Phase and Motor Protection Relays

Phase and motor protection relays are electronic relays that detect conditions such as phase loss, phase sequence error and voltage imbalance (asymmetry) in a three-phase supply and use the output contact to disconnect the motor contactor. They are used in the control circuits of pump, compressor, fan, elevator and air conditioner motors to prevent the windings from overheating when the motor is left on two phases or runs on unbalanced voltage. The category includes the Tense FK and FKM, Entes MK and MKC series, and Schneider Electric RM17, RM22 and RM35 monitoring relays; a fixed 30% or 40% or adjustable 5-25% asymmetry threshold, a PTC input and high-low voltage protection vary by model. Choose the required protection functions, whether the asymmetry threshold will be fixed or adjustable, connection with or without neutral, the supply voltage and the need for a PTC input according to the motor's wiring diagram. Besides DIN rail housings, there are also 96 × 96 mm panel-mount models. Phase protection relays that monitor only voltage do not measure motor current; they do not replace a thermal overload relay against overload.

Products

How to choose Phase and Motor Protection Relays

Phase and motor protection relays are electronic relays that detect conditions such as phase loss, phase sequence error and voltage imbalance (asymmetry) in a three-phase supply and use the output contact to disconnect the motor contactor. They are used in the control circuits of pump, compressor, fan, elevator and air conditioner motors to prevent the windings from overheating when the motor is left on two phases or runs on unbalanced voltage.

  • Separate overcurrent/leakage protection, motor starting and local isolation requirements.
  • Select rated current, pole count and breaking capacity (kA) from the installation calculation.
  • Match coil voltage, auxiliary contacts and tripping curve to the control circuit and load type.

Frequently asked questions

What is Phase and Motor Protection Relays and where is it used?

Phase and motor protection relays are electronic relays that detect conditions such as phase loss, phase sequence error and voltage imbalance (asymmetry) in a three-phase supply and use the output contact to disconnect the motor contactor. They are used in the control circuits of pump, compressor, fan, elevator and air conditioner motors to prevent the windings from overheating when the motor is left on two phases or runs on unbalanced voltage.

What should I compare when choosing Phase and Motor Protection Relays?

Choose the required protection functions, whether the asymmetry threshold will be fixed or adjustable, connection with or without neutral, the supply voltage and the need for a PTC input according to the motor's wiring diagram.

Where can I find technical details for Phase and Motor Protection Relays products?

Phase protection relays that monitor only voltage do not measure motor current; they do not replace a thermal overload relay against overload.

Phase and Motor Protection Relays Selection Guide: Technical Criteria and Engineering Practice