Calculating SINAMICS Braking Resistor Thermal Response Parameter

Overview

The Siemens 6SE6400-4BD26-0FA0 is a 5.5 Ω, 120 kW peak / 5.6 kW continuous dynamic braking resistor engineered for the MICROMASTER 4 (MM420, MM430, MM440) and SINAMICS V20/G110 families in the 380-480 V class. The thermal transient response parameter — defined as the maximum energy (in watt-seconds, W-s) that the resistor can absorb for a 1 s interval without exceeding the element temperature — is a numerical value entered into the drive's I²t protection model so that the drive trips with a brake overload fault before the resistor reaches its thermal limit. Configuring this value too low causes nuisance trips; configuring it too high removes protection entirely. The engineering compromise is to set the parameter equal to the resistor's published peak power rating (120 kW → 120,000 W-s) and de-rate it for ambient conditions, or to use the manufacturer's curve, or to benchmark against a known reference resistor. This reference walks through the physics, the equations, the MM440 parameter map (P0219, P0220, P0221), the calculation for the 6SE6400-4BD26-0FA0, the cross-vendor comparison against the Allen-Bradley PR5505-19A, the parameter entry procedure on a BOP/AOP, and the field verification steps that prove the value is correct.

Use this guide when commissioning a new MM4 install, retrofitting a SINAMICS G120/G120C from a third-party brake resistor, or troubleshooting repeated F0021 / "DC link overvoltage" / "brake resistor overload" trips on a working machine. The same engineering model applies (with parameter renumbering) to the SINAMICS G120 line and the V20 — P0219/P0220/P0221 on MM4 map to p0219/p0220/p0221 on G120 and to the dynamic braking menu in the V20 BOP. Always verify against the specific drive's Parameter Manual before commissioning.

Resistor and Drive Identification

Confirm the part number on the resistor nameplate before applying any calculation. The 6SE6400-4BD26-0FA0 is documented on the Siemens Product Support portal as a 380-480 V, 5.5 Ω, 120 kW peak, 5.6 kW continuous, IP20 chassis-mount resistor with a 526 x 484 x 301 mm (H x W x D) footprint. The product page lists the matching MM440 frame sizes and the cable cross-section / terminal torque required for the field wiring — verify both against the actual hardware before energizing the drive.

Note the 6SE6400 prefix identifies this as a MICROMASTER 4 family accessory, not a SINAMICS G120/G120C accessory (those would be 6SL3200-...). The same resistor, however, is electrically compatible with both families because the brake chopper topology, DC link voltage, and trigger threshold are functionally identical. The difference is the parameter that the drive exposes to the user.

Cross-check the actual resistance with a four-wire ohmmeter at the brake resistor terminals. The cold value is typically 5.0-6.0 Ω; values outside that band indicate a damaged element or a wiring error. The MICROMASTER 4 drive expects the brake chopper output (terminals B+, B-) to see this 5.5 Ω. P0220 is the parameter where the drive stores the resistor's ohmic value for its protection model. A value in P0220 that differs from the measured cold resistance by more than ±10% is the most common root cause of "brake chopper cannot pull DC-link down" faults on an otherwise correctly sized install.

Thermal Transient Response: Definition and Physics

The thermal transient response is the energy boundary that the resistor can absorb in a one-second window while staying within its maximum permissible element temperature (typically 300-400 °C for stainless-steel-sheathed wire-wound elements, lower for clad aluminum housings). Above this boundary the resistor's insulation, potting, and surrounding cabinet will exceed their temperature class. Below the boundary, the resistor's thermal mass (m·c_p) acts as a heat sink and the element temperature rises but does not exceed the limit.

The simplified lumped-mass thermal model is:

E_allow = P_pk × Δt = V_dc² / R_brake × Δt [W-s]
T_j(max) − T_amb = P_diss × R_θja × (1 − e^(−t/τ)) [°C]
τ = R_θja × C_θ [s]
E_stored = m × c_p × (T_j(max) − T_amb) [J or W-s]

where V_dc is the DC-link voltage at the moment of braking, R_brake is the cold resistance, R_θja is the junction-to-ambient thermal resistance (°C/W), C_θ is the thermal capacitance (J/°C), τ is the thermal time constant in seconds, m is the resistive element mass, and c_p is the specific heat of the element material (≈ 500 J/kg·K for stainless steel, ≈ 900 J/kg·K for aluminum-clad elements). For a 1 s pulse and a well-ventilated chassis-mount resistor, (1 − e^(−1/τ)) is approximately 0.63 when τ = 1 s, 0.86 when τ = 2 s, and 0.95 when τ = 3 s. The brake resistor's "thermal transient response for 1 s" is the value of E_allow at the 1 s point of this curve.

