Today’s ac drives are more reliable, more cost-effective, and last longer than their predecessors. With the addition of modern electronic controls, ac drives can provide performance that was not possible just a few years ago. These controls now allow ac drives to match the performance of dc drives in many applications.

But despite their reliability, things occasionally go wrong and the drive may operate erratically or stop running altogether. Fortunately, today’s drives are usually equipped with self-diagnostic controls that quickly indicate where the problem lies.

Typically, drive problems can be traced to improper setup or startup. Although specific error codes and the manner of transmitting them may differ for individual drives, all ac drives share common operating principles. Here’s how to interpret diagnostic codes and troubleshoot the problem for safe, reliable operation.

Safety First

It’s important to remember that dc bus capacitors retain hazardous voltages after input power has been disconnected. Therefore, wait five minutes for the dc bus capacitors to discharge once power has been disconnected. Check the voltage with a voltmeter to ensure that the capacitors have discharged before touching any internal components.

Problem Indicators

AC drives provide a number of different ways to determine their status and to troubleshoot problems. These include:

• LEDs on the front of the drive
  
• User-configurable and nonconfigurable alarms
  
• User-configurable and nonconfigurable faults
  
• Entries in the fault queue
  
• Drive status parameters. 

LEDs provide a quick indication of drive status, changing colors from green to yellow to red to indicate various conditions. Typical status LEDs are Power, Ready, Drive, Communications Module, and one or more Network Connections. Normally, steady glowing green status lights mean everything is running properly. However, there are occasions when status lights are green, yet the drive is not operating properly. Flashing yellow or red lights indicate a problem with the drive that should be checked. Consult the operator’s manual for the specific drive to determine what a particular flashing light means.

Alarms indicate conditions that may affect drive operation or application performance. They are cleared automatically when the condition that caused the alarm is no longer present. As mentioned above, alarms are either user configurable or user nonconfigurable:

• Configurable alarms alert the operator of conditions that, if left untreated, may lead to a drive fault. The drive continues to operate during the alarm condition, and the alarms can be enabled or disabled by the programmer or operator.
  
• Nonconfigurable alarms alert the operator of conditions caused by improper programming and prevent the drive from starting until the problem is resolved. These alarms can never be disabled.

Faults indicate conditions within the drive that require immediate attention. The drive responds to a fault by coasting to a stop and turning off output power to the motor. Faults can be auto-resettable, nonresettable or user-configurable:

• Auto-reset faults reset automatically if, after a preset time, the condition that caused the fault is no longer present. The drive then restarts.
  
• Nonresettable faults may require drive or motor repair; the fault must be corrected before it can be cleared.
  
• User-configurable faults can be enabled and disabled to enunciate or ignore a fault condition.

Fault queues normally retain a history of faults. Typically, queues hold only a limited number of entries; therefore, when the queue is full, older faults are discarded when new faults occur. The system typically assigns a time stamp to the fault so that programmers or operators can determine when a fault occurred relative to the last drive power up.

A complete listing of all the different types of faults and the appropriate corrective actions can typically be found in the operator’s manual for a specific drive.

When the Drive Fails

AC drive failures can be categorized according to a number of common problem areas. We’ll review the most common symptoms and corrective actions in this article.

Drive Does Not Start from Terminal Block Start, Run or Jog Inputs 
Indications for this condition are:

Flashing Red Ready LED, which signals that the cause is a faulted drive. The fault can be cleared by:

• Pressing Stop
  
• Cycling power
  
• Setting Fault Clear to the appropriate setting
  
• Clearing faults on the diagnostic menu.

Incorrect Operation from the Terminal Block, which can be caused by incorrect input wiring. Rewiring the input correctly or installing a jumper can correct this problem. Incorrect operation from the terminal block can also be caused by incorrect digital input programming in which:

• Mutually exclusive choices may have been made
  
• 2-wire and 3-wire programming may conflict
  
• Exclusive functions may be configured for multiple inputs.

If this setting is a factory default and is not wired or is open, stop the diagnosis here. These conditions can be corrected by programming the Digital Input Selection parameter for the correct input. In addition, check to ensure that the Start or Run programming is present.

Flashing Yellow Ready LED and Digital Input Conflict Indication on LCD. The Drive Status may also show an Under Voltage alarm. This condition is caused by incorrect digital input programming as described above. It can be corrected by programming the Digital Input Selection parameter to resolve conflicts, removing multiple selections for the same function, or installing a stop button to apply a signal at the stop terminal.

