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

Updated: 2 days ago

Can a Variable Frequency Drive cause a motor bearing failure?


Even though avoidable and not common, unchecked bearing currents can cause motor failure, but most commonly problems with bearing are caused by improper bearing lubrication, improper alignment (between motor & fan shafts), or by unbalanced fan.


Summary


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


Best wiring practice includes:

  • The cable between the VFD output and motor should be short (install the VFD close to the motor).

  • The cable between VFD output and motor should be a four wire cable in metallic conduit grounded at both ends (dedicated ground cable is always required) or a symmetrical shielded motor cable (especially if the cable is long).

  • Couple separate parts of a conduit together. Also bond the conduits to the drive PE (Protective Earth) and motor frame. Use separate conduits for input power, and for the motor.

  • Control cable should be Ecogate MASTER cable - the Modbus pair is properly twisted and shielded, and shielding must be connected at one point only (as it is described in our installation guide).

There are a lot of contradicting recommendations whether additional shaft grounding devices and optional filters (for example dv/dt) should be used; it really depends on who you talk to (and all of them will put the decision on you, as they do not like to take responsibility).



Is the Problem with Motor Bearing Current Common?


In reality these problems are not common; for example, here is a comment published by Jouni Ikaheimo, Technology Manager for ABB's Low Voltage Motors business: “Advances in design and installation procedures have gone a long way to solving bearing currents for large motors”.


However, recently there have been a few reports of bearing currents in motors with smaller frame sizes – up to around 55 kW (75 HP) – in variable frequency drive (VFD) applications. At ABB, we have received reports relating to some tens of installations at European production facilities.


Although this is not a high number (ABB makes over one million motors per year), we are committed to maintaining the highest possible reputation for quality and customer care. Therefore, we have carried out exhaustive testing to identify the cause of bearing currents and to develop solutions. It appears that the recent issues are due to capacitive induced bearing currents, resulting from the common mode voltage in the drive system. The possibility of damage arising from them is closely associated with the specific installation conditions. The statistical significance of this data is still quite low due to the small number of actual cases reported.


Most users of industrial motors need not to worry unduly about capacitive bearing current issues as they are very rare.

As you see, this is a very uncommon problem, therefore Ecogate recommends to follow carefully the best wiring practice, but not to use additional measures (as shaft grounding devices, insulated bearings, and VFD filters) to keep VFD installation costs reasonable (additional filters will improve VFD output, but also increase losses and cost). We are planning to measure Common Mode Voltage as a part of our commissioning procedure.



Why Motor Bearing Current Exits if Motor is Driven by the VFD?


If you’d like to understand what can potentially cause a motor bearing failure with the VFD, continue reading for technical details.


The voltage of the 3-phase power supply for AC (Alternating Current) motors looks like this:


The voltage is sinusoidal, changing sixty times per second (in USA) from positive values to negative and each phase is shifted in time (for rotational motors: 360 degree divided by three = 120 degree).


If you check “Common Voltage” = sum of the voltages of all three phases at any time, the sum is zero.


Model of 3 phase synchronous electric motor


The motor is running at frequency (speed) of this 3-phase voltage (technically minus so called slip). If we’d like to use a variable frequency drive to change the motor speed, the VFD must generate output voltage at different frequencies. It would be prohibitively expensive to generate sinusoidal output voltage, so instead output transistors of the VFD are switching at different intervals to achieve same effect (rotation of the motor shaft) as with sinusoidal output:


For different frequencies it looks like this:


The biggest problem with the fast rising voltage pulses and high switching frequencies output is that the sum of voltage (as generated by the VFD) is not zero, therefore now we need to handle common mode voltage. In other words, phase voltages are not cancelled out as with pure sinusoidal signal. If common mode voltage is present at the AC motor, the current will follow the path of least resistance to the common reference point (ground), which is unfortunately through the motor bearings. As the AC motor current passes through the bearings, electrical discharges are damaging the bearings (frostings or pits). This type of output is causing a lot of high frequency harmonics in output of the VFD.


The DC (Direct Current) current is flowing only in conductors, but AC current travels at surface of conductors, but also through capacitors (electrostatically coupled) and transformers (magnetically coupled), and by wireless transmission. The conductors to the motor act as a capacitor, a transformer, and as an antenna. In this case these effects are parasitic = unwanted; (sometimes these effects are wanted: our cell phones would not work without them). We call these unwanted effects EMI (Electro-Magnetic Interference). As the AC can travel outside the conductors, we need to use all mitigations possible to keep maximum of the AC current inside the cable, and inside the metallic conduit by following good wiring practice.



Good VFD Wiring Practice


The modern Variable Frequency Drives and VFD rated cables are designed in a way to limit unwanted common voltage and limit the electromagnetic interferences by optimizing the spectrum of the VFD output and by utilizing filters.


The VFD rated cables are shielded to limit the possibility of wires acting as an antenna, and will limit capacitance between phase wire and ground etc. Good wiring of the Variable Frequency Drive to limit Common Mode Voltage and EMI interferences looks like this:



  • The cable between the VFD output and motor should be short (install the VFD close to the motor).

  • The cable between VFD output and motor should be four wire cable in metallic conduit grounded at both ends (dedicated ground cable is always required) or a symmetrical shielded motor cable (especially if the cable is long).

  • Couple separate parts of a conduit together. Also bond the conduits to the drive enclosure and motor frame. Use separate conduits for input power, and for motor.

  • Control cable should be Ecogate MASTER cable. The Modbus pair of the Ecogate MASTER cable is properly twisted and shielded. The shielding must be connected at one point only (at greenBOX control unit as it is described in our installation guide).



Can Shaft Common Mode Voltage be Measured?


If you are an electronic engineer, you can actually measure Common Mode Voltage (by carefully following all safety precautions!) by using a battery operated oscilloscope: connect the probe to the motor shaft and switch the oscilloscope to automatic mode. There is no precise rule about what an acceptable shaft voltage is and this will vary according to the motor-drive package and the nature of the installation. The motor manufactures are not publishing what are maximum allowable voltages, but you can still compare motor to motor, and see changes over time.



What are the options if you see high Common Mode Voltage at a particular installation?


If you think you might have a problem, bearing currents are easy to check for and straightforward to solve. In most cases your first indication of a bearing current issue is an increase in noise and vibration. Sometimes problems with bearing are caused by improper bearing lubrication. The lubrication is the owner’s responsibility, the schedule is in the motor's user manual. Another possible issue is are improper alignment between motor & fan shafts, or by an unbalanced fan.


To eliminate bearing current you can install insulated bearings and shaft grounding devices, for example Baldor Shaft Grounding Brush Assembly (or Aegis bearing protection ring, or Inpro Seal Current Diverter Ring & Motor Grounding Seal).


Baldor Shaft Grounding Brush Assembly

Baldor Shaft Grounding Brush Assembly can be installed inside the motor (you can order a motor with shaft grounding installed)


Additionally, in special cases you can use various filters; they are available from manufacturers as an option (input and output reactors, EMC filters, dv/dt filters, sine wave filters).


The line reactors, sine filters, and dv/dt filters are improving output current - eliminating peaks, removing high frequencies i.e. signal is closer to pure sinusoidal 3-phase signal. It makes sense to use output filters if installations using old motors that are not VFD rated, for motors in aggressive environments or running at high temperatures, for applications with risk of arc flash; for applications with very short motor cables, or if motor braking is used.


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




Author: Ales Litomisky


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