In short. Unequal phase loading creates negative-sequence current, which rotates against the rotor, induces double-frequency current and overheats the rotor surface. Motors suffer too. The study quantifies unbalance, checks it against alternator and motor limits and sets protection to trip before damage.
Definitions
Voltage unbalance % = (maximum deviation from average ÷ average) × 100
More precisely, unbalance is the ratio of negative-sequence to positive-sequence voltage. Symmetrical components split any three-phase set into positive, negative and zero sequence parts.
Effects
| Equipment | Effect |
|---|---|
| Alternator | Rotor heating from negative-sequence current; has a continuous limit and an I²t withstand |
| Induction motor | Current unbalance several times voltage unbalance, extra heating, reduced torque |
| Rectifiers and drives | Extra ripple, trips on input unbalance |
Sources of unbalance
- Single-phase loads unevenly distributed.
- Open phase on a feeder, fuse or contact.
- Blown capacitor fuses.
- Unequal tap positions or impedances in transformers.
- Unsymmetrical faults.
Study steps
- Map single-phase loads by phase. Rebalance at design stage.
- Calculate negative-sequence current at the generator for the worst allowed unbalance.
- Compare with the alternator’s continuous negative-sequence rating and I²t limit from the manufacturer.
- Check motor derating for expected unbalance.
- Set protection: negative-sequence relay or phase-unbalance element. See protection coordination.
- Define periodic phase-current checks in maintenance.
Generator-specific cautions
- Generators are more sensitive than a utility because their negative-sequence impedance is higher and the rotor is smaller.
- Single-phase-to-neutral loads on a four-wire system deserve a separate check of neutral current and earthing. See neutral grounding.
Illustrative example
A building with a 250 kVA set has several single-phase floor distribution boards. A clamp-meter survey shows phase currents of 180 A, 120 A and 95 A. The study computes the negative-sequence component and finds it above the alternator’s continuous limit. Moving circuits to balance the phases brings the unbalance down to a small value.
Definition and calculation
Voltage unbalance factor = negative-sequence voltage ÷ positive-sequence voltage × 100
A simple approximation uses line voltages: unbalance ≈ maximum deviation from average ÷ average × 100.
Worked example (illustrative)
Line voltages 410, 418 and 424 V: average 417.3 V. Maximum deviation 7.3 V. Unbalance ≈ 1.7 percent.
Why a small voltage unbalance matters
Induction motors respond with negative-sequence current many times larger than the voltage unbalance. A 3.5 percent voltage unbalance can cause current unbalance of 20 to 30 percent, extra heating and derating of the motor.
| Unbalance | Typical consequence |
|---|---|
| Up to 1 percent | Negligible |
| 2 to 3 percent | Noticeable motor heating, derating suggested |
| Above 3 to 5 percent | Motor protection should trip, generator machine heating |
Check limits against the applicable standard and the machine maker’s guidance.
Sources of unbalance
- Uneven single-phase loads, particularly on a generator.
- Open phase or blown fuse.
- Unbalanced impedance in feeders.
Generator effects
Negative-sequence current heats the rotor surface. Alternators have a continuous negative-sequence limit and an I2²t limit. The study checks that unbalanced single-phase loading stays inside.
See negative-sequence protection and independent-phase regulation.
Common mistakes
- Phase balance checked once at design only.
- No negative-sequence protection on larger sets.
- Ignoring motor heating at low unbalance.
What to send us
Send phase-wise load data. Send your application details.
Questions engineers ask
What is negative sequence?
A component of current or voltage that rotates opposite to the normal direction.
Why does a small voltage unbalance cause large current unbalance in motors?
Motor negative-sequence impedance is low, so it draws large current for a small voltage.
What limit applies to alternators?
Manufacturer data gives continuous and short-time limits. Use those values.
How can I fix unbalance?
Rebalance loads, repair open phases and fix defective connections.
This guide gives general principles. Use manufacturer limits for design.