In short. Drives, UPS rectifiers, LED drivers and chargers draw distorted current, and that current distorts the voltage. A generator has higher source impedance than the grid, so the same load causes more voltage distortion on generator supply. Screen the share of non-linear load against the generator size, look for symptoms, and measure before deciding on filters, a larger alternator or a full study.
Who this page is for: For facility managers and engineers with drives, UPS or IT loads on generator supply.
Key facts
- Higher non-linear share means higher risk.
- Six-pulse drives without filters are the commonest source.
- Small generators relative to non-linear load suffer most.
- Symptoms: voltage regulator hunting, hot neutrals, flicker, capacitor failures.
- Plain capacitors can resonate with harmonics.
- Measure on both mains and generator supply.
Worked example: two sites screened (illustrative)
| Question | Office building | Water pumping station |
|---|---|---|
| Non-linear share | 20–50% (IT, LED, small UPS) | Over 50% (pump drives) |
| Six-pulse drives without filters? | No | Yes |
| Generator size against non-linear load | More than 2 times | Between 1 and 2 times |
| Symptoms | None | Voltage regulator hunting on generator |
| Plain capacitors? | No | Yes |
| Outcome | Low risk; measure once to confirm | High risk: measure, then filters and detuned capacitors |
At a site like the pumping station, voltage distortion is usually much higher on generator than on mains. Filters on the largest drives and detuned capacitors are the usual way to bring it under control. See harmonic studies and power factor correction.
Do not add plain power-factor capacitors to a site with large drives without a harmonic check. They can resonate and fail, or make distortion worse.
Mitigation options
| Option | Use |
|---|---|
| Line reactors or DC chokes on drives | Low cost first step |
| Passive or active filters | Large drive loads |
| Low-harmonic drives or UPS | New equipment |
| Larger alternator | Lowers distortion on generator supply |
| Detuned capacitor banks | Avoid resonance |
See power quality problem finder.
Screening sheet
| Question | Answer |
|---|---|
| Non-linear share of load | |
| Drives without filters | |
| Generator size against non-linear load | |
| Symptoms |
Common mistakes
- Measuring on mains only.
- Plain capacitors with drives.
- Generator sized on kW alone.
- Filters chosen without measurement.
What to send us
Send your load list with drives and UPS, and any measurements. WhatsApp 98852 01608 or use the application engineering enquiry.
Evidence note. The example is illustrative.
Questions engineers ask
What is THD?
Total harmonic distortion, the ratio of harmonic content to the fundamental, for voltage or current.
Why do generators suffer more than the grid?
Higher source impedance turns harmonic current into more voltage distortion.
Can a UPS upset a generator?
It can, through input current distortion and step loading. Check the UPS input specification.
Do I need a filter?
Only if measured or studied distortion exceeds limits or causes problems.
Thresholds are illustrative. Use applicable standards and manufacturer guidance.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and Service Engineer, Reddy Generators. Last updated 8 November 2026.