Editing Motor controller (section)

=== Reduced voltage starters ===
Reduced-voltage or soft starters connect the motor to the power supply through a voltage reduction device and increases the applied voltage gradually or in steps.<ref name=Siskind /><ref name=NEC430 /><ref name=Campbell /> Two or more contactors may be used to provide reduced voltage starting of a motor. By using an [[autotransformer]] or a series [[inductance]], a lower voltage is present at the motor terminals, reducing starting torque and inrush current. Once the motor has come up to some fraction of its full-load speed, the starter switches to full voltage at the motor terminals. Since the autotransformer or series reactor only carries the heavy motor starting current for a few seconds, the devices can be much smaller compared to continuously rated equipment. The transition between reduced and full voltage may be based on elapsed time, or triggered when a current sensor shows the motor current has begun to reduce. An [[Korndörfer autotransformer starter|autotransformer starter]] was patented in 1908.

Larger 3 phase induction motors can have their power reduced within the motor ! The motor is started 'DOL' with full voltage supplied to the field coils of the motor outer part ('stator'). The inner part ('rotor') has a current induced into it to once again react with the magnetic field generated by the stator. By breaking the rotor into parts and electrically connecting these parts to external resistances via slip rings and brushes as well as control contactors, the magnetic power of the rotor can be varied - i.e. reduced, for starting or low power running. Although a much more complex process, it means the currents (electrical loads) being switched are significantly lower than if reducing the power to the main feed of the motor.

A third way to achieve a very smooth progressive start is to dip resistance rods into a conductive liquid (e.g. mercury) which has a layer of insulative oil on the top. As the rods are lowered the resistance is gradually reduced.

A star delta starter is another type of Reduced-voltage starter in induction motor. A star delta starter will start a motor with a star connected stator winding. When motor reaches about 80% of its full load speed, it will begin to run in a delta connected stator winding. Star Delta Starter are two types. (1) Manual Operated Star Delta Starter, (2) Automatic Star Delta.

The manual operated star delta starter mainly consists of a TPDP switch which stands for Triple Pole Double Throw switch. This switch changes stator winding from star to delta. During starting condition stator winding is connected in the form of a star. Now we shall see how a star delta starter reduces the starting current of a three-phase induction motor.<ref>{{Cite web|last=Electrical4U|title=Star Delta Starter: What is it? (Working Principle & Circuit Diagram) {{!}} Electrical4U|url=https://www.electrical4u.com/star-delta-starter/|access-date=2021-10-26|website=www.electrical4u.com/|language=en-US}}</ref>

The above function achieved by using a power contactor and timer in automatic star delta starter. The automatic star delta starter is manufactured from three contactors, a timer and a thermal overload. The contactors are smaller than the single contactor used in a direct on line starter as they are controlling winding currents only. The currents through the winding are 1/root 3 (58%) of the current in the line. There are two contactors that are close during run, often referred to as the main contractor and the delta contactor. These are AC3 rated at 58% of the current rating of the motor. The third contactor is the star contactor and that only carries star current while the motor is connected in star. The current in star is one third of the current in delta, so this contactor can be AC3 rated at one third (33%) of the motor rating.<ref>{{Cite web|last=Portal|first=EEP-Electrical Engineering|date=2012-04-10|title=Star-delta motor starter explained in details - EEP|url=https://electrical-engineering-portal.com/star-delta-motor-starter|access-date=2021-10-26|website=EEP - Electrical Engineering Portal|language=en}}</ref>

The transition from star to delta can be an open transition or a closed transition. During open transition, the motor starter momentarily disconnects from the motor and reconnects in a delta configuration. In closed transition, the transition from the star to delta configuration is achieved without disconnecting the motor. In order to achieve that, an additional three-pole contactor and three resistors are required.<ref>{{Cite web |date=2020-12-25 |title=Star-delta starter (Wye-Delta Starters) - Circuit, working |url=https://www.electricalclassroom.com/star-delta-starter/ |access-date=2022-06-12 |website=www.electricalclassroom.com |language=en-US}}</ref>