Electric Motors 101
Electric motors are a widely misunderstood electronic component, especially
in the R/C community. I will try to describe here what the parts are, and
how they work.
Perhaps it is because the component can't be measured just sitting there
on the bench which is why it is so misunderstood. I know when I first
connected an ohmmeter to a motor so many years ago and found a resistance
of zero ohms that I was mystified -- that would mean a short circuit and
lots of smoke. So I knew then that the motion of a motor had something
to do with how you must measure it.
Let's first look at the major components of a motor so that we have some
common terminology with which to discuss our motor. This is a view of
a complete motor next to a completely disassembled motor. The parts are
described in order from left to right.
Click here for uncluttered
view
The piece of plastic with metal on it on the left is called the endbell.
The endbell supports the back end of the shaft on a
bearing, and holds the motor
brushes, and the power wires. The
endbell has brush hoods on it which serve as guides for the brushes, and
brush spring posts which hold the brush springs in
place.
Electric motors have commonly two types of bearings -- either ball-raced
bearings, commonly found in expensive motors equipped with hand-wound
armatures, or bronze bushing type bearings commonly
found in stock motors and motors equipped with machine wound armatures.
Ball type bearings are better than bushing type bearings for obvious
reasons -- longer life and closer tolerances. Friction is also mentioned
often as an advantage for ball-raced bearings, but the difference is not
as significant as most people think under operating conditions and loads.
There is one bearing in the endbell, and one in the
can.
Motor brushes are forged carbon and copper (usually), sometimes with
silver, which make a sliding electrical contact with the
commutator. They are pressed against the
commutator with brush springs. Brushes come in
different hardnesses, and can be cut to different shapes as tuning aids.
They are designed to have a low friction to reduce commutator abrasion.
They are a compramise between long wear life and long commutator life.
Brush springs press the brushes against the
commutator. This is to ensure a continuous
electrical contact between the brushes and the commutator.
The metal ring with two threaded holes and four notches in it is the timing
ring. This holds the endbell on with two screws,
and allows its rotational position to be adjusted relative to the motor
can. Stock motors do not have this piece -- instead, the
endbell is crimped on with tabs, and is considered non-removable. More about timing.
These shims are used to take up end-play in the
armature. A plastic one should be used adjacent to
the commutator, and a small diameter metal one
should be used adjacent to the bearing inside the
endbell. If a large diameter metal one is used
adjacent to the commutator, the commutator could short, which would be bad.
There is another set of shims between the armature and the bearing in the
can. The shims should be adjusted so that the armature
stack is centered in the magnetic field of the
magnets.
The armature is everything inside the motor that spins. The armature
consists of the shaft,
commutator, stack, and
windings.
The shaft is a piece of 1/8" rod that the armature
rotates on. It is supported at both ends by either bushing or ball type
bearings. Normally, a pinion
gear is mounted on one end.
The commutator is a segmented contact. In this case, it is constructed of
three pieces of copper mounted on mica. Each segment of the commutator
is connected to one end of each of two windings.
There are two styles of commutators in use -- large and small. Old
ones are small, new ones may be either large or small. The commutator
is the one component on the motor which wears with use and is quite expensive
to replace -- it is inseperable from the armature.
Commutators mostly wear from the arcing that happens between the
brushes and the commutator. The commutator is a
very important component in the motor, since it switches the polarity of
the windings at the appropriate time, and supplies power to the windings.
Comm drops are a lubricant that is used to lubricate the brush/commutator
interface. They tend to cause the brush dust to clump up and collect in the
slots in the commutator, sometimes causing a short-circuit between the
commutator segments. They can also soften the brushes, and cause accelerated
brush wear. The oils in comm drops tend to burn off fairly quickly, and
hence only have a limited useful lifespan.
