SINAMICS G120 PM240 External Braking Chopper Configuration and 570 V DC Link Diagnosis

Problem Overview

Engineers commissioning a SINAMICS G120 drive train with Control Unit CU240E-2 DP and a 200 kW PM240 Power Module (Frame Size G) frequently report an "abnormal" 540–580 V DC measurement across the external braking chopper terminals during no-motor bench test. The reading, although it triggers a stop-the-line reaction from operators accustomed to 480 V AC systems, is the expected intermediate-circuit (DC link) voltage of a 400 V-class PM240. The real fault is almost always one of three things: a measurement point error, a missing line-voltage entry, or a brake-chopper parameter set that has not been commissioned for the external unit. This reference explains the DC link physics, the parameter set that controls the brake chopper, and the commissioning sequence that separates a normal DC link reading from a true chopper failure.

The drive in question consists of:

• Control Unit: CU240E-2 DP (PROFIBUS variant, order number 6SL3244-0BB13-1PA0 or equivalent)
• Power Module: PM240 200 kW, 400 V 3-phase, Frame Size G (6SL3224-0BE38-8U..)
• External Braking Chopper: 100 kW / 185 A braking unit connected to the DC bus terminals
• Status: Bench test, no motor on the inverter output side

DC Link Fundamentals

The PM240 is a voltage-source inverter (VSI). The 3-phase AC mains is rectified to produce the DC link voltage that the IGBT bridge inverts back to variable-frequency AC. The DC link is the energy-storage reservoir between the rectifier and the inverter; without it, the inverter cannot commutate at low speed and the line-side harmonics become unacceptable.

For a 400 V 3-phase supply, the ideal no-load DC link voltage equals the peak line-to-line voltage:

V_dc(ideal) = V_LL × √2 = 400 V × 1.4142 = 565.7 V DC

Under load, the source impedance, the diode drops, and the DC link capacitor ripple reduce the average value to approximately 540 V DC. During regenerative braking, the DC link rises above the rectified mains because the motor acts as a generator pumping current back into the bus. The brake chopper is the controlled discharge path: it switches a power resistor across the DC bus when the bus voltage exceeds a fixed threshold, dissipating the regenerative energy as heat.

The expected DC link range for a 400 V-class PM240 is:

A 570 V DC reading at the chopper terminals during a no-motor test is therefore not a fault. It is the no-load DC link, sitting at the peak of the rectified mains. Confirm with the diagnostic parameter r0027 (DC link voltage actual) read on the IOP panel, STARTER, or TIA Portal Startdrive.

SINAMICS G120 PM240 Frame Size G Architecture

The 200 kW PM240 resides in Frame Size G (FSG). This is the largest frame in the PM240 family and is the first frame where the internal brake chopper transistor is not provided. FSG drives must use an external braking unit. The DC bus terminals (DCP, DCN) on FSG are heavy busbars, not stud terminals, sized for 1000 A continuous.

The frame layout is documented in the Siemens Industry Online Support portal under the SINAMICS G120 PM240 operating instructions. The relevant sections cover:

• Power Module connection overview (DCP, DCN, U/T1, V/T2, W/T3, PE)
• Minimum cable cross-section for the DC bus (≥ 95 mm² Cu at 200 kW, 105 °C rated)
• Cooling air requirements (FSG requires forced-air cooling with a 24 V DC fan)
• Touch-safe shrouding for the DC bus

For FSG, the wiring sequence is:

1. Confirm mains is locked out and the DC link is discharged (wait ≥ 5 minutes after power-off; the DC link LED on the CU must be off).
2. Connect the external brake chopper DC input cables between the PM240 DCP and DCN busbars and the chopper DC+ / DC- terminals.
3. Connect the braking resistor cables between the chopper output terminals and the resistor element.
4. Connect the chopper's fault/control signalling cable (typically a 24 V digital input signalling "chopper healthy") to a free DI on the CU240E-2.
5. Verify PE bonding on the chopper chassis and resistor housing.

