Three-phase electricity explained
Electrical Science · C&G 2365 electrical science · Free course section with an account
Three-phase supplies use three alternating displaced by a third of a cycle. A balanced load has equal phase with matching phase relationships; their vector sum is zero in the ideal sinusoidal case. Real installations need assessment of imbalance and harmonics rather than an assumption that a neutral never carries current.
In a balanced star connection, line is √3 times phase voltage and line equals phase current. In delta, line voltage equals phase voltage and line current is √3 times phase current. For a balanced load, real power is √3 × line voltage × line current × .
An induction motor needs slip to develop torque. Starting method, load torque, supply conditions and the motor’s ratings affect the design. Star-delta starting is suitable only for an appropriately rated motor and load; it is not a universal way to improve any motor installation.
The Electrical Science lessons introduce these distinctions before calculations. The scratchpad supports star/delta conversions, balanced power, an ideal neutral- model and motor slip.
Keep arithmetic separate from safe work
Updated
What to understand before practising
A balanced-load calculation describes a stated model. It does not establish that a conductor is safe to touch or that an actual circuit has the required protection. Use the result as part of design and verification, alongside the relevant installation information.
Sources
- HSE: electrical safety questions — Isolation, competence and precautions before electrical work.
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Common questions
- Can I try Three-phase electricity explained free on this page?
- This guide is free to read, but there is no public quiz specifically for this topic on this page. Browse public practice samples for other subjects, or create an account for the learning path. This course section is free with an account.
- Where does Three-phase electricity explained sit in the City & Guilds scheme?
- Three-phase electricity explained maps to electrical science. On TradeTraining it lives in the Electrical Science section as bite-size lessons with practice questions.
- Does balanced mean the neutral is always safe to touch?
- No. The ideal model is not a safe-isolation test. Actual imbalance, harmonics, faults and other supplies still need assessment.
- Does a zero calculated neutral current prove safe isolation?
- No. Safe work depends on the actual sources, circuit conditions and verified isolation, not on the ideal balanced-load calculation.
Explore key terms
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ACAlternating Current
Alternating current reverses direction repeatedly, unlike direct current. An AC voltage also changes polarity over its cycle. Frequency, measured in hertz (Hz), tells you how many complete cycles occur each second. In a three-phase system, the three AC waveforms are offset in time; three-phase does not mean three times the frequency.
Read the full AC glossary entryPower factor
The ratio of real power to apparent power in an AC circuit. Phase displacement and waveform distortion can affect it. Real power includes power dissipated as losses as well as useful output, so power factor is different from efficiency. A lower power factor means more RMS current is needed for the same real power at the same RMS voltage.
Read the full Power factor glossary entryElectrical power
Electrical power is the rate at which electrical energy is transferred, measured in watts (W) or kilowatts (kW). A 1 kW load transfers energy twice as fast as a 500 W load. In a DC circuit, power equals voltage multiplied by current. For AC loads, real power also depends on power factor, so multiplying voltage and current alone does not always give watts.
Read the full Electrical power glossary entry