Troubleshooting / Control Techniques trips

Control Techniques Trip Codes

The trip codes of Control Techniques drives, many still badged Emerson: Commander SK and the Unidrive M200 family. What each trip means, where the two generations differ, the likely causes, the first checks in order, and how to log it.

How trips show on the drive

When a Control Techniques drive trips, its output switches off, the motor coasts to a stop and the keypad shows the trip string, for example OI.AC.

On a Commander SK the left display shows the tripped status and the trip code sits in the right hand display, on an M200, M201, M300, C200 or C300 any sub-trip number flashes alternately with the trip string, and the M400 keypad spells the trip out as text such as Motor Too Hot.

The last ten trips are stored in Pr 10.020 (newest) to Pr 10.029 (oldest) on the Unidrive M and Commander C drives, with sub-trip numbers in Pr 10.070 to Pr 10.079, and in Pr 10.20 to Pr 10.29 on the Commander SK, where Pr 55 to Pr 58 in the basic menu also show the last four.

An alarm is only a warning: the alarm text (OV.Ld, d.OV.Ld or br.res on the M200 family, OVL.d, hot or br.rS on the Commander SK) alternates or flashes on the display while the drive keeps running, and if nothing is done the drive may go on to trip.

To reset, clear the cause and press the red STOP/RESET key, or open and close the enable/reset input (terminal B4 on a Commander SK), or reset over serial comms (Pr 10.038 set to 100 on the M200).

OI.AC and OI.br cannot be reset for 10 seconds, HF hardware faults need a power cycle, and automatic reset is set in Pr 10.034 and Pr 10.035 (Pr 10.34 and Pr 10.35 on the Commander SK, which never auto resets UV, Et, EEF or HF trips).

Control Techniques belonged to Emerson, trading in the US as Emerson Industrial Automation, until Nidec completed its purchase in January 2017, so older drives, labels and manuals carry the Emerson name while newer ones say Nidec Control Techniques, and the trip codes below apply to both.

Safety first.
Control Techniques warns that the drive contains capacitors that stay charged to a potentially lethal voltage after the AC supply is disconnected, so once a drive has been energized the supply must be isolated for at least ten minutes before work continues.

Disconnect the supply with an approved electrical isolation device before touching any electrical connection, because the STOP key and the Safe Torque Off function do not isolate dangerous voltages from the drive output.

Do not attempt internal repairs or fault finding beyond the drive's own diagnostic features; a faulty drive goes back to the supplier or an authorized Control Techniques distributor. Follow lockout/tagout and your site's safe-work procedures, and check the manual for your exact model before you act.

Quick lookup

Trips disable the drive output and the motor coasts to a stop, so in OEE every trip is an availability loss; the three alarms let the drive keep running until you act.

Type a trip or a word (over current, over voltage, motor overload, thermistor) to filter the list. The trip number is what the trip log keeps (Pr 10.020 to 10.029 on the M200 family, Pr 10.20 to 10.29 on Commander SK) and what the M400 text keypad goes by; warnings show as Alarm.

54 codes

CodeTrip no.Name
OI.AC3Instantaneous output over current
OI.br4Braking transistor over current
OV2DC bus over voltage
UV1DC bus under voltage
PSU5Internal power supply fault
Et6External trip
O.SPd7Over speed (over frequency)
U.OI8User over current trip
th.br10Braking resistor over temperature
tun.111Autotune: required speed not reached
tun.313Autotune: measured inertia out of range
tun.S18Autotune stopped before completion
It.br19Braking resistor overload (I2t)
It.Ac20Motor overload (I2t)
Oht.I21Inverter (IGBT) over temperature, thermal model
Oht.P22Power stage over temperature
th24Motor thermistor over temperature
thS25Motor thermistor short circuit
O.Ld126Digital output or 24 V overload
Oh.dc27DC bus over temperature, thermal model
cL.A128Analog input 1 current loss
SCL30Comms loss or control word watchdog timeout
EEF31Internal memory (EEPROM) failure, defaults loaded
PH.Lo32Supply phase loss
rS33Stator resistance out of range
PAd34Keypad removed in keypad mode
CL.bt35Trip from the control word
U.S36User save error
OI.Sn92Rectifier snubber over current
Out.P98Output phase loss
FAn.F173Fan failure
OI.A1189Analog input 1 over current
dESt199Destination parameter clash
SL.nF203Option module removed
SL.dF204Different option module fitted
tH.Fb218Internal thermistor failed
Oht.C219Control stage over temperature
St.HF221Stored hardware fault
So.St226Soft start relay failure
OI.SC228Output phase short circuit
Sto234Safe Torque Off board not fitted
r.b.ht250Hot rectifier or braking transistor
HF01 to HF23No numberInternal hardware fault (Unidrive M, Commander C)
C.Acc185Parameter card (SD card or SmartStick) access failed
O.ht121IGBT over temperature, thermal model (Commander SK)
O.ht222Heatsink over temperature (Commander SK)
O.ht327Drive over heat, thermal model (Commander SK)
O.ht4102Rectifier over temperature (Commander SK)
cL128Analog input 1 current loss (Commander SK)
PH32Input phase loss or imbalance (Commander SK)
HF20 to HF32220 to 232Hardware faults (Commander SK)
OV.LdAlarmMotor overload warning
d.OV.LdAlarmDrive over temperature warning
br.resAlarmBraking resistor overload warning

Each code: causes and first checks

OI.AC: Instantaneous output over current

Hits OEE: Availability

What it means: The output current spiked above the drive's hardware limit, so the drive shut its output off at once to protect the power transistors. It cannot be reset until 10 seconds after the trip.[1][2][3][4][5][6]

Also on: M200, M201, M300, C200, C300: OI.AC, trip 3. M400: OI ac, trip 3. Commander SK: OI.AC, trip 3.

Likely causes

  • Acceleration or deceleration ramp too short for the load
  • Short circuit or earth fault on the motor cable or in the motor windings
  • Motor, motor cable or motor data changed without re-running the autotune
  • Speed (frequency) loop or current loop gains set too high
  • Motor cable longer than the limit for the drive frame size

First checks, in this order

  1. Wait 10 seconds, reset, and note whether it trips at start, during a ramp or at steady speed
  2. Test the motor and motor cable insulation with an insulation tester
  3. Lengthen the acceleration and deceleration ramps
  4. Re-run the autotune if the motor or wiring changed; if it trips during autotune, reduce the voltage boost
  5. If it trips at steady speed, reduce the frequency loop and current loop gains

OI.br: Braking transistor over current

Hits OEE: Availability

What it means: Too much current flowed through the braking transistor and braking resistor circuit, so the drive tripped to protect it. It cannot be reset until 10 seconds after the trip.[1][2][3][4][5][6]

Also on: M200, M201, M300, C200, C300: OI.br, trip 4. M400: OI Brake, trip 4. Commander SK: OI.br, trip 4.

