Siemens LOGO! Two-Stage Cooling with Duty/Standby Rotation

System Overview

This reference documents the implementation of a two-stage cooling controller for a single conditioned room served by two independent package air-conditioner (AC) units arranged in a duty/standby configuration. The system uses a Siemens LOGO! 230RCE logic module to run the staging logic, weekly rotation, trip changeover, high-temperature override, and manual override. Two NTC 100k thermistors provide return-air temperature feedback, one per AC unit, with the higher reading used as the control variable to prevent short-cycling caused by a single failed probe.

The design targets the most common small-plant HVAC redundancy pattern: one operating unit and one standby unit, both identical, with automatic changeover driven by three independent triggers — a 7-day (168 h) duty cycle, a compressor or overload trip, and a sustained high-temperature event at 30 °C. The two compressor stages per unit allow partial-load operation and a recovery boost when load exceeds the first stage's capacity.

Functional Requirements and Operating Sequence

The setpoint is 23 °C. The first compressor stage energises when return-air temperature rises to SP + 1.5 °C = 24.5 °C and de-energises when it falls to SP - 0.5 °C = 22.5 °C. The second stage energises when temperature rises to SP + 3.0 °C = 26.0 °C and de-energises when it falls back to 24.5 °C, so that stage 2 always drops before stage 1 on the recovery side. The hysteresis bands are intentionally asymmetric to avoid simultaneous contactor dropout on small temperature movements.

Two interpretations of the stage-2 threshold appear in operator requirements — one places stage 2 at 24.5 °C (single dead-band control) and the other at 26.0 °C (two-step dead-band control). The two-step variant is recommended because it provides a true partial-load capability and reduces the number of compressor starts per hour under stable thermal load. If a single-dead-band implementation is required, set stage 2 ON to 24.5 °C and stage 2 OFF to 23.5 °C.

Controller Selection: Siemens LOGO! 230RCE and AM2 Extension

The 230RCE base unit (catalog number 6ED1052-2CC08-0BG0 for LOGO! 8.3 without display, 6ED1052-2MD08-0BG0 with display) is selected for sites where the control panel is fed from a 115/230 V AC distribution. It provides eight digital inputs rated for 115/230 V AC and four relay outputs rated 10 A resistive (8 A inductive) at 230 V AC. Crucially, the 230 V AC variant has no native analog inputs; an extension module is mandatory for temperature feedback.

The AM2 module is the correct analog input for this application provided the NTC 100k thermistor is paired with a signal conditioner that outputs 0-10 V (see next section). The AM2 RTD module is a viable alternative if the field sensor is changed to a PT1000 — the LOGO! then accepts the RTD directly without external conditioning. Up to eight extension modules can be stacked to the right of the base unit; only the AM2 (or AM2 RTD) is required for this application.

For the LOGO! Soft Comfort (LSC) development environment, version 8.3 or later is required for LOGO! 8.3 base units. LSC allows online monitoring, forcing of inputs/outputs, and offline simulation — all of which are used during the commissioning procedure described later. See the LOGO! 8 system manual and the LOGO! Soft Comfort online help on the Siemens support portal for the complete instruction set.

NTC 100k Sensor Interface to LOGO! AI

The NTC 100k thermistor has a nominal resistance of 100 kΩ at 25 °C with a B-value of 3950 K (typical for the 100k6 family). The AM2 analog input module accepts only 0-10 V or 0/4-20 mA. Direct connection of a resistive sensor is not supported, so a signal conditioner is required.

Three interface options are commonly used in the field:

1. External NTC-to-voltage transducer. Single-component, calibrated, isolated, and 24 V powered. Examples include Phoenix Contact MACX MCR-SL-NTC, WAGO 857-814, and Siemens SITRANS TR200/TK. Output is typically 0-10 V scaled to a user-defined range, e.g., 0-50 °C. Recommended for new installations because it removes calibration burden and provides sensor-break detection.
2. Discrete voltage divider + op-amp buffer. A 100 kΩ reference resistor in series with the NTC, buffered by a low-offset rail-to-rail op-amp (e.g., TLV2372) and scaled to 0-10 V. The divider source must be a precision 5 V reference (e.g., REF3030). This is acceptable for prototypes but adds components, drift, and EMC vulnerability.
3. Change the sensor. Replace the NTC 100k with a PT1000 RTD and use the AM2 RTD module. The LOGO! then scales directly to °C with no external components. This is the lowest-risk option if the field wiring is accessible.

