Drives, Motors, & MCC
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      2. Drives, Motors, & MCC

      Drive Overvoltage Fault

      Causes and mitigation

      The following is a list of items that might make the drive produce an Overvoltage fault:

      • The incoming Voltage is too high
      • Inertia from the load is overpowering the drive
      • Loss of motor load
      • Noise on the incoming line (relays contactors, brake coils, welders)
      • Long Motor lead lengths
      • Voltage transients on the line, possible from power factor correction caps
      • Arcing (shorting) in the motor
      • Output contactor opening (inductive kick)
      • Ground fault, such as a motor phase shorted to motor ground on an ungrounded (floating ground) or high resistive ground system
      • Bus Capacitors may be degrading
      • The line transformer does not have a solid ground, and the drive's power jumpers are installed (Common Mode Capacitors to Ground causing the DC bus levels to fluctuate and rise, causing a Fault when the drive is Idle)
      • During an autotune, the minimum speed is not 0 Hz
      • Unsuppressed motor brake coil
      • The motor is rotating when a start command is given
      • DC Bus Pump up Boost Converter Phenomenon of DC Bus Voltage See Technote 864938 - DC Bus Pump Up - Boost Converter Phenomenon
      • Open braking resistor
      • 7th IGBT is damaged
      • Motor bearing
      • CMC installed on an ungrounded distribution
      • The motor phase shorted to motor ground on an ungrounded (floating ground) or high resistive ground system
      • Bad motor

      Troubleshooting this fault depends on when it occurs.

      If it occurs when the drive isn't running:

      • Make sure the power wiring is correct. Input 3 phase on R S T, output motor leads on U V W
      • Monitor AC line for high line voltage or transient conditions phase-phase and phase-ground.
      • Use RSLogix trend or Drive Observer - monitor DC bus, speed, and current.
      • Disconnect motor leads and power cycle. If the F5 disappears. Check the motor/ground fault.
      • If the drive is on an ungrounded, high-resistance ground or dead-leg ground distribution system, double-check the power jumpers. This also may be a solution if a generator powers the drive.

      Refer to Knowledgebase note 823925 (see the end of this tech note) - PowerFlex Power Jumpers, Common Mode Capacitor, and MOV Jumpers for information on locating these jumpers on the various PowerFlex AC Drives.

      If it occurs when the drive is accelerating:

      • Check nameplate data
      • Tune the drive if needed (SV and FV modes)
      • In rare cases, the slip regulator can affect the bus regulator and not allow the dynamic brake to work properly. Set Slip RPM @ FLA parameter to a value of 0 to disable the slip regulator
      • Disconnect the motor leads and run in V/Hz mode at 60 Hz and measure the output voltage. All three output phases should be balanced. If not, replace the drive.
      • Check that power jumpers are installed properly
      • Change the PWM frequency - Lower carrier is Boost Converter Phenomenon of DC Bus Voltage is occurring.
      • If the motor is coupled to a moving load or other motors, ensure they are not pulling the motor faster than commanded.

      If it occurs when the drive is running at speed:

      • Check for an overhauling load
      • Transients on the input of the drive
      • If the motor is coupled to a moving load or other motors, ensure they are not pulling the motor faster than commanded.
      • Check for loss of load (i.e. belt slip)

      If it occurs when the drive is decelerating:
      • Bus overvoltage can also be caused by motor regeneration.
      • Extend the deceleration time or install a dynamic brake option.
      • Motor shorted to ground
      • In rare cases, the slip regulator can affect the bus regulator and not allow the dynamic brake to work properly. Set Slip RPM @ FLA parameter to a value of 0 to disable the slip regulator
      • Change Stop Mode to Coast

      If the fault occurs immediately when the drive is started:

      • Typically this indicates a problem with the motor
      • Megger the motor and the motor wiring
      • Try running the drive without the motor or another test motor
      • Have a motor shop test the motor

      Other Causes:

      • On a non-solidly grounded distribution system, there could be a ground fault anywhere in the distribution system (F05 is more likely to occur when the drive is stopped)
      • Bad capacitors
      • If a dynamic braking module or AFE system is used, there could be too much impedance in the system (the largest VFD should be close as possible to the AFE, and for a dynamic brake, the power wiring cables may be greater than ten linear feet).
      • Line transients due to poor power quality, across-the-line starters, or power correctional capacitors being switched in and out.
      • The drive could be misreading the bus voltage; verify the bus voltage reading with a meter.
      • If the fault occurs during an Autotune, then ensure the minimum speed is set to 0 Hz.
      • If the fault occurs even though a dynamic brake resistor is installed and the drive parameters are configured properly, then check to see if the drive's 7th IGBT has failed.

