Testing Shaft Voltages for Bearing Current Protection

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test for shaft voltage

Motor bearings are essential components in electric motors that provide support and facilitate smooth rotation of the motor’s rotating shaft. However, as electrical currents flow through motor bearings this can cause damage and shorten your motor’s lifespan.

Understanding the relationship between damaging electrical currents also known as shaft or common mode voltage, and their impact on motor performance is crucial. By recognizing this connection, you can take proactive measures to address damaging currents effectively, preventing costly repairs and machine downtime.

Understanding Common Mode Voltage and Its Effects on Motor

BearingsCommon mode voltages are high-frequency electrical currents that flow through the motor bearings in electric motors. These currents are typically caused by voltage variations on the motor shaft, especially in systems that use variable frequency drives (VFDs). The pulsed voltage output from VFDs can create imbalances and rapid switching between positive and negative states, leading to the flow of high-frequency currents through the motor bearings. These high-frequency electrical currents damage the motor bearings in two ways:

  1. Causes degradation of the bearing grease the roller assemblies are suspended in
  2. Through electrostatic machining (ESD) of the bearings and bearing raceways

The combination of the two can be devastating to the life of the motor bearing. Helwig Carbon offers a comprehensive solution to mitigate the detrimental effects of common mode voltage. With innovative technology, such as  Bearing Protection Kits™ (BPKs) and the BPK-Probe™, you can proactively safeguard your motors and ensure long-lasting durability.

A: Frosted ball bearings   B: Flutted bearing raceway   C: Bearing grease comparison (Black-Old vs. Blue-New)

How to Assess Motor Bearing Risk with Common Mode Voltage Measurements

We are often asked, “Why should we measure shaft voltage when current is the cause of damage to motor bearings?” While it is true that induced shaft current discharges cause damage to the motor bearings, measuring for current can be tricky, requiring specialized equipment and technical expertise. The only way to actually measure the discharge current in a motor is to electrically isolate the bearing from the motor frame, then add a wire from the bearings outer race to the motor frame.  The current through the wire can be measured and plotted along with the common mode voltage.  This type of testing is done by Ph.D.s in a lab not technicians in the field. 

By using Ohm’s Law (the flow of current is driven by voltage), the BPK-Probe™ can determine that with the presence of voltage, the current is also present. Therefore, the Probe determines the risk to the motor bearings by effortlessly measuring the voltage on the motor shaft.

testing shaft voltage, screenshots

A direct relationship with a single voltage discharge and a single subsequent current discharge captured at the same point in time. Voltage (yellow) measured by the BPK-Probe, Current (blue) measured by a Rogowski coil


example of the BPK Probe, which tests for shaft voltage


  • Color Display (LCD) for enhanced viewing capabilities
  • Durable silver graphite testing tips
  • Thin, compact design for tight spaces
  • Simple action test trigger
  • Micro SD card for recording test data
  • Rechargeable battery by Micro USB
  • BNC output for oscilloscope readings
  • Embedded Bearing Protection Kit


Get Started Measuring Shaft Voltage with Ease

If you would like to learn more about testing electric motor shafts for damaging electrical currents or how to utilize the BPK Probe, our team of expert engineers is here to assist you.

Brian Stone
Application Engineer II
[email protected]

Helwig Carbon Products, Inc. is the premier, family-owned, American manufacturer, of carbon brushes, brush holders, spring assemblies, bearing protection kits, metal graphite brushes, and mechanical carbons. Since 1928, we have forged innovations that have since become industry standards and requirements. We pride our self on manufacturing all of our products to the highest standard and performance because we know your application depends on it!


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