Electrical Projects The simplest method of controlling a motor is to interrupt the flow of current through it. This is done by means of a switch. The symbol is for an SPST (single-pole, single-throw) switch, and this may physically be a toggle switch or a light switch such as is found in most homes.

Switches come in many sizes and configurations, but basically they all perform the same function; switches interrupt or apply power to a circuit or series of circuits. Consider the speed control of a motor using a multiposition rotary switch and a series of field windings. As the rotary switch is turned towards the position marked "MAX," the number of field windings is increased, the magnetic field around the armature becomes greater, and the speed of the motor increases. The switch used is a five-position rotary switch; hence, there will be five different speed positions from "MIN" to "MAX."

Another way of controlling motor speed is by means of a resistor in series with the field or armature or both. A resistor is a device which adds resistance to the line and hence lessens the flow of current. Many small appliances use a fixed resistor to control the flow of current, but this does not permit speed control. Consider the use of a variable resistor, called rheostat, to control the speed of a motor. The resistance of the rheostat is variable, according to the position of a movable wiper along a coil of wire. When the rheostat is at a high resistance, less current flows through the motor field and armature, and motor speed drops.




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August 20, 2008