CMC Series AC Over Current Monitor with Reversing Relay

The CMC series AC over current monitor with reversing relay and fault detector counter from the ATC Diversified Electronics division of Marsh Bellofram is designed to detect a motor jam, providing the equipment with an opportunity to self-clear before going into a lockout and alarm mode, protecting machinery from damage due to a motor burnout condition.

Control voltage is continuously applied to the CMC series. Upon application of the control voltage, both the forward and alarm relays initiate. When AC current is initially applied, the inrush time delay initiates to disable the over current sensor during the inrush period. On over current, the forward relay de-energizes and the unit has a 5-second dead band, then the reverse relay energizes for the selected time delay. Finally, the reverse relay de-energizes, rests for the dead band and the forward relay energizes completing one full cycle.

After one cycle is complete, the forward relay remains energized if conditions have returned to normal. If an AC over current condition remains, the unit will go through the same operating sequence as before until jam is corrected or the number of preselected cycles has occurred. The number of cycles that occur before lockout is field selectable from 1 to 9 cycles to prevent the faults from accumulating and causing nuisance alarms, a time band is initiated upon sensing the first fault. Then, successive occurrences equaling the number of faults within the time band will cause alarm. When the number of cycles reaches the pre-selected count, the Forward, Reverse and Alarm Relays lock into the de-energized state. Reset is accomplished by pressing the reset button. The LED indicator glows when the forward relay is energized. The CMC series operates in the fail safe mode as the relays are energized in normal conditions. An external CT may also be used to extend the range of the current monitor.



  • AC over current monitor with reversing relay
  • Fault condition counter
  • Adjustments for:
    • Inrush delay time
    • Reverse operating time


  • Expressly designed to detect motor jams
  • Current demand level monitoring
  • Conveyor or motor load jam detection
  • Remote motor sensing