Siemens MIDIMASTER 1500 F002 Overcurrent on Multi-Motor Cold

Problem Overview

A Siemens MIDIMASTER Vector 1500/4 frequency inverter (15 kW, 575 V AC, 3-phase) drives six sets of four blower motors on a Tamglass tempering furnace. The drive ramps each set to set speed, transfers the load to the 575 V AC mains via a bypass contactor, then advances to the next set. Sets 1 through 5 are start-and-transfer; set 6 stays under VFD control for the full production run.

For three years the system ran 24/7 without fault. After the operator switched to daytime-only operation with overnight cooldown, the drive trips F002 (overcurrent) immediately on the cold-morning start command from the InTouch HMI. The fault clears only after cycling the input breaker. Once cleared, the drive restarts and runs normally for the rest of the shift. Motor insulation tests pass. Drive output terminals still measure 384 V AC with the run command removed, which indicates the output stage is not fully de-energized between attempts.

This article documents the multi-motor parallel-impedance root cause, the MIDIMASTER-specific parameter corrections (P077, P088, P089), the proper multi-motor sizing rule, and the DRIVEMON upload/download procedure required when replacing the unit.

System Topology and Operating Sequence

The blower bank is a sequential soft-start arrangement, not a continuous multi-motor parallel bus. Only one set of four motors is connected to the VFD output at any moment. The transfer to mains is a hard bypass at full speed; the VFD is then free to start the next set.

Because each set is independent, the failure mode is not steady-state parallel load sharing; it is the instant-of-energize transient when four stator windings in parallel demand magnetizing current plus locked-rotor current simultaneously.

Failure Mode: F002 Overcurrent on Cold Start

F002 on MIDIMASTER Eco / Vector is a hard instantaneous overcurrent trip sourced by the drive's internal IGBT module current sensors. It latches immediately and cannot be auto-reset through the standard P373/P374 auto-restart feature. The only recovery paths are a digital input programmed to the fault-acknowledge function, a STOP/RESET command, or a power cycle.

Two observations narrow the cause:

1. The trip occurs before the motors have rotated. Increasing P002 (ramp-up time) does not prevent it because the rotor never begins to turn. The trip is in the first 50–200 ms after the IGBTs close.
2. The fault only appears after a long cool-down. After the first successful start of the day, repeated start cycles succeed without fault.

These two signatures point to stator insulation leakage combined with DC-link residual charge, plus an aggressive flux model that overshoots magnetizing current on a cold stack of windings.

Root Cause Analysis

1. Parallel-motor impedance collapse

Four motors in parallel present one quarter of the single-motor locked-rotor impedance. The drive therefore sees a near-short at the first PWM edge. The actual locked-rotor current per phase approximates:

For a typical 4-pole 1.5 kW fan motor at 575 V, Z_motor at standstill is dominated by stator leakage reactance; per-motor starting current at 575 V typically lands in the 6–8× FLA range. With four motors the drive transient demand easily reaches 24–32× the FLA of one motor. If the VFD is rated for a single motor's FLA × 1.5, it has no headroom for the parallel-stack transient.

2. P077 = FCC control model amplifying the surge

MIDIMASTER Vector defaults P077 (Control Mode) to FCC (Flux Current Control). FCC is a sensorless vector strategy that measures stator resistance via P088 (Autocalibration) and writes the result into P089. With four dissimilar stators connected, P088/P089 produce a wrong resistance estimate, and FCC attempts to push a flux-establishing current pulse that is calibrated for one motor into four motors. The result is an overshoot of magnetizing current on the very first output cycle.

Switching P077 = 0 disables both FCC and the closed-loop vector model (P077 = 3) and reverts to linear V/f. Linear V/f is the only suitable strategy for multi-motor stacks on a fixed-ratio drive because it does not try to identify a "motor" that does not exist as a single entity.

3. P088 = 1 (autocalibration enabled) wrong for parallel motors

P088 = 1 (on) triggers a measurement of stator resistance on the first run command after power-up. With multiple stators the measurement cannot resolve correctly. Set P088 = 0 to disable autocalibration permanently on multi-motor applications.

