VFDs have many statuses when in operation including enabled, running, and stopped.  While this is common in the VFD space, many maintenance and engineering professionals are caught off guard by the intricacies involved in programming with these statuses in mind.  An exploration of drive status transitions will illuminate VFD programming best practices and benefit operators and technicians alike.

Variable Frequency Drive Construction

Pulse-Width Modulating drives achieve their speed and torque control with a three-component structure:  Converter/Bus/Inverter.  For the PowerFlex component and standard Drives, the converter is composed of diodes that take an incoming AC signal and convert it to DC at 1.414 x RMS AC supply voltage.  Once the voltage signal has been converted to DC, it is sent down the DC bus, which transmits the voltage to the inverter section.  For the PowerFlex 750-series and frame E PowerFlex 520-series drives, the DC bus contains a DC link choke.  Said choke acts in the same manner as a line reactor: limiting the DC bus voltage and mitigating harmonic distortion.  For the PowerFlex standard and component Drives (exclusive of the 755T series), both the converter and the bus are passive devices, providing the change to the incoming voltage signal without outside control or input beyond the incoming power itself.  The Inverter, however, is controlled by the drive’s central processor and actively switches insulated gate bipolar transistors (IGBTs) on and off to change the polarity of the DC signal received from the DC-bus.  This allows a drive to create a facsimile sine wave which is used to control an induction motor.     This controlling ability brings into play the Drive statuses in discussion.

Enabled

PowerFlex 525 IO Terminals

The “Enabled” or “Ready” status of a drive indicates that control of the inverter has, indeed, been enabled.  This means that the drive may produce a pulse-width modulated output when commanded to do so by way of digital input, HIM, or ethernet control.  A drive may enter a status of either “running” or “stopped” only after the drive has been enabled.  Here are three key things about the enabled status that will be of interest:
  1. The PowerFlex 750-series allows for either a software-enabled or hardware-enabled input.  “Software Enable” means that any programmable digital input may be made into an enable input.  It is also possible to manipulate the “enable” parameter by way of PLC or DeviceLogix.  Hardware enable indicates that one digital input must be sent a voltage signal for the drive to enter an enabled status.
  2. The “Enable Jumper” on the main boar on the PowerFlex 750-series may be removed to place the Drive into hardware enable mode.  In this mode, Di0 becomes the enable input for the Drive and must receive either a +24VDC or 115VAC signal to allow the Drive to enter and enabled status. 
  3. The PowerFlex 520 series has no dedicated hardware enable input nor does it have hardware enable jumpers.  This means that any of its digital inputs may be programmed as the enable input.  With that said, terminal 1 on the 520-series defaults to a stop input and will need to see +24VDC if the Drive is to enter a “running” status.

Connected to the concept of “enabled” is that of safe-torque-off (STO).  VFDs that feature safe-torque-off modules will go into a status of “safety ” which prevents the IGBTs from firing.  STO, therefore, puts a drive in a “not enabled” status that may be displayed on the HIM as “safety enabled” or “safety.”  Technologically, there is no difference between disabling a drive-by dropping out the enable signal and using STO.  However, the use of a safety card in a 750-series or the safety terminals on a 520-series endows the application with a certified safety rating and allows for the integration into safety zones.

If your drive is in a “safety” or “safety circuit open” status, check to see if attached safety circuits/e-stops are in fact open.  If operating a PowerFlex 525/527 and there is no associated safety circuit check that the safety jumper is in place on the associated safety terminals.

