digital inputs I7 and I8; if I7 or I8 are needed as digital, AI3/AI4 are shared with I1/I2 instead. Verify the relevant terminal layout in the LOGO! 8 System Manual for your exact base module.
2. Configure AI1 in LOGO! Soft Comfort as 0-10 V mode by adding an Analog Input block and selecting "0..10V" under the sensor type.
3. Build a resistor divider so the maximum expected battery voltage (e.g. 28.0 V for a 24 V lead-acid on float plus 10% tolerance) maps to 10.0 V at the AI terminal. A practical divider is R1 = 18 kΩ 0.1% (high side) and R2 = 10 kΩ 0.1% (low side) referenced to LOGO! 0 V analog common. Output V_out = V_bat · R2 / (R1 + R2).
4. Use a buffer op-amp (e.g. LM358, 1 MΩ input impedance) between divider and AI to avoid divider loading from the LOGO! input impedance. The 12/24 VDC LOGO! AI1 input impedance is > 100 kΩ nominal; a buffer is good practice but optional above 10 kΩ total divider impedance.
5. Apply software scaling: AI1 raw value in the 0-1000 range maps 0.0 V to 10.0 V. A second Arithmetic block multiplies by the inverse-divider factor to recover the real battery voltage in volts. Example: ax = (AI1 * 28) / 1000 produces 0-28 V scaled to 0-1000 internally.

Threshold and Hysteresis Logic

A single threshold (start at 23.5 V, stop at 27.0 V) without hysteresis causes chattering. The standard auto-start pattern uses two comparators and a small amount of logic:

Use the LOGO! Analog Threshold Trigger block with separate on/off thresholds; the block handles hysteresis in a single instruction. Wire the "threshold exceeded" output to a Set/Reset flip-flop (RS block) whose output is the "Generator Run" flag. RS prevents chatter from producing rapid start/stop cycles.

LOGO! Soft Comfort Program Skeleton

The block diagram below is the minimum viable auto-start program. The complete FBD uses four blocks plus an output.

1. B001 - Analog Input: AI1, 0-10 V sensor type. Output 0-1000 raw.
2. B002 - Arithmetic: ax = (B001 * 28) / 1000. Output 0-1000 representing 0-28 V.
3. B003 - Analog Threshold Trigger: On ≤ 700 (23.5 V), Off ≥ 760 (25.5 V). Output "Q_Start".
4. B004 - Off-Delay Timer: Input "Q_Start", time 30 s. Output "Q_Run" drives Q1 and persists for 30 s after the input drops to charge any downstream contactor coils.
5. Q1: Wired to B004 output. Energises the genset remote-start contact.

Save and download with LOGO! Soft Comfort using Ethernet (for LOGO! 8.3) or micro-SD card (for any LOGO! 8). Refer to LOGO! 8.3 Product Information for the download procedure and licence activation steps.

Run-On Cooling and Stop Sequencing

Stopping a diesel or gas engine by simply opening the start contact is a failure pattern. Most engine controllers require a start signal held throughout pre-heat, cranking, and running. Releasing it too early can fault the controller into "fail to stop" or leave the starter engaged. The correct sequence is:

1. Start request: Q1 closes; genset controller sees the closure and initiates pre-heat, then crank.
2. Crank cutoff is handled by the genset controller, not by the LOGO!. The genset opens its own starter once oil pressure rises.
3. Running: the LOGO! holds Q1 closed for the entire run. Releasing Q1 to the genset is interpreted as a stop request by most controllers (ComAp, DSE, PowerWizard, Woodward, etc.).
4. Cool-down: an off-delay timer in the LOGO! releases Q1 only 30-120 s after the battery threshold resets, allowing the genset to cool at idle before being told to stop.

Verify the specific controller: some need a pulsed stop (open-close-open) or a separate stop input. Refer to the manufacturer's ComAp or Deep Sea Electronics controller manual for the timing your genset requires.

Manual Override and Operator Interface

An automatic start logic without a manual override is a maintenance problem. Add three hardware inputs to a LOGO! with display:

Use the LOGO!'s onboard display or the LOGO! TDE text display (6ED1055-4MH08-0BA0) to show "Battery 24.1 V", "Genset RUN", and any active alarms. A text message block tied to the same analog value gives operators a one-line status read-out.

Verification and Commissioning

1. Download the program to the LOGO! and stop the program with the ESC > Stop menu before rewiring.
2. With the genset in manual mode at the genset controller, open the LOGO! wiring cover and verify that Q1 contacts are open at rest (use a multimeter on continuity).
3. Force Q1 from LOGO! Soft Comfort in online mode (Tools > Online > Force Output). The genset remote-start LED on the controller should illuminate. If not, the wiring polarity is reversed; swap the two wires at the genset terminal.
4. Unforce Q1 and let the controller return to standby.
5. Simulate a low-battery condition by reducing the divider input voltage (use a bench supply, 0-30 V) to 22.0 V. Confirm Q1 closes within one scan cycle (~70 ms typical for LOGO! 8 at default settings). The threshold value for "low battery" should be entered with the software engineering tool while the engine is in a known state.
6. Raise the bench supply to 27.0 V. Confirm Q1 opens after the configured cool-down time.
7. Cycle the bench supply three times to verify no chatter at the threshold, no latched flags, and that the off-delay timing is correct.
8. Switch the genset to auto mode and repeat steps 5-6 with the genset actually starting. Listen for two pre-heat cycles, one crank attempt, and a clean run-up to operating speed.
9. Trip a manual stop from the genset controller, confirm the LOGO! releases Q1 and the genset shuts down on its own stop sequence.

Troubleshooting Matrix

Safety and Code Considerations

• Generator start circuits are part of emergency power systems. In IEC jurisdictions, follow IEC 60364-5-55 for the wiring of low-voltage electrical installations, and the local fire-protection or building-code equivalent.
• Where a generator backs up life-safety loads (fire pumps, smoke-control, hospital circuits), the supervisory PLC must not be the only start command. Provide a redundant hard-wired start contact from a voltage-monitoring relay (e.g. ABB CM-UFD, Eaton EMR5-PFD, Finder 71 series) wired in parallel with the LOGO! output.
• Isolate the genset remote-start terminals from the LOGO! supply. If the genset controller uses 24 VDC on its remote-start input, use an interposing 24 VDC relay coil driven by the LOGO! Q1 contact; do not pass the genset's sensing voltage through the LOGO! contact.
• Apply the LOGO! output derating for inductive loads: for AC-3 (motor, solenoid, contactor) loads, the 10 A relay is rated to 3 A only. Pilot relays, contactors, and motor-operated valves count as inductive.
• Use a listed surge protective device on the supply to the LOGO! if the cabinet is fed from the same source as the genset battery charger; genset cranking generates significant voltage transients.

Alternative Controllers

The same pattern (single dry contact closure, battery threshold, hysteresis, cool-down) is portable to other small PLCs. On a Wago 750-8202 PFC200, drive a 750-523 1-channel relay output. On an Allen-Bradley Micro820, use output O:0.0.0. On an IDEC FT1A, use output Y00. None of them change the wiring: one dry contact closure, two wires, isolated from the LOGO! supply by the genset controller's own internal sensing.

FAQ