For the voltage-divider option the open-circuit transfer function at 24 °C with a 100 kΩ reference and 5 V excitation is:

V_out = 5 × R_fixed / (R_NTC + R_fixed) = 5 × 100k / (R_NTC + 100k)

At 25 °C, R_NTC = 100 kΩ and V_out = 2.50 V. At 30 °C, R_NTC ≈ 80 kΩ and V_out ≈ 2.78 V. This 280 mV swing across a 5 °C range must be amplified by a factor of ~7 to reach a 0-10 V range that uses the full AM2 input span. A typical scaling is 0 °C = 0 V and 50 °C = 10 V, giving a sensitivity of 200 mV/°C. The LOGO! analog amplifier block (B001 / B002) divides the 0-10 V value by 5 to obtain a 0.0-50.0 °C process variable.

Sensor cable must be shielded twisted pair, 0.5 mm² minimum, with the shield bonded to ground at the LOGO! panel end only. Maximum recommended run length is 30 m for an NTC; beyond that, the cable capacitance begins to affect divider dynamics. For runs up to 50 m, reduce the reference resistor to 47 kΩ to keep source impedance low.

I/O Assignment Table

All digital inputs are 230 V AC. The manual selector, trip signals, and reset pushbutton connect to the base 230RCE. The two NTC channels connect to the AM2 module in extension slot 0. Outputs Q1-Q4 drive compressor contactor coils via interposing 24 V DC relays where the coil current exceeds the LOGO! relay's inrush rating.

Two additional outputs are commonly wired to the optional DM8 230R extension for an alarm beacon and a common trip indicator. If a DM8 is not fitted, the spare LOGO! memory flag M8 can be used to drive a soft alarm through the LOGO! web server (LOGO! 8 with Ethernet).

Threshold Switch Configuration and Hysteresis

LOGO! provides dedicated analog threshold switch blocks (B006, B007, B008) and analog comparators that operate on the scaled process variable. The analog amplifier chain is:

1. AI1 raw value (0-1000 in LOGO! units, equivalent to 0-10 V) is amplified by B001 with gain 0.05 and offset 0 to obtain a 0-50 °C value.
2. AI2 is processed identically by B002.
3. B003 selects the higher of the two readings (max function), used as the controlling process variable for the high-temperature override.

The block properties dialog in LOGO! Soft Comfort allows the threshold to be entered as a scaled value (°C) directly when the input is a temperature signal. The hysteresis is implemented by the block itself, so no external OR of two thresholds is required. The output of each block is a digital flag (Hi/Lo) that is wired into the AND/OR gating that drives Q1-Q4.

Duty/Standby State Machine

The controller moves between five named states. Transitions are evaluated on every LOGO! scan (typically 5-20 ms); changeover is implemented as a non-overlapping transfer enforced by the min-stop timer.

Changeover rules in priority order:

1. Manual mode wins whenever I1=1 XOR I2=1. Both set to 0 returns to AUTO; the weekly timer and trip history are preserved.
2. Fault mode latches if BOTH Unit A and Unit B trip inputs are active simultaneously. The controller stops all Q outputs, raises a common alarm, and waits for reset.
3. High-temperature override transfers duty to the standby unit if the controlled temperature reaches 30 °C. The transfer is delayed by the min-stop timer to respect compressor restart requirements.
4. Trip changeover transfers duty to the standby unit on the rising edge of either trip input. The tripped unit is latched OFF until reset.
5. Weekly changeover transfers duty after 168 h of run time on the active unit. The counter is reset on transfer.

The current duty unit is stored in a latching (RS) relay (B013). The set input is driven by the OR of the three changeover triggers; the reset input is driven by the manual-mode return or the controller power-up initialise flag (M8 or LOGO! cold-start flag).

LSC / FBD Program Structure

The program is built in LOGO! Soft Comfort using the FBD (function block diagram) editor. A flat structure is used to keep scan time short and to make online forcing straightforward. The block list is shown below.

The text representation of the Q1 gating logic is:

Q1 = ( (ManualA AND NOT B_trip AND Stage1_demand)        // manual A path
     OR (Auto AND B013 AND NOT B_trip AND Stage1_demand)  // auto, A is duty
     OR (Auto AND NOT B013 AND B_trip AND Stage1_demand) // auto, B trip -> A runs
     OR (Auto AND NOT B013 AND HiTemp AND A_OK) )        // auto, high-temp, A is standby but healthy
     AND NOT A_trip
     AND (NOT MinStop OR MinRun_done)
     AND NOT ManualB
     AND NOT Fault

Q3 (Unit B stage 1) is the mirror image with A and B roles swapped. Q2 and Q4 add the B015 stage-2 enable delay to the corresponding Q1/Q3 path, and require that the same unit's stage 1 has been continuously ON for at least 30 s. This prevents simultaneous contactor pickup during high-inrush transitions.

