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Siemens SIMOREG K: converters for DC motors 6RA21 – 6RA26
SIMOREG K units are Siemens thyristor converters for driving DC motors, with part numbers from 6RA21 to 6RA26. For decades they equipped rolling mills, cranes, extruders, paper mills, winders and machine tool spindles, and in many plants they are still in service.
The E-Repair catalogue covers 323 SIMOREG K part numbers, all of which can be assessed for repair, plus 15 part numbers from the related SIMOREG M series (6RA50).
⚙️ How a SIMOREG K works
The converter rectifies the three-phase mains voltage through a thyristor bridge and controls the voltage applied to the armature of the DC motor, and therefore its speed and torque. It often also integrates or works alongside the field excitation supply.
- Single quadrant (B6C bridge): the motor runs in one direction only and does not return energy to the mains. In the catalogue: about 120 part numbers.
- Four quadrant (antiparallel (B6)A(B6)C bridge): the motor can reverse its direction of rotation and brake regeneratively. In the catalogue: about 120 part numbers.
The more recent versions of the K series have microprocessor control; the older ones have analogue electronics. There are also variants with dedicated technology boards, for example for cranes.
🔍 How to read the roots
| Root | Part numbers in the E-Repair catalogue |
| 6RA21 | about 40 |
| 6RA22 | 113, the most common root, including field supply units |
| 6RA23 | 35 |
| 6RA24 | 74, including microprocessor versions for the US market |
| 6RA25 | 8 |
| 6RA26 | about 50 |
| 6RA50 | 15, SIMOREG M series |
To identify a SIMOREG K with certainty you need the full part number and the rating plate data: input voltage, output current and bridge type.
👉 6RA2425-1FS22 – SIMOREG K with microprocessor, single quadrant
👉 Search the catalogue: 6RA22 · 6RA24 · 6RA26
Among the most frequent part numbers in the catalogue are single-quadrant converters with microprocessor (root 6RA2287), converters without closed-loop control for single-quadrant drives (root 6RA2487) and thyristor converters for current-controlled servo drives (root 6RA2625).
⚠️ Typical faults
- thyristors damaged by overcurrents or motor faults
- firing and control boards with intermittent faults due to age
- faulty field supply, with loss of speed control
- electrolytic components and transformers degraded after many years of service
- settings and calibrations lost after a board replacement
🔄 Keep or replace
SIMOREG K was replaced by SIMOREG DC Master (6RA70) and then by SINAMICS DCM (6RA80). A generation change, however, requires new engineering, wiring and commissioning. For many plants the most efficient choice remains repairing and refurbishing the existing converter, planning any upgrade around production schedules.
🧩 SIMOREG K supplementary boards (root 6RA82)
SIMOREG K 6RA21 and 6RA22 units are completed by supplementary boards that add functions to the converter. The E-Repair catalogue covers about 74 of them:
| Root | Content | Part numbers |
| 6RA8211 | Z2 – Z7 boards for 6RA21: braking with resistor or by field reversal, standstill monitoring, ramp-function generator | 6 |
| 6RA8212 | Z100 – Z104 modules for 6RA21 (brake control, setpoint and actual value monitoring), tension control for winders, SIMOREG tester for commissioning | 26 |
| 6RA8222 | Z701 – Z722 boards, for example for axial winders; signal converters, field supplies, SINEC L2 bus terminal | 30 |
| 6RA8232 | Z1004 – Z1025 boards for 6RA22, for example technology board and SINEC L1 interface | 9 |
| 6RA8210, 6RA8221 | Field current monitor, commutating reactors, Z1210 input and output expansion | 3 |
👉 Search the catalogue: 6RA8212 · 6RA8222 · 6RA8232
Supplementary boards often hold machine-specific calibrations and settings: before sending them, it is advisable to note down their configuration.
🛠️ E-Repair services for SIMOREG K
As Approved Partner Siemens – Industry Services, E-Repair supports the production continuity of DC drive plants:
🔧 Component-level repair in E-Repair's certified in-house laboratory, with diagnosis and repair where technically feasible.
♻️ Refurbishment with inspection and replacement of components subject to ageing, and functional testing according to the agreed scope of work.
🚚 Advance replacement with a refurbished product, if available in stock and after a compatibility check.
🛠️ On-site and remote technical assistance for diagnosis, calibration and support with recommissioning.
✅ Standard 12-month warranty on the services for which it applies, extendable up to 24 months under the applicable conditions.
❓ Frequently asked questions
✔️ Can a SIMOREG K still be repaired?
Yes, in most cases. E-Repair carries out a diagnosis and assesses a component-level repair; the proposal depends on the fault and on the condition of the unit.
✔️ Can I replace a SIMOREG K with a DC Master 6RA70?
It is possible, but it is not a direct replacement: dimensions, connections and parameters change. A technical assessment of the plant is needed.
✔️ What is needed for the diagnosis?
The full part number, the rating plate data, a description of the fault and, if available, the electrical diagrams of the machine.
📚 Further reading
ℹ️ The information provided is general and indicative. Before purchase, installation or commissioning, check the data against the official Siemens documentation or contact E-Repair.
