Verifying Simoreg 6RA70 Speed Accuracy vs Original DC Drive Specs

Overview: Retrofitting Legacy Analog DC Drives with Simoreg 6RA70 or Sinamics DCM

When replacing an imported analog DC drive with a Siemens SIMOREG 6RA70 or SINAMICS DCM, the most commonly overlooked acceptance criteria are static speed accuracy under torque disturbance and speed regulation range. The legacy nameplate often lists figures such as 2 RPM static error at a load change from 0.1 to 1.0 M_n and a 1:300 regulation range. The replacement drive must be evaluated against these numbers before commissioning, not just against line voltage, armature current, and field current.

This reference documents the engineering interpretation of those two specifications, the math required to confirm them on a SIMOREG 6RA70 (or its successor SINAMICS DCM 6RA80), the encoder/tacho selection logic that ultimately determines achievable accuracy, and the parameter-level verification procedure for a typical retrofit.

Definitions: Static Speed Accuracy and Speed Regulation Range

Both terms are taken from IEC 61800-1 and from manufacturer datasheet conventions used by Siemens for DC drive regulators.

Important: the 2 RPM figure is the static (steady-state) error, not the transient dip that occurs within the first 100–500 ms after a load step. The PI regulator in the 6RA70 will always overshoot/undershoot the setpoint first; the static number is the residual error after the integrator has wound up.

Translating the Legacy Specification into a Siemens-Compatible Number

Given the original data:

• Static speed accuracy at load change 0.1 → 1.0 M_n: 2 RPM
• Speed regulation range: 1:300
• Motor speed envelope: 10 RPM (min) to 3000 RPM (max)
• Feedback device: analog DC tachogenerator, 80 V / 1000 RPM (i.e., 80 V per 1000 RPM, or 4.8 V per 60 RPM)

The 1:300 range on a 3000 RPM base implies a minimum controllable speed of 10 RPM, which matches the motor envelope exactly. So the legacy drive was specified to hold ±2 RPM at 10 RPM (which is ±20 % of setpoint — already a wide band) and ±2 RPM at 3000 RPM (which is ±0.067 % of setpoint — extremely tight). Engineering practice treats the 1:300 figure as meaning the drive is specified to hold accuracy at every point between n_min and n_max, with the 2 RPM figure being the worst case at the lowest speed.

On a SIMOREG 6RA70 with digital encoder feedback, Siemens publishes a speed control accuracy of 0.01 % of rated speed (see the SIMOREG 6RA70 Operating Instructions entry in the Siemens Industry Online Support portal). For a 2000 RPM rated speed, that is 0.2 RPM, which is an order of magnitude tighter than the 2 RPM requirement. For a 3000 RPM rated speed, the achievable 0.01 % number is 0.3 RPM, still well inside the 2 RPM envelope.

Mathematical check:

• Required accuracy: 2 RPM / (target speed)
• At 3000 RPM: 2/3000 = 0.067 %
• At 10 RPM: 2/10 = 20 %
• Siemens 6RA70 with encoder: 0.01 % of n_rated ≈ 0.2–0.3 RPM across the full range

Conclusion: the 6RA70 easily meets the static accuracy requirement provided that the feedback resolution does not become the limiting factor at low speed.

Why the Feedback Device — Not the Drive — Sets the Real Accuracy

Although the SIMOREG 6RA70 is a fully digital 16-bit armature current regulator with an internal speed loop bandwidth of typically 30–50 Hz, the achievable static accuracy and the usable regulation range are bounded by:

1. The resolution of the feedback transducer at the minimum speed.
2. The noise floor of the feedback signal relative to the A/D converter range.
3. The linearity of the transducer across the operating range.
4. The maximum pulse frequency the SIMOREG encoder input can accept.

