Electric motors play a vital role in our everyday lives - they make almost everything that moves, move. Nearly 70 percent of electricity consumed by industry is used by electric motor systems. In an effort to reduce energy consumption – thereby reducing both operating costs and environmental impact – many industrial electric motors are paired with a variable speed drive. VSDs control a motor’s speed by varying the frequency and voltage supplied to the motor. By controlling a motor’s speed, a drive can reduce power consumption and provide an improvement in process control as well as a significant cost of operation savings over the life of the motor.

While VSDs save energy, they can lead to premature motor failure if not properly grounded. There are many causes of motor failure, but the most common issue when using a drive is bearing failure caused by common mode voltage. Common mode voltage is an imbalance between the three phases, or the voltage difference between the power source ground and the neutral point of the three-phase load. Common mode voltage induces voltage on the motor’s shaft, and this shaft voltage can discharge through the windings or through the bearings. Modern engineering designs, phase insulation and inverter spike-resistant wire can help protect the windings; however, when voltage spikes build on the rotor, the current seeks the path of least resistance to ground. In the case of an electric motor, this path usually runs directly through the bearings, then through the motor’s housing, to electrical earth ground. This movement of electrical current causes arcing in the bearings, which breaks down the bearing race, resulting in tiny pieces of metal breaking off inside the bearing. Eventually, the damaged material works its way between the bearing’s balls and races, causing grinding and pitting, noise, imbalances and, eventually, complete motor failure.

Proper grounding methods reduce the likelihood of common mode voltage; however, in some installations, robust grounding is not always available. In these instances, another method must be used to divert this current away from the bearings. The most common solution is the addition of a grounding device to the motor shaft. A shaft ground is a connection of the motor’s rotor to earth ground via the frame of the motor. Adding a shaft grounding device to the motor prior to installation (or buying a motor with one pre-installed) is a small price to pay when compared to the price tag of maintenance costs associated with bearing replacement, not to mention the high costs of downtime in a facility.

Shaft grounding devices and other bearing protection methods:

Carbon grounding brushes provide an electrical connection between the rotating and stationary portions of the motor’s electrical circuit and take the current from the rotor to ground so that the charge does not build up on the rotor to the point where it discharges through the bearings. Grounding brushes offer a practical and economical means to provide a low-impedance path to ground, especially for larger frame motors. See ABB’s HVAC motors with grounding brushes.

Shaft-grounding rings contain strands of conductive fibers arranged inside a ring around the motor’s shaft. The outside of the ring remains stationary, while the brushes ride on the surface of the shaft, directing the current through the brushes to ground. Unlike carbon brushes, shaft-grounding rings can be mounted inside the motor, allowing them to be used on washdown duty and dirty duty motors. See ABB’s HVAC motors with shaft-grounding rings.

Grounding bearing isolators combine a two-part, non-contact isolation shield and a metallic rotor and isolated conductive filament ring to divert shaft currents away from the bearings. Since these devices also prevent lubricant loss and contamination, they replace standard bearing seals and traditional bearing isolators.

Ceramic bearings feature ceramic-coated balls that protect the bearings by preventing shaft current from flowing through the bearings to the motor. Since no electrical current flows through the motor bearings, there is little chance of current-induced wear; however, the current will seek a path to ground, which means it will go through attached equipment. Since ceramic bearings will not remove the current from the rotor, only specific direct-drive applications are recommended for motors with ceramic bearings. See ABB’s critical cooling motors.

While protecting the bearings is the first step to preventing motor failure, ongoing maintenance is critical to the long-term health of the motor. ABB Ability™ Smart Sensors mount to any motor to monitor the health of the motor remotely. The sensor measures key parameters from the surface of the equipment, which can be used to gain meaningful information on its condition and performance. Alerts help identify potential causes of future motor breakdowns, such as bearing failures due to common mode voltage, and ABB Ability provides a safe way to retrieve information from motors or drives located in dangerous or difficult-to-access locations, allowing maintenance inspections to be conducted from a distance.