There are two types of DC motors – Brushed DC motor and Brushless DC motor. The direction of rotation of these motors is controlled by different methods.
Brushed DC Motor
The small DC motors (which has 12V or below 12V) made up of permanent magnet i.e. it contains a permanent magnetic field. If we want to change the direction of the shaft we change the polarity only. Because it contains only armature winding only.
Another method is to use an H-bridge to control the direction of a DC motor. It is a special circuit which allows motor rotation in both directions. From four terminals of an H bridge, you can control the direction of a DC motor. The method is briefly explained in this article.
High Voltage DC motors
If we want to change the direction of the shaft we need to change either field or armature supply. Take care, that you change either the field or the armature wires. If both are changed at the same time, the direction remains the same. Refer the figure above.
Brushless DC Motor
For a 3-phase brushless DC motor – you’d need to change commutation order, this is slightly more complex because how to do that depends on what kind of position sensor is being used.
For hobby-type brushless motors, like the ones you’d find in an RC car or quadcopter, and other motors that use sensorless control for position detection, you can simply swap any two of the phase connections. You can also reprogram the motor driver controller (if such facilities exist).
For devices that use hall sensors or other low-resolution position sensors, you’d have to also switch one phase connection and a corresponding hall sensor. Alternatively, you can reprogram the motor driver controller.
For 3-phase brushless DC motors, if it tries to commutate in the wrong order, you can risk damaging the motor or the motor driver.
Using a Microcontroller
As you can see in the figure above there are four switching elements named as “High side left”, “High side right”, “Low side right”, “Low side left”. When these switches are turned on in pairs motor changes its direction accordingly. Like, if we switch on High side left and Low side right then motor rotate in forward direction, as current flows from Power supply through the motor coil goes to ground via switch low side right. This is shown in the figure below.
|High Left||High Right||Low Left||Low Right||Description|
|On||Off||Off||On||Motor runs clockwise|
|Off||On||On||Off||Motor runs anti-clockwise|
|On||On||Off||Off||Motor stops or decelerates|
|Off||Off||On||On||Motor stops or decelerates|
H-bridge can be made with the help of trasistors as well as MOSFETs, the only thing is the power handling capacity of the circuit. If motors are needed to run with high current then lot of dissipation is there. So head sinks are needed to cool the circuit.