Why We Test For Shaft Voltage (And You Should, Too!)

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test for shaft voltage

Bearing discharges damage the motor bearings in two ways: the degradation of the bearing grease the roller assemblies are suspended in, and by electrostatic machining (ESD) of the bearings and bearing raceways. The combination of the two can be devastating to the life of the motor bearing and should be mitigated to avoid costly repairs and machine downtime.

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

Helwig Carbon Products offers a complete line of Bearing Protection Kits™ (BPKs) to mitigate the damaging electrical discharges from the beginning. Additionally, Helwig offers a diagnostic device to measure the risk to the motor bearings called the BPK-Probe™. This diagnostic device was designed for a technician to use in the factory or the field to measure the risk to the motor within seconds! The technician can also simulate the installation of a BPK with the rocker switch. The Probe’s embedded BPK will mitigate the problem and shunt induced shaft voltages to ground, proving its effectiveness with less than 1V on the shaft!


We are often asked,

“Why does the BPK-Probe™ measure voltage when current is the cause of damage to the 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 shaft voltage.  This type of testing is done by Ph.D.s in a lab not Technicians in the field.  By using Ohms Law (the flow of current is driven by voltage), the Probe can determine that with the presence of voltage, the current is also present. Therefore, the BPK-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


BPK-Probe™ Features:

  • 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

example of the BPK Probe, which tests for shaft voltage

The BPK-Probe™ is engineered for motor shaft voltage measurements without the need for expensive, specialized equipment or training.  Technicians in the field or factory can effortlessly measure and determine if there is a risk to the motor bearings. If requested, the captured test data or oscilloscope screenshots can be shared with Helwig Carbon’s Engineering Department for further evaluation and recommendations.

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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