Harmonics in industrial electric drives: causes, consequences and ESA diagnostics

At modern Kazakhstan plants the share of drives with variable frequency drives (VFDs) keeps growing. At the same time the network carries rectifiers, UPS units and arc furnaces in neighbouring shops. The result is harmonics in voltage and current. For the maintenance team the question is practical: is this "normal noise" or a sign of a problem that requires a shutdown?

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Where harmonics come from

Source	Typical frequencies	Where you meet it
VFD (inverter)	5th, 7th, 11th, 13th supply harmonics	pumps, conveyors, fans
Rectifiers	5th, 7th	electrolysis, chargers
Phase asymmetry	negative sequence	ageing network, damaged cables
Transformer saturation	odd harmonics	substations, CHP plants

Harmonics heat the windings, accelerate insulation ageing, reduce efficiency and can cause nuisance protection trips.

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How harmonics interfere with diagnostics

Classic vibration analysis is barely affected by supply quality. But current analysis that ignores harmonics produces false "bearing faults". That is why the system must:

1. Measure both current and voltage on all phases.
2. Separate the supply components from the mechanical ones (modulation at running speed).
3. Trend over time rather than take a single snapshot.

Artesis e-MCM and e-PCM for generators perform a full spectral analysis against a machine model.

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When harmonics are a symptom of a fault

Not all harmonics are external. A rise in certain components with a stable supply may mean:

• an inter-turn short in the winding;
• rotor bar damage — sidebands at slip frequency;
• a stator air gap problem — torque pulsation;
• mechanical resonance at certain VFD speeds.

ESA correlates spectrum changes with load and speed, separating process effects from electrical ones.

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VFD drives: specifics for maintenance

Typical mistakes on VFD-driven pumps:

• long operation at the minimum frequency (motor overheating);
• long motor cables — voltage reflections;
• wrong carrier settings — rising peak voltages.

Artesis records the operating point (power, cos φ, slip) and warns of running outside the allowable zone — see pump diagnostics and cavitation.

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Practical recommendations

1. Record a baseline spectrum after the VFD is commissioned — do not compare with a "classic" induction motor without a VFD.
2. If the current THD rises — check phase symmetry and the contacts in the cabinet (SDT ultrasound on electrical equipment completes the picture).
3. On critical drives use online ESA, not a one-off multimeter reading.
4. For generators and gensets — e-PCM taking load harmonics into account.

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Relation to other methods

Task	Method
Supply quality at the busbars	network analysers, oscilloscopes
Drive mechanics	vibration plus SDT ultrasound
Complete picture of the motor	Artesis ESA
Comparing ESA / MCSA / vibration	separate article

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Conclusion

Harmonics are no reason to abandon current-based diagnostics. On the contrary: proper ESA accounts for them and isolates mechanical faults against a background of VFDs and a "dirty" supply. For power and mining sites rolling out VFDs at scale, this is a critical advantage of Artesis over simplified MCSA.

See also: 5 motor faults, monitoring by industry.