Distribution Board humming or chattering?

Sunday, April 5, 2020

As a follow on from the Bearing fluting blog, I had a comment that DOL (Direct on line) systems also see issues. Data shown in this example is from my electical supply at my property.

In the bearing fluting blog I explained the electrical resonance between a Variable speed drive and the motor, resulting in induced voltage across the air gap onto the shaft.

Electrical resonance is not limited to VSD's, but fortunately most resonant frequencies are audible and can be heard relatively easily. In my case I had failures of applicances, two neutrals failed on circuits before remedies inacted.

This chart shows the peak current seen on a Ir 25 amp circuit.

When an electrical resonant event occurs the supply becomes very unstable and significant damage can occur in motor circuits. (Dominantly motor circuits), and the distribution board emits audible chatter. Induced  voltages from resonance can be many times line voltage and insulation material suffers.

As in mechanical resonance, a stiff strucutre with lots of mass is less prone to resonance. In an Electrical circuit a strong fault current and minimal cable capacitance is its equivalent. Unfortunately some of the these factors are controlled by the supply company.

An example of the when the damge can occur is when a "Ferranti Effect" event happens . This when a motor is stopped or supply fails and the flux feild collapses into the capacitance of the cable causing huge current spikes. My example 82 Amps on a 25 Amp circuit.

This effect is common in weak supplies, (long cable runs, small or overloaded transformers) and the natural resonance frequency of the circuit can be calculated easily.

Natural Freq = Supply Freqx sqrt (PSC/kVAr)

In my case it calculated at just over 350hz.

I collected data from a Hioki Power Analyser and the data used in this calculation being kVAr.

The negative kVAr indicates that current is in the direction of the transformer (Outgoing) from the "Ferranti Effect".

As in mechanical resonance a trigger must be present to start the resonance. So in the data collected I matched exactly the occurances of resonance with a 175hz signal.

In this example the circuit natural frequency is just over 350hz, 2x the trigger of 175hz being the ripple control of the supply company.

So I designed a high pass filter to dampen this trigger and the problem has been solved, and my power quality has improved and no more failures.

3Phi Reliability offers Electrical Supply Analysis, Harmonic Mapping and Recommendations.

Mark Gurney, Gerant 3Phi Reliability Sarl





No comments yet