I am fast realizing that Common Mode Voltage is the most dominant reliability issue of the modern world. As Inverters have taken 37% of our Motor systems (Measured by 3Phi Reliability) they have brought a nasty hidden consequence.

What is Common Mode Voltage?

In a Three Phase Balanced Motor the sum of the Voltages are close to Zero but that all changes when an Inverter is Utilized.

Common Mode Voltage is the unbalance, and its resembles a pushing or pulling of ALL three phases at typically six times per AC cycle.

This Voltage represents a saw tooth triangular signal which being sharp creates Harmonics of very high frequency.

The AC supply passes through a set of Diodes in the Rectifier of an Inverter and charges the DC Bus which acts as an Energy Store before switched into a reformed Output by Pulsed Wave Modulation.

The Diode acts as an One Way valve, when the Supply Voltage has a higher potential than the DC Bus + the forward Bias Voltage the DC Bus is charged. 

What this means is the Rectifier Clips the Wave using only the "Good Bits".

As the Charging Voltage is clipped, the sum of Voltages isn't Zero and this unbalance of current rushes into the DC Bus. It then peaks with the AC Cycles and reverses, cycling six times per Hertz.

This unbalance is the Common Mode Voltage in the shape of a Triangular Wave. Sharp signals act like impacts and when viewed in a Spectra (Fast Fourier Transform) are seen as Harmonics.

Any Voltage, Harmonics (5th especially) or Phase Imbalance in the Supply elevates this Common Mode Current therefore it is essential to check the Supply Quality.

High Frequency Voltages travel to the Motor where they "Hit a brick Wall" that is the Inductance of the Winding Coil. High Frequency Current passes through Capacitance like a dead short but Inductance is the opposite.

Bouncing off Inductance the Voltages reflect meeting the same voltage approaching them. In certain conditions these Voltages meet in Phase and double.  This is when cable damage can occur if the specification isn't right. Voltages of more than twice Line Voltage are experienced.

Motor Insulation being on the Capacitance side passing High Frequency Current gets Attacked.

By far the most effective remedy is to ensure Motors, Cable Screens, Cable Drain PE's, Inverters, and Supplies all have their Grounding verified. By adding earths the Capacitance is greatly reduced stopping this passing of High Frequency Current.   It's like grounding a capacitor to Planet Earth making that plate not charge up, and any charge is neutralized in a low impedance path to ground.

3Phi Reliability experience indicates (by Measurement with SKF TKED1) that 75% of Bearing Fluting, Lubrication & Motor Insulation Decay can be addressed by verification of Grounding.

Note: You need to be thorough in scanning, drive emissions can be quite concentrated in areas. One side of a Cable Gland can be OK, while the opposite have counts.

The Issue comes with the remaining Bearing Currents or in the case where Earthing can not be remedied immediately and you wish to "Stop the Attack at the Source".

Using Insulated Bearings or Grounding brushes work but have a limited life, and do not protect Motor Insulation from being Attacked.

3Phi Reliability database of Motor Tests shows the 37% of Motors on Drives account for 82% of the Winding defects through Motor Testing.

So it makes sense to stop the ATTACK AT THE SOURCE.

Fitting EMF (Electro Magnetic Frequency) Cores on the Inverter Output terminals act to Burn High Frequency Current.

These EMF Cores apply Impedance above 40kHz so not to affect normal 50/60 Hz supply and are effective means of Stopping these Bearing Currents.

It is very easy to see how effective these EMF Cores are, by taking a thermal Image it shows these High Frequency Currents being burnt and dissipated.

In nearly all case applying these techniques will eliminate more than 99% of Bearing Currents, but I can't stress enough most of the problem occurs in poor earthing.

www.3Phi-Reliability.com are a specialist in solving Bearing Fluting, EDM, Bearing Current issues.

The magnitude of Common Mode Voltage is minimized in a balance supply, problems in Neutrals, Earth Grids, Supply Voltage can amplify the issue and therefore must form part of the Root Cause.

Mark Gurney Motor Analyst

3Phi Reliability