LOGO! Three-Phase kWh Meter 400A Current Transformer Setup

Overview

A Siemens LOGO! logic module (0BA7 series or 0BA8 generation) can be configured as a three-phase kilowatt-hour meter when paired with split-core current transformers (CTs) that provide a 0-10 V or 0/4-20 mA analog output, a 0-250 V voltage transducer, and an AM2 analog input module. This article documents the signal chain, the scaling math, and the program structure required to derive instantaneous three-phase power (kW) and to integrate it over time into cumulative energy (kWh) for display on the LOGO! TD or the integrated Web server.

The target application is industrial energy sub-metering on a 230 V phase-to-neutral / 400 V phase-to-phase low-voltage system with line currents up to 400 A. With full-scale inputs, the achievable display resolution is 25 Wh per LSB of the integrated kWh counter, which is acceptable for machine-level and feeder-level energy monitoring but not for billing-grade metering.

Prerequisites

Hardware required for the build:

• LOGO! 0BA7 or 0BA8 base module. Recommended part numbers: 6ED1052-1MD08-0BA1 (LOGO! 8.3 with display, 230 V AC supply) or 6ED1052-1HB08-0BA1 (LOGO! 8.3 with Ethernet). For a LOGO! 7 (0BA7) retrofit use 6ED1052-2MD00-0BA7 or equivalent. Power supply: 12/24 V DC variant (6ED1052-1CC08-0BA1) if 24 V control power is already available on the panel.
• LOGO! AM2 analog input module, 6ED1055-1MA00-0BA2. The AM2 provides two universal analog inputs that accept 0-10 V or 0/4-20 mA signals. The base LOGO! 0BA8 has four onboard 0-10 V inputs (AI1, AI2, AI7, AI8) at 10-bit resolution (0-1000 counts), but to keep wiring short and isolated from the LV measurement side, the AM2 is the recommended path. Three phases of current plus one phase-to-neutral voltage require four analog channels (3 x CT + 1 x V), so the AM2 plus the two onboard 0-10 V inputs are the minimum allocation.
• Three current transformers, 0-400 A primary, with selectable 0-10 V or 0-20 mA secondary output. Carlo Gavazzi CTD-3V (split-core, 0-400 A, 0-10 V output) and MCR-V (solid-core, 0-400 A, 0-20 mA with burden resistor) are typical selections. The CT must be specified for the actual conductor size; for 400 A applications, verify the CT inner diameter accommodates the phase busbar or cable lug.
• One voltage transducer with 0-250 V AC input and 0-10 V DC output, with reinforced isolation between the LV measurement side and the LOGO! low-voltage analog side. Carlo Gavazzi VSP-1 (DIN-rail, 0-300 V AC input, 0-10 V output) is one option; alternatively, a Sentron 3VA-IC or Phoenix Contact MACX MCR-UI is acceptable.
• LOGO! Soft Comfort V8.x or later for offline program development and online monitoring.

For data export and remote reporting, the LOGO! 8 (0BA8) family supports the LOGO! CMR2020 or CMR2040 communication module (4G/GPS) and the integrated Ethernet port. Both enable remote polling of the kWh counter via the built-in Web server (Variable Table) or via Modbus TCP from a higher-level SCADA. Reference: Energy monitoring with LOGO! 8 and LOGO! CMR (Siemens, 109062859).

System Topology and Signal Chain

The measurement path is identical for all three phases; only the wiring terminal and AI block address differ. The per-phase chain is:

1. Phase conductor routed through the CT window (L1, L2, L3).
2. CT output (0-10 V or 0-20 mA) wired to the AM2 or LOGO! onboard AI terminal.
3. LOGO! AI block converts 0-10 V to a raw integer 0-1000 (10-bit ADC).
4. LOGO! Amplifier block scales the raw integer into engineering units (amperes) using Gain and Offset.
5. LOGO! Math block multiplies current by phase voltage (A x V) and divides by 100 to yield instantaneous kW with two decimal places.
6. LOGO! Math block sums the three kW values to yield total three-phase kW.
7. LOGO! Threshold trigger fires at a fixed kW tick (e.g. 0.025 kW = 25 W) and increments an Up/Down counter to accumulate kWh.