Likely causes

  • Braking resistor value below the minimum allowed for the drive
  • Excessive braking current in the braking resistor
  • Short circuit or damaged insulation in the braking resistor or its wiring

First checks, in this order

  1. Check the braking resistor wiring for shorts
  2. Measure the resistor and confirm it is equal to or above the minimum resistance for the drive
  3. Test the braking resistor insulation

OV: DC bus over voltage

Hits OEE: Availability

What it means: The DC bus voltage inside the drive rose above its limit, usually because the motor pushed energy back into the drive while slowing down. On the M200 family it trips at once on the peak level, or after 15 seconds above the continuous level.[1][2][3][4][5][6]

Also on: M200, M201, M300, C200, C300: OV, trip 2. M400: Over Volts, trip 2. Commander SK: trip 2, printed OV in the Getting Started Guide and OU in the Advanced User Guide; it trips at 415 V DC on 200 V drives and 830 V DC on 400 V drives.

Likely causes

  • Deceleration ramp too fast for the inertia of the machine
  • Mechanical load driving the motor
  • Braking resistor value too high, or no braking resistor where one is needed
  • AC supply voltage too high, or supply disturbances that lift the DC bus
  • Motor insulation fault

First checks, in this order

  1. Note whether it trips while stopping; if so, lengthen the deceleration ramp
  2. Check the nominal AC supply level and look for supply disturbances
  3. If a braking resistor is fitted, check its value is low enough while staying above the minimum
  4. Test the motor insulation with an insulation tester

UV: DC bus under voltage

Hits OEE: Availability

What it means: The DC bus voltage is too low for the drive to run, so the output is held off. On the Commander SK this is trip 1 and it clears itself once the voltage recovers; on the Unidrive M and Commander C drives it is shown as a status (UV or Under Voltage) with the output disabled, not as a logged trip.[1][2][3][4][5][6]

Also on: Commander SK: trip 1, printed UV in the Getting Started Guide and UU in the Advanced User Guide; self resetting. M200, M201, M300, C200, C300: UV status. M400: Under Voltage status.

Likely causes

  • Low AC supply voltage or a supply dip
  • Supply switched off or interrupted upstream
  • Low DC bus voltage when the drive is fed from an external DC supply

First checks, in this order

  1. Measure the AC supply at the drive input terminals
  2. Check upstream fuses, breakers and contactors
  3. If the drive is fed from DC, check the level is above the drive's restart level
  4. On a Commander SK, remember UV is only stored in the trip log if it happened while the drive was running

PSU: Internal power supply fault

Hits OEE: Availability

What it means: One or more of the drive's internal power supply rails is out of limits or overloaded. The sub-trip shows whether the control side or the power side raised it.[1][2][3][4]

Also on: M200, M201, M300, C200, C300: PSU, trip 5. M400: PSU, trip 5.

Likely causes

  • An option module loading the internal supply
  • Hardware fault inside the drive

First checks, in this order

  1. Remove the option module and reset
  2. If the trip stays with no option fitted, return the drive to the supplier

Et: External trip

Hits OEE: Availability

What it means: Something outside the drive told it to trip, normally a terminal wired to a safety chain, thermal switch or PLC output that controls the External Trip parameter.[1][2][3][4][5]

Also on: M200, M201, M300, C200, C300: Et, trip 6 (sub-trip 3 means Pr 10.032 = 1). M400: External Trip, trip 6. Commander SK: Et, trip 6.

Likely causes

  • External Trip parameter set to 1 (Pr 10.032 on the M200 family, Pr 10.32 on the Commander SK) by a digital input
  • Open contact or broken wire in the external trip circuit; on a Commander SK this input is normally set up inverted, so the terminal must be made for the drive to run
  • Serial comms or a PLC writing Pr 10.032, or writing 6 to Pr 10.038

First checks, in this order

  1. Find which terminal or device controls the External Trip parameter (on the M200 family set Pr 00 to dest or 12001 to list destination parameters)
  2. Check the external contact and its wiring back to that terminal
  3. Make sure serial comms or a PLC is not writing the trip
  4. Reset by hand: a Commander SK will not auto reset an Et trip

O.SPd: Over speed (over frequency)

Hits OEE: Availability

What it means: The motor frequency went above the over frequency threshold, typically because the load drove the motor faster than commanded.[1][2][3][4][5][6]

Also on: M200, M201, M300, C200, C300: O.SPd, trip 7. M400: Over Speed, trip 7. Commander SK: O.SPd, trip 7, raised when output frequency passes 1.2 times maximum frequency while regenerating in current limit.

Likely causes

  • Mechanical load driving the motor
  • Frequency overshoot from a speed loop proportional gain set too high (RFC-A mode)
  • Over Frequency Threshold Pr 03.008 set low; at 0 it equals 1.2 times the maximum frequency in Pr 01.006

First checks, in this order

  1. Check whether the load can push the motor, for example when lowering or on a downhill run
  2. Check Pr 03.008 and the maximum frequency setting
  3. In RFC-A mode, reduce the frequency controller proportional gain Pr 03.010
  4. Reduce the current controller Ki gain Pr 04.014

U.OI: User over current trip

Hits OEE: Availability

What it means: The output current went above the trip level the user set in Pr 04.041.[1][2][3][4]

Also on: M200, M201, M300, C200, C300: U.OI, trip 8. M400: User OI ac, trip 8.

Likely causes

  • Output current above User Over Current Trip Level Pr 04.041
  • Pr 04.041 set lower than the current the process normally draws

First checks, in this order

  1. Compare Pr 04.041 with the real running current
  2. Check the machine for extra load at the moment of the trip

th.br: Braking resistor over temperature

Hits OEE: Availability

What it means: The hardware thermal monitor on the braking resistor reports that the resistor is too hot.[1][2][3][4]

Also on: M200, M201, M300, C200, C300: th.br, trip 10. M400: Th Brake Res, trip 10. Searches for br.th usually mean this trip.

