July 15, 2013

Motor Madness - Part I

If it moves...it probably has a motor.  There are a number of different types of motors within lab instruments and while repair or diagnosis of many of them are beyond the expertise even the most savvy field service technician, it is helpful to know a bit about them and what makes them tick (or spin).

The most common motors are simple AC or DC motors. As their names imply, each uses a different current scheme to achieve basic rotation but a simple brushed DC motor has five parts:

  • Armature or rotortl_files/labsquad/blog_images/Motor Madness/BrushedDCMotor2_opt.jpeg
  • Commutator
  • Brushes
  • Axle (shaft)
  • Field magnet

In many motors, the outer metal housing contains at least two field magnets (North and South).

The armature, also called the rotor as it rotates about the axel, is an electromagnet made by coiling thin wire around  two or more poles of a metal core.

The commutator is a pair of plates attached to the axle.  These plates provide the two connections for the coil of the electromagnet.

The commutator and the brushes enable for the "flipping" of the electric field" part of the motor.tl_files/labsquad/blog_images/Motor Madness/motor-labels.gif

Brushed DC Motors have two coils of wire around a rotor in the middle. Surrounding the coil are two magnets, both facing in the same direction. When the coils are facing the magnets, electricity flows into them. When electricity flows into a coil, it creates a magnetic field, and this magnetic field pushes the coils away from their magnets. As the rotor turns, the current shuts off. When the rotor has turned 180 degrees, each rotor faces the opposite magnet. The coils turn on again, this time with the electricity flowing in the opposite direction. This creates another pulse, pushing the rotor around again. The rotor has electric contacts on it, and there are small metal brushes that bump against the contacts. The brushes send in electricity, turning the motor on and off at the right times.

Operationally, all you need to do is apply the proper DC voltage at the nominally rated current and the motor will spin. For simple devices this can be done via an on/off switch.

A brushless DC motor has a permanent magnet on the inside of the rotor, such that its north and south poles are perpendicular to the axle. Coils surround the rotor.  These coils function similar to a brushed motor in that hey give out timed pulses to push the magnet, spinning the rotor. Because there are no brushes however, the motor cannot control itself. Instead, it is attached to a speed controller ciruit, which gives pulses of electricity at a certain speed to control the motor. The faster the coils pulse, the faster the motor will spin.  This is called Pulse Width Modulation or PWM (more on that in Part 3).

On a final note, other than brushes, there reall isn't much that can be easily fixed on a DC motor.  For older devices that are no longer supported, you can find rebuild services that can repair toasted armature (more common on larger motors).   The most tempting way to test a DC motor is of course to apply power...but please, if you do this make sure to disconnect the motor from any mechanical drive components (pulleys, bests, chains, linkages...etc) first.   As always, if you choose to ignore this advice, please do not send nasty emails, legal notices or graphic images of your physical injuries...

Next Up: Part 2 - Stepper Motors