Practical Hints For Beginners - DC Motor Reversing

The Changeover Switch
DC motor reversing can be achieved with the use of a simple changeover switch, and when wired as shown in figure 1, does exactly what it says, it changes over the polarity of the voltage when operated therefore changing the direction of the motor. As DC motors can draw lots more current under loaded conditions, it's a good idea to ensure the DC Amps rating of your switch can handle the required loaded current of the motor. Figure 1 shows a typical hook-up with the changeover switch shown as SW1, the switch has six connections and when wired as shown the motor will change direction when the switch is operated. This simple solution may be enhanced by using a 3 position double pole changeover switch. This will then give FORWARD, REVERSE and OFF control without any additional wiring. When the DC Amperage of the motor exceeds 8 to 10 Amps it is often no longer practical to use a simple switch - this is when the use of a simple, but clever device called a relay comes to the rescue !

Relays
Relays are electro mechanical devices and basically consist of an electro magnet and a number of contact sets. In fact the circuit of the relay shown in figure 2 is exactly the same set-up as the changeover switch shown above - the only difference is that we are going to use electricity to change the direction of our motor instead of our finger ! The prime function of a relay is the switching of large currents from small currents. In fact currents of many hundreds or even thousands of Amps ( depending on type, they can get quite large ! ) can be switched by just a couple of hundred milli Amps. Many configurations of relay are available, but for our purposes, figure 2 shows a 2 changeover device. Our 2 changeover relay (bounded by the blue dotted line) shows the coil, marked CL1 and the two changeover contact sets A & B. Each set has three connections, the common being A2 & B2. The normally open contacts being A1 & B1 and the normally closed contacts A3 & B3. The 'normally' word in the context of relays always refers to the de-energised (no power to the coil) state. The circuit shows a battery B1, to supply power to operate the relay and a simple pushbutton switch SW1. SW1 is shown in the OPEN (not pressed) position so no Volts are connected to the relay coil. In this condition 2 separate circuits via contact sets A2 / A3 and B2 / B3 are electrically connected. When voltage is applied to the relay coil CL1, the magnet energises and closes the contact set changing the connected circuits to
A1 / A2 & B1 / B2. The pushbutton switch shown in the circuit could be replaced with a simple toggle switch if momentary action is not required.

24 Volt DC 2 changeover relay
Relay Selection Page

Now you understand the basic operation of a relay, take a look below at Figure 3. This shows the same relay but with the pushbutton in the CLOSED (button pressed) position. This supplies voltage to the coil, energising the magnet and moving the contacts to the energised state. The circuit also shows the wiring needed to make our motor change direction whenever the button is pressed. We have deliberately shown the DC supply to the motor as a separate connection. This highlights another advantage of relay use - isolation - this means that the coil voltage does not have to be the same as the voltage being switched via the contacts. For example we could use a relay with a DC coil voltage rating of 12 Volts (easy to drive from solid state devices such as PIC's or transistors) and use the contacts to switch a motor with a 48 Volt requirement. The two Voltages being completely isolated from each other.

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