Sinamics DCM Field Weakening Setup for Two-Speed DC Motor

1. Application Overview

This procedure covers the commissioning of a Siemens SINAMICS DCM (6RA80 series, firmware V1.5 and later) for a separately excited DC machine that must operate in two distinct speed regions:

The field supply is taken from a 380 VAC three-phase source that the DCM rectifies to 220 VDC for the shunt field winding. The DCM's integrated field converter (a single-phase half-controlled bridge) is sized to deliver the rated field current (5 A continuous, with short-time overload to ~6 A depending on the unit) and to be set back down to the weakened value of approximately 1.0–2.0 A at top speed.

The reference document for this configuration is the SINAMICS DCM List Manual (LH9) and the SINAMICS DCM Operating Instructions. Parameter numbers in this article follow the SINAMICS DCM p50000+ numbering scheme. Always verify the active parameter list against the firmware version installed on the Control Unit (CUD).

2. Motor Data Verification Before Commissioning

Enter the nameplate values into the motor data set before touching the field parameters. The DCM uses these values to calculate the back-EMF constant and the speed at which field weakening must begin.

With these values, the DCM internally computes the EMF at base speed:

E_base = V_a − I_a × R_a

For the example motor at base load (60 A, 220 V, Ra ≈ 0.10 Ω, I·R drop ≈ 6 V), E_base ≈ 214 V. The DCM uses this as the reference EMF (p50500) that the EMF controller will hold constant throughout the field-weakening range.

3. Field Circuit Topology and Supply Selection

The 6RA80 family uses an internal field bridge (one-phase B6 or B2H depending on unit rating). The field supply terminals on the SINAMICS DCM are typically 1U1/1V1/1W1 (3-phase field supply input) and 1C/1D (field output to the shunt winding).

Because the field coil is rated 220 VDC and the available AC supply is 380 VAC three-phase, the field bridge will rectify the line-line peak minus device drop:

V_fdc ≈ √2 × V_LL − 2 × V_F ≈ 1.414 × 380 − 1.4 ≈ 535 VDC (no-load)

That is well above the coil rating. The field current controller, not the field voltage, is what must be set. The field bridge fires at a delay angle α that regulates the field current to its setpoint. The field voltage across the coil is then the result of the current and the coil resistance:

V_fcoil = I_f × R_f (at steady state)

For a 220 VDC, 5 A coil, R_f ≈ 44 Ω (cold). At 5 A the coil sees 220 V; at 1 A it sees ~44 V. The 535 VDC open-circuit potential is therefore a non-issue because the firing angle increases dramatically as the controller regulates current down. The Operating Instructions, Section "Field circuit" confirms this: the field voltage specification is the coil rating, and the field converter simply delivers the current required.

4. Field Weakening Theory of Operation

For a separately excited DC machine, the back-EMF is:

E = k · Φ · ω

where k is a machine constant, Φ is flux (proportional to field current at low saturation), and ω is angular speed. With a fixed armature supply of 220 VDC and a small I·R drop, E is essentially clamped at the supply voltage once that voltage is reached. Above base speed, the only way to push ω higher is to reduce Φ, i.e., to weaken the field.

The DCM automates this in three regions:

1. Region 1 — Constant torque, n < n_base: Armature voltage ramps from 0 to 220 VDC. Field current is held at its rated value (5.0 A). EMF rises linearly with speed.
2. Region 2 — Constant EMF, n_base ≤ n ≤ n_max: Armature voltage is clamped at 220 VDC. The EMF controller (function diagram FP5050 in the List Manual) compares the actual EMF (r50114) to the setpoint (p50500) and reduces the field current setpoint until EMF matches the reference. As speed increases, the controller drives Φ down proportionally so that k·Φ·ω remains constant.
3. Region 3 — Field-weakened constant power, n = n_max: Field current is at its minimum (1.0–2.0 A in this application). Torque falls as 1/n; power is approximately constant.

