Troubleshooting Siemens MM420 Jerky Ramp-Up with SEW Motor

Problem Description and Symptoms

A Siemens MICROMASTER 420 (MM420) drive configured for fixed-speed operation at 40 Hz driving an SEW-Eurodrive induction motor exhibits the following fault signature on every run command:

1. Motor shaft begins to rotate as the MM420 ramps up.
2. Within the first 200-500 ms the rotation becomes visibly erratic and looks as if the motor is about to stall.
3. Rotation momentarily halts or hovers near zero rpm.
4. Drive current climbs to roughly 120-150 % of motor rated current.
5. Without warning, the motor "snaps" to the commanded 40 Hz speed and runs smoothly for the remainder of the run cycle.

The defect is reproducible on every enable, is independent of mechanical load variations, and persists even with the integrated holding brake released both electrically and mechanically. The signature is the classic fingerprint of an under-boosted V/Hz curve, mis-applied slip compensation, or a brake that is not fully releasing during the acceleration window. SEW-Eurodrive's official motor malfunctions troubleshooting guide lists jerky or stalling behavior on acceleration as a symptom of voltage sag, overload, or incorrect V/f characteristic.

The condition must be addressed. Setting the ramp-up time P1120 = 0 s only masks the defect by teleporting the motor past the unstable region, and prolonged stall-burst operation will trip the MM420's I²t model (warning A501 or fault F001 with subcode 11) if the load inertia is significant.

MM420 Architecture Constraints with SEW Motors

The MM420 is the entry-level member of the Siemens MICROMASTER 4 family. It is fundamentally a V/Hz (scalar) drive; no sensorless vector control (SLVC) is implemented in firmware. The supported control modes are P1300 = 0, 1, 2, 3, 5, and 6. The drive cannot estimate rotor flux position in real time the way an MM430, MM440, SINAMICS G110, or SINAMICS V20 can. Consequently, all low-speed torque deficiencies must be cured through:

• V/f curve voltage margins (P1310 continuous boost, P1311 acceleration boost, P1312 starting boost).
• Slip compensation scaling (P1335, P1333, P1334).
• Imax controller response (P1340, P1341, P1345, P1346).
• PWM frequency derating (P1800).

SEW-Eurodrive induction motors - typically from the DRN, DR, DT, or DV series described in the SEW Products and Solutions catalog - are 4-pole machines with rated slip between 3 % and 6 % at full load. They are wound for 230/400 V Δ/Y at 50 Hz or 265/460 V Δ/Y at 60 Hz. The rotor bars are deep-bar or double-cage optimized for high breakaway torque - exactly the regime where the MM420's fixed V/Hz curve is weakest.

Root Cause Analysis

The jerky-then-snap behavior is caused by the closed-loop interaction between three MM420 controllers operating at the limit:

Each of these loops has a settling time on the order of 50-200 ms. Their interaction at 40 Hz with a partially loaded SEW motor produces the visible jerk-then-snap pattern. The cure is to bring the V/Hz curve up to the motor's actual torque requirement before the slip estimator and Imax regulator have to take over.

MM420 Parameter Reference

The following parameters are critical for SEW motor commissioning. The values in the rightmost column are the working starting points that resolve the jerky-ramp fault in the majority of reported cases. Always adapt to the specific motor nameplate.

