A motor starter is a device for turning on electric motors, and consists of two main parts: a contactor and an overload. Modern motor starters might also have some circuitry built in for power quality detection, for example phase loss or grounding problems. The motor starter performs two functions: switching the electrical connection that powers the motor, and automatically shutting the motor off to prevent damage if there is a problem.
Single phase motor starters have two pole contactors, and three phase motor starters have three pole contactors. In this case, pole refers to the number of contact pairs in the contactor that are pulled together to establish an electrical connection to the motor. The contacts are typically pulled together with an electromagnetic solenoid (electric starter) or a spring loaded pushbutton (manual starter). Manual starters simply push the contactors together with the physical action of pushing the button, and the spring is there to allow the overload to automatically break the connection.
Electric starters use an electric signal to power a solenoid which pulls the contacts together by magnetic attraction. A spring separates the contacts when the solenoid is unpowered. Typically a button, switch, or PLC output provides the electric signal to engage the solenoid.
A common option on motor starters is an auxiliary contact switch: a smaller set of contacts that opens or closes along with the motion of the main contactors. This provides feedback to the rest of the system that the starter is indeed engaged and the motor therefore has power. Often these contacts are used to turn on a pilot light, provide a feedback signal to the PLC, or "latch" the signal to the solenoid. Latching means that the auxiliary contact bypasses the ON button so the solenoid remains energized, until a separate OFF button cuts the power.
Some motor starters have built-in latching. These starters have four terminals labeled 3, P, E, and C (historical leftovers from older models). The E and C terminals are for the hot and neutral wires, respectively, and remain powered. When power is applied to 3 and P at the same time, the starter will engage until power is removed from P. Power can be removed from 3 at any time without affecting operation.
The overload, noded in detail elsewhere, is a small heat-based piece of electronics which senses the amount of current going into the motor's windings. If the overload detects too much current, it disengages the starter to protect the motor's windings from damage. Overloads provide some protection from low levels of excessive current, but not high levels, such as those caused by short circuits; nor do they effectively protect the wiring. Therefore a circuit breaker or fuse must still be used to protect the system.
When the motor starter connects the electric motor to the power source, it reacts in different ways depending on the type of motor it is. A three-phase motor will simply start running, since the three phases produce a rotating magnetic field which gives the motor both power and a direction to turn. A capacitor start motor uses a capacitor to offset the phase of the incoming electric power slightly to provide a direction, and once the motor gets up to speed the capacitor typically disengages. A DC motor will power up both the rotor and the stator windings, and begin rotating according to the polarity of the windings.
It is important to note that all electric motors suffer from a condition called inrush current, which is similar in some ways to the inrush current an incandescent light bulb suffers when it is first turned on. When the motor is first turned on, there is a brief spike in current until the magnetic fields are set up, adding impedance to the system and reducing the current to steady-state levels. This inrush, although brief, can be several times the steady-state current, and the motor starter must be able to handle it without damage.