Motor Bearing Protection Guide

SineWave Guardian® and dV Sentry® Filters

Defining the added benefits of the dV Sentry™ versus a shaft grounding brush

What’s a Shaft Grounding Brush?
A common method across industries is to use a shaft grounding brush or ring to protect motor bearings from common mode over voltage. A shaft grounding brush is not a filter, so it does not help reduce or protect equipment from the actual problem. The current is detected around the motor bearing to an alternative route, thus the current still exists and can cause widespread damage in many other areas in the system.
Effects of Using a Shaft Grounding Brush
Due to their design and placement on motors, shaft grounding brushes have long term costs. The position and contact of these products with the shaft make them prone to wear and a shorter life span, and are not always easy to replace. A shaft grounding brush is also subjected to high levels of oxidation which causes the product to reduce its ability to divert currents from VFDs. All of which result in higher maintenance costs. One of the most problematic issues in using a shaft grounding brush is that it does not reduce the effects associated with common mode over voltage, fast rise time, and peak voltages. This issue can cause failures to surrounding equipment such as: erratic behavior of VFDs and PLCs, insulation failure leading to motor overheating, and unwanted ground faults.
Bearing Fatigue
One of the biggest problems is damage to motor bearings. When common mode current travels through the bearings it causes microscopic pitting. Since this occurs thousands of times per second, the pitting can be a serious problem which often times results in premature bearing failure.

Common Mode Over Voltage
VFDs give off power in a continuous generation of pulses, which on average achieve a sinewave. However, the sum at any point is not always zero, which results in damaging common mode over voltage. This can cause motors to break down over time.

Peak Voltages
The pulse of VFDs is not a clear square pulse. Each rise and fall of the pulse has an over shoot or transient over voltage. This phenomenon is also known as a “reflected wave.” These voltage spikes, especially in long cable leads, can reach dangerous levels and damage drives and motors.

Rise Time
Modern VFDs utilize IGBT systems that create extremely fast rise times which can increase motor performance. However, this can also result in increased motor insulation heating, which can reduce motor life in the long run.

The Best Solution for Motor Protection is the dV Sentry™
The dV Sentry™ is the only filter on the market that features a one of a kind patented Triple Defense Core that is proven to reduce common mode current by 50%. Also, the Triple Defense Core helps reduce peak voltages and rise time. No other filter