Motor starter
The unit that starts and stops a motor, usually a contactor with an relay. The relay looks for sustained excess , 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.
Using Motor 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 Motor starter beyond the definition?
- Explain what Motor 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 Motor starter fits in the learning path
The Commercial and Specialist Systems section covers these units:
- Three-Phase Distribution and Motors
- Fire Alarm and Emergency Lighting Circuits
See this section in the full curriculum
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- Three-Phase PowerElectrical Science
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Recall a related term
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An overcurrent in an otherwise electrically sound circuit, arising when the connected load exceeds the relevant rating. Several high-power appliances on one extension lead can cause an overload without a short circuit. Overload can cause overheating. An RCCB provides residual-current protection only; an RCBO combines that function with overload and short-circuit protection.
Overload
Read the Overload entryCompare related terms
Open a meaning to compare its role with Motor starter, or follow its full entry.
Overload
An overcurrent in an otherwise electrically sound circuit, arising when the connected load exceeds the relevant rating. Several high-power appliances on one extension lead can cause an overload without a short circuit. Overload can cause overheating. An RCCB provides residual-current protection only; an RCBO combines that function with overload and short-circuit protection.
Read the Overload entryStar-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 voltage, so starting current and torque are about a third of what direct delta starting gives. That means a motor needing high starting torque may not suit it.
Read the Star-delta starter entryPhase rotation
The order in which the three lines of a supply reach their peaks. It decides which way a three-phase motor turns, and swapping any two lines reverses it. BS 7671 calls the check phase sequence, and it's part of initial verification on multi-phase circuits, usually done with a phase rotation tester.
Read the Phase rotation 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 entryA definition is a start. Using it in a real question is where it sticks.