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Should Shaft Grounding be Used When a Motor is Used with a Variable Frequency Drive?

Can a Variable Frequency Drive Cause Motor Bearing Failure?


Although avoidable and uncommon, unchecked bearing currents can cause motor failure. Most common problems with bearings are due to improper lubrication, misalignment between motor and fan shafts, or an unbalanced fan.



Summary


If a Variable Frequency Drive (VFD) is installed carefully following the manufacturer's best wiring practices, it's not necessary to install shaft grounding devices and bearing insulation.


Best wiring practices include:


  • Keeping the cable between the VFD output and motor short (install the VFD close to the motor).

  • Using a four-wire cable in metallic conduit grounded at both ends between the VFD output and motor, or a symmetrical shielded motor cable (especially for longer cables).

  • Coupling separate parts of a conduit and bonding the conduits to the drive PE (Protective Earth) and motor frame. Utilize separate conduits for input power and motor.

  • Employing Ecogate Master Cable for control, ensuring proper twisting and shielding of the Modbus pair, and connecting shielding at one point only (as described in our installation guide).


There are conflicting recommendations on whether to use additional shaft grounding devices and optional filters (e.g., DV/DT); the decision ultimately depends on the specific situation.



How Common is the Problem with Motor Bearing Current?


In reality, these problems are not common. Jouni Ikaheimo, Technology Manager for ABB's Low Voltage Motors business, has stated that advances in design and installation procedures have significantly reduced bearing current issues for large motors. However, there have been a few reports of bearing currents in motors with smaller frame sizes in VFD applications. ABB's focus on quality and customer care has led them to conduct extensive testing to identify and address the causes of bearing currents.


As this is a very uncommon problem, Ecogate recommends carefully following best wiring practices, without using additional measures (such as shaft grounding devices, insulated bearings, and VFD filters) to maintain reasonable VFD installation costs.



Why Does Motor Bearing Current Exist if Motor is Driven by the VFD?


Voltage of the 3-Phase Power Supply for AC (Alternating Current) Motors - Graph
Voltage of the 3-Phase Power Supply for AC Motors

The power supply for AC (Alternating Current) motors works in a 3-phase system. This means that the voltage, which follows a wave-like pattern, flips back and forth sixty times per second if you're in the USA. Each of the three phases is delayed by 120 degrees compared to the last. Because of this delay, the combined voltage of all three phases - what we call the "Common Voltage" - adds up to zero at any given moment.


Model of 3-Phase Synchronous Electric Motor


VFDs alter the speed of a motor, generating output voltage at varying frequencies through the use of output transistors that switch at different intervals. This mimics the effect of a sinusoidal output for the rotation of the motor.


VFD Output Voltage & Current - Graph
VFD Output Voltage & Current
VFD Output at different frequencies - Graph
VFD Output at different frequencies

However, this process results in fast-rising voltage pulses and high switching frequencies, leading to a non-zero sum of voltage or common mode voltage. Unlike with a pure sinusoidal signal, the phase voltages do not cancel each other out. This voltage can find a path through the motor bearings causing them damage due to electrical discharges.


Unlike DC current, which flows only in conductors, AC current travels at the surface of conductors and even beyond through capacitors and transformers, or by wireless transmission. The conductors to the motor can, in this case, act as a capacitor, a transformer, and an antenna, creating unwanted effects known as EMI (Electro-Magnetic Interference). To mitigate these effects and keep the maximum of the AC current inside the cable, good wiring practice is crucial.



Good VFD Wiring Practice


Modern Variable Frequency Drives and VFD-rated cables are designed to limit unwanted common voltage and electromagnetic interferences (EMI) by optimizing the spectrum of the VFD output and utilizing filters. VFD-rated cables are shielded to reduce the likelihood of wires acting as antennas and limit capacitance between phase wire and ground.


Proper Variable Frequency Drive (VFD) Wiring - Diagram
Proper VFD wiring to limit Common Mode & EMI interferences

For optimal wiring of your VFD to limit common mode voltage and EMI, adhere to the following guidelines:


  • Keep the cable between the VFD output and motor short, installing the VFD as close to the motor as possible.

  • Use a four-wire cable in metallic conduit grounded at both ends, ensuring a dedicated ground cable. Alternatively, for longer cable lengths, use a symmetrical shielded motor cable.

  • Connect separate parts of a conduit and bond the conduits to the drive enclosure and motor frame. Maintain separate conduits for input power and the motor.

  • Use Ecogate's Master cable for control. This ensures the Modbus pair is properly twisted and shielded, with the shielding connected at one point only (at the greenBOX Control Unit, as outlined in our installation guide).



Can Shaft Common Mode Voltage be Measured?


If you're an electronic engineer, you can measure Common Mode Voltage, with caution and using a battery-operated oscilloscope (connect the probe to the motor shaft and switch the oscilloscope to automatic mode). While there's no precise rule about acceptable shaft voltage, it can vary according to the motor-drive package and the nature of the installation. Motor manufacturers typically don't publish maximum allowable voltages, but comparisons between motors and changes over time can be insightful.



What are the Options if High Common Mode Voltage is Detected at a Particular Installation?


If you suspect a problem, bearing currents are relatively easy to check for and solve. Initial indicators of a bearing current issue can include an increase in noise and vibration. Sometimes problems with bearings are due to improper lubrication, which is the owner’s responsibility as per the motor's user manual. Other potential issues include improper alignment between the motor and fan shafts or an unbalanced fan.


To eliminate bearing current, you can install insulated bearings and shaft grounding devices, such as the Baldor Shaft Grounding Brush Assembly or the Aegis bearing protection ring.


Baldor Shaft Grounding Brush Assembly
Baldor Shaft Grounding Brush Assembly

Baldor Shaft Grounding Brush Assembly (inside motor)
Baldor Shaft Grounding Brush Assembly (inside motor)

The Baldor Shaft Grounding Brush Assembly can be installed inside the motor (motors with shaft grounding installed can also be ordered).


In special cases, various filters (input and output reactors, EMC filters, DV/DT filters, sine wave filters) can be used. These improve the output current, eliminating peaks and removing high frequencies, making the signal closer to a pure sinusoidal 3-phase signal. Filters are particularly useful if installations use old motors not VFD rated, for motors in aggressive environments or running at high temperatures, for applications with a risk of arc flash, or if motor braking is used.


Please contact us if you have additional questions about Common Mode Voltage or motor bearing current.





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