The transition point (start of field weakening) is set by p50400 (speed threshold for field weakening). The lower limit of the field current is set by p50401.

5. Field Characteristic Curve (4-Point Linearization)

The DCM allows a 4-point linearization of the field characteristic so the controller can compensate for magnetic saturation. For most shunt machines with light saturation, three points (including the origin) are enough. The four points map speed to field-current setpoint.

The controller interpolates linearly between points. The slope from p50203 to p50204 (800 → 1500 RPM, 5.0 → 1.0 A) defines the field-weakening range. The DCM will then hold field current at the interpolated value whenever actual speed exceeds 800 RPM.

6. Base Speed and Top Speed Definition

The DCM needs two speeds to define the operating envelope. These are the parameters that the OP in STARTER asks for when you choose "Configure drive" → "Motor" → "Field weakening".

During guided commissioning with STARTER, the offline parameter screen for "Field weakening mode" offers the following selections, which correspond to drive parameter p50510.0 (field operating mode):

1. Fixed field (no weakening) — p50510.0 = 0: field current is held at p50100.4 regardless of speed. Not suitable for this application.
2. Field weakening via EMF control — p50510.0 = 1: correct selection for this motor. The DCM regulates armature voltage to maintain constant EMF above base speed, reducing field current to track speed.
3. External field setpoint — p50510.0 = 2: field setpoint is written by a higher-level controller via PROFIBUS/PROFINET. Useful if the line PLC computes the optimal flux profile, but unnecessary for this stand-alone application.

7. Step-by-Step Configuration in STARTER

The following procedure assumes STARTER V5.4 SP3 or later and a SINAMICS DCM with firmware V1.5.x or V1.6.x. Equivalent steps in SINAMICS Startdrive follow the same parameter tree.

7.1 Prerequisites

• STARTER installed and able to go online with the CUD (USB or Ethernet PROFINET).
• Motor nameplate data: 220 V, 60 A, 800/1500 RPM, field 220 V / 5 A, insulation class, ambient temperature.
• Measured cold resistances: R_a (between A1 and A2 with brushes lifted) and R_f (between F1 and F2). The DCM's automatic armature resistance measurement requires the motor to be uncoupled from the load.
• Encoder connected to terminals X9 (HTL/TTL encoder on the CUD) or pulses from a tacho. The EMF controller can run sensorless, but closed-loop speed control requires feedback. For this 2-speed application, an incremental encoder is recommended; tacho is also acceptable.
• Line supply connected: 380 VAC three-phase to U1/V1/W1 (armature) and 1U1/1V1/1W1 (field), protective earth to PE.

7.2 Drive Configuration

1. Open STARTER, navigate to your SINAMICS DCM, and click Configure drive.
2. Select Drive → Function modules → Field. Tick "Field weakening active".
3. Choose field operating mode "Field weakening via EMF controller". This sets p50510.0 = 1.
4. On the Motor page, enter the nameplate data (Section 2 of this article). Click Calculate control parameters — STARTER will pre-populate p50300 (EMF controller Kp) and p50301 (Tn) based on motor data.
5. On the Field characteristic page, accept the default 4-point linearization or enter the points from Section 5.
6. On the Limits page, set:
   • Minimum field current p50401 = 1.0 A (weakened value at 1500 RPM).
   • Maximum field current p50402 = 5.0 A (rated, do not exceed without consulting motor thermal curves).
   • Field current ramp p50410 = 2.0 s (prevents inrush when raising field from 1.0 A to 5.0 A on start).
7. Click Finish. STARTER writes the parameters to the offline project.
8. Right-click the drive → Download to drive target. Confirm the safety prompt — the drive will load parameters and the CUD will perform a brief reset.

7.3 Manual Parameter Entry (if Commissioning Wizard Skips Steps)

Some STARTER versions do not show the field-weakening page until the field module is explicitly activated. To enter the data directly in the parameter list view:

1. Open Parameter list on the drive object.
2. Filter by parameter group p50500–p50599 (field/EMF).
3. Set the values from the table in Section 6.
4. For the field characteristic, navigate to p50201–p50208 and enter the four (speed, current) pairs.
5. Press F5 (Download to drive).

