MCU 101: How does a DC Motor work?

Document created by GUO XIAOLI Employee on Jul 12, 2012Last modified by johnmc on Sep 26, 2012
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How does a DC Motor work?

The DC motor is a machine that transforms electric energy into mechanical energy in form of rotation. Its movement is produced by the physical behavior of electromagnetism. DC motors have inductors inside, which produce the magnetic field used to generate movement. But how does this magnetic field changes if DC current is being used?

Electromagnet.jpg

An electromagnet, which is a piece of iron wrapped with a wire coil that has voltage applied in its terminals. If two fixed magnets are added in both sides of this electromagnet, the repulsive and attractive forces will produce a torque.

motor_revolution.JPG

Then, there are two problems to solve: feeding the current to the rotating electromagnet without the wires getting twisted, and changing the direction of the current at the appropriate time. Both of these problems are solved using two devices: a split-ring commutator, and a pair of brushes.

TwoPole_motor.JPG

As it can be seen, the commutator has two segments which are connected to each terminal of the electromagnet, besides the two arrows are the brushes which apply electric current to the rotary electromagnet. In real DC motors it can be found three slots instead of two and two brushes.

dc-motor-tear-down.JPG

This way, as the electromagnet is moving its polarity is changing and the shaft may keep rotating. Even if it is simple and sounds that it will work great there are some issues which make these motors energy inefficient and mechanically unstable, the principal problem is due to the timing between each polarity inversion.
Since polarity in the electromagnet is changed mechanically, at some velocities polarity is changing too soon, which result in reverse impulses and sometimes in changing too late, generating instantaneous “stops” in rotation. Whatever the case, these issues produce current peaks and mechanical instability.

 

How a DC motor can be controlled?

DC motors have only two terminals. If you apply a voltage to these terminals the motor will run, if you invert the terminals position the motor will change its direction. If the motor is running and you suddenly disconnect both terminals the motor will keep rotating but slowing down until stopping. Finally if the motor is running and you suddenly short-circuit both terminals the motor will stop.
So there is not a third wire to control a DC motor, but knowing the previous behaviors it can be designed a way to control it, and the solution is an H-bridge.

motor_switching.JPG

Look at the last evolution of the DC Motor above, you can observe that there are four gates and a motor connected between them. This is the simplest H-bridge, where the four gates represent for transistors. By manipulating these gates and connecting the upper and lower terminals to a voltage supply, you can control the motor in all the behaviors as below.

Hbridge_dc_motor.JPG

Things to Consider When Using Motors

With the Motor and Line scan Camera hooked up to the same board there is a significant problem with noise.
The higher you turn the PWM on your drive motors the noise produced and the worse the data will appear from the camera. TO significantly reduce this noise you can simply solder an inductor directly across the 2 drive motors. This will allow you to increase the speed of the car without significantly affecting the data you receive back from the camera.

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