Editing Motor controller (section)

=== Motor starters ===
{{See also| Motor soft starter}}
A small motor can be started by simply connecting it to power. A larger motor requires a specialized switching unit called a motor starter or motor contactor. When energized, a direct on line (DOL) starter immediately connects the motor terminals directly to the power supply. In smaller sizes a motor starter is a manually operated switch; larger motors, or those requiring remote or automatic control, use magnetic contactors. Very large motors running on medium voltage power supplies (thousands of volts) may use power circuit breakers as switching elements.

A '''direct on line''' (DOL) or ''across the line'' starter applies the full line voltage to the motor terminals. This is the simplest type of motor starter. A DOL motor starter often contains protection devices (see below), and in some cases, condition monitoring. Smaller sizes of direct on-line starters are manually operated; larger sizes use an electromechanical contactor to switch the motor circuit. Solid-state direct on line starters also exist.

A direct on line starter can be used if the high inrush current of the motor does not cause excessive voltage drop in the supply circuit. The maximum size of a motor allowed on a direct on line starter may be limited by the supply utility for this reason. For example, a utility may require rural customers to use reduced-voltage starters for motors larger than 10&nbsp;kW.<ref name=Summers87>[[Terrell Croft]] and Wilford Summers (ed), ''American Electricans' Handbook, Eleventh Edition'', McGraw Hill, New York (1987) {{ISBN|0-07-013932-6}} pages 78-150 through 7-159</ref>

DOL starting is sometimes used to start small [[Pump|water pumps]], [[compressor]]s, [[Fan (machine)|fans]] and [[conveyor belt]]s. In the case of an asynchronous motor, such as the [[AC motor#Squirrel Cage rotors|3-phase squirrel-cage motor]], the motor will draw a high starting current until it has run up to full speed. This starting current is typically 6-7 times greater than the full load current. To reduce the inrush current, larger motors will have reduced-voltage starters or [[adjustable-speed drive]]s in order to minimise voltage dips to the power supply.

A reversing starter can connect the motor for rotation in either direction. Such a starter contains two DOL circuits&nbsp;— one for clockwise operation and the other for counter-clockwise operation, with mechanical and electrical interlocks to prevent simultaneous closure.<ref name=Summers87 /> For three phase motors, this is achieved by swapping the wires connecting any two phases. Single phase AC motors and direct-current motors often can be reversed by swapping two wires but this is not always the case.

Motor starters other than 'DOL' connect the motor through a resistance to reduce the voltage the motor coils get on start up. The resistance for this needs to be sized to the motor - and a quick source for a good resistance to use is another coil in the motor - i.e. series/parallel. In series gives a gentler start then switched to parallel for full power running. When this is done with three phase motors, it is commonly called a star-delta (US: Y-delta) starter. Old star-delta starters were manually operated and often incorporated an ammeter so the person operating the starter could see when the motor was up to speed by the fact the current it was drawing had stopped decreasing. More modern starters have built-in timers to switch from star to delta and are set by the electrical installer of the machine. The machin's operator simply presses a green button once and the rest of the start procedure is automated.

A typical starter includes protection against overload, both electrical and mechanical, and protection against 'random' starting - if, for instance, the power has been off and has just come back on. An acronym for this type of protection is TONVR - Thermal Overload, No Volt Release. It insists that the green button is pressed to start the motor. The green button switches on a solenoid which closes a contactor (i.e. switch) to primarily power the motor. It also powers the solenoid to keep the power turned on when the green button is released. In a power failure, the contactor opens turning itself and the motor off. The only way the motor can then be started is by pressing the green button. The contactor can be quickly tripped by the starter passing a very high current due to an electrical fault downstream of it in either the wiring to the motor or within the motor. The thermal overload protection consists of a heating element on each power wire which heats a bimetallic strip. The hotter the strip, the more it deflects to the point it pushes a trip bar which disconnects power to the contactor solenoid, turning everything off. Thermal overloads come in different range ratings and this should be chosen to match the motor. Within the range, they are adjustable enabling the installer to set it correctly for the given motor.

Which type for specific applications? DOL gives a quick start so is used more commonly with generally smaller motors. It is also used on machines with an uneven load such as piston type compressors where the full power of the motor is needed to get the piston past the compression stage - the actual working stage. Star-delta is generally used with larger motors or where either the motor is under no load at starting, very little load or a consistent load. It is particularly suited to motors driving machinery with heavy flywheels - to get the flywheels up to speed before the machine is engaged and driven by the flywheel.