DC MOTOR in the application of Current Electricity

1. 

A  DC motor is an electric motor that runs on direct current (DC) electricity.
Brushed
The brushed DC motor generates torque directly from DC power supplied to the motor by using internal commutation, stationary permanent magnets, and rotating electrical magnets. It works on the principle of Lorentz force, which states that any current carrying conductor placed within an external magnetic field experiences a torque or force known as Lorentz force. Advantages of a brushed DC motor include low initial cost, high reliability, and simple control of motor speed. Disadvantages are high maintenance and low life-span for high intensity uses. Maintenance involves regularly replacing the brushes and springs which carry the electric current, as well as cleaning or replacing the commutator. These components are necessary for transferring electrical power from outside the motor to the spinning wire windings of the rotor inside the motor.
Synchronous
Synchronous DC motors, such as the brushless DC motor and the stepper motor, require external commutation to generate torque. They lock up if driven directly by DC power. However, BLDC motors are more similar to a synchronous ac motor.
Brushless
Brushless DC motors use a rotating permanent magnet in the rotor, and stationary electrical magnets on the motor housing. A motor controller converts DC to AC. This design is simpler than that of brushed motors because it eliminates the complication of transferring power from outside the motor to the spinning rotor. Advantages of brushless motors include long life span, little or no maintenance, and high efficiency. Disadvantages include high initial cost, and more complicated motor speed controllers.
Uncommutated
Other types of DC motors require no commutation.
• homopolar motor-A homopolar motor has a magnetic field along the axis of rotation and an electric current that at some point is not parallel to the magnetic field. The name homopolar refers to the absence of polarity change.
Homopolar motors necessarily have a single-turn coil, which limits them to very low voltages. This has restricted the practical application of this type of motor.
• ball bearing motor-A ball bearing motor is an unusual electric motor that consists of two ball-bearing-type bearings, with the inner races mounted on a common conductive shaft, and the outer races connected to a high current, low voltage power supply. An alternative construction fits the outer races inside a metal tube, while the inner races are mounted on a shaft with a non-conductive section (e.g. two sleeves on an insulating rod). This method has the advantage that the tube will act as a flywheel. The direction of rotation is determined by the initial spin which is usually required to get it going.

Principles of operation

In any electric motor, operation is based on simple electromagnetism. A current-carrying conductor generates a magnetic field; when this is then placed in an external magnetic field, it will experience a force proportional to the current in the conductor, and to the strength of the external magnetic field. As you are well aware of from playing with magnets as a kid, opposite (North and South) polarities attract, while like polarities (North and North, South and South) repel. The internal configuration of a DC motor is designed to harness the magnetic interaction between a current-carrying conductor and an external magnetic field to generate rotational motion.
Let's start by looking at a simple 2-pole DC electric motor (here red represents a magnet or winding with a "North" polarization, while green represents a magnet or winding with a "South" polarization).
                                             
Every DC motor has six basic parts -- axle, rotor (a.k.a., armature), stator, commutator, field magnet(s), and brushes. In most common DC motors (and all that BEAMers will see), the external magnetic field is produced by high-strength permanent magnets1. The stator is the stationary part of the motor -- this includes the motor casing, as well as two or more permanent magnet pole pieces. The rotor (together with the axle and attached commutator) rotate with respect to the stator. The rotor consists of windings (generally on a core), the windings being electrically connected to the commutator. The above diagram shows a common motor layout -- with the rotor inside the stator (field) magnets.
The geometry of the brushes, commutator contacts, and rotor windings are such that when power is applied, the polarities of the energized winding and the stator magnet(s) are misaligned, and the rotor will rotate until it is almost aligned with the stator's field magnets. As the rotor reaches alignment, the brushes move to the next commutator contacts, and energize the next winding. Given our example two-pole motor, the rotation reverses the direction of current through the rotor winding, leading to a "flip" of the rotor's magnetic field, driving it to continue rotating.

In real life, though, DC motors will always have more than two poles (three is a very common number). In particular, this avoids "dead spots" in the commutator. You can imagine how with our example two-pole motor, if the rotor is exactly at the middle of its rotation (perfectly aligned with the field magnets), it will get "stuck" there. Meanwhile, with a two-pole motor, there is a moment where the commutator shorts out the power supply (i.e., both brushes touch both commutator contacts simultaneously). This would be bad for the power supply, waste energy, and damage motor components as well. Yet another disadvantage of such a simple motor is that it would exhibit a high amount of torque "ripple" (the amount of torque it could produce is cyclic with the position of the rotor).
You'll notice a few things from this -- namely, one pole is fully energized at a time (but two others are "partially" energized). As each brush transitions from one commutator contact to the next, one coil's field will rapidly collapse, as the next coil's field will rapidly charge up (this occurs within a few microsecond). We'll see more about the effects of this later, but in the meantime you can see that this is a direct result of the coil windings' series wiring:




                                                   