      There are many reasons why a Drive can get a Bus Overvoltage fault (F05). This particular document will address one reason why this can occur every time the drive is started.

      If the drive is attached to a distribution system that is not solidly grounded, Meaning the XO of the transformer is not grounded, or the ground wire of the motor does not come directly back to the drive PE/GND terminal, it will affect the Drives ability to detect ground fault conditions. In cases like this, Rockwell Automation recommends that a certified motor shop meggers and/or surge test the motor for internal winding failure. Attempt the following to resolve the issue:

      • Megger motor.
      • Check motor wiring. If this is a dual voltage motor (for example, 230/460VAC motor), make sure the wiring is correct for the voltage you are providing
      • See if it runs without the motor connected.
      • Try running with a different motor, if practical.

      For more references, verify Grounding techniques in Chapter 2, Power Distribution of the Wiring and Grounding Guidelines for Pulse Width Modulated (PWM) AC Drives Installation Instructions, Publication DRIVES-IN001.

      Note: If the drive faults when starting and there is no motor attached, it’s very likely that the drive is bad and will need to be replaced.

      In some cases, it is found in the acceleration time is set too short, the drive may have an overvoltage fault immediately upon receiving a start command.

      The DC bus overvoltage fault trip level for PowerFlex 750-Series drives is as follows:

      • 380/400VAC and 480VAC drives will produce an overvoltage fault when the voltage reaches 815VDC.
      • 600VAC and 690VAC drives will produce an overvoltage fault when the voltage reaches 1167VDC.

      This is usually caused by one of the following programming/setup errors:

      1. The drive is not actually being commanded to stop, but the reference is being decreased. This typically occurs when the PLC logic is written in such a way that it ramps down the speed reference or sets the speed reference to 0. Sometimes the intent of the logic is to set the speed reference to 0 at the same time the drive is commanded to stop, but the sequencing of the logic actually sets the speed reference to 0 before the drive is commanded to stop.
      2. The start/stop/run logic is written in such a way that the command(s) chatter. If a drive is commanded to coast to a stop and flying start is not enabled, and logic commands the drive to run again before the motor stops, the drive will try to begin running at zero speed. This causes regeneration and usually a very quick overvoltage trip. Pay attention to any permissions that are included in the start/stop/run logic.
      3. The same situation described above can occur with anything that may cause chatter on digital input start/stop/run/enable commands.

      MOV and Common Mode Cap Removal Recommendations

      Question:

      • When are MOV and Common Mode Cap removal recommended?
      • Do we still need to remove the Common Mode Capacitor from PF700AFE frame 13 when the input transformer Neutral point is grounded?
      • Do we need to remove common mode capacitors on the Phase V module of the inverter for Frame 13 PowerFlex 700S DC input drive installed with PowerFlex 700AFE on a high resistive ground system?

      Environment

      Answers:

      If any PowerFlex 4-class or PowerFlex 7-class drives are connected to a distribution transformer that is NOT configured as a wye winding secondary with the neutral or star point solidly grounded to the facility grounding electrode and safety ground system the MOV and common mode capacitor jumper is to be removed.

      This includes a transformer secondary that is configured as; not grounded, a resistive ground, or a delta secondary that is B-phase corner grounded.

      There is a small increased risk of drive faults such as overvoltage faults and drive failures when the PowerFlex drive is connected to a distribution transformer secondary winding that is configured as ungrounded, high resistive ground, or delta B phase or corner grounded.

      Drive faults or failures can happen when there are power quality events, such as voltage transients caused by primary and secondary voltage phase-to-phase or phase-to-ground shorts, lightning strikes, and drive load (motor) ground faults.

      When any frame size of PowerFlex 700AFE has IP21 Rittal enclosure, then common mode capacitors must be disconnected, regardless of the application in which the AFE is used.

      For any other enclosure of the LCL Filter of the PowerFlex Active Front End, common mode capacitors must be disconnected only if the PowerFlex 700AFE is installed on a resistive grounded distribution system or an ungrounded distribution system.

      Notes:

      When the Active Front End is used with drives that have common mode capacitors (for example, PowerFlex 7-Class or PowerFlex 750-Series drives), the common mode capacitors of these drives must be disconnected.