Starting up small motors is usually through a manual starter that can make or break the power supply line to the motor. The method is also known as DOL or direct online start. If the motor gets too hot due to an overload, a thermal protection circuit in the starter opens and disconnects the motor. DOL starters are the most common method of starting and stopping single-phase motors up to 5 HP, 230 VAC, and three-phase motors up to 15 HP, 600 VAC.
Magnetic starters can have controls such as float switches, pressure switches, timers, relays, limit switches, and push buttons, as they have a separate mechanism for closing and opening a set of contacts for the motor circuit. They also include a thermal overload protection device. The mechanism consists of a coil, which, when energized, closes contacts to complete the electrical circuit of the motor. Likewise, de-energizing the coil opens the contacts, switching off the motor.
However, one of the problems with DOL or magnetic starters is both allow the motor to start with a high current. Under normal conditions, motors must start with a current that is nearly 6 to 7 times the rated running current of the motor. This is necessary for the motor to overcome the initial torque due to friction. However, for some motors, the starting current can go up to 9-10 times the rated current.
Reversing any two phases of a three-phase induction motor results in the motor reversing its direction of rotation. Adding an extra set of contacts to a basic starter can turn it into a reversing starter. Appropriate electrical and mechanical interlocking mechanisms must also be present for safeguarding the motor operations.
A soft starter applies a low voltage to the motor, ensuring a low starting current and torque. The torque gradually increases as the soft starter begins to apply higher voltage. Semiconductor switches such as thyristors, inside the starter, accomplish the gradual increase in the voltage that the starter applies to the motor.
A slow start is essential to prevent stress on the internal components of the motor, and to the machinery, the motor is driving, especially belts and gear drives. The soft starter also features soft stopping. This is essentially helpful for stopping conveyor belts and pumps, where a sudden stop may cause water hammering in the pipe system.
Multispeed induction motors have multiple windings that require special starters. For instance, two-speed motors with separate windings need starters with two built-in standard starters within a single enclosure with mechanical and electrical interlocks.
Consequent-pole two-speed motors need a three-pole starter unit or a five-pole starter unit. The design of the motor winding determines whether the three- or five-pole unit makes a slow-speed or fast connection.
Delta-type multi-speed motors require different power circuits for the currents circulating within the unconnected and inactive windings. Two-speed motors with separate open-delta windings require a pair of four-pole starter. For each speed, a different four-pole starter is necessary. Therefore, very complex starters are necessary for motors with open-delta windings capable of running at three or four speeds.