Published February 2000

MOTORS: A LOST ART

by Bob Perrin

Start • A Few Words on Words • The DC Motor • Polyphase AC Motors • Single-Phase AC Motors • Winding Down • Sources and PDF

THE DC MOTOR

One of the simplest motors to understand is the commutated DC motor. This type of motor is so simple that back in the days of junior high school electronics shop, building a simple DC motor on a pine board was a required project.

The How Stuff Works website (www.howstuffworks.com/motor) gives a great explaination of a simple DC motor. Because I’m sure most of you remember playing with this type of project, I’ll skip over the basics and move on to the more interesting part.

The motor shown in Figure 1 is not a significantly different concept than that shown in a simple science project motor. The rotor in Figure 1 simply has more poles providing a smoother and more efficient interaction between the armature’s fields and the stator’s field.

Motors that use brushes have two disadvantages. The first is brush wear. The second is arcing and the resultant EMI generated. Depending on the application, these disadvantages may or may not be significant.

Nowadays, electromagnetic compliance (EMC) is a big issue for many. That is the primary reason solid-state commutated DC motors are finding their way into common use. This type of motor is often referred to as a brushless DC motor.

Photo 3 shows a brushless DC motor from a laser printer. The polygonal mirror that scans the laser’s beam can be seen on top of the rotor in Photo 3a.

a)

b)

Photo 3—(a) The motor from a laser printer is deceptively simple in appearance. (b) Notice the zigzag pattern on the stator PCB—it’s a rotational speed encoder. The windings are actually on the backside of the PCB, and are energized by external switches that connect to the solder pads at the bottom of the board.

Photo 3b shows the rotor removed and turned to expose the interior. The dark areas on the rotor are permanent magnets. The poles alternate north and south along the circumference of the rotor. Also in Photo 3b, you can see the stator consists of a green PCB.

Photo 3c shows the PCB removed and flipped over to expose the stator’s six coils (windings). Also three white Hall-effect sensors can be seen 120^o apart on the stator. The Hall-effect sensors are used to determine the rotor’s position. Solid state switches (BJTs or MOSFETs), located externally, are used to energize the stator coils appropriately.

Photo 3c—shows the windings and the Hall-effect sensors.

The zigzag pattern seen on the stator’s PCB in Photo 3b is a magnetic speed encoder. The frequency produced by the encoder is proportional to the rotor’s speed.

With the advent of inexpensive silicon, brushless DC motors have become feasible for widespread use. Low RDS on MOSFETs don’t dissipate much heat, and an $0.89 PIC or CPLD can close the feedback loop between the sensors and the commutator MOSFETS.

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