Likely causes

  • Braking resistor overheating
  • Braking resistor or thermal monitor wiring fault
  • Braking resistor value below the minimum allowed
  • Trip left enabled on a drive that uses no braking resistor

First checks, in this order

  1. Check the braking resistor and its thermal monitor wiring
  2. Confirm the resistor value is equal to or above the minimum
  3. Test the resistor insulation
  4. If no braking resistor is used, disable the trip with bit 3 of Pr 10.037

tun.1: Autotune: required speed not reached

Hits OEE: Availability

What it means: During a rotating autotune or mechanical load measurement the motor did not reach the speed the test needed.[1][2][3][4]

Also on: M200, M201, M300, C200, C300: tun.1, trip 11 (sub-trip 2). M400: Autotune 1, trip 11.

Likely causes

  • Motor not free to turn, for example the mechanical brake is still applied
  • Mechanical Load Test Level Pr 05.021 set wrong

First checks, in this order

  1. Make sure the shaft is free and the brake is released
  2. Check Pr 05.021
  3. Run the autotune again

tun.3: Autotune: measured inertia out of range

Hits OEE: Availability

What it means: A rotating autotune or mechanical load test could not identify the motor inertia, or the measured value was outside the parameter range (RFC-A mode only).[1][2][3][4]

Also on: M200, M201, M300, C200, C300: tun.3, trip 13. M400: Autotune 3, trip 13.

Likely causes

  • Measured inertia exceeded the parameter range (sub-trip 1)
  • Load test unable to identify the motor inertia (sub-trip 3)
  • Motor cable wiring incorrect

First checks, in this order

  1. Read the sub-trip number
  2. Check the motor cable wiring is correct
  3. Run the test again

tun.S: Autotune stopped before completion

Hits OEE: Availability

What it means: The autotune was interrupted because the drive enable or the run command was removed before the test finished.[1][2][3][4][5][6]

Also on: M200, M201, M300, C200, C300: tun.S, trip 18. M400: Autotune Stopped, trip 18. Commander SK: tunE, trip 18.

Likely causes

  • Drive enable removed during the autotune
  • Run command removed during the autotune

First checks, in this order

  1. Keep the drive enable (terminal 11 on the M200) on for the whole test
  2. Keep the run command on until the test ends (check Pr 08.003 or Pr 08.004)
  3. Run the autotune again

It.br: Braking resistor overload (I2t)

Hits OEE: Availability

What it means: The drive's software model of braking resistor heating reached 100 percent, so it tripped to protect the resistor.[1][2][3][4][5][6]

Also on: M200, M201, M300, C200, C300: It.br, trip 19. M400: Brake R Too Hot, trip 19. Commander SK: It.br, trip 19.

Likely causes

  • Too much braking energy for the resistor
  • Resistor power, thermal time constant or resistance entered wrong (Pr 10.030, Pr 10.031, Pr 10.061)
  • Resistor value or power rating not suited to the duty

First checks, in this order

  1. Check Pr 10.030, Pr 10.031 and Pr 10.061 match the actual resistor
  2. Check the resistor value and power rating
  3. If an external thermal device already protects the resistor, the software trip can be disabled by setting one of those parameters to 0

It.Ac: Motor overload (I2t)

Hits OEE: Availability

What it means: The drive's motor thermal model, based on motor rated current and thermal time constant, reached 100 percent. This overload builds up over time; it is not a sudden spike like OI.AC.[1][2][3][4][5][6]

Also on: M200, M201, M300, C200, C300: It.Ac, trip 20. M400: Motor Too Hot, trip 20. Commander SK: It.AC, trip 20. Warning before it: OV.Ld on the M200 family, OVL.d on the Commander SK.

Likely causes

  • Load jammed or sticking
  • Load on the motor has increased
  • Motor rated current or thermal time constant set wrong (Pr 05.007, Pr 04.015)
  • High impedance phase to phase or phase to earth fault at the drive output
  • Drive needs re-autotuning to the motor

First checks, in this order

  1. Watch Pr 04.019 (motor temperature as a percentage of the trip level) while running
  2. Check the machine for jams, binding or extra load
  3. Check the motor rated current matches the nameplate and is not zero
  4. Re-run the autotune if the motor or wiring changed
  5. In RFC-A mode, tune the motor rated speed Pr 05.008

Oht.I: Inverter (IGBT) over temperature, thermal model

Hits OEE: Availability

What it means: The drive's software estimate of IGBT junction temperature reached the trip point, 145 degrees C on the M200, and it will not reset until it falls to 139 degrees C.[1][2][3][4]

Also on: M200, M201, M300, C200, C300: Oht.I, trip 21. M400: OHt Inverter, trip 21. Commander SK equivalent: O.ht1, trip 21. Searches for OHt.i or Oh.Inv usually mean this trip.

Likely causes

  • Switching frequency too high for the load
  • Automatic switching frequency reduction disabled (Pr 05.035)
  • Heavy duty cycle or motor load
  • Very short acceleration or deceleration ramps
  • Missing or unbalanced input phase, or high DC bus ripple

First checks, in this order

  1. Reduce the switching frequency
  2. Set Pr 05.035 to OFF so the drive can lower the switching frequency itself
  3. Reduce the duty cycle or motor load
  4. Lengthen the acceleration and deceleration ramps
  5. Check all three input phases are present and balanced

Oht.P: Power stage over temperature

Hits OEE: Availability

What it means: A thermistor in the power stage measured a temperature above the trip level for that drive size (95 degrees C on M200 frames 1 to 4). The sub-trip shows which thermistor.[1][2][3][4]

Also on: M200, M201, M300, C200, C300: Oht.P, trip 22. M400: OHt Power, trip 22. Nearest Commander SK trip: O.ht2, trip 22 (heatsink).

Likely causes

  • Drive or enclosure fans not working
  • Blocked ventilation paths or dirty enclosure door filters
  • High switching frequency, duty cycle or motor load
  • Very short acceleration or deceleration ramps
  • Drive undersized for the application

First checks, in this order

  1. Check the drive and enclosure fans are running
  2. Clean the door filters and clear the airflow paths
  3. Force the heatsink fans to full speed and increase ventilation
  4. Reduce the switching frequency or the load, or use an S ramp
  5. Check the derating tables and confirm the drive is sized for the job

th: Motor thermistor over temperature

Hits OEE: Availability

What it means: The thermistor built into the motor windings reports that the motor is too hot.[1][2][3][4][5][6]

Also on: M200, M201, M300, C200, C300: th, trip 24 (terminal 14, digital input 5). M400: Thermistor, trip 24. Commander SK: th, trip 24 (terminal B7, trips at 3 kohm and resets at 1.8 kohm).