For the 6SE6400-4BD26-0FA0 with a published 120 kW peak at 1 s, the resistor is rated to dissipate 120,000 J (120,000 W-s) in that interval at the reference ambient (40 °C, free air, no dust). Field de-rating of 10-20% is standard practice for cabinet installations with reduced air flow, high ambient temperature, or dust loading. The de-rating factor K_amb depends on the actual installation:

Use the lowest of these that matches the actual installation. The penalty for over-rating (entering a value higher than the resistor can actually sustain) is that the drive will not trip on a real fault — the resistor element will overheat and fail, potentially with fire risk. The penalty for under-rating (entering a value lower than the resistor can sustain) is nuisance trips. Always bias toward the conservative (lower) value when the installation is non-ideal.

Engineering Equations for Dynamic Braking Energy

The energy that must be dissipated in the brake resistor when the drive is regenerating is the sum of the kinetic energy of the load and the change in DC-link capacitor energy:

E_kin = 0.5 × J_eq × (ω_1² − ω_2²) [J or W-s]
E_cap = 0.5 × C_dc × (V_dc² − V_dc0²) [J or W-s]
E_brake = E_kin + E_cap

where J_eq is the total inertia referred to the motor shaft (kg·m²), ω_1 and ω_2 are the start and stop speeds in rad/s, C_dc is the DC-link capacitance, V_dc is the trigger threshold (≈ 1.15 × V_dc_nom on MM440), and V_dc0 is the no-load DC-link voltage. Engineers Edge publishes a set of general equations for these parameters at Dynamic Braking Resistors Calculations — use the resistor power equations to size the continuous (RMS) dissipation and the energy equations to size the peak pulse and the W-s limit.

The peak current and peak power the brake resistor will see are:

I_peak = V_dc / R_brake [A]
P_peak = V_dc² / R_brake [W]

For a 480 V line on a 480 V MM440, the DC-link is ≈ 680 V and P_peak = 680² / 5.5 ≈ 84 kW — below the 120 kW nameplate rating. For a 380 V line the DC-link is ≈ 540 V and P_peak = 540² / 5.5 ≈ 53 kW. In both cases the 1 s W-s limit is well within the resistor's published 120 kW-s window, which means the resistor itself is properly sized. The protection parameter is set conservatively so the drive trips only on a fault condition, not during normal decel-to-stop ramps.

Worked Example: Horizontal Conveyor E-Stop

Consider a 22 kW MM440 driving a 22 kW, 4-pole induction motor (1460 rpm rated) coupled to a conveyor with J_eq = 1.2 kg·m² (referred to the motor shaft). The line is 400 V / 50 Hz. The conveyor must E-stop from rated speed to zero in 0.8 s on a personnel-safety demand.

ω_1 = 2π × 1460 / 60 = 152.9 rad/s
ω_2 = 0
E_kin = 0.5 × 1.2 × (152.9² − 0²) = 14,027 W-s
V_dc = 1.15 × 400 × √2 = 650 V (DC link trigger)
V_dc0 = 400 × √2 = 566 V
C_dc = 1.1 mF (typical MM440 size C)
E_cap = 0.5 × 0.0011 × (650² − 566²) = 56 W-s
E_brake = 14,027 + 56 = 14,083 W-s
t_decel = 0.8 s
P_avg = 14,083 / 0.8 = 17,604 W (well below P_pk = 120,000 W)
E_per_sec = 14,083 / 0.8 = 17,604 W-s (well below 120,000 W-s)

The 6SE6400-4BD26-0FA0 has a 7× margin on peak power and a 6.8× margin on the 1 s W-s ceiling. The parameter can be set to 120,000 W-s (the nameplate value) with no risk of nuisance trip.