Analog Tachogenerator (Original Configuration)

An 80 V / 1000 RPM analog tacho is a reasonable choice at high speed. At 3000 RPM the tacho produces 240 V DC, which must be scaled into the 6RA70 analog input. The problem is at the low end. At 10 RPM, the tacho produces 0.8 V DC. The SIMOREG 6RA70 analog speed actual input (terminals at the basic unit) has a typical ±10 V or ±20 V range with 14-bit resolution. With a 240 V full-scale at n_max and a 0.8 V signal at n_min, the LSB of the A/D converter at the bottom of the range corresponds to:

1 LSB = 20 V / 2¹⁴ = 0.00122 V ≈ 0.015 RPM of tacho signal at the high end. At 10 RPM the tacho signal is 0.8 V which is only 65 LSBs, meaning the resolution is poor. Thermal drift of the tacho (typically 0.02–0.05 %/K) further degrades accuracy. The legacy drive was able to claim 2 RPM largely because the analog tacho was filtered and because the load on the motor at 10 RPM was negligible (low torque demand at low speed on most process lines).

Digital Pulse Encoder (Recommended for 6RA70 Retro)

Replace the analog tacho with a digital incremental encoder. The minimum recommended pulse count for a 1:300 range is given by:

N_ppr ≥ (2 × range × desired_resolution_RPM × 60) / 1

For a 1:300 range at 3000 RPM max, with a target of 1 RPM resolution at n_min = 10 RPM:

N_ppr ≥ 3000 / 10 × 1 = 300 pulses per revolution (minimum theoretical)

In practice, a 4× safety margin against quadrature decoding losses, electrical noise, and the maximum pulse-frequency limit of the SIMOREG 6RA70 encoder input (typically 200–500 kHz depending on option board) is recommended. The 6RA70 manual lists the maximum input frequency for the standard encoder board at 200 kHz; for higher pulse rates a special option module is required.

Recommended encoder: incremental HTL or TTL encoder with 2048 PPR (pulses per revolution) as a baseline. This gives:

• Quadrature decoding: 4 × 2048 = 8192 counts/rev
• Resolution at 3000 RPM: 3000 / 8192 = 0.37 RPM per count
• Resolution at 10 RPM: 10 / 8192 = 0.0012 RPM per count
• Pulse frequency at 3000 RPM: 2048 × 3000/60 = 102.4 kHz — within the 200 kHz limit

This is well above the 2 RPM static accuracy requirement at any operating point.

SIMOREG 6RA70 vs SINAMICS DCM: Feature Comparison

For a retrofit where the existing motor, cabling, and tacho/encoder are retained, both drives deliver the same order of accuracy. The migration driver in most cases is spare-part availability, firmware support lifecycle, and the commissioning tool the plant team is trained on.

Parameter Mapping: From Legacy Drive to SIMOREG 6RA70

Key parameters that must be set on the 6RA70 to honor the original specification. Reference parameter numbers are from the 6RA70 parameter list (see Operating Instructions, chapter "Parameter List").

Commissioning Procedure for Static Accuracy Verification

1. Verify motor data and feedback wiring. Armature, field, encoder channels A, B, and Z must be wired per the wiring diagram. Shield the encoder cable and ground one end only.
2. Run the drive auto-tuning routine (P051 = 1 on 6RA70). This calculates the field current setpoint and the armature resistance for the current loop.
3. Set the encoder PPR parameter (P083) to the actual encoder fitted. Setting this wrong will cause the drive to read a wrong speed, even though the rotation looks normal.
4. Configure the speed PI controller with conservative initial values (P151 = 1.0, P152 = 1000 ms) and then perform a step-load test.
5. Apply a step load from 0.1 to 1.0 M_n with a controlled brake, generator, or by using the actual process load. Measure the static speed error after the transient has settled (typically > 3 seconds).
6. Record results at 5 speed setpoints: 10, 100, 500, 1500, 3000 RPM. The 10 RPM test is the most demanding.
7. Tune the speed controller if the static error exceeds 2 RPM at any point. Increase K_p (P151) first; only reduce T_n (P152) if K_p is at its limit.
8. Document the result and compare against the original specification. Save the parameter set to EEPROM (P052 = 1 on 6RA70).

