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Tech Tip 02

Line Reactors Stop Adjustable Frequency Drive Trip-Outs


  • Adjustable Frequency Drives Shut Down for No Apparent Reason.
  • Drive Diagnostics Report Overvoltage Condition.
  • Failures Often Occur About the Same Time Each Day.


  • Capacitor Switching Transients.
  • Drives Too Small Compared to Available Short Circuit Capacity.


  • Install Line Reactors or Isolation Transformers Ahead of the Drive.
  • Use Less Sensitive Drives.

A manufacturing facility installed new adjustable frequency drives to improve production. Unfortunately, the new drives started tripping off line for no apparent reason. This often happened on clear days with no visible power interruptions or other problems. Only the new drives experienced the problem.

As the problem continued, the plant electrician noticed the tripouts often occurred about the same time each day. Furthermore, the electrician checked the drive diagnostics and always found an overvoltage indication.

Examination of the situation revealed the customer was served from a 12.47kV distribution feeder which had switched capacitors for voltage and var control. Also, the drive horsepower ratings were very small compared to the short circuit capacity available at the drive terminals. Testing confirmed that the drives would trip off almost every time a certain capacitor was switched on.

A capacitor creates a momentary short circuit at the instant it is energized. Current surges into the capacitor and line voltage collapses for a few millionths of a second. As the current surge declines, it causes a ringing transient with overshoot on the peak voltage. Since the dc bus voltage is a function of the peak voltage, the overshoot causes the dc bus voltage to exceed the overvoltage setting which results in the drive tripping off line.

The solution in this application was to install a properly sized line reactor in the plant's electrical supply to the drive. The reactor's high impedance to the transient cuts the voltage overshoot and lowers inrush current to the drive. (Isolation transformers also do this, but they cost more.) Experience has shown that a 3% impedance reactor is effective at protecting against nuisance tripping. In order to achieve the 3% impedance, it is necessary to specify the voltage and horsepower or KVA rating for the drive it is to be installed on. Voltage recordings below show the overshoot reduction achieved by the installation of reactors. This transient reduction was enough to prevent the drives from tripping.

Source Side Voltage

 Figure 1: Voltage on the Source Side of the Reactor

Load Side Voltage

Figure 2: Voltage on the Load Side of the Reactor

This also shows that all drives are not created equal. Many drives in the plant survived capacitor switching with no problems. Careful selection of new drives helps prevent production outages from normal utility operations.

The information and diagrams presented herein are for general educational purposes only, and should not be relied upon as instructions for customer self-wiring. Customers should at all times seek the assistance of qualified electricians or utility personnel for all wiring projects.