Drive Does Not Start or Jog from OIM (Operator Interface Module) 
If there is no indicator on the control panel, the cause is a drive programmed for 2-wire control while the OIM start is disabled for 2-wire control. If 2-wire control is required, no action is necessary. If 3-wire control is required, program the digital input selection parameter for the correct inputs. Other indications for this condition are:

Flashing or Steady Red Ready LED, which indicates an active fault. Reset the fault and restart the drive.

Flashing Yellow Ready LED. There are a number of causes for this condition:

• Open Enable Input – Correct by closing the terminal block enable input
  
• Terminal block stop input is open and control source is set to All Ports – Correct by closing the terminal block stop input
  
• Start Inhibit bits are set – Check the status of these bits in the Start Inhibits function.

Drive Does Not Respond to Speed Change Commands 
Indications for this condition are:

LCD OIM Status Line Indicates “At Speed” and Output is 0 Hz, which signals that no value is coming from the command source. If the source is an analog input, check the wiring and use a meter to check for the presence of a signal. Then, check the Commanded Speed parameter for the correct source.

No Indication on the control panel shows that an incorrect reference source has been programmed. Check the Speed Reference Source parameter for the correct source, and reprogram the Speed Reference Selection parameter for the correct source. No indicator on the control panel might also signal that an incorrect Reference Source is being selected via a remote device or digital inputs. The steps to correct this condition are:

• Check Drive Status bits for unexpected source selections
  
• Check Digital Input Status to see if inputs are selecting an alternative source
  
• Reprogram digital inputs to correct Speed Selection parameter.

Motor or Drive Will Not Accelerate to Commanded Speed 
Indications for this condition are:

Excessive Acceleration Time, caused by an incorrect value in Acceleration Time input. Correct by reprogramming the Acceleration Time Input.

Drive Forced into Current Limit, Slowing, or Stopping Acceleration, which is caused by excessive load or short acceleration time. Fix this condition by checking the Drive Status bit to see if the drive is in current limit. Also, remove excessive loads or reprogram the Acceleration Time input.

Speed Command Source or Value is Not Expected, which is caused by an improper speed command. Check the Operator’s Manual for the proper speed command and reset.

Incorrect Programming Prevents Drive Output from Exceeding Limiting Values. Check the Maximum Speed Input and Maximum Frequency Input to ensure speed is not limited by programming.

Unstable Motor Operation 
There usually is no indicator for this condition, which is caused by incorrect motor data being entered or failure to perform an autotune procedure. The first step is to input the motor nameplate data correctly. Then, if necessary, perform a static or rotational autotune procedure.

Drive Will Not Reverse Motor Direction
Again, there usually is no indicator for this condition. Causes include:

Digital Input Not Selected for Reversing Control. Corrective action is to check the Digital Input Selection parameter, choose the correct input and program the drive for reverse.

Digital Input Wired Incorrectly. Check the input wiring.

Direction Mode Parameter Programmed Incorrectly. Reprogram for analog bipolar or digital unipolar control.

Motor Wiring Improperly Phased for Reverse. Switch the two motor leads.

Bipolar Analog Speed Command Input Wired Incorrectly or Signal Absent. Use a meter to check that an analog input is present. Also, check the wiring. A positive voltage commands forward direction; a negative voltage commands reverse direction.

Stopping Drive Results in Deceleration Inhibit Fault 
Here the Diagnostic Screen displays a Deceleration Inhibit fault. The LCD status line may also indicate a Fault. This condition can be caused if a bus regulation feature is enabled and halts deceleration due to excessive voltage. The excessive voltage, in turn, is normally caused by excessive regenerated energy or unstable ac line input voltages. Another cause for this condition is that an internal timer has halted drive operation. The following steps will correct the problem:

• Reprogram bus regulation to eliminate any Adjust Frequency selection
  
• Disable bus regulation and add a dynamic brake
  
• Correct ac input line instability or add an isolation transformer.
  
• Reset drive.

Conclusion

AC motor drives are among the most reliable components in use today. They typically run for long periods with no problems. Occasionally, problems do arise, but modern drives have built-in diagnostic tools that help maintenance personnel trace the source of the problem quickly and easily to get the system up and running again.