Formula Reference: Static Error and Regulation Range

For reference, the static speed error of a PI-controlled DC drive in steady state under a constant load torque is theoretically zero (the integrator eliminates the steady-state offset), but in practice the achievable static error is bounded by the feedback resolution:

n_{stat_error} = (1 LSB of speed actual) / K_feedback

where K_feedback is the transfer gain of the feedback path (V per RPM for analog tacho, or counts per RPM for encoder).

The maximum speed for a given encoder PPR and input frequency is:

n_max = (60 × f_max) / N_ppr

For 2048 PPR and 200 kHz input: n_max = (60 × 200 000) / 2048 = 5859 RPM. Comfortably above 3000 RPM.

The minimum controllable speed (subject to a defined static accuracy) is:

n_min = (60 × f_min) / N_ppr

where f_min is the lowest frequency the 6RA70 encoder input can detect without quantization error (typically 1–5 Hz, depending on filter settings).

For f_min = 1 Hz and 2048 PPR: n_min = 60 / 2048 = 0.03 RPM. Well below the 10 RPM requirement.

Troubleshooting Matrix: When the Static Accuracy Test Fails

Selecting the Right 6RA70 Variant for a Retrofit

The 6RA70 family is sized by armature rated current. Common variants:

For sizing, three-phase apparent power is kVA = sqrt(3) × V_LL × I_line / 1000. If the source current is given as a single-phase value (for example, a single-phase 220 V supply on a small bench drive), use kVA = V × I / 1000. Always confirm whether the manufacturer datasheet quotes line current, per-phase current, RMS, or peak before sizing transformer feeders.

Overload: the 6RA70 typically allows 150 % of rated current for 60 s in cold state. Verify against the actual load duty cycle of the process. The IEC 61800-1 standard provides the framework for sizing; the actual permissible overload for a given drive is a manufacturer-published value and must be checked against the relevant catalog page, not assumed.

Field-Proven Notes from Real Retrofits

• Most analog DC drive retrofits to a digital 6RA70 or SINAMICS DCM improve static accuracy simply because the digital speed loop eliminates drift in the analog PI network. The bottleneck is almost always the feedback device.
• When the original drive claims a 1:300 range on a 3000 RPM motor, it is the 10 RPM end that is hard. Do not test only at high speed; a passing high-speed test does not validate the 1:300 claim.
• If the existing analog tacho is in good condition and the budget is constrained, the 6RA70 can be used with the tacho, but expect the static accuracy to be limited to 0.1 % rather than 0.01 %. Confirm this is acceptable against the 2 RPM target before commissioning.
• Field current must be re-tuned when changing the drive, even if the motor is unchanged. The 6RA70 field current loop is digital and will not necessarily match the analog drive's field setting.
• Save the parameter set in EEPROM (P052) at the end of commissioning, and back it up via DriveMonitor or a memory card on SINAMICS DCM.

Summary: Decision Path for "Same Features Available?"

For a 2 RPM static error requirement on a 1:300 range motor with 3000 RPM max:

1. Compute the required accuracy as % of n_rated: 2/3000 = 0.067 %.
2. Compare with the SIMOREG 6RA70 published figure of 0.01 % with encoder feedback — 6.7× margin.
3. Verify the feedback device: 2048 PPR encoder gives 0.37 RPM per count, well within target.
4. Verify encoder input frequency limit: 102.4 kHz at 3000 RPM, within the 200 kHz board limit.
5. Confirm field current, armature current, and voltage ratings per motor nameplate.
6. Run the static accuracy commissioning procedure above and document at five speed points.

The 6RA70 (or its SINAMICS DCM successor) will meet the original specification in virtually every case where the original drive is a 1980s/1990s analog DC regulator. The retrofit acceptance is not blocked by the drive capability; it is blocked only by failure to upgrade the feedback to a digital encoder.