Motor Starting Resistors: How They Prevent Overheating

In an industrial plant, controlling electric current is essential to ensure system safety and reliability. One of the most critical aspects concerns overcurrent situations—when the current exceeds the limits allowed for cables, panels, and electrical components.

These anomalies can have serious consequences: overheating of insulating materials, system degradation, fire risks, damage to motors and equipment, interference on other circuits, and, of course, financial losses due to downtime and extraordinary maintenance.

In this context, motor starting resistors play a key role in limiting inrush current during the startup of electric motors. They also help both AC and DC motors gradually increase torque and voltage, avoiding harmful peaks and allowing the motor to safely reach its operating speed.

Benefits of Motor Starting Resistors

Using starting resistors allows you to:

  • reduce motor overheating;
  • extend motor lifespan;
  • keep voltages within safe ranges.

While modern electronic starting systems have simplified torque and speed control, many industrial facilities still rely on dedicated resistors—especially in heavy-duty sectors like mining, steel production, or large rotating machinery.

How Starting Resistors Prevent Overheating

Motor starting resistors are designed to handle high currents for very short durations—typically only during the initial startup phase (a few seconds). Once the motor reaches a stable speed, the resistors are automatically disconnected from the circuit, usually via contactors.

This temporary use significantly reduces the risk of overheating. Additionally, Fairfild resistors are built with high thermal capacity materials and are engineered to withstand power peaks without damage. Their shapes, geometries, and supports (such as fins or ceramic saddles) help dissipate heat quickly.

Another key element is the duty cycle: resistors must respect rest periods between starts to avoid thermal buildup. For more demanding applications, natural or forced ventilation can be integrated to aid cooling. Everything starts with proper sizing—factoring in current, voltage, ambient temperature, and usage frequency.

Fairfild Motor Starting Resistors

Fairfild designs and manufactures custom industrial resistors, including a full range of motor starting resistors, available in both custom configurations and standard series:

  • RXT – Bare wire RMS resistors, with galvanized steel frame (IP23, also available in IP20, AISI 304 and 316L). Max voltage 1500 V. Nickel-plated brass terminals and integrated thermal pad;
  • RM/RMX – Galvanized steel resistors with IP20 protection, available in 6 models;
  • RNP – Ribbon resistors wound on ceramic saddles, mounted on a metal bar. Excellent thermal capacity and customizable ohmic values;
  • RME – Box resistors with RHP rods. Galvanized steel casing, IP20. Terminals accessible via lift-up cover. UL certified with 50% power derating;
  • RCPO – Corrugated ribbon resistors on ceramic tubular supports, coated with phosphate cement. Larger exchange surface for high dissipated power.

Choose Fairfild Quality

If you’re looking for high-performance motor starting resistors, Fairfild is the ideal partner. With over 40 years of industry experience, we design tailor-made solutions for every industrial need—ensuring reliability, safety, and thermal performance.

Contact us for a technical consultation or visit the section dedicated to motor starting resistors to explore all available configurations.