Motor Contactor Applications
A motor contactor is designed specifically to handle the rough operating conditions (e.g. dust, moisture, etc.) and high power loads of an electric motor. Most motor contactors are 3-pole electrically operated switches, and you can use a three phase contactor in a single phase motor with some simple rewiring.
Do I need a motor contactor for motor starting? Technically, no. A simple toggle (on-off) switch, such as those used for household lights, could be used to start any size motor.
Why should I use a motor contactor? A motor contactor allows you to remotely turn on motors using a fraction of the power it takes to start the motor, which provides two major advantages: safety and cost.
Higher horsepower motors draw more current at any given voltage. However, high voltages increase the possibility of electric shock, while high currents increase the actual damage from getting shocked. For example, the buildup of a high voltage as a person walks across a carpet may cause him to get shocked while reaching for a doorknob, but the amount of current flowing from finger to doorknob is so small there is little damage done.
A motor contactor is safer to use when starting up motors because current cannot flow from the circuit powering a motor contactor to the main circuit being switched. Thus, the circuits are electrically isolated from each other, and contactors (and relays) never touch the circuit being switched.
Because the control circuit uses much less power than the main circuit being switched, the cables supplying power to the control circuit can be much smaller, which lowers the cost of installation.
For example, if you wanted to turn on a large electric motor in the basement of a building from another floor you would need some sort of remote switch. You could just run cables directly from the power supply to the switch and back down to the motor, but you would need bigger, more expensive cables to handle the higher inrush (startup) and normal operating currents drawn by large motors. Over large distances the materials, cost, and labor saved by using smaller cables is substantial.