SINAMICS G120C Braking Resistor Overheating Sizing & Selection

1. Problem Overview

A 5.5 kW SINAMICS G120C drive (MLFB 6SL3210-1KE21-7AF1, 400 V three-phase) is equipped with a Siemens catalog braking resistor MLFB 6SL3201-0BE21-0AA0 (140 Ω, 4 kW peak). During dynamic braking of the connected load, the resistor housing reaches 142 °C at the surface, but the integrated bi-metallic (Klixon) thermal switch has not tripped, and the drive has not registered an external fault.

This symptom is the classic signature of a mismatched braking resistor: the ohmic value is too high for the G120C frame size, the continuous/peak power rating is too low, and the thermal sensor inside the housing is sized for the resistor's design cycle, not for the energy being forced into it. The resistor is technically functioning, but it is dissipating far more energy per cycle than Siemens rated the part for. Surface temperatures in the 140 °C range also exceed common cabinet-component derating limits and create a real risk of cable insulation damage, terminal discoloration, and premature failure of the drive's internal brake chopper IGBT.

2. Hardware Identification & Mismatch

Siemens assigns braking resistor MLFBs to a specific G120/G120C frame/power band. The two part numbers above look similar, but they are not drop-in alternatives:

• 6SL3201-0BE21-0AA0 (140 Ω / 4 kW peak) is the catalog braking resistor typically listed for a much smaller G120C output (around 0.55–0.75 kW class) on a 400 V line.
• 6SL3201-0BE21-8AA0 (75 Ω / 7.5 kW peak) is the catalog match for a 5.5 kW G120C on a 400 V three-phase line.

Using the smaller resistor in a larger drive inverts the safety design: the chopper is sized to dump into 75 Ω, so 140 Ω forces the chopper to a higher on-time duty, raises the resistor's V²/R heating per pulse, and pushes more energy per cycle into a device that can only safely sink about half the rated peak.

3. Root Cause Analysis

Three compounding factors explain the 142 °C reading:

1. Ohmic value too high for the chopper. The G120C internal brake chopper is designed to switch at a fixed DC-link threshold (default around 760 V DC for a 400 V unit, configurable via P1244). With R = 140 Ω the instantaneous current at the threshold is I = V/R ≈ 760/140 = 5.43 A, giving an instantaneous power P = V²/R ≈ 4.13 kW. With the recommended R = 75 Ω, I = 760/75 = 10.13 A and P = V²/R ≈ 7.7 kW. The higher resistance limits the chopper's ability to bleed energy quickly, so the DC bus climbs more slowly and the chopper stays active for a longer total on-time per stop. The energy that should leave the bus in 0.5 s takes 1.5–2 s, and the resistor sees that extra time as additional heating.
2. Peak power rating exceeded by application. A 5.5 kW drive connected to a high-inertia load (fan, mixer, conveyor with large flywheel, hoist in regenerative mode) can easily demand more than 4 kW of peak braking power, especially during fast decel ramps (P1121 / ramp-down time). Each cycle the resistor is being asked to dissipate more than its rated peak, with no margin for thermal accumulation.
3. Continuous (average) power is also wrong. Catalog "peak power" is a short-cycle (typically 5 % duty) rating, not a continuous rating. The continuous power for the 6SL3201-0BE21-0AA0 is in the 100–200 W range. Any application with decel more than a few times per minute will drive the resistor past its continuous rating even when the peak is nominally OK.

The Klixon thermal switch is typically a normally-closed contact that opens somewhere in the 130–155 °C range inside the resistor housing (Siemens does not publish an exact cut-out temperature for this specific MLFB in the catalog datasheet — verify against the device label or the operating instructions supplied with the resistor). At 142 °C surface, you are sitting right at or just above the switch's calibration band, which explains why the contact has not yet opened cleanly: surface temperature is not identical to internal element temperature, and the thermal mass of the housing delays the trip.