Likely causes

  • Excessive motor temperature
  • Thermistor trip threshold set low (Pr 07.048 on the M200 family)
  • Break in the thermistor circuit

First checks, in this order

  1. Check the real motor temperature and its cooling
  2. Check thermistor continuity and wiring (terminal 14 on the M200, terminal B7 on the Commander SK)
  3. Check the threshold in Pr 07.048
  4. Let the motor cool before resetting

thS: Motor thermistor short circuit

Hits OEE: Availability

What it means: The motor thermistor input reads a short circuit or very low resistance (under 50 ohm).[1][2][3][4][5]

Also on: M200, M201, M300, C200, C300: thS, trip 25. M400: Th Short Circuit, trip 25. The Commander SK does not trip on a shorted thermistor.

Likely causes

  • Short circuit in the thermistor wiring
  • Failed motor thermistor

First checks, in this order

  1. Check thermistor continuity and wiring
  2. Replace the motor thermistor, or the motor, if the sensor has failed

O.Ld1: Digital output or 24 V overload

Hits OEE: Availability

What it means: Too much current is being drawn from the drive's 24 V supply or its digital outputs.[1][2][3][4][5][6]

Also on: M200, M201, M300, C200, C300: O.Ld1, trip 26. M400: I/O Overload, trip 26. Commander SK: O.Ld1, trip 26.

Likely causes

  • Excessive load or short circuit on the +24 V output
  • Total load on the digital outputs too high
  • Damaged control wiring
  • AI Adaptor 24 V load too high (M200 sub-trip 2)

First checks, in this order

  1. Add up the loads on the 24 V supply and the digital outputs
  2. Check the control wiring for shorts and damage
  3. On a Commander SK reset with the STOP/RESET key, because the enable/reset terminal will not clear this trip

Oh.dc: DC bus over temperature, thermal model

Hits OEE: Availability

What it means: The drive's model of DC bus component heating, which includes output current and DC bus ripple, reached 100 percent (shown in Pr 07.035). The drive tries to stop the motor first and trips at once if the motor has not stopped within 10 seconds.[1][2][3][4]

Also on: M200, M201, M300, C200, C300: Oh.dc, trip 27. M400: OHt dc bus, trip 27. Commander SK equivalent: O.ht3, trip 27. Searches for OHt.dc usually mean this trip.

Likely causes

  • Unbalanced or low AC supply
  • High DC bus ripple
  • Heavy duty cycle or motor load
  • Unstable output current from motor settings that do not match the motor

First checks, in this order

  1. Check the AC supply voltage level and balance
  2. Check the DC bus ripple level
  3. Reduce the duty cycle or motor load
  4. If the output current is unstable, check the motor map against the nameplate and do a rotating autotune with the load disconnected

cL.A1: Analog input 1 current loss

Hits OEE: Availability

What it means: Analog input 1 is set to 4-20 mA or 20-4 mA and the signal fell below 3 mA, so the drive treats it as a lost signal.[1][2][3][4][5][6]

Also on: M200, M201, M300, C200, C300: cL.A1, trip 28. M400: An Input 1 Loss, trip 28. Commander SK: cL1, trip 28.

Likely causes

  • Broken or disconnected current signal wiring
  • Transmitter or PLC output not supplying current
  • Analog Input 1 Mode Pr 07.007 not matching the signal

First checks, in this order

  1. Measure the current at terminal 2; it must be above 3 mA
  2. Check the control wiring is correct and undamaged
  3. Check Pr 07.007

SCL: Comms loss or control word watchdog timeout

Hits OEE: Availability

What it means: The drive stopped hearing from whatever controls it. On the Unidrive M and Commander C drives the control word watchdog was not refreshed within 1 second; on the Commander SK the serial link to a remote keypad or controller timed out.[1][2][3][4][5][6]

Also on: M200, M201, M300, C200, C300: SCL, trip 30. M400: Watchdog, trip 30. Commander SK: SCL, trip 30.

Likely causes

  • Control word watchdog enabled (Pr 06.042 bit 14) but not serviced at least once a second
  • Loss of communication while the drive is under remote control
  • Remote keypad disconnected from the drive's comms port (Commander SK)

First checks, in this order

  1. Check the comms cable and the remote keypad connection
  2. Check the PLC or master is still toggling the watchdog bit
  3. Re-enable the watchdog after the reset, because the trip disables it
  4. On a Commander SK, power cycle the drive after an SCL from the keypad

EEF: Internal memory (EEPROM) failure, defaults loaded

Hits OEE: Availability

What it means: The parameters saved in the drive's memory could not be used, so the drive loaded default settings and your set-up may be gone.[1][2][3][4][5][6]

Also on: M200, M201, M300, C200, C300: EEF, trip 31. M400: EEPROM Fail, trip 31. Commander SK: EEF, trip 31; cannot be reset until defaults are loaded and never auto resets.

Likely causes

  • Power removed while parameters were being saved
  • Corrupted parameter data in the drive's memory
  • Drive firmware image, power stage or control board changed (read the sub-trip on the M200 family)

First checks, in this order

  1. Note the sub-trip number
  2. Load defaults (Pr 00 = 1233 or 1244 on the M200 family, Pr 29 on the Commander SK), then reset
  3. Re-enter the parameters or reload them from a backup card or SmartStick
  4. Give the drive time to finish a save before switching it off
  5. If the trip keeps coming back, return the drive to the supplier

PH.Lo: Supply phase loss

Hits OEE: Availability

What it means: The drive sees too much ripple on its DC bus, which points to a missing input phase or a large supply imbalance. It tries to stop the motor before tripping.[1][2][3][4][5][6]

Also on: M200, M201, M300, C200, C300: PH.Lo, trip 32. M400: Phase Loss, trip 32. Commander SK: PH, trip 32. Searches for PH.Ac usually mean this trip.

