Grounding the Motor Shaft Improves Reliability of Inverter-Powered Motors

Maintenance engineers on the tops of commercial buildings or industrial plants are regularly relubricating motors and checking for other signs of fatigue, and without preventive maintenance tools or advanced predictive control software to provide alerts, engineers may stop and think, “What are those motors that are getting worse?” Is it getting louder, or is this just my imagination?” The experienced engineer’s internal sensors (hearing) and hunches (predictive alarms) of the motor may be correct, over time, the bearings are in the middle of no one’s awareness. Premature wear in the case, but why? Be aware of this “new” cause of bearing failure and know how to prevent it by eliminating common mode voltages.

Why do motors fail?

While there are many different causes of motor failure, the number one cause, time and time again, is bearing failure. Industrial motors often experience a variety of environmental factors that can adversely affect the life of the motor. While contamination, moisture, heat or incorrect loading can certainly cause premature bearing failure, another phenomenon that can cause bearing failure is common mode voltage.

Common mode voltage

Most motors in use today run on cross-line voltage, which means they are connected directly to the three-phase power entering the facility (via a motor starter). Motors driven by variable frequency drives have become more common as applications have become more complex over the past few decades. The benefit of using a variable frequency drive to drive a motor is to provide speed control in applications such as fans, pumps and conveyors, as well as running loads at optimum efficiency to save energy.

One disadvantage of variable frequency drives, however, is the potential for common mode voltages, which can be caused by an imbalance between the drive’s three-phase input voltages. The high-speed switching of a pulse-width-modulated (PWM) inverter can cause problems for motor windings and bearings, the windings are well protected with an inverter anti-spike insulation system, but when the rotor sees voltage spikes accumulating, the current seeks Path to least resistance to ground: through bearings.

Motor bearings are lubricated with grease, and the oil in the grease forms a film that acts as a dielectric. Over time, this dielectric breaks down, the voltage level in the shaft increases, the current imbalance seeks the path of least resistance through the bearing, which causes the bearing to arc, commonly known as EDM (Electrical Discharge Machining). Over time, this constant arcing occurs, the surface areas in the bearing races become brittle, and tiny pieces of metal inside the bearing can break. Ultimately, this damaged material travels between the bearing balls and the bearing races, creating an abrasive effect that can cause frost or grooves (and potentially increase ambient noise, vibration, and motor temperature). As the situation worsens, some motors can continue to run, and depending on the severity of the problem, eventual damage to the motor bearings may be inevitable because the damage has already been done.

based on prevention

How to divert the current from the bearing? The most common solution is to add a shaft ground to one end of the motor shaft, especially in applications where common mode voltages may be more prevalent. Shaft grounding is basically a way of connecting the rotating rotor of a motor to ground through the motor frame. Adding a shaft ground to the motor (or buying a pre-installed motor) prior to installation can be a small price compared to the maintenance costs associated with bearing replacement, not to mention the high cost of facility downtime.

Several types of shaft grounding arrangements are common in the industry today. Mounting carbon brushes on brackets is still popular. These are similar to typical DC carbon brushes, which basically provide an electrical connection between the rotating and stationary parts of the motor circuit. . A relatively new type of device on the market is the fiber brush ring device, these devices work in a similar way to carbon brushes by laying multiple strands of conductive fibers in a ring around the shaft. The outside of the ring remains stationary and is usually mounted on the motor’s end plate, while the brushes ride on the surface of the motor shaft, diverting current through the brushes and safely grounded. However, for larger motors (above 100hp), regardless of the shaft grounding device used, it is generally recommended to install an insulated bearing on the other end of the motor where the shaft grounding device is installed to ensure that all voltages in the rotor are discharged through the grounding device.

in conclusion

Variable frequency drives can save energy in many applications, but without proper grounding, they can cause premature motor failure. There are three things to consider when trying to reduce common mode voltages in variable frequency drive applications: 1) Make sure the motor (and motor system) is properly grounded. 2) Determine the proper carrier frequency balance, which will minimize noise levels and voltage imbalance. 3) If shaft grounding is deemed necessary, select the grounding best suited for the application.