4. Correct Braking Resistor Selection

For a G120C 6SL3210-1KE21-7AF1 (5.5 kW / 400 V 3-ph), the Siemens catalog and the SINAMICS G120C Operating Instructions (entry ID 109758076 on Siemens Industry Online Support) list the following options:

How the ohmic value is selected:

• Lower bound (R_min): Determined by the chopper's peak current rating. Going below R_min will fault the chopper with F3001 / overcurrent on the brake chopper. Siemens publishes R_min in the operating instructions for each frame.
• Upper bound (R_max): Determined by the DC-link threshold and the available peak current. R_max = V_DC,brake² / P_{peak,required}. If R is too high, the bus clamps at a higher voltage and the chopper duty cycle stretches.
• Target (R_nom): The catalog value sits between R_min and R_max and is the value Siemens has validated for thermal, lifetime, and EMC behavior.

For 5.5 kW / 400 V 3-ph, the validated catalog value is 75 Ω. Do not substitute 140 Ω on a 5.5 kW frame.

5. Braking Energy & Power Sizing Calculations

Before swapping hardware, calculate the energy that the load will return to the DC bus. This determines whether the catalog 7.5 kW peak resistor is enough or whether you need a larger engineered bank.

Kinetic energy of a rotating load:

Or in motor terms:

where J_red is the total inertia referred to the motor shaft in kg·m² and n is the motor speed in rpm.

Braking power during a decel ramp:

Set by the drive's ramp-down time (P1121). A 5.5 kW drive on a high-inertia fan with J_red = 0.5 kg·m² decelerating from 3000 rpm in 5 s:

This single decel pulse is already above the 4 kW peak rating of the 6SL3201-0BE21-0AA0 and at the edge of the 7.5 kW peak rating of the 6SL3201-0BE21-8AA0. If decel happens more than once per minute, average power becomes the binding constraint, and the 7.5 kW peak unit will still be too small.

Repeat-cycle derating:

Compare P_avg to the resistor's continuous rating. If P_avg > P_cont, the resistor will thermal-soak upward over time and surface temperature will climb continuously until the Klixon trips or the element fails open.

6. Parallel / Series Configurations for Higher Capacity

If the application legitimately needs more than 7.5 kW of peak or more than ~400 W of continuous dissipation, you can build a network of identical Siemens DBR elements to keep the catalog ohmic value at 75 Ω while increasing power. Two equivalent topologies work:

Two rules:

1. Always preserve the catalog R value (75 Ω for this drive). Any deviation in R is a deviation from the chopper's validated operating point.
2. Wire all thermal switches in series into a single fault input. All elements must be inside the cabinet's safety loop; if one overheats, the drive must trip.

For 3-phase 400 V / 5.5 kW applications with decel duty of roughly 1 stop per minute and J_red up to ~0.5 kg·m², a single 6SL3201-0BE21-8AA0 is normally sufficient. For heavier cycles, build a 2×2 series-parallel bank of 6SL3201-0BE21-8AA0 elements. Verify with P1240/P1237 parameter observation during a controlled full-speed stop.

7. Thermal Protection: Klixon Switch Behavior

Siemens catalog braking resistors (suffix AA0) include a bi-metallic (Klixon-style) thermal switch embedded in the resistor housing. The contact is typically a normally-closed connection that opens when the internal element temperature exceeds the calibration point of the switch. Siemens does not publish a single universal calibration temperature for the entire DBR family — the exact cut-out is model-specific and is documented in the operating instructions shipped with the resistor (and in the matching section of the G120C Operating Instructions).

What is generally true across the family:

• Cut-out is a one-shot bi-metallic; the contact resets only when the resistor cools below the calibration reset point (lower than the cut-out by a small differential).
• Reset time is many seconds to minutes, depending on the resistor's thermal mass.
• The switch is rated for low-voltage signaling (typically 250 V AC / a few amps), not for carrying the brake current. It is a fault input, never a brake-current path.