Likely causes

  • Input phase lost, for example a blown fuse or loose supply terminal
  • Large supply imbalance or high supply impedance
  • Severe output current instability or mechanical resonance with the load
  • Drive run from DC or single phase with phase loss detection still on

First checks, in this order

  1. Measure the AC supply voltage balance and level at full load
  2. Check upstream fuses, breakers and terminals
  3. Check the DC bus ripple with an isolated oscilloscope
  4. Reduce the duty cycle or motor load
  5. For DC or single phase supply, set the detection mode in Pr 06.047

rS: Stator resistance out of range

Hits OEE: Availability

What it means: During autotune, or at start in some open loop modes, the drive measured a motor stator resistance outside the range it can work with.[1][2][3][4][5][6]

Also on: M200, M201, M300, C200, C300: rS, trip 33. M400: Resistance, trip 33. Commander SK: rS, trip 33.

Likely causes

  • Motor much smaller than the drive; drive to motor size ratios above 15 to 1 are likely to cause this
  • Motor cable disconnected during the measurement
  • Damaged motor winding or connection
  • Out of range value typed into Stator Resistance Pr 05.017

First checks, in this order

  1. Check the motor cable and connections at both ends
  2. Measure phase to phase resistance at the drive terminals and at the motor terminals
  3. Test the stator winding with an insulation tester
  4. Check Pr 05.017 was not set out of range by hand
  5. Confirm the motor suits the drive rating

PAd: Keypad removed in keypad mode

Hits OEE: Availability

What it means: The drive takes its speed reference from the keypad and the keypad has been removed or disconnected.[1][2][3][4]

Also on: M200, M201, M300, C200, C300: PAd, trip 34. M400: Keypad Mode, trip 34.

Likely causes

  • Keypad unplugged or disconnected
  • Reference Selector Pr 01.014 set to keypad (4 or 6)

First checks, in this order

  1. Refit the keypad and reset
  2. If keypad control is not wanted, change Pr 01.014 to another reference source

CL.bt: Trip from the control word

Hits OEE: Availability

What it means: A PLC or fieldbus set bit 12 of the control word (Pr 06.042) while the control word is enabled, which commands the drive to trip.[1][2][3][4][5]

Also on: M200, M201, M300, C200, C300: CL.bt, trip 35. M400: Control Word, trip 35. Commander SK: CL.bt, trip 35.

Likely causes

  • Controller set control word bit 12
  • Control word enabled (Pr 06.043 = On) with unexpected data

First checks, in this order

  1. Check the value in Pr 06.042
  2. Check the PLC logic that writes the control word
  3. Clear bit 12 to reset, or disable the control word in Pr 06.043 if it is not used

U.S: User save error

Hits OEE: Availability

What it means: The last user parameter save in memory is corrupt, often because power was removed during the save. The drive falls back to the last good save.[1][2][3][4]

Also on: M200, M201, M300, C200, C300: U.S, trip 36. M400: User Save, trip 36. The related Pd.S (trip 37) is a power down save error.

Likely causes

  • Power removed while a user save was running
  • Error in the user save data in non-volatile memory

First checks, in this order

  1. Do a user save (Pr 00 = Save or 1001, then press reset)
  2. Let the save finish before switching the drive off

OI.Sn: Rectifier snubber over current

Hits OEE: Availability

What it means: The snubber circuit on the input rectifier saw over current, usually from supply disturbances, long motor cables or insulation problems.[1][2][3][4]

Also on: M200, M201, M300, C200, C300: OI.Sn, trip 92. M400: OI Snubber, trip 92. Searches for OI.SnS usually mean this trip.

Likely causes

  • Internal EMC filter not installed
  • Motor cable longer than the maximum for the selected switching frequency
  • Supply voltage imbalance
  • Supply disturbance such as notching from a DC drive
  • Motor or motor cable insulation fault

First checks, in this order

  1. Check the internal EMC filter is installed
  2. Check the motor cable length against the switching frequency limit
  3. Check the supply for imbalance and disturbances
  4. Test the motor and cable with an insulation tester
  5. Consider an output line reactor or sinusoidal filter

Out.P: Output phase loss

Hits OEE: Availability

What it means: The drive found one of its output phases U, V or W disconnected from the motor.[1][2][3][4]

Also on: M200, M201, M300, C200, C300: Out.P, trip 98. M400: Out Phase Loss, trip 98.

Likely causes

  • U, V or W phase disconnected when the drive was enabled (sub-trips 1, 2, 3)
  • A phase disconnected while running above 4 Hz (sub-trip 4)

First checks, in this order

  1. Read the sub-trip to see which phase
  2. Check the motor and drive connections
  3. If the check is not wanted, set Output Phase Loss Detection Enable Pr 06.059 to 0

FAn.F: Fan failure

Hits OEE: Availability

What it means: The drive's cooling fan has failed. The trip cannot be reset until 10 seconds after it occurred.[1][2][3][4][5]

Also on: M200, M201, M300, C200, C300: FAn.F, trip 173. M400: Fan Fail, trip 173. On a Commander SK fan faults show as HF29 or HF31.

Likely causes

  • Fan not fitted or not connected correctly
  • Fan obstructed
  • Failed fan

First checks, in this order

  1. Check the fan is fitted and plugged in
  2. Clear anything blocking the fan
  3. Contact the supplier to replace the fan

OI.A1: Analog input 1 over current

Hits OEE: Availability

What it means: The current flowing into analog input 1 went above the limit of the input.[1][2][3][4][5][6]

Also on: M200, M201, M300, C200, C300: OI.A1, trip 189. M400: An Input 1 OI, trip 189. Commander SK: O.cL, trip 189.

Likely causes

  • Current input on analog input 1 above 24 mA
  • On a Commander SK, current loop input above 25 mA

First checks, in this order

  1. Measure the current into analog input 1
  2. Check the transmitter and its wiring
  3. Check the analog input mode matches the signal

dESt: Destination parameter clash

Hits OEE: Availability

What it means: Two or more drive functions are set to write to the same parameter.[1][2][3][4][5]

Also on: M200, M201, M300, C200, C300: dESt, trip 199 (menus 7, 8, 9, 12 or 14). M400: Destination, trip 199. Commander SK: dESt, trip 199.

