Three phase motors may be controlled by a three pole switch, or a contactor which itself is controlled by a simple on/off switch, this is commonly known as two wire control; or by a contactor controlled by push buttons with maintaining circuits which is commonly known as three wire control.
In this article the use of a switch to control a motor is mentioned for completeness only as it is a manual device which will have a fuse or circuit breaker backup. It is normally only used for very small totally enclosed motor driven devices.
A contactor in its basic form is an electrically operated switch, it ‘switches’ on when a voltage is applied to its coil and ‘switches’ off when the voltage is removed from its coil. A contactor will have a number of high current main contacts and a number of auxiliary contacts the numbers and ratings of the main and auxiliary contacts and their operation are dependant on the equipment to be switched. The energy absorbed by the coil will depend on the contactor rating, it takes more energy to energise a three pole 250A contactor than it does to energise a three pole 25A contactor.
A motor operated by two wire control runs when switched on and stops when switched off.
In the above diagram the switch positions are shown as O and X, the O indicates switch contact open and X switch contact closed, thus in the off position the switch is open and in the on position the switch is closed. The switch is connected to a contactor coil which switches the main contacts on and off which starts and stops the motor.
The switch may be augmented by protection devices, interlocking circuits and automation systems.
A motor operated by three wire control runs when the start button is pressed and stops when the stop button is pressed.
In the above diagram the start button, which is shown as green, has a ‘normally open’ (NO) contact on terminals a3-a4, and the stop button, which is shown as red, has a ‘normally closed’ (NC) contact on terminals a1-a2. Thus when the start button is pressed it’s contact on terminals a3-a4 closes to energise the contactor via it’s operating coil C. The main contacts of the contactor close and the motor starts and accelerates to it’s full speed. A normally open auxiliary contact, on terminals 13-14, of the contactor also closes when the contactor energises, this contact is connected in parallel with the start button contact; thus when the start button is pressed and the contactor energises the circuit is maintained to keep the coil energised when the start button is released.
In series with the supply side of the start button is the stop button with it’s ‘normally closed’ contact on terminals a1-a2, when this button is pressed the contact opens which de-energises the contactor operating coil, the main and auxiliary contacts of the contactor open and the motor stops.
The stop and start buttons may be augmented by protection devices, interlocking circuits and automation systems.
The contactor and its associated auxiliary components are known as a starter.