Field caveat: surface temperature measured with an IR pyrometer or thermocouple on the outside of the housing can lag the internal element temperature by 20–50 °C during a transient. A 142 °C surface reading on a 4 kW resistor that has been cycling for 30 s strongly implies internal element temperature well past 200 °C, which is why the Klixon is on the verge of opening.

8. G120C Parameter Configuration

Once the correct 75 Ω / 7.5 kW resistor is installed, the following parameters should be confirmed on the G120C. Settings apply to firmware V4.7 SP3 and later (verify with the firmware version shown in r0018 on the BOP-2/IOP). Adjust to match the specific firmware installed in your unit.

For a Commissioning Engineer using the IOP-2 or Startdrive, navigate to Setup → Braking and select "Brake resistor (chopper)" to populate these values automatically.

9. Wiring the Thermal Switch as an External Fault

Wire the Klixon switch as a fail-safe, normally-closed loop into one of the G120C's digital inputs, and map that DI to the external fault function. A typical wiring topology is shown below.

Figure 1 — Klixon wired NC into a digital input mapped to external fault; brake circuit is the B+/B- terminals of the G120C.

Recommended mapping:

• DI source: any free digital input on the G120C control unit (CU230P-2 / CU240E-2 / CU250S-2 depending on variant).
• Configure the DI as active low so that a wire break also produces a fault.
• Map the DI via P2103 / P2104 → External fault 1 (F0721).
• Set P2103 to the DI number (e.g. 722.x for DI 0 on a CU240E-2).

10. Cabinet & Ambient Temperature Considerations

Even with the correct 75 Ω / 7.5 kW resistor installed, a 5.5 kW G120C driving a high-inertia load is a significant heat source inside the cabinet. Plan for the following:

A surface reading of 60 °C on the resistor in a 40 °C cabinet is normal and healthy. A surface reading of 142 °C is not — it indicates either a sizing error (this case) or blocked ventilation, or both.

11. Verification & Commissioning Procedure

After installing the correct 6SL3201-0BE21-8AA0:

1. Visual: Confirm the resistor housing is vertical, on a flat metal surface, with the ventilation slots unobstructed and at least 50 mm of free air on all sides.
2. Electrical: Verify B+ / B- are connected to the G120C brake terminals (B+/B-) and that the Klixon loop is wired to the configured DI and 24 V common.
3. Parameter: Confirm P1237 = 1, P1240 = 1, and P1244 matches the supply voltage. Read the DI status on r0722.x — the DI should be logic "1" with the Klixon healthy (closed), "0" with the Klixon hot (open).
4. Functional: Run the drive to 100 % speed, command a full stop via the configured ramp (P1121), and observe the DC-link voltage in r0026. It should rise toward 760 V, the chopper should engage, and the bus should hold at the threshold until the kinetic energy is dumped.
5. Thermal: Clamp a thermocouple on the resistor housing. Run a full-speed-stop cycle every 60 s for 10 cycles. The housing temperature should stabilize below 120 °C and the Klixon should not open.
6. Fault test: Briefly force the Klixon open (or pull the wire) and verify the drive trips to F0721 with the configured stop reaction (OFF2 by default).
7. Fault reset: Confirm F0721 clears only after the Klixon has cooled and re-closed, not via a generic reset command.

12. Preventive Maintenance

• Inspect the DBR housing every 6 months for discoloration, bulging, or cracked ceramic. These are signs of repeated over-temperature cycling.
• Verify the Klixon continuity with a cold ohmmeter reading on a de-energized unit; the contact should read < 1 Ω.
• Clean dust from the ventilation slots. Dust acts as an insulator and raises the housing temperature without changing the load.
• Re-check the decel ramp time (P1121) after any mechanical change to the driven equipment. Adding a coupling, sheave, or gearbox can change J_red by an order of magnitude.
• Log DC-link voltage (r0026) and chopper duty cycle trends in the PLC if a data historian is available — a rising trend on a constant load indicates resistor degradation.

13. Frequently Asked Questions