Likely causes

  • Two analog or digital I/O functions routed to the same destination parameter
  • Two logic, threshold, variable selector or PID functions writing the same parameter

First checks, in this order

  1. Set Pr 00 to dest (12001) so only destination parameters are shown
  2. Find the two that point at the same parameter and change one
  3. Save and reset

SL.nF: Option module removed

Hits OEE: Availability

What it means: The drive remembers an option module in slot 1 that is no longer there. The sub-trip gives the ID of the missing module.[1][2][3][4][5]

Also on: M200, M201, M300, C200, C300: SL.nF, trip 203. M400: Slot 1 Not Fitted, trip 203. Commander SK: SL.nF, trip 203 (Solutions Module).

Likely causes

  • Option module removed since the last power up
  • Module not seated correctly

First checks, in this order

  1. Refit the option module and check it is seated
  2. If the module is no longer needed, save parameters in Pr 00 to accept its removal

SL.dF: Different option module fitted

Hits OEE: Availability

What it means: The module in option slot 1 is a different type from the one fitted when parameters were last saved.[1][2][3][4][5]

Also on: M200, M201, M300, C200, C300: SL.dF, trip 204. M400: Slot 1 Different, trip 204. Commander SK: SL.dF, trip 204.

Likely causes

  • Option module swapped for a different type
  • Module fitted where none was before (sub-trip 1)
  • Same module type but its menu changed, so defaults were loaded for that menu

First checks, in this order

  1. Power off and fit the correct module
  2. If the new module is intended, set its parameters and do a user save in Pr 00

tH.Fb: Internal thermistor failed

Hits OEE: Availability

What it means: A temperature sensor inside the drive reads open circuit or short circuit. This is in the group of trips that cannot be reset.[1][2][3][4][5]

Also on: M200, M201, M300, C200, C300: tH.Fb, trip 218. M400: Temp Feedback, trip 218. Commander SK equivalent: HF27 or HF28.

Likely causes

  • Failed internal thermistor (the sub-trip shows its location)
  • Internal hardware fault

First checks, in this order

  1. Note the sub-trip number
  2. Contact the supplier of the drive, this is a hardware fault

Oht.C: Control stage over temperature

Hits OEE: Availability

What it means: The control section of the drive is too hot while Cooling Fan Control Pr 06.045 is set to 0.[1][2][3][4]

Also on: M200, M201, M300, C200, C300: Oht.C, trip 219. M400: OHt Control, trip 219.

Likely causes

  • Cooling Fan Control Pr 06.045 set to 0
  • Not enough ventilation for the control stage

First checks, in this order

  1. Set Pr 06.045 above 0 so the fan helps cool the drive
  2. Check airflow around the drive

St.HF: Stored hardware fault

Hits OEE: Availability

What it means: An HF hardware trip happened, the drive was power cycled, and it now reports that stored fault. The sub-trip is the original HF number.[1][2][3][4]

Also on: M200, M201, M300, C200, C300: St.HF, trip 221. M400: Stored HF, trip 221.

Likely causes

  • Hardware trip HF01 to HF18 before the last power down
  • Possible control board failure

First checks, in this order

  1. Note the sub-trip, which is the HF code
  2. Enter 1299 in Pr 00 and press reset
  3. If the HF trip comes back, contact the supplier

So.St: Soft start relay failure

Hits OEE: Availability

What it means: The soft start relay that bypasses the charging circuit failed to close, or its monitoring circuit failed.[1][2][3][4][5]

Also on: M200, M201, M300, C200, C300: So.St, trip 226. M400: Soft Start, trip 226. Commander SK equivalent: HF26.

Likely causes

  • Soft start failure (sub-trip 1)
  • DC bus capacitor failure on a 110 V size 2 drive (sub-trip 2)

First checks, in this order

  1. Note the sub-trip
  2. Contact the supplier of the drive, this is a hardware fault

OI.SC: Output phase short circuit

Hits OEE: Availability

What it means: Over current was detected on the drive output as soon as it was enabled, which points to a short or an earth fault on the motor side.[1][2][3][4]

Also on: M200, M201, M300, C200, C300: OI.SC, trip 228. M400: Output phase s/c, trip 228.

Likely causes

  • Short circuit in the motor cable
  • Motor earth fault or failed motor insulation
  • Motor cable longer than the limit for the frame size

First checks, in this order

  1. Check the output cabling for a short circuit
  2. Test the motor insulation with an insulation tester
  3. Check the motor cable length against the frame size limit

Sto: Safe Torque Off board not fitted

Hits OEE: Availability

What it means: The drive cannot find its internal Safe Torque Off board fitted correctly.[1][2][3][4]

Also on: M200, M201, M300, C200, C300: Sto, trip 234. M400: STO Error, trip 234.

Likely causes

  • Internal STO board not fitted correctly
  • Internal hardware fault

First checks, in this order

  1. Do not open the drive to repair it
  2. Contact the supplier of the drive, this is a hardware fault

r.b.ht: Hot rectifier or braking transistor

Hits OEE: Availability

What it means: Over temperature was detected on the input rectifier or the braking transistor.[1][2][3][4][5][6]

Also on: M200, M201, M300, C200, C300: r.b.ht, trip 250. M400: Hot Rect/Brake, trip 250. Commander SK equivalent: O.ht4, trip 102.

Likely causes

  • Input rectifier or braking IGBT running too hot
  • Cooling Fan Control Pr 06.045 set to 0
  • Supply imbalance (Commander SK guidance for O.ht4)

First checks, in this order

  1. Set Pr 06.045 above 0 to increase ventilation
  2. Check airflow around the drive
  3. Check the supply for imbalance and lengthen the ramps

HF01 to HF23: Internal hardware fault (Unidrive M, Commander C)

Hits OEE: Availability

What it means: The main processor found an internal error it cannot recover from and stopped every drive function. Most of these codes mean the control board has failed.[1][2][3][4]

Also on: M200, M201, M300, M400, C200, C300: HF01 to HF23, which are not given trip numbers except HF08, HF11, HF12 and HF18. The Commander SK uses its own HF list, see the next rows.

Likely causes

  • Control PCB failure such as CPU, bus, watchdog or memory errors
  • Firmware CRC check failed (HF19), the drive waits in bootloader mode for new firmware
  • Internal flash or non-volatile memory error (HF11, HF18)

First checks, in this order

  1. Write down the HF number
  2. Power cycle the drive; after restart it shows St.HF
  3. Clear St.HF by entering 1299 in Pr 00 and pressing reset
  4. For HF19, reprogram the firmware with Unidrive M Connect
  5. If the fault returns, contact the supplier

C.Acc: Parameter card (SD card or SmartStick) access failed

Hits OEE: Availability

What it means: The drive could not read or write its parameter storage card. If this happened during a transfer, the data may be incomplete.[1][2][3][4][5][6]

Also on: M200, M201, M300, C200, C300: C.Acc, trip 185 (NV Media Card). M400: Card Access, trip 185. Commander SK: C.Acc, trip 185 (SmartStick read/write fail).