Current Measurement Scaling

The CT is the most common cause of measurement error in a LOGO! kWh build. The two critical specifications are the primary-to-secondary ratio and the linearity class. A Carlo Gavazzi CTD-3V 400 A type has a stated linearity of +/-1% of full scale at 50 Hz, which dominates the overall error budget. The CT secondary is a current output (typically 0-20 mA or 4-20 mA) terminated in a burden resistor to convert to a voltage that the LOGO! AI can read. The burden value is selected so that 20 mA x R_burden = 10 V, which means R_burden = 500 ohm. A 0.1% precision, 500 ohm resistor is the minimum spec; a 0.5 W rating is sufficient because the burden dissipates only 200 mW at full scale.

If the CT is a 0-10 V output type (e.g. CTD-3V with built-in burden and voltage output), the burden resistor is internal and the 0-10 V output connects directly to the AM2 input. The polarity marking on the CT must be observed; reverse polarity inverts the current reading and produces negative kW values.

LOGO! amplifier block configuration for the current channel:

• Sensor type: 0-10 V (4-20 mA if using a current-output CT, scaled with internal burden)
• Gain: set so that full-scale input (10 V) yields a 0-40 representation. The 0-40 range is a deliberate trick to keep the integer math within the 16-bit signed limit of +/-32767. The 0-40 value is then multiplied by 10 in the display math to recover the real 0-400 A reading.
• Offset: 0 V is the nominal zero, so offset = 0.
• Decimal places on the display: 1 (so 400.0 A is shown as 400.0).

The Gain=4 amplifier settings are derived as follows. The onboard AI block reports 0-1000 for 0-10 V input. The desired full-scale intermediate value is 40 (representing 40 tens of amps, or 400 A). Therefore Gain = 40 / 1000 = 0.04, but LOGO! amplifier blocks accept integer gains in the range -1000 to +1000 multiplied by a 0.01 scale factor, so the equivalent gain of 4 corresponds to multiplying the input count by 4/1000 x 1000 = 4. Concretely, set Gain = 4 and the displayed AI value ranges 0-40; the display expression multiplies this by 10 to yield 0-400 A.

Voltage Measurement Scaling

The voltage transducer must be line-powered (typically 24 V DC) and isolated. The output is 0-10 V DC, scaled to the AC input range. For a 0-250 V AC input transducer, full-scale output is 10 V. The amplifier settings are:

• Offset: 0
• Decimal places: 2 (so 230 V is shown as 230.00)

For a true RMS-responding voltage transducer, the LOGO! program does not need to perform RMS calculation; the transducer outputs a DC voltage proportional to the true RMS of the input. The transducer's stated accuracy class (e.g. class 0.5 or class 1) determines the voltage channel error contribution. For the three-wattmeter method (three CTs, three voltage transducers), each phase voltage is read independently; for the single-voltage method, the assumption is a balanced 3-phase voltage, which is reasonable for utility-supplied industrial feeders but degrades on heavily loaded islanded or generator-fed systems.

Three-Phase Power Calculation

The instantaneous three-phase real power is computed as the sum of per-phase contributions. With one shared voltage reading and three CT readings (single-voltage method):

P_total = V_phase_to_neutral x (I_L1 + I_L2 + I_L3) / 1000 [kW, assuming unity PF]

For accurate per-phase real power at non-unity power factor, the cosine of the phase angle between voltage and current must be known. The LOGO! platform does not perform phase-angle math natively; the program approximates P_total by the sum of per-phase apparent powers divided by an assumed power factor. A common field assumption is PF = 0.85 for industrial mixed loads; substitute this as a fixed constant in the program and accept the +/-10% error this introduces.