Running/Active

When a start/run command has been successfully delivered to a VFD that is enabled, said Drive will enter the “Running” or “Active” status.  This simply means that the drive’s IGBTs are activated, and the motor is being controlled.  Drives will not enter a running status if they are not enabled or have a start-inhibit in play.  Start inhibits are conditions/programming that prevent a drive from entering a running status.   For the PowerFlex 750-series, parameter 933 will list the start-inhibits currently in place.
The start-inhibits shown in Parameter 933 for the PowerFlex 750-series are as follows:
  • Faulted – Drive is in a faulted state. See P951 [Last Fault Code].
  • Alarm – A Type 2 alarm exists. See P961 [Type 2 Alarms].
  • Enable – An Enable input is open.
  • Precharge – Drive is in precharge. See P321 [Prchrg Control], P11 [DC Bus Volts].
  • Stop – Drive is receiving a stop signal. See P919 [Stop Owner].
  • Database – Database is performing a download operation.
  • Startup – Startup is active and preventing a start. Go to Start-Up Routine and abort.
  • Safety – The safety option module is preventing a start.
  • Sleep – Sleep function is issuing a stop. See P350 [Sleep Wake Mode], P351 [SleepWake RefSel].
  • Profiler – The profiler function is issuing a stop. See P1210 [Profile Status].
  • CommutNotCfg – The associated PM motor commutation function has not been configured for use.
While the PowerFlex 520-series does not have a start-inhibit parameter, parameter B012 will list the active start source for the drive.  If the drive is stopped, then the stop command would be coming from said start source.
The PowerFlex 753/755 will take a start command from any source unless programmed not to do so.  This means that an operator may start the drive from the HIM, digital inputs, or Ethernet-based commands at any time.  To isolate the start source for a 753/755, access parameter 324 and unselect the ports that you do not wish to provide a start command.  For the HIM, this will be port 1 if in the drive cradle, and port 2 or 3 if in a door bezel.  The mainboard is always port 0, and the built-in ethernet port on the 755 is always port 13.
The PowerFlex 523/525, in contrast to the 750-series, will only receive a start command from the currently programmed start source.  The 523/525 can maintain three different start sources and will default to start source 1.  To switch between these sources, a digital input will need to be programmed to allow a transition.  To do this, choose the digital input of choice and program it for either “Start Src 2” or “Start Src 3.”  When the digital input sees 24VDC, the start source will switch from source 1 (the default) to the chosen start input.   This transition may be achieved through digital input from a PLC or physical switch.  You may also change the status of a digital input by writing to a said parameter using PLC logic.  Selection of the Start and/or Frequency Source can be done in multiple ways if the Drive is connected via a network.  For an example of switching between start sources, check out this article.

Stopped/Stop Asserted

The delivery of a stop command to a drive will send the drive to a “stopped” status.  Both the PowerFlex 750 and 520-series will take a stop command from any attached source, whether that be the HIM or Ethernet-based communication.   For the PowerFlex 750-series, manipulating parameter 324 can deny a start source, but will not deny a stop source.  For the PowerFlex 520, The drive will take a stop command from any properly configured and connected stop input, regardless of the currently selected start-source.   As previously stated, a drive will not enter a “stopped” status unless it has already been enabled.
To add a further dimension to the stopped status:  If a Drive has received and enabled signal but has not yet received a start or stop command, then the Drive will be in a status of “stopped” which indicates that the drive has not yet received either a stop or start command but is also not active.  If, however, the Drive is receiving an active stop command from a control source such as the HIM or a PLC, then the drive will display a status of “Stop Asserted.”  When in a “stop asserted” status, the drive will not run until the active stop command is relinquished.
If your VFD is stopped and you are not sure why checking out the start inhibits for the drive will be of use.  For the PowerFlex 750-series parameter 933 will reveal the source of the start inhibit.  For the PowerFlex 520 series, there are three configurable start commands and any one of them might deliver a stop command as can the stop button on the HIM.  Noting which start source is in control can illuminate where a stop signal is originating from.

A Note On Clearing Faults

It is common for a stop button input to be used to simultaneously clear a fault.  In the PowerFlex 750-series, the red stop button on the HIM will clear faults automatically without the need for special programming.  In the PowerFlex 523/525 the stop mode chosen signifies whether a stop input will also clear a fault (parameter P045).  When operating a PowerFlex 753/755, if a start source has been eliminated using parameter 324, the stop input associated with said start source will still stop the Drive but will not clear a fault.

Understanding the different drive statuses is integral to proper drive programming and operation.  A careful review of start inhibits and stop sources can quickly shed light on many troubleshooting endeavors.   As always, reference your VFDs manual to better understand your Drive product.

For more information about drives and drive systems, check out the Variable Frequency Drive Main Help Page