Likely causes

  • Card not installed or not located correctly
  • Faulty card
  • Card removed during an access

First checks, in this order

  1. Reseat the card
  2. Replace the card
  3. If a transfer into the drive was interrupted, power cycle the drive to get the original parameters back

O.ht1: IGBT over temperature, thermal model (Commander SK)

Hits OEE: Availability

What it means: The Commander SK estimate of IGBT junction temperature passed 145 degrees C and the drive could not lower its switching frequency any further.[5][6]

Also on: Commander SK: O.ht1, trip 21. M200 family equivalent: Oht.I, trip 21. Warning before it: hot.

Likely causes

  • Overheat in the drive's software thermal model from high load
  • High switching frequency selected (Pr 5.18)
  • Automatic switching frequency change disabled (Pr 5.35 = On), so the drive trips instead of slowing the switching

First checks, in this order

  1. Reduce the motor current (load)
  2. Lower the switching frequency
  3. Leave automatic switching frequency change enabled
  4. Reduce the ambient temperature around the drive

O.ht2: Heatsink over temperature (Commander SK)

Hits OEE: Availability

What it means: The heatsink temperature on the Commander SK went above its maximum (95 degrees C on the Pr 7.04 sensor). It can only be reset once the temperature falls below the reset level.[5][6]

Also on: Commander SK: O.ht2, trip 22. Nearest M200 family trip: Oht.P, trip 22.

Likely causes

  • Heatsink temperature above the allowed maximum
  • High ambient temperature or high motor current
  • Power circuit cooling fan failure on size 6

First checks, in this order

  1. Check the heatsink fan and the airflow
  2. Reduce the ambient temperature or the motor current
  3. Wait for the heatsink to cool before resetting

O.ht3: Drive over heat, thermal model (Commander SK)

Hits OEE: Availability

What it means: The Commander SK model of internal component heating, which looks at output current and DC bus ripple, reached 100 percent (Pr 7.35). The drive tries to stop the motor and trips at once if it has not stopped within 10 seconds.[5][6]

Also on: Commander SK: O.ht3, trip 27. M200 family equivalent: Oh.dc, trip 27.

Likely causes

  • High output current for long periods
  • High DC bus ripple

First checks, in this order

  1. Reduce the motor load
  2. Check the supply phases for balance, since imbalance raises DC bus ripple
  3. Watch Pr 7.35 while running

O.ht4: Rectifier over temperature (Commander SK)

Hits OEE: Availability

What it means: The power module rectifier on the Commander SK is above its allowed temperature.[5][6]

Also on: Commander SK: O.ht4, trip 102. M200 family equivalent: r.b.ht, trip 250.

Likely causes

  • Power module rectifier temperature above maximum
  • Supply imbalance

First checks, in this order

  1. Check the supply for imbalance
  2. Lengthen the acceleration and deceleration ramps
  3. Check the cooling around the drive

cL1: Analog input 1 current loss (Commander SK)

Hits OEE: Availability

What it means: Analog input 1 on the Commander SK is set to 4-20 mA or 20-4 mA and the input current fell below 3 mA.[5][6]

Also on: Commander SK: cL1, trip 28. M200 family: cL.A1, trip 28. M400: An Input 1 Loss, trip 28.

Likely causes

  • Broken current signal wire
  • Transmitter or PLC output not supplying current
  • Current mode selected for a signal that is not a current loop

First checks, in this order

  1. Measure the current at terminal T2
  2. Check the wiring and terminals
  3. Check the analog input 1 mode (Pr 7.06)

PH: Input phase loss or imbalance (Commander SK)

Hits OEE: Availability

What it means: The Commander SK sees DC bus ripple that points to a missing input phase or a strong supply imbalance. A motor load between 50 and 100 percent of drive rating is normally needed to trigger it.[5][6]

Also on: Commander SK: PH, trip 32. M200 family: PH.Lo, trip 32. M400: Phase Loss, trip 32.

Likely causes

  • One input phase disconnected from the drive
  • High phase imbalance on the input voltage

First checks, in this order

  1. Measure all three input phases under load
  2. Check upstream fuses, breakers and terminals
  3. Set Pr 10.37 if you want the drive to stop before it trips

HF20 to HF32: Hardware faults (Commander SK)

Hits OEE: Availability

What it means: Internal faults in the Commander SK power stage, sensors or fans. HF20 to HF32 are stored in the trip log as trips 220 to 232 and cannot be reset; HF01 to HF19 stop the drive completely and are not logged.[5][6]

Also on: Commander SK: HF20 to HF32, trip numbers 220 to 232. The M200 family reports similar faults as So.St, tH.Fb, FAn.F and HF01 to HF23.

Likely causes

  • Power stage code error or unrecognised frame size (HF20, HF21)
  • Output short circuit or output bridge failure seen at power up (HF22)
  • Soft start relay failure or braking IGBT short circuit at power up (HF26)
  • Power stage thermistor or fan faults (HF27, HF28, HF29, HF31) or a DCCT wire break (HF30)

First checks, in this order

  1. Write down the HF number
  2. For HF22, check the output wiring for a short circuit
  3. For HF29 or HF31, check the fans
  4. If the fault repeats, return the drive to the supplier

OV.Ld: Motor overload warning

Warning: act before it becomes a stop

What it means: The motor thermal model has reached 75 percent of the It.Ac trip level while the load is above 100 percent. The drive keeps running, but it will trip on It.Ac if this goes on.[1][2][3][4][5][6]

Also on: M200, M201, M300, C200, C300: OV.Ld. M400: Motor Overload. Commander SK: OVL.d, flashing when the accumulator passes 75 percent with load above 105 percent.