LOGO! Math block configuration per phase:

// Math block A1: per-phase kW
Inputs: AI_current (0-40 representing 0-400 A), AI_voltage (0-25000 representing 0-250.00 V)
Operator: Ax
Constant: 1
Decimal places: 2
Result: I x V, scaled such that 400 A x 250 V = 100,000 raw, divided by 1000 = 100.00 kW

For the LOGO! implementation, set the Math block to:

• Operator 1: Multiplication
• Value 1: I_raw (0-40)
• Operator 2: Multiplication
• Value 2: V_raw (0-25000, scaled as 250.00 with 2 decimal places)
• Final divide: / 1000 (or built into the constant)

Sum the three per-phase kW values with an additional Math block (addition) to yield P_total in kW. The display shows P_total with two decimal places, e.g. 123.45 kW. The P_total value drives both the display and the kWh integrator.

kWh Integration Logic

LOGO! does not have a native time-integral function for kWh accumulation, so the integration is implemented as a fixed-tick pulse counter. The principle is to fire a counter increment every time a fixed amount of energy has passed through the load. For 25 Wh resolution:

At full load (e.g. 100 kW = 100,000 W), 25 Wh = 25 / 100,000 hours = 0.00025 h = 0.9 s. The increment timer must be faster than this worst case, so the timer period is set to 0.1 s (100 ms). For lower power, the kWh increment rate slows down proportionally; at 1 kW the increment fires every 90 s, at 100 W every 15 minutes.

Implementation: use the LOGO! Asynchronous Pulse Generator block or the Pulse Relay with a high-frequency base. The math is:

// kWh tick generator
Inputs:
  P_total_kW x 100  (e.g. 123.45 kW becomes 12345 internal units, scaled with 2 decimal places)
Outputs:
  Count = P_total_kW / TickEnergy_kW  (pulses per second)
  Pulse period (s) = TickEnergy_kW / P_total_kW x 3600

Because LOGO! has no direct floating-point divide-by-time block, the practical implementation uses a fixed-frequency pulse (e.g. 1 pulse per second from the on-board real-time clock) gated by a Threshold Switch. The threshold is set to fire when P_total x pulse_count / 3600 >= 0.025 kWh. Concretely, the Counter block is incremented once for every 3600 / (P_total / 0.025) = 90 / P_total[kW] seconds. At P_total = 100 kW, increment every 0.9 s; at P_total = 1 kW, increment every 90 s.

For the LOGO! 8 (0BA8) family, the Math block with operator x and constant, combined with the Counter and Threshold blocks, implements this without requiring a custom FB. The Up/Down counter block (C_U/D in LOGO! notation) maintains the kWh total; the count is preserved across power cycles if the LOGO! is set to retain on power failure. To set retention, in LOGO! Soft Comfort open the Counter block properties, tick the "Retentivity" box, and assign it to one of the 16 (0BA7) or 64 (0BA8) retentive flag words.

The 32767 Boundary Limitation

For the kWh total, the same bound applies. 32767 kWh is approximately 32.7 MWh, which fills up quickly on a 100 kW load that runs 24/7. A 100 kW continuous load accumulates 100 x 24 = 2400 kWh per day, hitting the 32767 kWh limit in approximately 13.6 days. For longer-term aggregation, the program must either:

1. Reset the counter at a fixed interval (e.g. weekly) and forward the total to a higher-level recorder via Modbus TCP.
2. Use a MWh counter and a kWh counter in parallel, both incrementing from the same tick.
3. Use the LOGO! 8 retentive memory and accept periodic manual reset.

LOGO! Program Structure

The complete program in LOGO! Soft Comfort consists of the following blocks. Block numbers in parentheses correspond to a typical 0BA8 program, with AI1, AI2, AI3, AI4 as the four input sources.