External Braking Chopper Topology

An external braking chopper is a self-contained unit containing:

• DC bus input (DCP, DCN) — wired in parallel with the PM240 DC link
• IGBT switching element — pulsed at low duty to control resistor current
• Braking resistor output (R+, R-) — wired to the external resistor
• Control logic / power supply — derives gate drive from the DC bus, monitors the threshold
• Fault relay or transistor output — wired back to the CU digital input

The chopper is electrically parallel to the DC link. When the DC link voltage exceeds the chopper's internal threshold (typically 760–780 V DC for 400 V class), the chopper IGBT begins to pulse, switching the braking resistor across the bus. This clamps the DC link at the threshold and dissipates the regenerative energy as heat in the resistor.

Two design philosophies exist:

For the user's 100 kW / 185 A third-party chopper, the self-contained design is assumed. The chopper's own threshold is set on the chopper hardware; the drive's role is to:

1. Disable the (non-existent on FSG) internal chopper.
2. Wire the chopper's fault output to a digital input.
3. Map the digital input to fault code F06902 so the drive trips on a chopper-side fault.
4. Enable the Vdc controller so the drive limits the DC link rise by reducing the inverter's regen torque.

Expected DC Link Voltage by Line Class

The PM240 supports both 400 V and 480 V AC input. The activation threshold p1231 is auto-calculated from p0210 and the unit's voltage class. The two major variants:

The 570 V DC reported in the source is therefore consistent with a 400 V class PM240 with mains on, no load, and the Vdc controller inactive (because there is no motor connected to demand a regen event). If the user ever sees 695 V DC under the same conditions, the drive is set to the 480 V class and p0210 = 480 was entered by mistake.

Brake Chopper Parameter Set

The SINAMICS G120 brake chopper parameter family is p1230 through p1245, with status read-back in r0027, r1238, and r1239. The relevant entries:

For an external chopper application, the drive-side commissioning values are:

p0210 = 400         ; (or 480 if 480 V class)
p1230[0] = 0        ; internal chopper disabled (FSG has none)
p1232[0] = 0        ; drive does not generate brake pulses
p1235 = 0           ; no internal IGBT thermal model used
p1240[0] = 1        ; Vdc controller enabled
p2100 = 722.0       ; fault source for F06902 = DI 0 (adjust to actual DI)
p2101 = 724         ; fault source inverted = 0 (active high fault)

The mapping in p2100 and p2101 is the often-missed step. Without this, a chopper hardware fault will not surface as a drive fault. The drive continues to push the bus, the chopper saturates, and the bus climbs to F30004 overvoltage trip — by which point the resistor has been over-stressed.

Commissioning Procedure

1. Verify the drive rating and chopper sizing. The 200 kW PM240 with a 100 kW chopper is a common mismatch used when the load's peak braking power is < 100 kW. Confirm the load's worst-case regen does not exceed 100 kW for more than the chopper's rated duty cycle. If the load can demand 200 kW of regen (a free-falling vertical axis for example), the chopper is undersized and the Vdc controller will not save it.
2. Set p0210 to the actual line-to-line voltage. For 380–415 V class, use 400. For 440–480 V class, use 480. This single parameter sets the auto-derived chopper threshold p1231 and the Vdc controller bands.
3. Disable the internal chopper. FSG does not have an internal chopper, but set p1230[0] = 0 explicitly to remove the drive-side pulsing logic. The drive will not pulse the (non-existent) internal IGBT.
4. Enable the Vdc controller. Set p1240[0] = 1. The Vdc_max controller will then actively reduce the inverter's regen torque when the DC link approaches p1231, providing a smooth hand-off to the external chopper.
5. Map the chopper fault input. Wire the chopper's fault contact to a free digital input (e.g., DI 0 on the CU240E-2). Map it via p2100[0] = 722.0 and set the inversion in p2101[0] so that an open contact (chopper fault) trips F06902.
6. Save to EEPROM. p0971 = 1 or copy RAM to ROM via STARTER / Startdrive / IOP.
7. Apply mains and check r0027. With mains on, no motor, no enable: r0027 should read 565–580 V DC for 400 V class. This is the no-load DC link and is the value the user is reading at the chopper input terminals. There is no fault.
8. Verify chopper idle. r1239[0] bit 0 = 0 (chopper not active), bit 1 = 0 (no fault). r1238[0] = 0 A. The voltage on the braking resistor terminals of the external chopper should be 0 V DC (chopper IGBT is open).
9. Perform a controlled braking test. With a motor connected, accelerate to a known speed, then command a fast stop. Monitor r0027, r1238, and r1239 in the trace. The DC link should rise to ~774 V (or 967 V for 480 V class), the chopper should engage (r1239[0] bit 0 = 1), and the resistor current should peak at the chopper rating (185 A × duty factor). The Vdc controller should clamp the peak just above p1231 and the chopper should regulate around it.
10. Verify fault path. Manually trip the chopper (open the fault contact). The drive should immediately post F06902 and coast to stop. Reset only after the chopper is healthy. This proves the safety path end-to-end.

