ELEC 3105 Basic EM and Power Engineering Rotating DC Motor PART 1 Mechanical

ELEC 3105 Basic EM and Power Engineering Rotating DC Motor PART 1 Mechanical
ELEC 3105 Basic EM and Power
Engineering
Rotating DC Motor
PART 1 Mechanical
Rotating DC Motor
Linear motors are good for certain tasks, but
industrial and transportation applications
usually demand rotating motion.

v
Linear motor
Assume external applied B field much
greater than B field generated by current
in bar.
V
bat


R

B
I
app

B
I
app

v
Metal fixed rail
Magnetic flux density into page
External applied field
Movable metal bar
The DC rotating motor is commonly
constructed with an armature on the rotor
and a field generated by a permanent
magnet instead of a stator winding.
Stator: non-moving coil
Rotor: rotating part
motor, electric,
machine that converts electrical energy into mechanical energy. When an electric current is passed
through a wire loop that is in a magnetic field, the loop will rotate and the rotating motion is transmitted to a
shaft, providing useful mechanical work. The traditional electric motor consists of a conducting loop that is
mounted on a rotatable shaft. Current fed in by carbon blocks, called brushes, enters the loop through two
slip rings. The magnetic field around the loop, supplied by an iron core field magnet, causes the loop to turn
when current is flowing through it.
In an alternating current (AC) motor, the current flowing in the loop is synchronized to reverse
direction at the moment when the plane of the loop is perpendicular to the magnetic field and there is no
magnetic force exerted on the loop. Because the momentum of the loop carries it around until the current is
again supplied, continuous motion results. In alternating current induction motors the current passing
through the loop does not come from an external source but is induced as the loop passes through the
magnetic field. In a direct current (DC) motor, a device known as a split ring commutator switches the
direction of the current each half rotation to maintain the same direction of motion of the shaft. In
any motor the stationary parts constitute the stator, and the assembly carrying the loops is called
the rotor, or armature. As it is easy to control the speed of direct-current motors by varying the field or
armature voltage, these are used where speed control is necessary. The speed of AC induction motors is set
roughly by the motor construction and the frequency of the current; a mechanical transmission must
therefore be used to change speed. In addition, each different design fits only one application. However, AC
induction motors are cheaper and simpler than DC motors. To obtain greater flexibility, the rotor circuit can
be connected to various external control circuits. Most home appliances with small motors have a universal
motor that runs on either DC or AC. Where the expense is warranted, the speed of AC motors is controlled
by employing special equipment that varies the power-line frequency, which in the United States is 60 hertz
(Hz), or 60 cycles per second. Brushless DC motors are constructed in a reverse fashion from the
traditional form. The rotor contains a permanent magnet and the stator has the conducting coil of
wire. By the elimination of brushes, these motors offer reduced maintenance, no spark hazard, and
better speed control. They are widely used in computer disk drives, tape recorders, CD drives, and
other electronic devices. Synchronous motors turn at a speed exactly proportional to the frequency. The
very largest motors are synchronous motors with DC passing through the rotor.
Rotating DC Motor
Terming this device a DC motor is not
entirely clear, in fact, the current in the
armature coil alternates in polarity, even
though the supply is DC.
WHY ?
Armature coil
Brushes
Stator: non-moving coil
Rotor: rotating part
97.315 Basic EM and Power
Engineering
Rotating DC Motor Theory
Rotating DC Motor
Consider a rectangular coil rotating in a
uniform magnetic field B
Axis
2r
Magnetic field and
current in loop
interact in such a way
as to generate a
torque on the loop.

B

Figures extracted from Lecture 20 Slide 38
Rotating DC Motor
Torque on a magnetic dipole
r

m


B
r
Side view
We will consider a dipole in a uniform
magnetic field. We can use any shape
we want for the dipole.
2r
I out of page

I
I into page
Wire loop
Top
view
Rotating DC Motor
Taken from Lecture 20 Slide 40
Torque on a magnetic dipole

m

F


B
r
Side view
Torque
  
G = r F

F
Pivot point
Torque attempts to align dipole
moment

m
with

B.
Rotating DC Motor
Taken from Lecture 20 Slide 41
Torque on a magnetic dipole

F

m

r
Side view
G = 2Fr sin( )

F
Pivot point
F => Magnetic force on
wire of length 
F = IB Through postulate 1 for magnetic fields
Then
G = 2rIB sin ( )

B
Rotating DC Motor
Axis
If current always
flows in the same
direction then loop
will only oscillate
rather than rotate
2r

B


F
Stable equilibrium
G

m

Unstable equilibrium
r
0
180
360


F
ELEC 3105Basic EM and Power
Engineering
Rotating DC Motor Torque Taming
Rotating DC Motor
We need to reverse the direction of the current on each half cycle.
Transform original torque versus angle curve from:
G
0
180
360

TO
G
0
180
360
Now torque always applied in same direction
inducing loop to spin continuously in same direction.