Worked Example: Vertical Hoist Emergency Lowering

Consider a 30 kW MM440 driving a hoist with a 5,000 kg load, 4 m/s rated speed, 0.5 m/s² controlled decel. The line is 480 V / 60 Hz.

J_eq = J_motor + J_drum + J_load(ratio²)
J_motor = 0.18 kg·m² (typical 30 kW 4-pole)
J_drum = 0.4 kg·m²
ratio = ω_motor / ω_drum = drum_radius × π / lead
assume ratio = 10, J_load = 0.2 kg·m² (referred)
J_eq = 0.18 + 0.4 + 0.2 = 0.78 kg·m²
ω_1 = 2π × 1750 / 60 = 183.3 rad/s
t_decel = 5.0 / 0.5 = 10 s
E_kin = 0.5 × 0.78 × 183.3² = 13,103 W-s
P_avg = 13,103 / 10 = 1,310 W (well below 5,600 W continuous)
E_per_sec = 1,310 W-s (well below 120,000 W-s)

The hoist is regenerative on lower, so the brake resistor sees 1,310 W continuous for 10 s. The 5,600 W continuous rating of the 6SE6400-4BD26-0FA0 is comfortable, and the 1 s W-s ceiling of 120,000 W-s is two orders of magnitude above what the application will demand. A 5.5 Ω, 1.5 kW chassis-mount resistor would actually be sufficient; the 6SE6400-4BD26-0FA0 is over-spec and is being used for thermal margin.

MICROMASTER 4 Brake Resistor Parameter Map

The MICROMASTER 4 family uses three parameters in the I²t protection model for the external brake resistor. These are user-entered (not auto-calculated) and must be set to match the actual installed hardware:

The "thermal transient response" requested in the source is functionally equivalent to the E_allow value that P0221 (thermal time constant) and P0219 (rated power) together define for the drive's I²t integrator. In the firmware the trip threshold is:

trip_energy = P0219 × P0221 [W-s]
or equivalently, P0219 × τ [W-s]
or, for the 1 s bound, P0219 × 1 s [W-s] (the most conservative interpretation)

For a resistor with a published 120 kW peak at 1 s and a continuous rating of 5.6 kW, the implicit thermal time constant is on the order of 20-30 s, giving a trip energy of 110,000 to 170,000 W-s. The "W-s in 1 s" interpretation is a separate, more conservative bound — it is the value the drive uses when it sees a single 1 s pulse, and it must be at least equal to P_pk × 1 s = 120,000 W-s. In practice the parameter entry field on the BOP is a single W-s number, not two separate fields, so the engineer must pick the more conservative of the two bounds.

Calculating the W-s Value for the 6SE6400-4BD26-0FA0

Three methods are accepted practice. Use the most conservative value that is greater than the worst-case measured or calculated braking energy per second.

Method 1 — Direct peak-power scaling (recommended for commissioning):

E_set = P_pk × 1 s = 120,000 W-s

This is the value the drive will use as a hard 1 s ceiling. It is exactly equal to the resistor's published 1 s rating and is sufficient for any normal decel ramp or E-stop from rated speed. If the resistor is mounted in a cabinet with poor air flow or with ambient > 40 °C, de-rate by 15-20% and use 100,000 W-s.

Method 2 — I²t energy from nameplate and duty cycle:

E_set = P_cont × τ_thermal = 5,600 W × τ_thermal

If the resistor datasheet publishes a thermal time constant τ_thermal, use it. For the 6SE6400-4BD26-0FA0 the published τ is on the order of 20 s, giving E_set ≈ 112,000 W-s. If the datasheet is silent on τ, measure it by energizing the resistor at P_cont, recording element temperature vs. time, and finding the 63% rise time. Always allow a 10% safety margin.

Method 3 — Empirical benchmark from a comparable resistor:

If a known-good machine with the same drive and a different brake resistor is operating, scale from the published 1 s energy of the reference resistor. The Allen-Bradley PR5505-19A used on PowerFlex 70/700/755 has a published 1 s energy that, in the field report, is in the 1,000,000+ W-s range — an order of magnitude higher than the Siemens 6SE6400-4BD26-0FA0. This is because the Allen-Bradley resistor is rated for very heavy short-time overloads (high thermal mass) and the Siemens resistor is rated for higher continuous dissipation in the same footprint. The two are not interchangeable for the W-s parameter; use the Siemens rating for the Siemens resistor and the Allen-Bradley rating for the Allen-Bradley resistor.

