Is the Siemens G120 a VFD? Architecture, Power Modules & Fault Codes

Walk up to a G120 cabinet for the first time and it does not look like the single-block VFDs you might be used to. Two pieces bolted together, a keypad that pops off — it raises a reasonable question: is this thing actually a variable frequency drive?

What Is the SINAMICS G120?

The SINAMICS G120 is Siemens' workhorse general-purpose VFD platform. It is used in pumps, fans, conveyors, compressors, and any application where you need precise motor speed control on a three-phase AC induction or permanent magnet motor. You will find it in food and beverage, water treatment, HVAC, and general manufacturing across North America and globally.

What distinguishes it from a conventional single-block drive is the modular architecture. The drive is always two parts:

• Control Unit (CU): The brain. Handles parameter storage, fieldbus communication (PROFIBUS, PROFINET, CANopen, depending on variant), I/O, and the control algorithms. Common variants include the CU230P-2, CU240B-2, CU240E-2, and others — each supports different fieldbus protocols and feature sets.
• Power Module (PM): The muscle. Converts incoming AC supply to DC, then inverts it back to variable-frequency AC for the motor. This is the piece with the heat sink, DC bus capacitors, and IGBT output stage.

Together they function as a single VFD. Separately, neither one runs a motor.

What Does the G120 Power Module Actually Do?

The Power Module is the electrical converter stage of the G120. Here is what happens inside it when the drive is running:

1. Rectification: Incoming three-phase AC line voltage is rectified to a DC bus. The PM handles the inrush and maintains the DC bus voltage.
2. DC Bus filtering: Capacitors smooth the rectified DC and provide ride-through during momentary dips.
3. IGBT inversion: Six IGBTs (insulated-gate bipolar transistors) switch the DC bus at high frequency (carrier frequency) using pulse-width modulation (PWM) to synthesize the variable-frequency, variable-voltage output your motor sees.
4. Protection: The PM monitors its own temperature, DC bus voltage, and output current. It feeds fault data up to the Control Unit.

Power Modules come in different frame sizes to cover a wide range of motor ratings — from fractional to multi-hundred kilowatt applications. Each frame size is matched to a motor current rating. Always verify the exact kW/HP and ampere rating for your specific PM part number against the Siemens catalog or the rating plate on the unit itself; frame letter alone does not guarantee a rating match across all voltage classes.

G120 Modular Architecture vs. a Conventional VFD

Why does the modular split matter on the plant floor? Two practical reasons:

• Faster spare parts strategy: If an IGBT fails, you swap only the Power Module. All your tuned parameters stay on the Control Unit (or on the BOP/IOP operator panel, which stores a parameter set). No re-commissioning from scratch.
• Fieldbus flexibility: You can upgrade communication protocol by swapping the Control Unit to a different CU variant, reusing the same Power Module in the cabinet.

Common G120 Fault Codes — How to Read Them

The G120 signals problems with two types of messages:

• Faults (F codes): Drive has tripped and stopped. Must be acknowledged before the drive can restart.
• Alarms (A codes): Warning condition. Drive continues running but something needs attention.

The BOP-2 or IOP operator panel displays the code directly. On a fieldbus system, the code appears in the status word readable from the PLC.

The most common fault codes technicians encounter in the field:

Troubleshooting Workflow: G120 Won't Run

1. Read the fault code first. Write it down. Press the information button on BOP-2 for the built-in help text if available.
2. Check the diagnostic buffer. Parameter r0945 logs the last several faults in sequence — useful when a nuisance trip clears itself before you arrive.
3. Verify supply voltage at the PM input terminals. Confirm all three phases are present and balanced.
4. Check motor wiring. Confirm U, V, W output connections are tight and there is no phase-to-ground fault on the motor cable (use an insulation tester with power OFF and LOTO applied).
5. Inspect cooling. Clear any debris from the heat sink fins and verify the internal fan spins on power-up.
6. Verify motor nameplate data matches drive parameters. Rated voltage, current, frequency, and speed parameters must match. A mismatch causes nuisance overcurrent and overtemperature faults.
7. Check CU–PM communication. The Control Unit and Power Module communicate over a backplane connector. If that connector is dirty or the CU is not fully seated, you may see erratic fault behavior.

When to Replace Rather Than Repair

Some faults point to hardware that cannot be field-repaired. Replace the Power Module when:

• F0052 (power stack fault) returns immediately after reset with no load on the output.
• Any output phase reads significantly different resistance to DC bus than the other two (measured with a diode checker — LOTO applied, capacitors fully discharged).
• The drive runs but output current is severely unbalanced with a known-good motor.
• Visible burn marks or failed capacitors are present inside the PM.

Replace the Control Unit when:

• F0051 returns after a factory reset and re-download of parameters.
• Fieldbus communication faults persist after confirming the network and option board are good.
• The display is blank or garbled and the BOP-2 shows normal on another drive.

The modular architecture means you replace only the failed half — not the entire drive. That is a significant cost advantage on larger frame sizes.

Where to Find Official G120 Documentation

Fault code definitions, parameter lists, and wiring diagrams live in the official Siemens documentation. Always pull the manual that matches your exact CU and PM part numbers — details differ across CU variants and firmware versions.

Start at the Siemens Industry Online Support portal and search by your exact CU or PM part number. The G120 has separate operating instructions for each Control Unit variant, a separate PM hardware installation manual, and a List Manual for all parameters and fault codes. The List Manual is the authoritative source for F and A codes.

Disclaimer: This guide is provided for general information only. Procedures vary by model, series, and firmware version. Industrial equipment can cause serious injury, equipment damage, or downtime if serviced incorrectly. Always consult the manufacturer's official documentation and a qualified professional before performing work, and follow your facility's lockout/tagout and safety procedures. PLC Department is an independent surplus supplier and is not affiliated with or endorsed by the manufacturers referenced. Use this information at your own discretion.