Rotating DC Motor
This is accomplished by using a commutator
(either mechanical or electronic cycle).
G = 2rIB sin ( )
G
0
180
360

How it Works….
Metal ring attached
to shaft split in two
sections
Rotating DC Motor
The figure illustrates one method by which the commutation function might be
accomplished. Rather than hard wiring the current source to the coil, the current is
conducted through sliding contacts (brushes) connected to the current source. The
brushes ride on the ends of the coil wires, thus conducting current through the coil.
In this simplified motor, the brushes switch coil connections about once every 180o
of rotation. Therefore, the direction of current flow remains fixed with respect to the
magnetic field.
Rotating DC Motor
The torque produced by this design momentarily goes to zero
every half cycle. Stall is possible, also start up may require a
small push. In addition to this the torque versus rotation angle
is not uniform
bad
better
best
Rotating DC Motor
Link to related site
Link to related site
Rotating DC Motor
How to get rid of undesirable torque fluctuations and momentary
zero values?
Answer: Redesign several features of the simple loop motor.
ELEC 3105 Basic EM and Power
Engineering
Rotating DC Motor Field Taming
Rotating DC Motor

B

B
To obtain a more even
torque, the magnetic field
lines should look something
like:

F

B

B

F

B

B
And how do you get a magnetic field with that shape?
Rotating DC Motor
poles of magnet
To obtain a more even
torque, the magnetic field
lines should look something
like:

B
Rotor
B field lines follow the path of least reluctance, so the curved poles create
roughly a radial field pattern.
Rotating DC Motor
Redesign the permanent magnet poles.
Insert soft iron rotor
Rotating DC Motor
Radius r
Rotating DC Motor
Consider a rectangular coil rotating in a
uniform magnetic field B
Motor dimensions:
Axis
2r
Magnetic field and
current in loop
interact in such a way
as to generate a
torque on the loop.

B

Figures extracted from Lecture 20 Slide 38
Depth
Torque in radial field
G = 2rBI

Recall slide 9 of this lecture for
parameters of the wire loop.

B
r
Rotor
ELEC 3105 Basic EM and Power
Engineering
Rotating DC Motor / Generator
Motor / Generator Action
As the motor turns, a back emf is produced:
V = 2vB
emf
velocity of the outer edge of the
rotor v = r
V = 2rB
emf
There are two conductors of length
in the loop.

Motor / Generator Action
bat
G = 2rBI


I
R


V
emf

v
Expression of Vemf
V = 2rB
terminal
emf
V = IR  V
bat

Loop
Equivalent circuit
V

emf
G
V =
R  2rB
2rB
bat
Slide extracted from linear motor and modified for loop motor.
Motor / Generator Action
Slide extracted from linear motor and modified for loop motor.
G
V =
R  2rB
2rB
Linear relation between
speed and torque
bat


V
=
2rB
bat
noload
Current
flows in a
direction to
charge the
battery.
Link
Generator
 =0
2rBV
G=
R
bat
Stall torque
Motor
G
Rotating DC Motor
In a practical motor design, use many turns of wire on the rotor
(rather than just one) to increase the torque.