4. Stator insulation leakage when cold

575 V class insulation in older blower motors can develop low-grade absorption currents when cold. After a thermal soak the windings are dry and warm and the leakage is negligible. After an overnight cool-down, moisture absorption plus thermal contraction produce a brief inrush of leakage current on top of the magnetizing transient. A 500 V or 1000 V megohmmeter test may show acceptable values, but the VFD's IGBT sensors detect the sub-cycle spike and trip F002.

5. Output not fully de-energized between attempts

The 384 V AC residual measured on the output terminals with no run command is abnormal. After a STOP command the MIDIMASTER ramps the output to 0 Hz and then enters "ready" state, but the output stage should be in high-impedance. Persistent voltage indicates either:

• A stuck output contactor / bypass interlock back-feeding the drive output.
• A DC bus pre-charge circuit not discharging through the normal bleed path.
• A failed IGBT module short-circuiting one phase to DC+.

Any of these will load the next start command with additional stress.

6. Drive undersizing for the application

The MIDIMASTER 1500/4 is rated 15 kW at 575 V, which corresponds to roughly 24 A continuous. Four motors of 1.5 kW (≈ 2 A each at 575 V) draw 8 A running, well within rating. The 24 A figure is not the issue; the issue is the peak: a 15 kW drive does not have the short-time overload margin to absorb a 4× locked-rotor transient cleanly.

The field-proven sizing rule for multiple identical motors on one VFD is:

For four motors (N = 4):

The VFD must be rated for at least 130% of the summed nameplate FLA. If the original selection was "drive kW = sum of motor kW", it sits at 100% and has zero margin for transient inrush or aging motors.

Drive Sizing Calculation

Worked example for the Tamglass blower bank:

The continuous current margin is fine. The deficit is in short-time overload capability. A 15 kW drive at 575 V typically offers 150% for 60 s, which is insufficient to ride through the four-motor magnetizing transient when stators are cold. Options to recover overload margin:

1. Step up to a 22 kW or 30 kW MIDIMASTER (Micromaster 440 or SINAMICS G120 successor platform) and derate the current parameter to 24 A to retain the bypass sequence.
2. Add a 5% line reactor on the drive output. This adds 5% impedance between the IGBTs and the motor stack, raising the seen impedance by 5%, which is typically enough to push the cold-start transient below the F002 trip threshold without changing the running voltage by more than 5%.
3. Reduce the drive's maximum current clamp (P077 in linear V/f interacts with current-limit parameters) so the IGBTs cannot command more than the per-motor FLA × 4.

Diagnostic Procedure

Before changing parameters, gather the data below so that any change is verifiable.

1. Read the actual parameter set with DRIVEMON. Export to a .dnl file. Note the firmware version reported on the drive's nameplate sticker versus the parameter file header — a mismatch causes the "versions of the device configured and the device identified do not match" error during download.
2. Verify P077, P088, and P089 readings. If P089 shows an unusually low or unusually high stator resistance (compare to a single-motor benchmark), autocalibration has corrupted the value.
3. With the drive stopped and isolated, measure output voltage phase-to-phase and phase-to-ground with a true-RMS multimeter. Anything above 5 V indicates a stuck contactor, failed IGBT, or pre-charge bleed fault.
4. Megger each motor winding to ground at 1000 V. Reading should be > 100 MΩ hot; cold readings < 5 MΩ indicate moisture absorption and require bake-out or space-heater retrofit.
5. Record the DC-link voltage at the test points after a STOP command. Should bleed to < 50 V within 5 minutes per the operating-instructions safety note.
6. Capture the F002 trip current value (r0054 or equivalent on the parameter set) the moment the fault occurs using the drive's fault buffer.