There's probably no better way to see how an average DC motor is put together, than by just opening one up. Unfortunately this is tedious work, as well as requiring the destruction of a perfectly good motor.
Luckily for you, I've gone ahead and done this in your stead. The guts of a disassembled Mabuchi FF-030-PN motor (the same model that Solarbotics sells) are available for you to see here (on 10 lines / cm graph paper). This is a basic 3-pole DC motor, with 2 brushes and three commutator contacts.
The use of an iron core armature (as in the Mabuchi, above) is quite common, and has a number of advantages2. First off, the iron core provides a strong, rigid support for the windings -- a particularly important consideration for high-torque motors. The core also conducts heat away from the rotor windings, allowing the motor to be driven harder than might otherwise be the case. Iron core construction is also relatively inexpensive compared with other construction types.
But iron core construction also has several disadvantages. The iron armature has a relatively high inertia which limits motor acceleration. This construction also results in high winding inductances which limit brush and commutator life.
In small motors, an alternative design is often used which features a 'coreless' armature winding. This design depends upon the coil wire itself for structural integrity. As a result, the armature is hollow, and the permanent magnet can be mounted inside the rotor coil. Coreless DC motors have much lower armature inductance than iron-core motors of comparable size, extending brush and commutator life.
   

         
                                                         
                                                               Diagram courtesy of MicroMo


The coreless design also allows manufacturers to build smaller motors; meanwhile, due to the lack of iron in their rotors, coreless motors are somewhat prone to overheating. As a result, this design is generally used just in small, low-power motors. BEAMers will most often see coreless DC motors in the form of pager motors.

                                               


Again, disassembling a coreless motor can be instructive -- in this case, my hapless victim was a cheap pager vibrator motor. The guts of this disassembled motor are available for you to see here (on 10 lines / cm graph paper). This is (or more accurately, was) a 3-pole coreless DC motor.

I disembowel 'em so you don't have to...

To get the best from DC motors in BEAMbots, we'll need to take a closer look at DC motor behaviors -- both obvious and not.




DC MOTOR BEHAVIOR

At a simplistic level, using DC motors is pretty straightforward -- you put power in, and get rotary motion out. Life, of course, is never this simple -- there are a number of subtleties of DC motor behavior that should be accounted for in BEAMbot design.
High-speed output
This is the simplest trait to understand and treat -- most DC motors run at very high output speeds (generally thousands or tens of thousands of RPM). While this is fine for some BEAMbots (say, photopoppers or solarrollers), many BEAMbots (walkers, heads) require lower speeds -- you must put gears on your DC motor's output for these applications.
Back EMF
Just as putting voltage across a wire in a magnetic field can generate motion, moving a wire through a magnetic field can generate voltage. This means that as a DC motor's rotor spins, it generates voltage -- the output voltage is known as back EMF. Because of back EMF, a spark is created at the commutator as a motor's brushes switch from contact to contact. Meanwhile, back EMF can damage sensitive circuits when a motor is stopped suddenly.
Noise (ripple) on power lines
A number of things will cause a DC motor to put noise on its power lines: commutation noise (a function of brush / commutator design & construction), roughness in bearings (via back EMF), and gearing roughness (via back EMF, if the motor is part of a gear motor) are three big contributors.
Even without these avoidable factors, any electric motor will put noise on its power lines by virtue of the fact that its current draw is not constant throughout its motion. Going back to our example two-pole motor, its current draw will be a function of the angle between its rotor coil and field magnets:

Since most small DC motors have 3 coils, the coils' current curves will overlay each other:

Added together, this ideal motor's current will then look something like this:

Reality is a bit more complex than this, as even a high-quality motor will display a current transient at each commutation transition. Since each coil has inductance (by definition) and some capacitance, there will be a surge of current as the commutator's brushes first touch a coil's contact, and another as the brushes leave the contact (here, there's a slight spark as the coil's magnetic field collapses).
As a good example, consider an oscilloscope trace of the current through a Mabuchi FF-030PN motor supplied with 2 V (1ms per horizontal division, 0.05 mA per vertical division):
 
In this case, the peak-to-peak current ripple is approximately 0.29 mA, while the average motor current is just under 31 mA. So under these conditions, the motor puts about less than 1% of current ripple onto its power lines (and as you can see from the "clean" traces, it outputs essentially no high-frequency current noise). Note that since this is a 3-pole motor, and each coil is energized in both directions over the course of a rotor rotation, one revolution of the rotor will correspond to six of the above curves (here, 6 x 2.4 ms = 0.0144 sec, corresponding to a motor rotation rate of just under 4200 RPM).
Motor power ripple can wreak havoc in Nv nets by destabilizing them inadvertently. Fortunately, this can be mitigated by putting a small capacitor across the motor's power lines (you'll only be able to filter out "spikey" transients this way, though -- you'll always see curves like the ones above being imposed on your power). On the flip side of this coin, motor power ripple can be put to good use -- as was shown above, ripple frequency can be used to measure motor speed, and its destabilizing tendencies can be used to reverse a motor without the need for discrete "back-up" sensors.
To scope out what motor is best for a given BEAM application, we'll need to do some math -- let's move on to DC motor performance parameters.