Star-delta starter
A starter that connects a three-phase motor's windings in star to start, then changes them to delta to run. In star each winding sees less , so starting and torque are about a third of what direct delta starting gives. That means a motor needing high starting torque may not suit it.
Using Star-delta starter in context
Reviewed
A distinction worth remembering
A calculation can go wrong even when the arithmetic is correct if line voltage is substituted for phase voltage. Draw the supply and load connection, label the quantities and state whether the load is balanced before choosing a formula.
Questions to take further
- How can I revise Star-delta starter beyond the definition?
- Explain what Star-delta starter describes, compare it with a related term and use it in a topic question. State any assumptions and what evidence you would need before applying the idea to real work.
Sources
- Professional Electrician: single-phase and three-phase power supplies (Enkin, 2024) — The basic difference between the two supplies, smoother power, higher capacity and balancing loads.
- Voltimum: basic three-phase supplies (Scolmore, 2025) — Where three-phase supplies come from: windings spaced 120° apart.
- legislation.gov.uk: Electricity Safety, Quality and Continuity Regulations 2002, regulation 27 — Declared low voltage supply of 230 V between phase and neutral in Great Britain, unless agreed otherwise.
- Professional Electrician: demand for three-phase supply in the UK (GivEnergy) — Most UK homes on a single-phase supply with one fuse, and why a battery maker says demand is growing.
- NIE Networks: connections FAQs — Northern Ireland: machines up to 17 kW on a single-phase supply, three-phase above that.
- IET: current BS 7671 edition and updates — BS 7671:2018+A4:2026 and the main subject areas updated by Amendment 4.
- Open University: relationships between electrical quantities — The relationship between voltage, current and resistance and the limits of an ohmic model.
Where Star-delta starter fits in the learning path
The Electrical Science section covers these units:
- Circuit Fundamentals
Curriculum mapping: C&G 2365 Units 202/302.
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- Three-Phase PowerElectrical Science
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- Electrical Science — quiz previewFree · Electrical Science
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The unit that starts and stops a motor, usually a contactor with an overload relay. The relay looks for sustained excess current, such as from a jammed machine, and its setting comes from the maker's data. A fuse or circuit breaker upstream deals with fault current and has to ride through the start, so the two are chosen together.
Motor starter
Read the Motor starter entryCompare related terms
Open a meaning to compare its role with Star-delta starter, or follow its full entry.
Motor starter
The unit that starts and stops a motor, usually a contactor with an overload relay. The relay looks for sustained excess current, such as from a jammed machine, and its setting comes from the maker's data. A fuse or circuit breaker upstream deals with fault current and has to ride through the start, so the two are chosen together.
Read the Motor starter entryThree-phase supply
A supply with three line conductors whose voltages reach their peaks one after another. On a standard 400/230 V system there's 400 V between any two lines and 230 V from any line to neutral. It's used for bigger loads such as motors and large boards, and the lines are identified by brown, black and grey.
Read the Three-phase supply entryPhase voltage
The voltage across one phase of a three-phase system. On a star-connected 400/230 V supply that's line to neutral, so 230 V. In a delta-connected load there's no star point, and each winding takes the full line voltage. Mixing the two up is an easy way to get a three-phase sum wrong.
Read the Phase voltage entryA definition is a start. Using it in a real question is where it sticks.