Many issues can affect AC and DC motors, leading to equipment inefficiency or failure if their root causes are left unresolved. These problems can develop in both used and new motors regardless of manufacturer, so it is vital to know which issues you should look out for to enable early troubleshooting, conduct preventive maintenance, or know when it is time for servicing, like an ex proof motor rewind.
Read on to know the most common faults in AC and DC motors and identify their possible causes.
AC Motor Issues
1. Mechanical faults
The main mechanical AC motor faults to watch out for are incorrect bearing installation, shaft overdrive mechanical tolerance, and bearing housing ID measurements. The last one, in particular, can impede the bearing from running smoothly during its life cycle.
Apart from proper installation, motor bearings and their protective shielding also need regular lubrication and maintenance, respectively. End users must know how much grease is required in the grease cavity within the bearing or the bearing retainer to avoid over-lubrication.
2. Electrical faults
Old or poor insulation is the leading cause of motor electrical issues. Surge testing motor windings to check if they are at 75% of the recommended surge values of a rewound or brand-new winding is a good indicator of their insulation capacity and turn-to-turn integrity. This is especially beneficial for motors operating for quite some time already.
Electrical overloading is another concern due to the insulation breaking down rapidly when the motor is running in overload conditions. In these situations, the motor will certainly fail to ground, suffer a turn-turn short, or short phase-to-phase. Thankfully overload failure and costly repairs can easily be avoided with the help of current protection.
3. Excessive vibration
Motors must be checked frequently for any excess vibration as it can be a mechanically and electrically destructive force, with a higher risk of the latter if the vibration is severe. The typical causes of excessive vibration are:
- Loose mounting base
- Poor alignment
- Imprecise rotor balancing when conducting repairs
- Compacted dirt is lodged in the rotor assembly
- The driven equipment transfers energy back into the motor through the coupling
DC Motor Issues
1. Worn-out carbon brushes
When working with DC motors, applying the right amount of load is crucial for current density and brush conductivity on the commutator to enable a certain degree of lubricity to the brushes while in contact with the commutator under load.
Premature deterioration of the brushes will inevitably occur if the applied load is inconsistent and the motor uses the wrong grade of brushes, leading to carbon dust accumulating inside the machine and irreversibly damaging the commutator bars. Hence, make sure the brushes to the commutator receive constant pressure so that full current density gets delivered at the point of contact, preventing premature wear on the brushes.
2. Excessive arcing
Excessive arcing may arise while the motor is under load if the brushes are not in a neutral position. Adjusting the brush rack to this proper position involves placing the adjacent brushes in an area on the commutator in between or outside the polarity reference of the armature coils.
3. Broken brush springs or brushes
Broken brushes and springs may occur if the commutator’s eccentricity is not within acceptable parameters. Therefore, it is vital that the commutator also receives maintenance from time to time, starting with machining the segmented commutator bars properly. When doing this maintenance process, the main goal will be to ensure the roundness of the commutator with no run-out on the surface. Otherwise, the commutator may potentially result in high bars that can ruin the brushes and cause excessive arcing where the brushes meet the copper bars.
4. Faulty DC drive
DC drives are another possible reason behind inefficient DC motors. Problems with the drive’s silicon-controlled rectifiers can be identified at the commutator through excessive arcing. This arcing occurs at the brushes to the nearest metal surface or ground within the end frame at the side of the motor where the commutator is.
Conclusion
Encountering AC and DC electrical motor issues is inevitable for the length of their service life. Therefore, it is vital to engage in companies that provide mechanical and electrical engineering services to conduct proper and timely maintenance work on these motors to quickly resolve or prevent these issues from happening and avoid inefficiencies or failures that will impact your business’s bottom line.
