Troubleshooting SIMOREG 6RA70 DC Drive: Motor Running at Low Speed

The Siemens SIMOREG DC-MASTER 6RA70 series converter (frame size 75, model 6RA7075-6DV62-0) is a fully digital three-phase armature and field converter rated up to 600 A DC armature current at 575 V or 720 V line supply. It is widely deployed on planers, rolling mills, paper machines, hoists, and printing machinery where 4-quadrant DC motor control is required. One of the most common field complaints is a motor that suddenly runs at a fraction of its commanded setpoint while the drive remains online and free of alarms. This reference consolidates the official Siemens diagnostic procedure, parameter-level fault isolation, and field-proven corrective actions for this exact symptom.

1. Problem Definition

Symptom reported on a 5-unit fleet of 6RA7075-6DV62-0 drives fitted to a planning machine (planer):

• Drive reports no fault (no F-code, no A-code alarm).
• Run enable and operating state are active (r000 = 0107 / ready-to-run / run).
• Motor spins but at a speed significantly below the speed setpoint commanded from the OP1S, DriveMonitor, or analog input.
• All other four drives on the same machine continue to run normally.
• Failure appears suddenly during running, without parameter changes or operator intervention.

Because the drive does not trip, the root cause is almost always a degradation in the closed-loop speed chain: setpoint, ramp generator, speed controller, armature current, or speed feedback (tachometer / encoder).

2. SIMOREG 6RA70 Control Architecture Overview

The 6RA70 uses a multi-loop control structure. Understanding which loop is failing is essential to short-circuiting the diagnosis.

Every loop above has a connector-type visualization parameter (r000 / r005 / r045 / ...) accessible in chapter 11 of the SIMOREG 6RA70 Operating Instructions. The speed-loop connectors most relevant to this fault are:

If K0167 tracks K0170, the speed controller is closed-loop healthy and the fault is mechanical or in the field. If K0167 falls behind, the fault is in feedback, controller, or current limit.

3. Root Cause Matrix for "Sudden Low Speed"

4. Diagnostic Procedure (No Oscilloscope Required)

When a portable oscilloscope or a DriveMonitor PC connection is not available, the OP1S plain-text panel supports full parameter and connector read-out. The 6RA70 also exposes four internal test sockets routed to the front-panel BNCs (terminals 14-17) via parameter selection.

4.1 Enable digital test outputs

Set the following parameters to route the most important speed signals to the front-panel BNCs and to the connector monitors:

These DAC outputs are accessible at the front-panel terminal block:

• Terminals 14 (+), 15 (-) - DAC1 (P750)
• Terminals 16 (+), 17 (-) - DAC2 (P755)
• Terminals 18 (+), 19 (-) - DAC3 (P760) - depending on hardware version
• Terminals 20 (+), 21 (-) - DAC4 (P765)

With a handheld DMM set to DC volts (10 V range), the diagnosis below can be performed by any technician in the field.

4.2 Read connectors on OP1S

From the OP1S front panel:

1. Press P to enter parameter mode, then 5 0 0 0 0 to display connector r045 segment, or press SHIFT + >> until r045 is visible.
2. Use the index button to step through connectors and read:
   • r045.001 ≈ K0166 (raw setpoint)
   • r045.002 ≈ K0167 (speed actual)
   • r045.003 ≈ K0168 (current setpoint)
   • r045.004 ≈ K0170 (filtered setpoint at speed controller)
3. Press SHIFT + >> again to reach r000 and confirm drive state (e.g. 0107 = run, 0105 = ready-to-run).

DriveMonitor (Siemens 6RA70 commissioning software) presents the same values on the Connectors tab, but with a real-time trace. If a PC with DriveMonitor is available, this is the preferred method as it eliminates the manual reading error.

4.3 Interpret the connector values

4.4 Speed feedback (tacho) verification

The 6RA70 supports analog tachogenerator (default), pulse encoder, or EMF-based internal feedback. Selection is via P741 and the rear-panel DIP switches on the CUD1 board.

1. Stop the drive (terminal 37 OFF) and lock the cabinet.
2. At terminals 17 (+), 18 (-) for analog tach, measure DC voltage with motor hand-turned at known RPM. The voltage should match the tachogenerator scaling. Typical: 60 V / 1000 RPM.
3. Check P741 (tacho type): 0 = analog, 1 = pulse encoder, 2 = EMF model only.
4. Check P742 / P743 / P744 (tacho scaling, peak speed, filter).
5. Inspect terminal block for oxidation, loose screw, or wire break. Re-torque to 0.6 Nm.
6. If the motor is DC shunt with separate excitation, verify the field is at rated value (see 4.5) before declaring the tachogenerator faulty.