Verification Parameters

After commissioning, monitor the following signals continuously for the first 10 operating cycles:

The combination of r0027 not exceeding p1231 and r1239[0] bit 0 toggling only during regen events is the canonical proof that the chopper is correctly parameterized.

Fault and Warning Codes

The most common code on FSG is F30004 when a third-party chopper has been wired but not mapped. The drive does not know the chopper exists, the bus rises above 800 V DC, and the IGBT bridge self-protects by tripping. The fix is the p2100 mapping plus verifying p1240[0] = 1.

Troubleshooting Matrix

Field-Proven Caveats

From repeated commissioning of PM240 FSG systems with external choppers, the following caveats avoid the most common pitfalls:

• Do not measure at the resistor terminals with a standard DMM while the drive is running. The PWM pattern from the chopper can give misleading RMS readings. Use an oscilloscope with a 1000 V DC / high-bandwidth differential probe if a resistor-side measurement is required.
• Frame Size G is air-cooled with a 24 V fan. A missing or failed fan is a common cause of "drive trips after 20 minutes" because the chopper overheats and trips F06901. Check the fan before anything else.
• Wire the DC bus in a tight, equal-length loop. The stray inductance of the DC bus cable couples to the IGBT collector. A 1-metre difference between DCP and DCN paths can add 200 nH and cause a 50 V overshoot on every regen pulse. Use twisted or co-ax arrangement.
• PE bond the chopper chassis to the drive PE bar, not to the building steel. A floating chassis can develop 100 V common-mode with respect to the drive ground and inject noise into the CU.
• Set p1235 = 0 for external choppers. The default value of p1235 is the internal IGBT's continuous rating, which is 0 W for FSG. Leaving the default value can trigger false I²t warnings.
• Verify the line-side voltage class with the order code suffix. PM240-2 has order number suffix letters that indicate voltage class: 0B = 400 V, 0C = 480 V, 0D = 690 V. The p0210 value must match the unit's voltage class, not the actual mains. A 480 V unit on 400 V mains will undervoltage-fault; a 400 V unit on 480 V mains will overvoltage-fault within 50 ms of mains close.

Safety Considerations

Additional safety requirements for the chopper installation:

• The braking resistor must be mounted outside the cabinet on a non-flammable surface, with a minimum 200 mm clearance to any cable or combustible material.
• Wire the chopper's fault contact through a safety-rated digital input if the axis is a vertical load. A failed chopper on a vertical axis is a free-fall hazard.
• Use a thermal switch on the resistor, wired to a drive DI mapped to a fault. A resistor that has lost its cooling (fan failure) is a fire risk.
• Do not bypass the chopper's internal I²t protection with p1235 = 0 on internal-chopper PM240 frames. FSG has no internal chopper so this is a non-issue, but a common error when migrating a 75 kW PM240 (FSF) configuration to 200 kW (FSG).

Document References

The following Siemens documents and tools are required for a proper PM240 FSG commissioning. They should be available in the cabinet's documentation pocket:

• SINAMICS G120 PM240 Operating Instructions (Function Manual, Frame Sizes FSD to FSG)
• SINAMICS G120 Parameter Manual (complete p-parameter reference including p0210, p1230–p1245, p2100)
• SINAMICS G120 List Manual (complete fault and warning list including F06901, F06902, F30004, A06901, A06902)
• TIA Portal Brake Chopper Commissioning Reference
• Dynamic Brake Connection Reference (cross-vendor comparison)