Trip Handling, Manual Override, and Reset

Trip inputs I3 and I4 are wired to normally-closed contacts of the compressor overload relay and the high-pressure cut-out switch in series. Under healthy conditions the contact is closed and the input reads 1. A fault opens the contact, the input reads 0, and the rising edge (1->0 transition with NOT input) is fed to the set terminal of the corresponding RS latch (B010/B011). The latched output drives three downstream effects:

1. The corresponding Q1/Q3 gating is forced to 0, removing the run command from the tripped unit.
2. The changeover request (B014) is asserted, transferring duty to the other unit if that unit is healthy.
3. A latched memory flag (M1 for unit A, M2 for unit B) is set to drive a panel-mounted trip indicator.

Reset is implemented as a single pushbutton on I5. The input is fed to an edge detector (rising-edge block in LSC) and then to the reset terminal of both B010 and B011. After reset, the trip flags clear and the previously tripped unit is allowed to start on the next valid run request. Operators are required to inspect the compressor and confirm the cause of the trip before pressing reset; there is no auto-clear timer.

Manual mode is selected by either I1 (Manual A) or I2 (Manual B). The B012 mode decoder is:

ManualA = I1 AND NOT I2
ManualB = I2 AND NOT I1
Auto    = NOT I1 AND NOT I2

While in manual mode, all auto logic (weekly rotation, trip changeover, high-temperature override) is disabled. The selected unit runs at its staging thresholds (24.5 °C / 26.0 °C) with no changeover even if it trips. If the manually selected unit trips, the trip latch engages, the unit stops, and a fault alarm is raised; the controller does NOT auto-transfer to the other unit because the operator has assumed explicit responsibility. Returning both I1 and I2 to 0 exits manual mode and resumes auto operation on the current duty unit.

The 30 °C override behaviour: in auto, a sustained reading >= 30 °C transfers duty to the standby unit after the min-stop delay. If the standby unit is itself tripped, the duty unit continues to run with both stages ON, and a high-temp alarm is raised. The override clears when the controlled temperature falls back below 28 °C.

Commissioning and Verification Procedure

Commissioning is performed in LOGO! Soft Comfort online mode with the controller fitted to the panel but before the contactor coils are wired. The panel is energised with all AC unit isolators open and all Q outputs verified by forcing.

1. Power-on sanity. With the AM2 and DM8 (if fitted) connected, apply 230 V AC to the base and 24 V DC to the AM2. Check the LOGO! display (if fitted) or LSC online view for the program name and version. Confirm the analog inputs read a sensible value (typically 1-3 V for a 5-15 °C room).
2. Sensor calibration. Compare the LOGO! displayed temperature against a calibrated reference thermometer placed next to each NTC probe. If the reading is more than +/- 0.5 °C off, adjust the analog amplifier offset (B001/B002) accordingly. The gain is factory-trimmed on the transducer and should not need adjustment.
3. Output forcing. In LSC online mode, force Q1, Q2, Q3, Q4 individually. Verify the corresponding contactor pulls in and the auxiliary contact closes. Release each output and confirm the contactor drops out within 50 ms.
4. Threshold verification. Using a precision voltage source on AI1 (or by adjusting the transducer offset), step the input through 22.0, 24.0, 24.5, 26.0, 30.0, 28.0, 22.5 °C equivalents. Confirm Q1 turns on at 24.5, Q2 turns on at 26.0 (with the 30 s delay), Q2 turns off at 24.5, and Q1 turns off at 22.5.
5. Trip simulation. Open the I3 circuit (simulate Unit A trip). Confirm Q1 and Q2 drop out, Q3 and Q2 (of unit B) take over after min-stop, and the M1 trip flag sets. Press and release the reset pushbutton (I5) and confirm M1 clears.
6. Weekly timer. The 168 h interval is impractical to test in real time. Set B009 threshold to 0.01 h in LSC simulation mode and trigger a transfer. Return the threshold to 168 h before downloading to the live controller.
7. Manual mode. Set I1 = 1, I2 = 0. Confirm only Q1/Q2 are active and that weekly and trip changeover are inhibited. Force a trip on I3 and confirm Q1 drops out without auto-transfer to Q3. Return I1 and I2 to 0 and confirm auto mode resumes.
8. 30 °C override. Force AI1 (or AI2) to 6.0 V (equivalent to 30 °C). Confirm duty transfers to the standby unit after min-stop delay. Reduce the input to 5.5 V (equivalent to 27.5 °C) and confirm the override clears only when the input drops below 5.6 V (equivalent to 28 °C).
9. 24-hour soak test. Close the AC unit isolators and let the system run for 24 h with normal load. Log staging activations and any fault conditions through the LOGO! web server (LOGO! 8 with Ethernet) or the LSC trend view.

Troubleshooting Matrix

Frequently Asked Questions