MM440 Axial Fan Reverse Rotation: Flying Start and DO3 Relay

Problem Overview: MM440 Axial Fan Will Not Restart After Reverse Coast-Down

An axial-flow fan driven by a 30 kW four-pole induction motor and fed by a Siemens MICROMASTER 440 (MM440) inverter rated 37 kW was being operated at approximately 580 r/min with the maximum output frequency capped at 30 Hz. After a normal stop command, the fan was observed to free-wheel in the reverse direction (opposite to the commanded rotation) before coming to rest. On the subsequent start command from the plant DCS (PLC program), the drive either refused to accelerate, tripped on overcurrent during the ramp, or stalled silently with no fault indication on the MM440 BOP (Basic Operator Panel) or AOP (Advanced Operator Panel).

Six identical fans were installed on the same duct system. Five of the six started normally without any firmware intervention; only one fan exhibited the no-start-after-coast-down symptom. The fault therefore had two distinct layers:

1. Aerodynamic / mechanical layer: residual airflow across the blades drives the impeller backward (reverse rotation) when the drive is off.
2. Control / electrical layer: the digital output used to feed back the drive's "running" state to the DCS had failed intermittently.

Both layers must be addressed to obtain a reliable restart sequence. This article documents the diagnostic procedure, the MM440 parameter set required for bidirectional flying restart (P1200 family), and the digital-output (DO) wiring / parameter logic that ultimately proved to be the root cause of the no-start symptom.

Mechanical and Aerodynamic Root Cause: Windmilling Reverse Rotation

Axial fans installed in ventilation, exhaust, or process-air systems are subject to significant windmilling effects. When the drive is off and the duct is pressurised or depressurised by an adjacent running fan, or by natural draft, the impeller rotates passively in whichever direction the net airflow dictates. With an upstream or downstream fan running in the same direction as the stopped fan, the stopped fan will rotate backwards.

At 580 r/min in the reverse direction, the rotating mass of the impeller stores a non-trivial amount of kinetic energy. If the drive is asked to start the motor directly across the line equivalent of the inverter output (without any resynchronisation routine), the inverter will encounter:

• A rotating rotor whose slip is effectively 200 % (negative-sequence rotation at full mechanical speed).
• A back-EMF that opposes the inverter's applied voltage vector at the wrong sequence.
• High instantaneous line currents as the inverter attempts to force the rotor to synchronise.

Without a flying restart function the MM440 will report F0001 (overcurrent) or F0002 (overvoltage) on the DC bus during the first few hundred milliseconds of the start ramp, depending on the magnitude and direction of the residual speed. In some cases the drive will simply refuse to develop torque and remain stalled with no fault displayed because the current limit (P0640) clamps the output before the trip threshold (P0290 action) is reached.

Reference material on this physical effect for industrial fan drives is provided in the Siemens application note "Fans and Pumps — Energy-Saving Applications" available through the Siemens Industry Online Support portal.

MM440 Flying Restart Function (P1200 Family)

The MM440 firmware provides a flying restart (also called "catch-on-the-fly" or auto-restart) function that searches for the actual rotor speed and direction, then re-synchronises the V/f curve to the existing rotation. The function is enabled by parameter P1200 and refined by P1202 and P1203.

Parameter P1200 — Flying Restart Operating Mode

For an axial fan that may be windmilling in either direction, the correct selection is P1200 = 1 (always active, bidirectional search). Values 2 and 3 also perform a bidirectional search but are gated by the OFF2 / fault condition, which is less robust for an application where the operator expects a start on demand from any state.

Parameter P1202 — Flying Restart Search Current

P1202 sets the percentage of the motor rated current (P0305) that the drive injects during the speed-search ramp. The default value is 100 %. Reducing P1202 lowers the mechanical stress on the coupling and the audible noise during the search, at the cost of longer search time. Recommended values for axial fans in the 30–37 kW class:

• P1202 = 100 % (default) — fastest search, suitable when the fan inertia is moderate.
• P1202 = 70 % — quieter, recommended when the search is audibly disturbing the mechanical envelope.
• P1202 = 50 % — minimum acceptable for most 4-pole motors; do not go lower as the back-EMF detection becomes unreliable.

Parameter P1203 — Flying Restart Search Time / Ramp

P1203 defines the rate at which the drive sweeps the search frequency. The default of 100 % corresponds to roughly 3–5 seconds for a full sweep across the speed range set by P1082 (maximum frequency). For a 30 Hz-limited fan the effective sweep time is therefore only 1.5–2.5 seconds. If the search terminates too early (false-zero-speed detection), increase P1203; if the audible kick is objectionable, decrease it.

Additional Parameters Required for Reliable Flying Restart

Digital Output Fault: Why the Motor "Stopped Without a Fault"

In the field, enabling flying restart on the single affected inverter produced an unexpected result: when the operator attempted to start the motor, the drive itself was perfectly capable of catching the rotating rotor, but the start command never reached the drive's logic because the digital output that signalled "drive running" to the DCS had failed.

The MM440 base unit provides three programmable digital outputs on terminals 19, 20 and 21 (DO1, DO2, DO3) plus a relay output on terminals 30/31 (NO), 32/33 (NC). On the affected unit, DO3 had been assigned (via parameter P0731 = 52.3, "Drive running") to feed back to the DCS. The relay contacts inside the DO3 output driver were found to be intermittent: closed on some commands, open on others, with no consistent pattern. The DCS therefore received a "not running" feedback even though the drive was commanded to run, and the PLC logic interpreted this as a fault condition that called for a stop.

The drive did not display any fault code because no internal protection threshold was exceeded — the unit simply obeyed the OFF1 it received via the fieldbus or terminal command after the PLC dropped its start output.