8. EMF Controller Tuning

The EMF controller is the closed loop that drives field current down to keep back-EMF constant. It is functionally identical in structure to the speed controller: a parallel PI with Kp (p50300) and Tn (p50301). STARTER's automatic calculation gives conservative defaults; refine them by step response.

1. Run the drive to base speed (800 RPM) at no load. The field current should stabilize at 5.0 A.
2. Step the speed setpoint from 800 to 1000 RPM (50% of the weakening range). Observe r50081 (actual speed) and r50114 (actual EMF) on the trace recorder.
3. Check that EMF remains at the p50500 reference (~214 V). If EMF dips and recovers with a damped overshoot, the controller is well tuned.
4. If EMF oscillates: reduce p50300 by 30% (lower gain) and repeat.
5. If EMF recovers slowly (> 2 s): increase p50300 by 30% and/or reduce p50301 by 30% (lower integral time).
6. Final check: step from 1000 to 1500 RPM. The field current should drop smoothly from ~3.5 A to 1.0 A, and EMF should stay within ±2% of the reference throughout the transient.

Typical first-pass values for a 60 A / 5 A DC motor: p50300 = 0.8, p50301 = 0.40 s. The exact value depends on the field winding time constant τ_f = L_f / R_f (often 0.3–1.5 s for a 220 V / 5 A coil).

9. Field Current Limiting and Protection

10. Commissioning Verification Procedure

1. Static checks (no rotation): With the line on and the enable removed, verify in STARTER's control panel that r50101 (field current actual) tracks r50040 (field current setpoint) at 1.0 A, 3.0 A, and 5.0 A. Each step should reach the setpoint within p50410 + p50420 seconds.
2. Armature pre-charge: Apply the ON command but keep the speed setpoint at 0. Verify the armature voltage r50039 = 0 V and the field current r50101 = 5.0 A (full field, ready for base-speed run-up).
3. Base speed ramp: Step the speed setpoint to 800 RPM with a ramp time of 5 s. Confirm the armature voltage rises to 220 VDC and the field current stays at 5.0 A. Measure actual speed with a handheld tachometer; r50081 should match within ±10 RPM.
4. Field-weakening transition: Step the setpoint from 800 to 1500 RPM. Monitor r50114 (EMF) — it should remain within ±2% of p50500. Field current should fall smoothly to 1.0 A. Speed should reach 1500 RPM within 10 s for a 5 s ramp + 2 s settling.
5. Reverse transition: Step from 1500 to 800 RPM. Field current should rise from 1.0 to 5.0 A. Watch for F07910 "field current not reached" — if it trips, lengthen p50420.
6. Load step at top speed: Apply 60 A load at 1500 RPM. Field current will briefly dip below 1.0 A as the EMF controller compensates; the lower clamp p50401 will hold it at 1.0 A. Torque falls as 1/n relative to base — this is normal constant-power behavior.
7. Trip test: Manually command the drive OFF (OFF1). Verify the field current ramps down on p50411, the armature voltage collapses, and the contactor (if any) drops after the configured delay (typically 0.5–2.0 s).

11. Troubleshooting Matrix

12. Field Weakening Verification on a Loaded Motor

The no-load check confirms the control loop. The loaded check confirms the torque and thermal envelope. With the motor coupled to its load (or a dynamometer), apply 60 A armature current at the following test points and let the drive stabilize for 5 minutes at each:

Constant-power behavior at the top end is by design: with EMF held constant, power is constant, and torque falls as 1/n. For a 60 A / 220 V motor, the rated mechanical power at base speed is approximately 12.5 kW (assuming 90% efficiency). At 1500 RPM, the same 12.5 kW corresponds to ~80 N·m, not the 160 N·m available at 800 RPM. This is the inherent tradeoff of the field-weakening region and is not a tuning error.

13. FAQ