While AC and DC motors convert electrical energy into mechanical energy to power your equipment, they operate on different principles and use different internal components. While both motors have a stator, rotor, and shaft, DC motors require a commutator and brushes to reverse the current’s direction. These electric motors are typically used where precise speed and torque control are required, such as.
- Permanent Magnet Motors: which use permanent magnets to create the fixed magnetic field.
- Series Motors: where the magnetic field is produced by a winding in series with the motor.
- Shunt Motors: where a winding in parallel with the motor produces the magnetic field.
- Compound Motors: that combine the series and shunt motor characteristics together.
Understanding the most common DC motor failures and how to avoid them can help you maximize uptime and profitability. Most DC motor failures can be classified into mechanical, electrical, or operational/environmental categories.
5 Common Mechanical Failures for DC Motors
Bearing failure can be caused by general wear, improper assembly (like excessive axial or radial loading), or lack of lubrication. Symptoms include noise, grinding, and excessive heat from the bearing housing.
Brush and commutator wear naturally over time, but the commutator can become dirty, burned, or worn, disrupting electrical contact. This can be exacerbated by high speeds or consistently overdriving the motor.
Gear damage can result from many different issues, including overloading the motor, shock loading from repetitive hard stops, general wear, or debris in the gear train.
Misalignment and vibration: Misaligned or bent shafts, as well as mechanical or electrical imbalances, can cause excessive vibration, which generates heat and can lead to further damage to components like bearings and windings.
Contamination: Dirt, dust, oil, and moisture can enter the motor, causing corrosion, damaging insulation, and blocking ventilation. Oil on the commutator, for example, can lead to black spots from burning.
How to Identify a Mechanical Failure
Listen for unusual noises, such as a rough sound, grinding, or squealing, which can indicate potential problems with bearings or gears.
Check for excessive vibrations during operational cycles. This could indicate an imbalance, shaft misalignment, or failing bearings.
A hot motor casing or bearing could indicate excessive mechanical friction due to a lack of lubrication or contamination.
Look for visible signs of grease leakage, excessive dirt buildup, and damage or excessive wear on the commutator and brushes.
Common Electrical Failures for DC Motors
Brush and commutator issues: Worn brushes or a dirty, contaminated commutator can lead to poor electrical contact, arcing, and even catastrophic flashovers where a short circuit occurs between the brushes.
Winding failures fall into two categories:
- Electrical overload can generate excessive heat, leading to insulation breakdown in the windings and creating short circuits.
- Insulation breakdown: High temperatures, electrical stress, or physical damage can degrade insulation, leading to winding shorts and reduced motor efficiency.
Overheating: Can be caused by a variety of factors, including electrical overload, poor ventilation, or motor contamination, and can lead to insulation breakdown and subsequent component damage.
Short circuits: These can be caused by issues with the windings, such as insulation breakdown, or by contamination between commutator segments, leading to a sudden surge of current.
Field coil failure: These failures can result from overheating, mechanical stress, contamination, or age, and can be caused by electrical overloads or external factors.
Armature problems: Can include turn-to-turn shorts, which may be indicated by a burn mark on the commutator.
How to Identify an Electrical Failure
Frequent tripping of circuit breakers can indicate a short in the power supply or the motor windings, or an insufficient ground.
Decreased DC motor performance and efficiency can result from loose connections, degraded insulation, or excessive overheating.
Excessive energy consumption is normally a sign of electrical issues within the motor
Irregular operation or stalling can indicate a fluctuating power supply or damaged internal electrical components
The discoloration of motor components can usually be traced back to regular overheating episodes.
Environmental and Operational Failures
Contamination: Dust, dirt, and moisture can enter the motor and cause issues with the bearings, commutator, and windings.
Misalignment: If the motor shaft is misaligned, it can transmit damaging forces and excessive stress to the bearings and other motor components.
Improper Lubrication: Too little or too much lubrication can cause bearings to fail, while too much grease creates additional friction and heat buildup.
Identifying an Environmental or Operational Failure
Check voltage and current: Use a multimeter to ensure the correct voltage is being supplied and monitor the current draw. Excessive current can indicate an overload or a short circuit.
Test winding resistance: Measure the resistance of the windings. A low resistance could mean a short circuit, while an open circuit (very high resistance) would prevent the motor from working.
Inspect the commutator and brushes: Look for wear, flat spots on the commutator, or worn-out or poorly contacting brushes.
Check bearings: Try to turn the motor shaft by hand. If it’s stiff or gritty, the bearings may be bad. Check for signs of lubricant leakage or excessive heat from the bearing housing.
How to Protect Your DC Motors From Unexpected Failures
The best way to protect the DC motors essential to your business’s success is through regular inspections and testing as part of a predictive maintenance plan. By reviewing the condition-monitoring data, maintenance can be scheduled proactively to prevent unexpected failures, optimize performance, and improve overall equipment reliability.
To learn more about reducing downtime, lowering maintenance costs, and extending the motor’s lifespan, please contact the Hi-Speed team at 1-800-713-0103 or via email.
Additional Electric Motor Resources
Based on your interest in repairing an electric motor, here are some recent blog posts you might find interesting or helpful.
Best Electric Motor Repair Practices
The Essential Guide to Getting Your Electric Motors Serviced or Repaired