Likely causes

  • Motor running above its rated load
  • Motor Protection Accumulator Pr 04.019 rising toward the trip level

First checks, in this order

  1. Reduce the motor current (load)
  2. Check the machine for jams or binding
  3. Check the motor rated current setting against the nameplate

d.OV.Ld: Drive over temperature warning

Warning: act before it becomes a stop

What it means: The drive's own thermal level is above 90 percent of its trip point (Pr 07.036). It keeps running, but an over temperature trip will follow if nothing changes.[1][2][3][4][5][6]

Also on: M200, M201, M300, C200, C300: d.OV.Ld. M400: Drive Overload. Commander SK: hot (heatsink or IGBT temperature too high).

Likely causes

  • High ambient temperature around the drive
  • High motor current

First checks, in this order

  1. Reduce the ambient temperature in the panel
  2. Reduce the motor current
  3. Check the fans and the airflow

br.res: Braking resistor overload warning

Warning: act before it becomes a stop

What it means: The braking resistor thermal accumulator (Pr 10.039) has reached 75 percent of the It.br trip level. The drive keeps running for now.[1][2][3][4][5][6]

Also on: M200, M201, M300, C200, C300: br.res. M400: Brake Resistor. Commander SK: br.rS.

Likely causes

  • Heavy or frequent braking
  • Braking resistor parameters not matched to the fitted resistor

First checks, in this order

  1. Reduce the braking duty or lengthen the deceleration ramp
  2. Check Pr 10.030, Pr 10.031 and Pr 10.061 against the resistor

Log it, or the same alarm comes back

A drive trip stops the motor and with it the conveyor, pump or fan the line depends on, so in OEE terms it is a breakdown. Log the trip, the drive and the minutes lost.

  • Reason code: the trip plus the drive, for example M200 It.AC, mixer drive, so a motor overload that trips every afternoon stands out.
  • Loss bucket: a stop that needs a reset, a part or a technician is an availability loss (breakdown). Stops shorter than your micro-stop threshold go to performance (minor stops). The six big losses page shows where each one belongs.
  • What to count: how often each code fires per week and the minutes it costs. Two numbers a week turn "that alarm comes up sometimes" into a ranked list. The MTBF and MTTR calculator and the downtime cost calculator do the math.

The free OEE tracker logs each stop with its reason and turns the week into a downtime Pareto, so the code that costs the most rises to the top.

For drive faults across brands, fabrico.io has a VFD fault codes guide. A fault code tells you what the drive measured, not what happened at the machine in the seconds before. Fabrico's camera-based OEE software syncs line camera video with machine data, so you can replay the moment the line stopped next to the trip.

Frequently asked questions

Where can I see the last trips on a Commander SK or a Unidrive M200?

On the M200, M201, M300, M400, C200 and C300 the last ten trips sit in Pr 10.020 (most recent) to Pr 10.029 (oldest), with sub-trip numbers in Pr 10.070 to Pr 10.079 and the date and time of each trip from Pr 10.041 onward. On a Commander SK the log is Pr 10.20 to Pr 10.29, and Pr 55 to Pr 58 in the basic menu show the last four trips. A Commander SK only logs a UV trip if it happened while the drive was running.

How do I reset a Control Techniques drive after a trip?

Fix the cause first, then press the red STOP/RESET key; on a Commander SK, opening and closing the enable/reset terminal B4 also works, except for O.Ld1. OI.AC, OI.br and FAn.F cannot be reset until 10 seconds after the trip. An M200 can also be reset over serial comms by writing 100 to Pr 10.038. HF hardware faults need a power cycle, and on the M200 family the St.HF that follows is cleared by entering 1299 in Pr 00 and pressing reset.

What is the difference between OI.AC and It.AC?

OI.AC is instantaneous: the output current spiked above the hardware limit, typically from a short, an earth fault or a ramp that is too fast. It.AC (It.Ac on the M200, Motor Too Hot on the M400) builds up over time as the drive's motor thermal model reaches 100 percent, usually from overload, a jam or wrong motor current settings. An OV.Ld or OVL.d warning normally flashes before an It.AC trip, which helps tell the two apart.

Why does my drive trip on OV when the machine stops?

When the motor is slowed faster than the load wants to slow down, it works as a generator and pushes energy back into the drive's DC bus. Control Techniques lists a deceleration ramp that is too fast for the machine inertia and a load that drives the motor as the main causes. Lengthen the deceleration ramp and check the supply voltage; if a braking resistor is fitted, a lower value helps as long as it stays above the minimum for the drive.

My drive says Emerson. Do these Nidec Control Techniques codes still apply?

Yes. Control Techniques was part of Emerson, trading in the US as Emerson Industrial Automation, until Nidec completed its purchase in January 2017, so older drives and manuals carry the Emerson name. The Commander SK is now in maintenance status, with repairs and selected spare parts still offered, and Nidec names the Commander C200 as its replacement. The C200 and M200 keep most trip names, but some changed, for example cL1 became cL.A1, PH became PH.Lo and O.ht1 became Oht.I.

Sources

Codes and meanings follow the manufacturer documentation below. Wording is ours, simplified for the shop floor; the manual for your exact model and software version wins if it differs.

  1. Control User Guide Unidrive M200/M201, Part Number 0478-0351-03, Issue 3 (Nidec Control Techniques)
  2. Control User Guide Unidrive M300, Part Number 0478-0350-03, Issue 3 (Nidec Control Techniques)
  3. Control User Guide Unidrive M400, Part Number 0478-0349-03, Issue 3 (Nidec Control Techniques)
  4. Control User Guide Commander C200/C300, Part Number 0478-0535-02, Issue 2 (Nidec Control Techniques)
  5. Advanced User Guide Commander SK, Part Number 0472-0001-10, Issue 10 (Nidec Control Techniques)
  6. Getting Started Guide Commander SK Model sizes 2 to 6, Part Number 0472-0064-10, Issue 10 (Nidec Control Techniques)

See the moment the drive tripped

Fabrico, a light MES with computer vision, syncs camera footage with machine and timeline data, so you can replay the exact moment the line stopped and review the cause next to the drive's fault. Cameras also catch short stops and a line that is switched on but not producing, the losses nobody writes down. Machines connect through a PLC, a Fabrico IoT device or an AI camera.

Book a Fabrico demo
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Related tools: full troubleshooting directory · OEE calculator · downtime cost · MTBF / MTTR · glossary

OEE essentials: OEE calculator · What is OEE · How to improve OEE · Six big losses · Availability · TEEP · Cost of downtime · OEE benchmark · Free OEE tracker · Free OEE software · How to track downtime · How to implement OEE · Light MES vs full MES