The armature stack is a laminated piece of "soft
iron" (or iron that does not stay magnetized) which enhances the
magnetic field produced by the windings. There
are three poles, or places to wind the windings. The use of three poles
ensures that the motor can always start; if an even number of poles were
used, then the motor will not start from its resting position. The use of
a laminated piece of iron is for the same reason laminated iron is used in
transformers; it is to prevent magnetic eddy currents from forming and
reducing the efficiency. Holes are drilled in the stack to ballance the
armature after it has been wound and laquered. There are several different
styles of armature stacks around; there is the full stack (pictured above),
and slotted stacks. Slotted stacks tend to have either one or two slots
in the stack, and will increase the RPM of a motor at the expense of both
efficiency and torque. Modern stock motors also change the shape of the
stack in order to increase the timing, again at the expense of efficiency.
The windings are what differentiate one motor from the next. The different
windings have a great deal to do with the performance of the motor. There
is one set of windings per pole on the armature
stack. The poles are wound with magnetic copper
wire; wire with a laquered finish for insulation. The laquer is quite thin,
so the insulation is not appropriate for high voltages. This is
appropriate, because the windings only see small voltages when the motor is
in operation. The poles can be wound by hand, or by machine. Hand wound
armatures tend to be higher quality than machine wound armatures. Often
times on high quality hand wound armatures, the windings are laquered after
being completed to ensure the windings stay in place. The armature is
ballanced after being wound and/or laquered. Sometimes, poorly wound
motors will "toss a wind," which is when a piece of wire comes loose and
often will wedge itself between the armature and the
magnets. One end of each coil (or set of windings)
is attached to each commutator segment. Each coil
is made up of a certain number of turns of the biggest wire that can be used
to make that number of turns. For example, a 12 turn motor has 12 turns
per pole. Single, double, tripple, etc. indicate how many strands the
wire is made up with. For example, a single can be thought of as solid
core wire, a double as multi-strand wire with two strands, tripple three
strands, etc. Most are wound as if the strands are seperate, however
some motors have the strands twisted together to form a wire before
winding. Doublets, tripplets, quadruplets, etc. are motors where the
multiple strands are of different wire gauges.
More about windings.
The can houses the magnets and supports the end of the
armature shaft in a ball or
bushing type bearing. Shims
take up the remaining end-play in the armature between the armature and the
bearing. The can also affixes the endbell and is what
is mounted into the car.
The magnets are what attract and oppose the poles on the
armature in order to produce differential movement between them. The
stronger the magnets are, the faster the motor will go, and the motor will
also have more torque. There are two major types of magnets available
for racing use; dry type and wet type. Dry magnets are formed by taking
a powder and forging the power into the correct shape. These magnets are
not as strong as the wet type, and as such have gone the way of the Dodo
bird. Wet magnets are formed by taking a paste and forging this into the
magnet shape. The number describing the type of magnet is a measurement of
the thickness; the thicker the magnet the stronger it is. Some motors boast
Cobolt or other rare earth magnets, but these are prohibited by most
sanctioning bodies, since they are quite expensive. It is the positioning
of the magnets relative to the commutator which
determine the neutral point of the motor, and the position of the brushes
relative to this neutral point adjusts the timing of the motor. Wet magnets
are very good at keeping their magnetism, so magnet strength will not diminsh
a great deal throughout a motors lifetime. However, any decrease in magnet
strength will result in diminshed performance. This is normally an
insignificant amount. It is heat which does the most damage to these
magnets; if you severely overheat your motor, you may experience diminished
magnet strength, and hence degraded motor performance. Imapcts can also
cause magnets to loose magnetism, but the impact force would destroy a motor
before causing the magnets to loose their magnetism.
A pinion gear is any spur gear which drives another spur gear. A spur gear
is just a plain ol' simple gear. The pinion gear is used to take the power
out of the motor, convert it to a usefull range of torque and RPM, and
do something with it. A motor is fairly useless without a gear on it. . .
You can email me here with any
suggestions, comments, or whatever. Give me some feedback so that I know
what I've done right, wrong, okay, or whatever. Let me know what I need to
include that I haven't yet, or whatever.
Click here for uncluttered
view