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Why High Frequency Drive Emissions are Deadly for Electric Motor Insulation.

Why High Frequency Drive Emissions are Deadly for Electric Motor Insulation.

Sunday, October 30, 2022

Electrical Cancer affects Motor Reliability

Basic electrical theory states that Impedance of a circuit made up of Resistance, Inductance Reactance, and Capacitance Reactance.

The frequency of the Alternating current greatly affects this Impedance.

Xl Inductance Reactance is stated as 2fL, meaning increasing frequency makes the Impedance rise in a linear relationship.  So when High frequency current from Inverter emissions enter a Motor winding being mostly Inductance the Impedance is approaching an finite amount as the frequency is extremely high. Typical drive emissions are in the Mhz range.

This means High Frequency Current may enter a Motor winding but stops within a few turns due to Inductance.

The opposite occurs for Xc Capacitance Reactance which is 1/2fc. Impedance is goes down in a linear relationship with frequency and High Frequency current freely flows.

In an electric motor the Insulation acts as a Capacitor, in fact measuring Capacitance is a good indicator of Insulation health if trended. What this means is High frequency Current see Motor Insulation as low resistance and attracts this deadly current, as I call Electrical cancer.

The other effect with High Frequency is the Skin affect, meaning as the frequency increases the carrying conductor has surface current flow. The center of the conductor is carrying very little current.

The combination of the two affects Skin, and Capacitance Reactance means High Frequency current flows on on the surface trying to find a path to ground. Often this current flows on the surface of insulators.

Insulators become Conducting with Higher Frequencies
Exit Point of High Frequency Current
High Frequency Current tracks down the cable causing decay.

These are examples of insulation decay that can be easily seen, but the damage occurs within Motor Insulation. 3Phi Reliability database shows 37% of Motors in industry are driven by inverters but account for 82% of Winding defects.

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