For pulse-encoder feedback (P741 = 1), the encoder is wired to X1 on the CUD1 (terminals 1-6). Verify 24 V supply at terminals 1-2, A/B signals at 3-4, and Z reference at 5-6. P744 sets the pulse count per motor revolution (e.g. 1024, 2048).

4.5 Field current verification

A motor running with the correct % setpoint but reduced absolute RPM is almost always under-fluxed. With the Siemens 6RA70 EMF control loop active, the drive will hold EMF constant above base speed by reducing field. Below base speed, field should be at rated value.

1. Read r029 (armature EMF actual) - should be ~85-95% of P101 (line voltage) at base speed.
2. Read r026 (field current actual) - should equal P102 (rated field current) at zero speed, then weaken above base speed.
3. Measure DC voltage across the field winding at terminals 3U / 3C. Compare to P102 × field-winding resistance.
4. Check C2 / C3 capacitors on the field power board. Aging electrolytics (older than 8-10 years) reduce ripple, and the field current dips on the average value.
5. Replace the field board (Siemens part number 6RY1703-0DA01 or 6RY1703-0DA02 depending on rating) if field actual is more than 5% below P102 at base speed.

5. Mechanical and Brush-Related Causes

Because the user reports the failure on a planer (wood or metal planing machine), the mechanical load has high cyclic peaks. The most common mechanical causes of low-speed operation are:

1. Brush wear: Minimum brush length is 8 mm. Below this, contact resistance spikes under load, increasing armature voltage drop and reducing effective EMF.
2. Commutator scoring: Visible black bands indicate poor commutation; the drive may be in I²t foldback (r019 reduced) due to high ripple current.
3. Coupling slip: Planers use a flexible coupling. Check the rubber element for cracking. A slipping coupling gives constant K0168 saturation.
4. Planer tool jam: Wood chips jammed in the cutter head present a torque load above r019 limit. Speed will sit at 30-50% of setpoint with full current. Clear jam, then test.

6. Parameter Reset and Autotune Procedure

If the diagnostic shows the speed controller is unstable or has been corrupted, perform a structured re-tune. Note: parameter set is a free read/write area; parameter set 2 is the active set. Verify with P069 which set is active.

1. Back up current parameter set using P051 = 21 (RAM → EEPROM of parameter set 1) or P052 = 22 (parameter set 2). Note the backup date.
2. Run armature autotune: P051 = 25. Drive will measure armature resistance Ra, commutation inductance La, and current controller optimum gains.
3. Run speed controller autotune: P051 = 26. Motor will run at the speed set in P046 / P047 momentarily to identify moment of inertia.
4. Run field autotune: P051 = 27. The field is energized, and the controller identifies field resistance and inductance.
5. Verify: P051 = 28 shows the calculated Ra, La, Tn, etc. Record the new P225, P226, P228, P155, P156.
6. Save to EEPROM: P051 = 21.

7. Firmware Considerations (CUD1 / CUD2)

The 6RA70 uses the CUD1 (basic) or CUD2 (advanced) control board. The firmware version is shown in r060 / r061. Known issues that map to "low speed without alarm":

8. Verification Checklist

Once a corrective action is applied, validate with this structured check:

1. Drive reaches r000 = 0107 on command, no alarm.
2. K0167 follows K0170 with less than 2% steady-state error at 25%, 50%, 75%, 100% setpoint.
3. Step response: from 0 to 100% setpoint, motor reaches 95% speed within the time set in P303 + P304 + 1 s.
4. No overshoot greater than 5% on a step from 0 to 50% (P230 should be configured for non-oscillatory response on planer loads).
5. Field current at zero speed is within ±2% of P102.
6. EMF actual (r029) at base speed is within ±3% of P101 × 0.9.
7. Armature current ripple (visible on OP1S r019 trend) is less than 10% of rated.
8. Drive runs at full speed for 30 minutes at rated load without thermal warning (F011, F012, F013).

9. Spare Parts Recommendation

For a fleet of five 6RA7075-6DV62-0 units, Siemens recommends the following critical spares to be kept on site:

10. Escalation Path

If all the above checks pass and the motor still runs low, escalate to Siemens Industry Online Support with the following data set attached to the ticket:

• Drive type 6RA7075-6DV62-0 and serial number (r060 / r061)
• Firmware version (r065)
• All connector values (K0166, K0167, K0168, K0170, K0268, K0269) at no-load, half-load, full-load
• Parameter dump: DriveMonitor Parameter → Upload all to file (.dnl)
• Fault buffer (P053 / P054) and alarm buffer (P055 / P056)
• Mechanical decoupled test result

Submit through Siemens Industry Online Support with the Service Request number auto-issued by the support form. The technical data above matches the standard escalation template in the SIMOREG DC-MASTER 6RA70 Operating Instructions chapter 13.

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