1. B01 - Amplifier (Current L1): Input AI1 (0-1000 from AM2/0-10 V), Gain = 4, Offset = 0, Sensor 0-10 V. Output: 0-40 internal.
2. B02 - Amplifier (Current L2): Input AI2, Gain = 4, Offset = 0. Output: 0-40.
3. B03 - Amplifier (Current L3): Input AI3, Gain = 4, Offset = 0. Output: 0-40.
4. B04 - Amplifier (Voltage): Input AI4 (or AI7/AI8 if AM2 channel 2 is used), Gain = 25, Offset = 0. Output: 0-25000 (representing 0.00-250.00 V).
5. B05 - Math (P_L1): A x B / 1000. Inputs: B01 output x B04 output. Output: 0-10000 (0-10.00 kW at 40 x 25000 / 1000).
6. B06 - Math (P_L2): A x B / 1000. Inputs: B02 x B04.
7. B07 - Math (P_L3): A x B / 1000. Inputs: B03 x B04.
8. B08 - Math (P_total): A + B + C. Sum of B05, B06, B07. Output: P_total in 0.01 kW units (e.g. 12345 = 123.45 kW).
9. B09 - Display (Current L1): Show B01 x 10, format 4 digits + 1 decimal, suffix "A".
10. B10 - Display (Current L2): B02 x 10.
11. B11 - Display (Current L3): B03 x 10.
12. B12 - Display (Voltage): B04 / 100, format 5 digits + 2 decimals, suffix "V".
13. B13 - Display (P_total): B08 / 100, format 5 digits + 2 decimals, suffix "kW".
14. B14 - Pulse Generator: Period = 1 s, On/Off ratio variable. Drives the kWh tick logic.
15. B15 - Counter (kWh): Counts kWh ticks, retentive. Output to display B16.
16. B16 - Display (kWh total): Show B15 count, format 5 digits, suffix "kWh".

The kWh tick logic (B14-B15) can be implemented as a Wiping Relay (off-delay) feeding a Counter. The off-delay time is computed dynamically by dividing 0.9 s by P_total (in kW); for low power the delay extends and counter ticks are sparse, for high power the delay shortens and ticks are frequent. An alternative that avoids dynamic timing is the PI controller block, but LOGO! does not integrate over time, so the Wiping Relay / pulse counter method is the standard pattern.

Display Configuration

The LOGO! TD (Text Display) supports up to four lines per page and 16 message pages. Configure the message pages as follows for the kWh build:

Use the ESC + OK key combination on the LOGO! TD to scroll between message pages. For LOGO! 8 with built-in display (e.g. 6ED1052-1MD08-0BA1), the same message pages are shown on the integrated 6-line LCD.

For remote viewing, enable the Web server in the LOGO! Soft Comfort project (Project > Properties > Web Access). The kWh variable is accessible as VW0 or whichever address is assigned to the counter block. The Web URL is http://<LOGO_IP>/<variable_name>.shtm. For Modbus TCP export, the holding register holding the kWh total can be polled at register address 300 + block_number, depending on the LOGO! version.

Resolution and Accuracy

The achievable resolution and accuracy are dominated by the LOGO! 10-bit ADC and the CT linearity. The error budget for the build is:

The 25 Wh resolution is the LSB of the kWh counter. If the real load operates at less than the full 250 V / 400 A range, the resolution improves proportionally. For a typical industrial feeder that operates at 70% of full scale (175 A average, 230 V L-N), the resolution becomes 25 x (400/175) x (250/230) = 62 Wh per LSB, which is still acceptable for sub-metering.

Verification and Commissioning

Commission the kWh meter in five steps. The verification is the same regardless of whether the LOGO! 0BA7 or 0BA8 generation is used.

