Voltage unbalance ng supply
Voltage o current unbalance mula sa tatlong line reading.
Subukan ang isang job 4
Nilalagyan ng halimbawang value ang bawat job. Palitan ang mga ito ng sarili mong value.
- Check the voltage unbalance at a motorThe unbalance of 415, 402 and 409 V at a motor starter.
- Voltage unbalance of a rural supplyHow far out a long rural line is at 400, 384 and 396 V.
- Check the current unbalance of a motorThe current unbalance of a motor drawing 31, 27 and 29 A.
- Find the motor heating from unbalanceHow much a 3 % unbalance adds to the winding temperature rise, by the rule of thumb.
Mga input
Worked example gamit ang mga default na input.
- Mga reading
- Mga line voltage
- Boltahe A hanggang B
- 405 V
- Boltahe B hanggang C
- 398 V
- Boltahe C hanggang A
- 411 V
Paglihis sa unbalance
1.65%
- Sequence unbalance
- 1.86%
- Mean ng tatlo
- 404.67V
- Pinakamalaking paglihis mula sa mean
- 6.67V
- Pagtaas ng temperature rise ng winding, ayon sa rule of thumb
- 5.4%
Design aid lang ang resultang ito. I-verify ito laban sa kasalukuyang standard.
Mga hakbang ng kalkulasyon
- Mean ng tatlo404.67V
- Pinakamalaking paglihis mula sa mean6.67V
- Kahulugan ng deviation: 100 × 6.67 ÷ 404.67Resulta1.65%
- Sequence definition: 100 × negative ÷ positive sequence, mula sa tatlong magnitude1.86%
- Pag-init ng motor: 2 × 1.65²5.4%
Tungkol sa calculator na ito
How this is calculated
A three‑phase supply is balanced when its three line voltages are equal and 120° apart. Long rural lines, single‑phase loads spread unevenly and a failing connection all pull the three apart. The tool puts a number on how far apart they are, in the two ways the trade uses.
The deviation definition is the simple one. The tool takes the mean of the three readings and finds the reading furthest from it. The unbalance is that deviation over the mean, as a percent. This is the definition NEMA uses for motors.
The sequence definition comes from symmetrical components. Any three phasors split into a positive sequence, which turns a motor forward, and a negative sequence, which turns against it. The unbalance is the negative sequence over the positive sequence, as a percent. This is the IEC definition. The tool works it from the three magnitudes alone, with a closed formula that holds for line-to-line voltages. For a small unbalance the two definitions are close.
For voltages the tool also gives a motor heating figure. A widely published rule of thumb puts the percent increase in the winding temperature rise at twice the square of the percent unbalance. It is an estimate, not a standard's curve. AmpSize holds no standard for unbalance, so the tool cites none.
The same sums work on three line currents. Current unbalance is often several times the voltage unbalance behind it.
| Step | Table or clause | What it decides |
|---|---|---|
| Mean | The sum of the three readings over three | The reference the deviation is taken from |
| Deviation definition | Largest deviation over the mean | The NEMA style unbalance |
| Sequence definition | Negative over positive sequence, from the magnitudes | The IEC style unbalance |
| Motor heating | Twice the square of the unbalance | A rule of thumb for the increase in the winding temperature rise |
| A reading that cannot close | One reading larger than the other two together | No sequence figure, check the readings |
Worked examples
- A motor starter reading 415, 402 and 409 V is 1.63 % out by the deviation definition and 1.84 % by the sequence definition.
- A rural supply at 400, 384 and 396 V is 2.37 % out by the deviation definition and 2.43 % by the sequence definition.
- A supply at 400, 388 and 412 V is 3 % out by the deviation definition and 3.47 % by the sequence definition. The rule of thumb puts the increase in the winding temperature rise at 18 %.
- A motor drawing 31, 27 and 29 A has a current unbalance of 6.9 % by the deviation definition and 7.99 % by the sequence definition.
Limits
The tool takes three readings at one moment. A supply that swings through the day needs readings at its worst hour, or a logger.
It does not give a motor's rating at an unbalance. That curve is in NEMA MG 1 and in the maker's data, which AmpSize does not hold. Use the maker's figure for a motor that runs near its full load.
Phase-to-neutral readings can carry a zero sequence part that the sequence formula leaves out. Enter line-to-line voltages.
See also: Neutral current, Three phase power and Motor.
Mga tanong
Design aid ang resulta. I-verify ito laban sa kasalukuyang standard.
Paano kinakalkula ang voltage unbalance?
Kunin ang mean ng tatlong line voltage. Hanapin ang reading na pinakamalayo sa mean. Ang unbalance ay ang deviation na iyon na hinati sa mean, bilang porsyento. Ang 405, 398 at 411 V ay may mean na 404.67 V at 1.65 % ang layo.
Ano ang pagkakaiba ng deviation at sequence definitions?
Ginagamit ng deviation definition ang pinakamalaking deviation mula sa mean. Hinahati ng sequence definition ang negative sequence voltage sa positive sequence voltage. Kapag maliit ang unbalance, magkalapit ang dalawa. Ibinibigay ng tool ang pareho.
Bakit nagpapainit ng motor ang voltage unbalance?
Ang negative sequence voltage ay nagtutulak ng current na kumokontra sa rotor. Nagdaragdag ito ng loss at init nang walang trabahong nagagawa. Ayon sa inilathalang rule of thumb, ang porsyentong pagtaas ng winding temperature rise ay humigit-kumulang 2 × ang square ng percent unbalance.
Dapat ko bang sukatin ang line o phase voltages?
Gamitin ang line-to-line voltage. Kailangan ng sequence formula ang tatlong phasor na bumubuo ng triangle, gaya ng line voltage. Ang phase-to-neutral reading ay maaaring may zero sequence part na hindi kasama sa formula.
Kaya ba nitong kalkulahin ang current unbalance?
Oo. Pumili ng line current at i-enter ang tatlong reading. Kadalasan, ilang beses na mas malaki ang current unbalance kaysa sa voltage unbalance na sanhi nito, kaya ang maliit na voltage unbalance ay puwedeng magpakita bilang malaking current unbalance sa motor.
Mga kaugnay na tool
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