How Field Coils Work and How to Prevent Common Failures

Introduction

Field coils are essential components in electrical machines such as motors, generators, and alternators. They create the magnetic field necessary for the machine’s operation by producing magnetism when energized by an electric current. Proper functioning of the field coil ensures efficient torque generation in motors and stable voltage regulation in generators.

Function of Field Coils

The primary function of field coils is to generate a magnetic field that interacts with the armature winding. In a DC motor or generator, the field coil is usually mounted on the stator or pole core. When DC excitation current passes through the coil, it magnetizes the poles, creating a strong and stable magnetic flux. This magnetic field is crucial for:

• Producing the necessary electromagnetic force for motor rotation.
• Regulating voltage output in generators and alternators.
• Maintaining stable operation under varying load conditions.

Common Failures of Field Coils

Despite their robust design, field coils are susceptible to several types of failures due to electrical, mechanical, and thermal stresses. The most common failures include:

1. Coil Burnout

Excessive current, poor insulation, or inadequate cooling can cause overheating, leading to insulation breakdown and coil burnout. This is often accompanied by a burning smell, discolouration, or open circuit in the winding.

2. Insulation Deterioration

Over time, insulation materials degrade due to heat, vibration, or moisture ingress. Once the insulation weakens, it can cause short circuits between turns or to the ground, resulting in unstable operation or complete failure.

3. Loose or Broken Connections

Mechanical vibration or corrosion at terminals can lead to loose or broken coil connections, increasing resistance and causing arcing or uneven magnetic field distribution.

4. Contamination and Moisture

Field coils exposed to dust, oil, or moisture may experience reduced insulation resistance. This leads to leakage currents and, in severe cases, electrical shorting or flashover between coil turns.

Solutions and Preventive Measures

Preventive maintenance is key to avoiding costly coil failures. Recommended practices include:

• Regular insulation resistance testing using a megohmmeter to detect early deterioration.
• Ensuring proper ventilation and cooling to prevent overheating.
• Tightening electrical connections and applying anti-corrosion treatments where necessary.
• Periodic cleaning to remove dust, oil, and moisture from windings and terminals.
• Maintaining proper excitation voltage and current to avoid overloading the coil.

Conclusion

Field coils are critical for maintaining the performance and reliability of electrical machines. Proper inspection and maintenance can significantly extend their lifespan. However, if a field coil is found to be burnt or severely damaged, it should not be reused or repaired with temporary fixes. The recommended action is to rewind or replace the coil using appropriate insulation materials and follow manufacturer specifications. Professional rewinding ensures correct resistance values, insulation strength, and mechanical fit, restoring the equipment to safe and reliable operation.