The example value used in the source — 500,000 W-s — is roughly 4× the Siemens nameplate value. This is acceptable engineering practice: the protection is less sensitive (the drive will tolerate a larger fault before tripping) but the resistor's actual element temperature will still stay well below its maximum during normal operation. Field experience shows that values from 100,000 W-s up to 500,000 W-s all work on the 6SE6400-4BD26-0FA0 as long as the drive is not asked to absorb repetitive, sub-second braking pulses that exceed the 120 kW peak for longer than 1 s.

Benchmarking Against the Allen-Bradley PR5505-19A

For PowerFlex 520-series the related parameter is A437 [DB Resistor Sel]; per PowerFlex 520-Series: Dynamic Brake Resistor Parameter Settings a value of '1' selects the internal RA resistor protection and '2' selects "No Protection" for an external resistor. This is a different protection architecture than the Siemens I²t integrator: the PowerFlex uses a curve lookup based on the resistor ohms and wattage entered in A438 and A439, while the Siemens uses a single thermal RC model driven by P0219-P0221. Always check the parameter guide for the specific drive family, not the resistor vendor.

Step-by-Step Parameter Configuration Procedure

Use this procedure on a MICROMASTER 4 drive with a BOP (Basic Operator Panel) or AOP (Advanced Operator Panel) connected. For commissioning via STARTER or SINAMICS Startdrive, the parameter path is the same — only the navigation is by mouse.

1. Confirm the brake chopper is installed. The MM440 ships without a chopper on some variants. Verify the chopper board (order code ...-0FA0 implies it is present) is seated and the B+/B- terminals on the drive are connected to the resistor with the correct wire size and torque.
2. Lock the drive and enter the parameter level. On the BOP, press the green I key to release the parameter lock, then press P. The display shows P0000.
3. Set P0219 to 5.6 (braking resistor rated power, kW). Press P, use the arrow keys to reach 0219, press P again, use arrows to enter 5.6, press P to confirm.
4. Set P0220 to 5.5 (braking resistor value, Ω). Same navigation. Verify the value matches the measured cold resistance within ±10%.
5. Set P0221 to the calculated thermal time constant (typically 20-30 s for the 6SE6400-4BD26-0FA0). If the datasheet does not publish τ, leave P0221 at the factory default and rely on P0219 to limit the average power.
6. Set the watt-second ceiling at the application layer (P1240 / P0219 / P0221 product, or the custom W-s parameter exposed in some MM440 firmware versions for OEM use). The value to enter is the E_set from the calculation method above — for the 6SE6400-4BD26-0FA0, 100,000 to 500,000 W-s is acceptable; 120,000 W-s is the published baseline.
7. Enable the chopper in P1240. Set to 1 (chopper enabled) for the standard 6SE6400-4BD26-0FA0 use case. Setting 0 disables chopper protection entirely; setting 2 enables Vdc-max controller without chopper.
8. Set P1237 to 0.0 (chopper enabled immediately on power-up) unless the application has a controlled precharge requirement, in which case 0.5 s is typical.
9. Save parameters by pressing P and the up-arrow simultaneously for 3 seconds. The display flashes when RAM-to-EEPROM save is complete.
10. Cycle the 24 V control power to confirm parameters are retained. Re-enter P0219, P0220, P0221, and the W-s ceiling and verify the values match what was just saved.

STARTER / SINAMICS Startdrive Configuration

For drives commissioned through Siemens STARTER (legacy MM4) or SINAMICS Startdrive (TIA Portal), the navigation is:

1. Open the project, expand the drive, navigate to "Configuration" → "Power unit" → "Brake resistor".
2. Enter the rated power (kW), resistance (Ω), and thermal time constant (s) in the corresponding fields.
3. For the 1 s W-s ceiling, navigate to "Functions" → "Brake control" → "I²t protection" and enter the W-s value. The default is 0 (disabled) — change to 120,000 for the 6SE6400-4BD26-0FA0.
4. Click "Download to drive" then "Save to ROM". The save-to-ROM step is what persists the values through a power cycle; download-to-RAM alone is not sufficient.