Step-by-Step Commissioning Procedure

1. Verify supply and motor rating match. Confirm the MM420 mains voltage rating (1AC 200-240 V or 3AC 380-480 V) matches the SEW motor's stator connection. A 400 V star motor on a 230 V drive will produce roughly 57 % of rated torque at 40 Hz, exactly the under-voltage condition the SEW troubleshooting matrix describes.
2. Factory reset to known state. Set P0010 = 30 and P0970 = 1. Wait for the drive to revert (about 10 s) and the green LED to return to ready state.
3. Set access level. Set P0003 = 3 to expose all expert-level parameters.
4. Enter quick commissioning. Set P0010 = 1 to start the guided commissioning wizard.
5. Enter motor nameplate data exactly as printed on the SEW nameplate. P0304, P0305, P0307, P0308, P0310, P0311. Do not round. Do not use drive defaults.
6. Enter the source of command and setpoint. For the bench test, set P0700 = 1 (keypad) and P1000 = 1 (keypad MOP). Press the green Run button to verify the drive responds to the keypad.
7. Run motor identification. Set P340 = 1, leave P0010 = 0, then issue the Run command. The motor will not spin; the drive will inject DC and pulsed signals to measure stator resistance. Allow 30-60 s; the drive will exit the routine automatically. If P340 returns to 0, the identification succeeded. If it returns to 2, an error occurred - check P0947 for the fault value.
8. Set the V/Hz curve and boost values. Refer to the table above. Start with P1310 = 60, P1311 = 15, P1312 = 20, P1300 = 0.
9. Set ramp parameters. P1120 = 5-8 s, P1130 = 0.5 s, P1131 = 0.5 s. Do not leave P1120 at the default 10 s; with a loaded conveyor, the long ramp keeps the motor in the low-voltage corner too long.
10. Run a no-load bump test. Command 40 Hz, watch the motor response on the BOP-2 keypad. The acceleration should be smooth; current should peak at 1.2-1.4 × rated during ramp and fall to 0.6-0.8 × rated at speed.
11. Restore field wiring. If the test was successful, set P0700 = 2 (terminal) and P1000 = 2 (analog) and reconnect the production wiring.

V/Hz Curve and Boost Configuration

The MM420 builds its V/Hz curve from three base points: P1320/P1321 (first corner), P1322/P1323 (second corner), and P1324/P1325 (third corner). Below the first corner the boost voltages P1310, P1311, and P1312 are added. For a SEW 4-pole motor at 50 Hz, the linear V/Hz law is:

V(f) = P0304 × (f / P0310) + V_boost

where V_boost is the sum of P1310 (always active below the first corner), P1312 (active for the first 200 ms after enable), and P1311 (active only during ramp). All three are expressed as a percentage of P0304.

For a SEW DRN motor driving a loaded conveyor at 40 Hz, the no-load voltage is 320 V (80 % of 400 V). With a loaded rotor the slip increases from 3 % to perhaps 5 %, demanding roughly 5-8 % more voltage to maintain the same air-gap flux. If the V/Hz curve is under-volted, the rotor drops out of the linear region of the torque-slip curve and approaches the breakdown-torque point at 200 % slip. The motor loses torque, decelerates, and stalls. The MM420's Imax controller then raises voltage, the motor recovers, and the snap occurs.

Recommended boost values for a SEW conveyor application:

• P1310 continuous boost: 60-80 % of rated voltage. Provides constant extra flux below 5 Hz and during steady-state low-speed running.
• P1311 acceleration boost: 15-25 %. Active only while the ramp is in motion, this overcomes the higher breakaway friction during startup.
• P1312 starting boost: 20-30 % for the first 200 ms after the enable edge. The highest of the three values because cold grease, belt tension, and initial slip are at their worst.

Set P1312 = 0 only if the SEW motor is a small frame (≤ 0.55 kW) and the load is light. Large frames with high inertia absolutely need a starting boost or the first 200 ms of every run will be a stutter.

Slip Compensation Tuning

The MM420 computes the motor's rated slip from P0310 and P0311:

n_synchronous = 120 × P0310 / number_of_poles
slip_nominal = n_synchronous - P0311
slip_comp_factor = P1333 × P1335 / 100

For a SEW 4-pole motor with P0310 = 50 Hz and P0311 = 1430 rpm:

n_synchronous = 120 × 50 / 4 = 1500 rpm
slip_nominal = 1500 - 1430 = 70 rpm
f_slip = 70 × 50 / 1500 = 2.33 Hz

At 40 Hz, the drive should add approximately 2.33 × (load_factor) Hz of frequency to maintain constant air-gap flux. With P1335 = 100 % and P1333 = 100 %, the MM420 will attempt to add the full 2.33 Hz at full load. If the load is partially loaded (50 %), the drive will add only 1.17 Hz, which is correct.