Corrective Actions

A. Parameter corrections on the existing drive

1. Set P077 = 0 (linear V/f). This removes the FCC flux model and the closed-loop vector option. Linear V/f does not try to identify a single motor's stator resistance.
2. Set P088 = 0 (autocalibration off). Prevents the drive from re-measuring the wrong stator resistance on the next power-up.
3. Verify P089 is no longer being overwritten by autocalibration. If the value looks suspect, write a nominal value manually based on the per-motor nameplate.
4. Reduce the starting voltage boost parameters (P078 / P079 range) until the cold start no longer trips F002. Increasing voltage boost worsens the cold-start transient because it raises the first-cycle voltage step.
5. Set ramp shape to S-curve if available (P002 / P003 ramp profile). The S-curve delivers lower dI/dt at the first PWM edge compared to linear ramp, giving the rotors time to begin rotating before full voltage is applied.
6. Configure an S-curve accel of at least 30 s with rounded initial and final 20% segments to relieve the magnetizing transient.

B. Output-stage residual voltage fault

1. With drive isolated and locked-out, measure DC-link voltage at the test points. If > 50 V after 5 min, the internal bleeder is open; the unit must be replaced or sent for repair.
2. Disconnect the bypass contactor output leads and re-measure drive output voltage at no load. If 384 V persists, the IGBT module has failed short. Replace the drive.
3. If the residual disappears with the contactor leads disconnected, the contactor is back-feeding the drive through the motor windings as generators. Check that the bypass contactor is electrically and mechanically interlocked with the drive run command.

C. Sizing upgrade or output reactor

1. If parameter changes alone do not clear F002 on cold start, the drive's short-time overload is genuinely insufficient. The minimum-cost fix is a 5% motor-side reactor rated for the drive's continuous current. Reactor impedance raises the seen impedance at the first PWM edge and clips the cold-start peak.
2. If a reactor is impractical, replace the drive with a unit one frame size larger (22 kW class). Confirm the new unit supports the same parameter map or migrate to a successor (SINAMICS G120) with the appropriate parameter transfer tool.

D. Replacement drive parameter loading procedure

The Siemens Tech Support workflow for the "versions of the device configured and the device identified do not match" error on DRIVEMON:

1. Upgrade DRIVEMON to v5.3 (current revision). v5.2 cannot download parameter files created on newer firmware revisions.
2. Connect to the new drive via the RS-485 port on the front of the unit. Use the standard USS protocol (P700–P704 must match the new drive's defaults before DRIVEMON will handshake).
3. Read the existing parameter set from the new drive first; do not attempt to push the old file blindly.
4. Diff the parameter sets and manually key the values that differ into the new drive. For most multi-motor applications this is < 20 parameters.
5. Verify P077 = 0 and P088 = 0 on the new drive before any run command.

Reference: MICROMASTER Eco & MIDIMASTER Eco Reference Manual.

Verification

1. After applying P077 = 0 and P088 = 0, perform three consecutive cold starts with at least 4 hours cooldown between each. F002 must not trip.
2. Monitor r0054 (output current) on the first start. Peak should not exceed 1.5× the drive's continuous rating. If it does, increase the output reactor or step up the drive frame.
3. After the third successful start, repeat the output voltage measurement with the drive stopped. Terminals must show < 5 V phase-to-phase and phase-to-ground.
4. Capture the parameter set with DRIVEMON and store as the new baseline file. Label with the drive serial number and date.

Fault Code Reference

Field-Commissioning Notes

• Linear V/f (P077 = 0) gives up torque accuracy below 5 Hz. Multi-motor blower stacks typically operate above 20 Hz, so the loss is irrelevant.
• If set 6 must run continuously for hours, monitor drive temperature. The MIDIMASTER 1500/4 is rated 24 A continuous at 575 V with the stock fan; if ambient exceeds 40 °C, derate to 20 A or add cabinet ventilation.
• The bypass contactor must be mechanically and electrically interlocked with the drive's run permissive output. Without this interlock, the 384 V back-feed observed on the original incident will recur.
• When migrating to a SINAMICS G120 replacement, the equivalent parameter for P077 is P1300 (control mode) and for P088 is P1900 (motor data identification). Set P1300 = 0 (linear V/f) and P1900 = 0 (identification disabled) for multi-motor service.