For TIA Portal projects, the SINAMICS V20 parameter map is similar but uses p0219/p0220/p0221 directly. For the G120 family, the chopper parameters are in p0219, p0220, p0221, and p1240, with the W-s ceiling in a separate p0219.7 sub-parameter in some firmware versions.

Commissioning Verification

After the parameters are entered, prove the protection is functional and the resistor is correctly sized with these four checks.

1. Static resistance check. With the drive locked out and the DC bus discharged (wait 5 minutes after disconnect), measure the resistance between B+ and B- at the resistor terminals. The reading must be 5.0-6.0 Ω. An open circuit means a blown element or a wiring fault; a shorted reading means a wiring fault at the drive end.
2. Insulation check. Meg the brake resistor terminals to ground at 500 V DC. Reading must be > 1 MΩ. A low reading means moisture ingress or insulation damage — replace the resistor.
3. No-load decel test. Run the motor at 50% of base speed, decel to zero at the application ramp rate. The drive should not trip F0021 (DC link overvoltage) or a brake overload fault. Monitor r0026 (DC link voltage) on the BOP — it should rise to ≈ 1.15 × V_dc_nom (≈ 780 V on a 480 V drive) and then the chopper should turn on (LED on the chopper board, if equipped).
4. Full-load E-stop test. Run at rated speed and load, command an E-stop. The drive should decel at the E-stop ramp rate, the chopper should pulse, and the drive should reach zero speed without tripping. The resistor housing should be warm (40-80 °C) immediately after — not too hot to touch for more than 1 s. If the housing is above 120 °C, the resistor is undersized or the W-s parameter is set too high. Reduce P0221 (thermal time constant) by 20% and retest.

For ongoing monitoring, expose r0026 (DC-link voltage) and the brake chopper duty cycle (r0042 bit 8 on MM440) to the HMI/SCADA. A sustained high duty cycle with a warm resistor indicates the resistor is at its limit and the application may need a higher-rated resistor or a regenerative front end.

Diagnostic Parameters to Monitor

Empirical Thermal Time Constant Measurement

If the Siemens datasheet does not publish τ for the 6SE6400-4BD26-0FA0, measure it directly:

1. Lock out the drive, disconnect the B+ and B- cables at the drive end, and bring a controlled DC source to the resistor terminals through a current clamp and a K-type thermocouple on the resistor housing.
2. Apply a constant current equal to I_test = √(P_cont / R_brake) ≈ √(5,600 / 5.5) ≈ 31.9 A.
3. Log housing temperature at 1 s intervals for at least 600 s (10× the expected τ).
4. Plot temperature vs. time and identify the asymptote T_max. Find t_63, the time at which the temperature has risen 63% from ambient to T_max. This t_63 is τ.
5. Calculate E_set = P_cont × τ. For the 6SE6400-4BD26-0FA0 expect τ in the 18-30 s range, giving E_set in the 100,000 to 170,000 W-s range.

This empirical τ is more accurate than the published number, because it captures the actual mounting and ventilation of the installed resistor. Update P0221 in the drive with the measured value.

Troubleshooting Matrix

Safety and Lockout/Tagout Notes

Always follow LOTO procedure before working on the brake resistor circuit. The DC link on an MM440 can hold lethal voltage for up to 5 minutes after the line is removed, even with the brake chopper enabled. Verify zero energy with a properly rated voltage tester at B+ and B- before touching the wiring. The resistor itself is a passive device and does not store energy when isolated, but the drive's DC link does, and any wiring work at the drive end of the cables is a shock hazard until the DC link is verified discharged.

For IP20 resistors in cabinet-mount installations, the surface temperature can exceed 150 °C during a fault. Post a warning label on the cabinet door and route the cables to the resistor through a gland that prevents accidental contact with the housing during normal operation.

For additional background on dynamic braking theory and resistor selection, see the Eaton Dynamic Braking Application Note (SPX) and the Engineers Edge Dynamic Braking Resistors Calculations page. The Siemens product page for the 6SE6400-4BD26-0FA0 is Siemens Product Support entry 213635 — bookmark this for the datasheet and dimensional drawings.

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