SMC Flex (150-F) soft starter fault codes: reading the LCD and fault buffer, Shorted SCR, Line Loss and fixes

An SMC Flex soft starter (Bulletin 150-F) shows a fault on its built-in three-line, 16-character LCD: the first line says the unit has faulted, the second gives the fault number and the third the fault text, for example "Faulted / Fault #1 / Line Loss A" (150-UM008J-EN-P, p. 99). The unit trips and stops on every fault in the fault definitions table (p. 101). The full list is Table 42, "Fault Code Cross-reference" (p. 100), and the causes and fixes are in Table 44, "SMC Fault Display Explanation" (pp. 104–105).

Read the display first

• Fault on the LCD: stays on screen as long as control power is applied. Pressing Esc moves you to other menus, but the controller stays faulted (p. 99).
• Alarm: many protections have an alarm (pre-fault) level as well as a fault level, for example undervoltage, overvoltage, unbalance and jam. An alarm shows on the LCD, the HIM, over communication if used, and on an alarm contact (pp. 19–20). The "Diagnostics" menu holds "Alarms", "Faults" and "Device Revision" (p. 78).
• Auxiliary contacts: the four outputs Aux #1 to Aux #4 can be programmed as Fault or Alarm, N.O. or N.C., so the PLC or a lamp can see the trip (pp. 25, 101).
• Display blank, "Stopped 0.0 Amps" or "Starting" with no rotation: not a fault code; see the symptom tables (p. 105). If all three phases are missing when a start is given, the LCD shows "Starting" and the motor does not turn (p. 101).

Fault buffer

The controller keeps the five most recent faults. View them in the fault queue, or read Fault 1 (Parameter 124) to Fault 5 (Parameter 128); Fault 1 is the current fault and Fault 5 the oldest (p. 100). Write them down before cycling control power, because cycling control power clears the active fault (p. 99).

Common fault codes

Phase loss, overvoltage and undervoltage protection are off during braking; jam and underload are off during slow speed and braking (p. 101). Jam and underload only work when Motor FLC is programmed (p. 86).

Checks, step by step

1. Record before resetting: the LCD text, Fault 1 to Fault 5 (Parameters 124–128) and what the machine was doing.
2. Supply faults (1–3, 35–37, 41–43): line and load connections, fuses, line leads; for 41–43 also line distortion and high-impedance connections on the line or motor side (pp. 104–105).
3. Shorted SCR (4–6): with main power disconnected and the wires marked, measure line to load on each phase (L1-T1, L2-T2, L3-T3) with an ohmmeter; the reading should be above 5,000 Ω (p. 106).
4. Check Line Load (energized test): only authorized, electrically qualified persons, with power applied, no start command and no motor connected. Measure L1…T1, L2…T2 and L3…T3: on a 480 V system each reads 277 V; if phase A is shorted the readings are 0, 480, 480. For 600 V: 347 V, or 0, 600, 600 (p. 107).
5. Temperature (10–12): controller or motor ventilation, duty cycle, fan running, ambient; for Motor PTC, the PTC resistance. The PTC input trips above 3400 Ω ± 150 Ω and also on a short circuit below 25 Ω ± 10 Ω (pp. 22–23, 104).
6. Load protections (22–25): overload class and Motor FLC against the nameplate, then the machine: belts, pump, binding (pp. 104–105).
7. Ground Fault (33): needs an external core balance sensor, for example 825-CBCT on 5…480 A units, and the level (Parameter 73), delay (74) and inhibit time (75) set for the application (pp. 21–22, 104).
8. Communication (27–32): the cable at each DPI port. Control through DPI is disabled by factory default; to give control, set the Logic Mask in the Communication group (p. 101).

Clearing faults safely

Correct the cause first: resetting does not fix it (p. 99). The manual lists these ways to clear a fault (p. 99):

• "Clear Fault" in Main Menu / Diagnostics / Faults.
• Hold Esc on the control module keypad for 3 seconds (firmware 6.001 or later).
• Press Stop on a connected HIM. A stop from the HIM always stops the motor and clears the fault, regardless of Logic Mask.
• A N.O. reset push button on Option Input #2 (terminal 15) programmed for Clear Fault.
• Cycle control power.

An overload fault cannot be reset until Motor Thermal Usage (Parameter 12) is below 75 % (p. 99). Overload Reset (Parameter 47) can be Manual or Auto (p. 17), and Figure 17 gives an auto reset time for each trip class (p. 18). Before choosing Auto, check what the start circuit does once the fault resets itself (the manual does not describe this), and follow the plant's permit and lockout rules.

The manual's hazard notes: hazardous voltage is present in the motor circuit even when the controller is off, so disconnect main power before working on the controller, motor or control devices (p. 103). Hazardous voltage is present on L1, L2, L3, T1, T2, T3, T4, T5 and T6 (p. 28). Disconnect the controller from the motor before an insulation resistance test of the windings; IR test voltage can destroy the SCRs, and no megger measurement should be made on the controller (p. 103).

When it is hardware

• Shorted SCR test below 5,000 Ω, or Shorted SCR that does not clear: replace the power module (pp. 104, 106). A power pole includes the SCRs and the bypass contactor (p. 109).
• Open Gate after the resistance check, and Open Bypass from an inoperable bypass: replace the power module (p. 104).
• SCR Overtemp or PTC Pwr Pole with a failed thermistor: power module or control module (p. 104).
• Hall ID with the wrong power module installed (p. 105).
• NVS Error that returns after loading defaults, V24 or V Control faults, and system faults 128…209: replace the control module (p. 105).

Renewal part numbers for control modules and power poles are in Appendix A (p. 109).

Before calling for support

Note the catalog number, the firmware in "Device Revision" (p. 78), the five faults in the buffer, the parameter values, and readings from the checks above. Where an SMC Flex feeds a two-speed motor or several motors, the manual calls for separate overload relays for each speed, or individual overload protection for each motor (p. 32); if that relay is an E300, see E300 trip and warning codes. Need an engineer on site: breakdown support.