1. Sensor wiring check: Disconnect the load. All three CT outputs should read 0.0 A on the LOGO! TD. If any CT reads negative, reverse the CT orientation on the phase conductor. If a CT reads a small residual (0.1-0.5 A) with no load, this is normal CT zero-drift; add a small offset in the Amplifier block to null it out.
2. Voltage reference check: Apply a known reference voltage (e.g. 230 V L-N from a calibrated variac). The displayed V should match within 1%. If not, adjust the Gain in B04. Document the corrected Gain in the program header comment.
3. Current reference check: Apply a single-phase known load on L1 (e.g. a 10 kW resistive heater drawing 43.5 A at 230 V). The I_L1 display should read 43.5 A +/- 1 A. Adjust Gain in B01 if needed. Repeat for L2 and L3.
4. Power reference check: With the same 10 kW single-phase load on L1, P_total should read approximately 10.00 kW. The error should be within +/- 3% (the CT and PF assumption contributions).
5. kWh reference check: Run the load for a timed period (e.g. 1 hour at 10 kW = 10 kWh). The displayed kWh should advance by 10 +/- 0.1 kWh. If it advances faster, the tick threshold is set too low; if slower, the threshold is too high.

Troubleshooting Matrix

Safety Considerations

The CTs and voltage transducer are the only components connected to the LV power system. The following safety rules apply:

• CT secondary must never be open-circuited while the primary is energized. An open CT secondary generates a high voltage (potentially several kV) that is a lethal hazard and will damage the CT. Always short the CT secondary terminals before disconnecting the wiring.
• The voltage transducer must be installed behind a short-circuit protective device (fuse or MCB) sized per the transducer installation manual.
• The LOGO! and AM2 module operate at SELV/PELV. The reinforced isolation in the voltage transducer (2500 V AC test voltage typical) is the safety boundary; do not bypass it with a direct connection.
• All wiring to AM2 and the CTs must be segregated from power wiring per IEC 61131-2 / EN 61131-2 installation guidelines. Use shielded twisted pair for the CT secondary wiring and ground the shield at the panel end only.
• The LOGO! BM should be de-energized before inserting or removing the AM2 module; hot-swapping is not supported.

Advanced: Modbus TCP Export

LOGO! 8 (0BA8) base modules with Ethernet expose a Modbus TCP server on port 502. The kWh total, per-phase currents, voltage, and total power can all be polled by a SCADA, energy management system, or building automation controller. Holding register mapping is project-specific; in LOGO! Soft Comfort, open Project > Properties > Modbus, tick "Enable Modbus TCP", and assign VM (Variable Memory) addresses to each block output. Typical VM address assignments for the kWh build:

Reference the LOGO! 8 system manual (Siemens order number 6ED1050-1AA08-0BA1) for the full Modbus register map and the list of supported function codes. Function code 3 (read holding registers) and function code 6 (write single register) are the only universally supported codes on LOGO! 8.

Alternatives to a LOGO!-Based kWh Meter

For higher resolution, larger current, or true three-wattmeter accuracy, consider the following alternatives. They are mentioned here to set realistic expectations for the LOGO! build, not as a recommendation against using LOGO!.

• Siemens SENTRON PAC4200 with integrated Web server and Modbus TCP; supports up to 690 V and 0-20000 A via external CTs, 0.2S accuracy class, and is MID-certified for billing.
• Siemens 7KT PAC1600 three-phase energy meter with S0 pulse output; 0.5S accuracy class, suitable for sub-metering with the S0 pulse aggregated by a LOGO! digital input.
• Siemens LOGO! 8 with the LOGO! AM2 RTD module and 4G CMR for remote energy reporting on a feeder where the load is non-unity PF. The CMR publishes a JSON record of the kWh total over HTTPS at intervals defined in the LOGO! program.

The LOGO!-based build is best suited to machine-level energy monitoring, where the 1-2% accuracy and 25 Wh resolution are sufficient and the existing LOGO! 0BA7/0BA8 can be reused without adding a dedicated energy meter.

FAQ