Setting P1333 too high (>130 %) will cause overshoot and hunting at steady state. Setting P1335 = 0 disables slip compensation entirely - which is the worst case for a loaded SEW conveyor. The correct procedure is to leave P1335 = 100 % and P1333 = 100 %, then adjust P1333 upward in 5 % increments if the motor still bogs at 40 Hz under load.

P1334 (slip compensation limit) caps the maximum frequency offset. The default 250 % is excessive; reduce to 200 % to prevent the drive from commanding frequencies above the motor's mechanical limit on a transient load spike.

Brake Interaction and Timing

SEW-Eurodrive motors are commonly delivered with an integrated holding brake (the BE series, BME, or BMG). The brake is spring-applied and electrically released. A brake rectifier is typically supplied from the motor terminal box and rated for the supply voltage: 230 V AC, 400 V AC, or 24 V DC, depending on configuration. SEW also produces a half-wave rectifier that releases the brake at 170 V DC and a full-wave bridge that releases at 100 V DC.

On a VFD-driven system, the brake must be released before the motor develops torque. The sequence must be:

1. Drive receives Run command.
2. Drive begins outputting voltage and frequency at 0 Hz / 0 V (DC brake active or boost voltage at standstill).
3. Brake release voltage is applied (either from the drive output at low frequency, or from a separate 24 V DC source through DOUT).
4. Brake release time elapses (typically 30-100 ms for SEW BE1-BE5 brakes, 100-200 ms for BE10 and larger).
5. Drive begins ramp-up.

If the drive begins ramping before the brake has released, the motor will develop torque against a held rotor. The motor will jerk, the Imax controller will clamp, and the observed symptom is identical to the under-boost case. The cure is to introduce a brake-release delay using the MM420's digital output and P0731/P0732 to interlock the ramp.

Recommended MM420 brake sequencing:

• Set P0731 = 25 (drive ready / no fault active).
• Use a separate 24 V DC source to release the brake through the drive's DOUT or through an interposing relay.
• Set P0347 (de-magnetization time) to 1-2 s to allow residual flux to decay before brake re-engagement on stop.
• For SEW brakes, set a minimum 100 ms delay between Run command and the start of the ramp by using P1120 = 5 s plus P1130 = 1 s, or by using a digital input edge-triggered run command through P0701 = 1 (run enable) with a 200 ms software debounce.

SEW-Eurodrive's motor malfunctions troubleshooting guide explicitly notes that if the brake does not release cleanly, the motor exhibits "jerky startup" - matching the observed symptom exactly.

Factory Reset and Baseline Test

When the parameter set has been altered by multiple service interventions, the most efficient diagnostic is to revert the drive to factory defaults and verify the issue on a known configuration. The MM420 factory reset is a two-step process:

1. Set P0003 = 3 (expert access).
2. Set P0010 = 30 (factory reset).
3. Set P0970 = 1 to initiate the reset.

The display will blank, the drive will beep once, and after roughly 10 s the green LED will return to ready state. The motor will be re-parameterized to a Siemens 4-pole standard motor at the drive's rated power. The SEW motor will not run optimally on this baseline, but it will run, and the question of whether the jerky behavior is drive-parameter related or motor-mechanical related can be answered cleanly.

Baseline test sequence on factory defaults:

1. Wire the SEW motor to the MM420 output.
2. Set P0700 = 1 and P1000 = 1 (keypad control).
3. Issue Run from the BOP-2 keypad with the motor setpoint at 40 Hz (set via the MOP increment button).
4. Observe the rotation. If the jerky behavior persists, the cause is mechanical (load, alignment, gearbox), not the drive parameters.
5. If the motor runs smoothly on the baseline, restore the SEW-specific parameters one block at a time and re-test after each block to identify the parameter that introduces the instability.

