Motor starting resistors allow to limit overcurrent and help AC and DC motors to gradually increase torque and voltage, thus reaching the desired speed through a smooth acceleration without absorbing too much current. Among their advantages:

  • Reduced overheating in electric motors
  • Extended life-span of the motor
  • Voltages kept at safe levels

After the development of speed drives and soft start systems, the control over speed and torque in AC and DC electric motors has been made easier. But many motors still around did not benefit from these technological improvements and still require dedicated resistors.


Motor starting resistors are employed in many industries relying on large electric motors (such as AC slip-ring motors, Squirrel-cage induction motors, DC motors and so on) for their operations. Among these, there are heavy industries such as mining and steel. Other applications include:

  • Lifting (as in crane control systems)
  • Pumping
  • Rolling


Fairfild developed several custom motor starting resistors in the past few years, all designed and realized on the base of the client’s needs. Fairfild’s catalogue also features many standard series and models perfectly suited for this application.

  • RXT series: RMS nude wire-wound spark plugs (9 models)
    Power ratings: from 3 kW to 120 kW.
    Ohmic values: from 0.13 Ω to 560 Ω.
    The frame of the enclosure is made of galvanized steel and has an IP23 degree of protection (IP20, AISI 304 and AISI 316L enclosures are also available). The power terminals are nickel plated brass screw terminals and are found inside a terminal box. The box is equipped with thermal pad with proper terminal on the outside of the metal strap. The maximum voltage is 1500 V.
  • RM and RM/X series (6 models in total):
    Power ratings: from 0.9 kW to12 kW
    Ohmic values: from 0.43 Ω to 360 Ω
    Maximum voltage: 700 V
    In both series, the enclosure is made of galvanized steel and has an IP20 protection degree. The terminals are available inside the framework through nickeled brass cable gland on terminal board
  • RNP series: stripped edgeways on ceramic U-bolts mounted on a metal rod (3 models):
    Power ratings: from 800 W to 4.4 kW
    Ohmic values: from 0.1 Ω to 4.3 Ω
    Maximum voltage: 1500 V
    Thanks to its construction model, the thermal resistance and the ohmic values are very low. These make the series perfect for absorbing high energy pulses.
  • RME series (7 models):
    Power ratings: from 0.6 kW to 6 kW
    Ohmic values: from 1.5 Ω to 270 Ω
    Maximum voltage: 800 V
    The casing uses RHP tubular cemented spark plugs. The frame of the enclosure is made of galvanized steel and has an IPO20 degree of protection. Power terminals are available on a clam and are located within the lid, which can be opened. The UL certification provides for a derating of the rated power of approximately 50%.
  • RCPO series: Rectangular wire resistor wound on a ceramic tubular support coated with inorganic phosphate cement (11 models).
    Power ratings: from 152 W to 1500 W
    Ohmic values: between 0,068 Ω and 120 Ω.
    The greater heat exchange surface area, due to the winding with ribbed rectangular wire, allows reaching higher values of dissipated power. The RCPO series is available in versions with adjustment slider. In this case, it is called RCPOR. The maximum voltage of use varies depending on the model.