MM440 Digital Output Assignment Parameters

For the affected fan, the operator reassigned the "Drive running" function (state word 1, bit 3 = 52.3) from DO3 to DO2 by setting P0731 = 52.3 and freeing DO3 for diagnostic use. This is consistent with the standard MM440 commissioning pattern where critical feedback signals should not be concentrated on a single output that also drives an alarm function.

Recommended DO Assignment Strategy for Fan Arrays

For an array of identical fans with shared DCS, the following assignment matrix minimises the probability that a single failed output can take a fan out of service:

The exact mapping between r0052 (status word 1) bits and the hex value entered into P0730–P0733 is documented in the MM440 parameter list. The relevant bits are:

• Bit 0 (52.0): Drive ready
• Bit 1 (52.1): Drive ready to run
• Bit 2 (52.2): Drive running
• Bit 3 (52.3): Drive fault active
• Bit 4 (52.4): OFF2 active
• Bit 5 (52.5): OFF3 active
• Bit 6 (52.6): ON inhibit active
• Bit 7 (52.7): Drive running (legacy, duplicates bit 2 in some FW versions)

Parameter Reference for the 30 kW Fan Application

Commissioning Procedure

1. Verify nameplate data. Read motor nameplate and confirm P0304, P0305, P0307, P0308, P0310, P0311 match exactly. Mismatches in P0311 (rated speed) are the most common cause of poor flying restart performance.
2. Run motor data identification. Set P1910 = 1, issue an ON command with the motor uncoupled if possible (or with the fan blades isolated), and allow the drive to perform the static + rotating autotune. P1910 resets to 0 when complete. Record P0350 (stator resistance), P0354 (rotor resistance), P0356 (leakage reactance) and P0358 (magnetising reactance) for the maintenance file.
3. Set flying restart. Configure P1200 = 1, P1202 = 100, P1203 = 100. Save with P0971 = 1.
4. Reassign digital outputs. Move the "drive running" function from DO3 to DO1 or DO2. Confirm wiring by measuring voltage at the terminal with the drive in run state. The MM440 DO outputs are open-collector, rated 24 V DC / 50 mA; an external pull-up or sinking input is required.
5. Test with the fan stopped. Issue a run command from the DCS. The motor should accelerate along P1120 to 30 Hz. Verify current does not exceed P0305 × P0640 / 100.
6. Test with the fan windmilling in the reverse direction. With the fan coasting at ~580 r/min in the reverse direction, issue a run command. Observe the drive for the search sequence (audible frequency sweep from P1082 down to 0 Hz). The drive should detect the reverse rotation, resynchronise, then ramp to the setpoint. Typical search time is 1.5–3 seconds.
7. Test with the fan windmilling in the forward direction. Repeat step 6 with the fan being driven in the forward direction by an adjacent fan. Confirm that the drive still starts reliably.
8. Verify DCS feedback. With the drive in run state, measure the DO1 output state and confirm the DCS sees a "running" signal within 200 ms of the run command.

Verification Checklist

Troubleshooting Matrix

Hardware Considerations: Why DO3 Failed

The MM440 digital outputs are implemented as open-collector transistors with shared common at terminal 22. The absolute maximum ratings per the operating instructions are:

• Voltage: 30 V DC
• Current per output: 50 mA continuous
• Total current across DO1 + DO2 + DO3: 100 mA continuous

Common causes of premature DO failure include:

1. Overvoltage from an inductive load (relay coil, contactor) without a flyback diode across the coil.
2. Reverse polarity of the external 24 V supply at terminal 9 (DO common return).
3. Sustained overcurrent (e.g., short to ground on the field wiring).
4. Thermal cycling in high-ambient cabinets that fatigues the bond wires inside the I/O module.

If any DO output on the MM440 shows intermittent operation, the recommended field action is:

1. Remove the field wiring from the suspect terminal.
2. Verify the 24 V supply at terminal 9 is between 19.2 V and 28.8 V.
3. Connect a known-good load (24 V relay or 4.7 kΩ resistor to +24 V) directly to the terminal.
4. Force the output ON via P073x = 99.0 (forced active) and OFF via P073x = 99.1 (forced inactive). Measure the voltage drop across the output. A healthy output shows < 0.5 V in the ON state; a failing output shows > 2 V or floating voltage.

Replacement of the I/O module is the definitive repair. On the affected unit, the operator planned to replace the entire I/O board (Siemens part number 6SE6400-1PC00-0AA0 or equivalent, depending on the variant) and to verify that all inductive loads driven from the MM440 DO outputs have flyback protection.

Preventive Recommendations for Similar Fan Installations

• Configure P1200 = 1 on every fan drive in the array, regardless of whether the fan currently windmills, because adjacent fans or process conditions can change over time.
• Use the relay output (terminals 30/31/32/33) for hard-wired safety signals (fault, ready) and reserve the open-collector DO outputs for non-safety signalling to the DCS.
• Add a hardware interlock on the DCS side that uses both a digital input from the MM440 status word (via fieldbus) and a hard-wired contact from the relay output. The DCS start command should be latched on the rising edge of the relay contact and cleared on the falling edge, not on the absence of a signal.
• Install a mechanical anti-rotation device (backstop brake or ratchet) on fans that are known to windmill heavily. This decouples the aerodynamic root cause from the electrical restart requirement.
• Verify that the MM440 firmware version supports P1200 = 1 through 6 in the required way. All firmware versions from 1.0 onward support the function, but some early 1.x versions had a different mapping for P073x functions. Confirm the firmware on the BOP main screen before commissioning.

Safety Notes

Frequently Asked Questions