Pulse Frequency, Cable Effects, and EMC

The MM420's PWM output at 8 kHz default (P1800) produces voltage rise times of approximately 200-400 ns. On long motor cables, the characteristic impedance mismatch causes reflected-wave overshoots at the motor terminals, which can reach twice the DC bus voltage. SEW motors with their deep-bar rotors are particularly sensitive to this; the reflected wave stresses the inter-turn insulation and produces stray rotor currents that manifest as torque ripple and audible noise.

Recommendations:

• For cable lengths up to 25 m, the default P1800 = 8 kHz is acceptable.
• For 25-50 m cable runs, reduce P1800 to 4 kHz. The lower PWM frequency reduces the reflected-wave amplitude at the cost of higher acoustic noise.
• For 50-100 m, install a Siemens dv/dt filter (6SE6400-3TD00-4AD0) or a sinusoidal filter (6SE6400-3CC00-4CA0) between the drive and the motor. Never operate the MM420 with an SEW motor on cable runs longer than 100 m without filtering.
• Verify the motor cable is a symmetrical 4-core (3 phases + PE) shielded cable, with the shield bonded to the MM420 ground stud and the motor PE terminal at both ends. A floating shield introduces common-mode currents that destabilize the V/Hz curve.

If the application requires quiet operation at 40 Hz, the lower PWM frequency will produce more acoustic noise. The trade-off is between audible noise and torque smoothness. For a conveyor application where acoustic noise is not a concern, 4 kHz is the most stable choice.

Verification and Acceptance Test

After parameter adjustments, verify the fix with the following sequence:

1. Visual check. Motor rotation is smooth throughout the ramp. No audible clicking, no visible cogging, no hesitation between 0 and 40 Hz.
2. Current check. With a clamp meter or the MM420's r0025 (output current) display, confirm the starting current is 1.2-1.4 × motor rated current for 1-2 s, then falls to 0.6-0.8 × rated within 3-5 s of reaching 40 Hz. Spikes above 1.5 × rated for more than 10 s indicate the boost is still under-set.
3. Voltage check. Measure line-to-line voltage at the motor terminals with a true-RMS meter. At 40 Hz the voltage should be roughly 320 V + 30 V (boost) = 350 V on a 400 V base. If the measured voltage is below 300 V, the boost values are not taking effect - check that P1310, P1311, P1312 are not being overwritten by the V/Hz curve points P1320-P1325.
4. Frequency check. Measure output frequency with a counter or frequency meter on the drive's analog output. The frequency should track the ramp exactly. Slip compensation should hold the motor at 40 Hz at full load.
5. Thermal check. After 30 minutes of continuous running, the SEW motor housing should be warm but not hot to the touch. If the housing exceeds 80 °C, the motor is overloaded or the boost is too high - refer to SEW's motor malfunctions troubleshooting for derating guidance.
6. Fault log check. Read r0947 and r0948 on the MM420 for any faults or warnings during the test. A501 (I²t warning) or F001 (overcurrent) indicates the boost values are still too low for the load.

When to Migrate to a More Capable Drive

If the SEW motor is rated above 7.5 kW, the MM420 becomes a poor choice. The MM420's scalar V/Hz control cannot maintain rotor flux at the higher stator currents demanded by larger motors, and the jerky-ramp defect becomes chronic. Migrate to:

• SINAMICS V20 for 0.12-15 kW applications, which supports sensorless vector control and a more sophisticated boost model.
• MICROMASTER 430 for 7.5-250 kW, which adds SLVC and FCC mode.
• MICROMASTER 440 for 0.12-250 kW, which adds closed-loop vector with encoder feedback and is the recommended platform for SEW MOVIMOT or DRN..K geared motors in demanding conveyor applications.

The MM420 should be retained only for low-inertia fan, pump, and light-conveyor applications where the breakaway torque demand is below 120 % of motor rated torque.

Frequently Asked Questions