B
r
Rotor
DC Motor Simulation
ELEC 3105 Basic EM and Power
Engineering
Rotating DC Motor and Regenerative Braking
Regenerative Braking
Since the DC motor and a DC generator are virtually the same
machine mechanically, it was immediately realized that a train
could use its motors to act as generators and that this would provide
some braking effect if a suitable way could be found to dispose of
the energy. The idea formed that if the power could be returned to
the source, other trains could use it. Trains were designed therefore,
which could return current, generated during braking, to the supply
system for use by other trains. Various schemes were tried over
many years with more or less success but it was not until the
adoption of modern electronics that reliable schemes have been
available.
Regenerative
braking
Extra slides after this one
Simple
Motor
PURPOSE: To illustrate possibly the world's simplest motor.
DESCRIPTION: A small coil is mounted across the terminals of a battery as shown. The
enamel is scraped off half of the coil wire where it contacts the battery terminals. The magnet is
oriented such that when the coil is rotating it either pushes away or pulls toward the magnet in
the appropriate part of its cycle. The other half-cycle the enamel prevents the coil from being
activated; if it were it would counteract the torque which produces the desired rotation.
SUGGESTIONS:
REFERENCES: (PIRA unknown.)
EQUIPMENT: Mounted battery and rotation coil with carefully polished lead wires.
SETUP TIME: None.
Simple Motor
ST. LOUIS MOTOR
PURPOSE: To demonstrate the structure and operation of a simple motor.
DESCRIPTION: This is a two-pole DC motor with a split-ring commutator and
permanent magnets. Operates with a 1.5 volt battery.
SUGGESTIONS:
REFERENCES: (PIRA 5K40.10)
EQUIPMENT: St. Louis motor.
SETUP TIME: None.
ELEC 3105 Basic EM and Power
Engineering
Rotating DC Motor Dissection
Inside the DC Motor
Link by clinking
on figure
"How Electric Motors Work" describes how an electric
motor works and explains the basic components found in
any simple DC electric motor. In this article we will take
apart an actual electric motor and see what's inside. The
motor being dissected is a simple electric motor that you
would typically find in a toy:
Inside the DC Motor
You can see that this is a small motor about as big around as a dime. From the
outside you can see the steel can that forms the body of the motor, an axle, a
nylon end cap and two battery leads. If you hook the battery leads of the motor
up to a flashlight battery the axle will spin. If you reverse the leads it will spin in
the opposite direction. Here are two other views of the same motor. Note the
two slots in the side of the steel can in the second shot - their purpose will
become more evident in a moment:
Inside the DC Motor
The nylon end cap is held in place by two tabs that are part of the steel can. By
bending the tabs back you can free the end cap and remove it. Inside the end
cap are the motor's brushes. These brushes transfer power from the battery to
the commutator as the motor spins:
Inside the DC Motor
The axle holds the armature and the commutator. As described in "How Electric
Motors Work", the armature is a set of electromagnets, in this case three. The
armature in this motor is a set of thin metal plates stacked together, with thin
copper wire coiled around each of the three poles of the armature. The two ends
of each wire (one wire for each pole) are soldered onto a terminal, and then each
of the three terminals is wired to one plate of the commutator. The figures below
make it easy to see the armature, terminals and commutator:
Inside the DC Motor
The final piece of any DC electric motor is the field
magnet. The field magnet in this motor is formed by the
can itself plus two curved permanent magnets:
One end of each magnet rests against a slot cut into the
can, and then the retaining clip presses against the other
ends of both magnets.
ELEC 3105 Basic EM and Power
Engineering
Rotating DC Motor Building at Home
Home Made DC Motor
While this motor is very crude and inefficient, it cost me less than $5.00 to build from
parts I mostly had around the house, and total construction time was under four hours.
The hardest part was winding the field magnet and the armature coils.
Note that you can click on many of the smaller images on this page to see larger
versions.
The wooden frame of the motor was constructed from various bits of scrap lumber I had
laying around. If you build your own, look through all of these pictures and you can rig
something up based on what YOU have laying around.
Home Made DC Motor
I could have used permanent magnets for the fields on this motor,
but this is a section on electromagnetism - and I couldn't find
any. I made my field coil by winding 75 feet of 26 gauge
enameled magnet wire onto the "U" of a 2.5 in iron Muffler
clamp. Note the fence staple on the left to provide a route for the
wires. The coil is wound in several neat overlapping layers, with
a layer of electrical tape between each. Wind a layer, then wrap
with a single layer of electrical tape, and wind back over the coil
you have already wound. Just make sure that you always wind in
the same clockwise or counterclockwise direction in which you
started.
The arms of the "U" bolt are passed up through holes drilled in
the bottom of the wooden frame. The whole assembly is held in
place by gravity and by the nuts on the top of the frame
assembly.
Home Made DC
Motor
The bearings for the shaft are simply screw eyes screwed in to
the sides of the wooden frame. The brushes, which will transfer
current to the slip rings in the commutator assembly are made
from 22 gauge solid copper wire with a couple of inches of the
insulation stripped from each end. Note that one is mounted on
the top of one wooden cross piece, while the other is mounted
to the bottom of the other. This wire must be stiff enough to
hold a shape, but not so stiff that it puts too much friction on the
commutator assembly.
Home Made
DC Motor
The armature is made from a section of iron nail which was cut to fit cleanly
between the arms of the "U" bolt. Before winding the coil for the armature, wrap
one turn of 12 gauge solid copper insulated house wiring around the very center of
the nail. Bend the wire in such a way that it comes straight off the piece of nail,
and that the nail is positioned in a ninety degree angle to the wire. Place the nail
and wire on a chunk of waxed paper and place some two part epoxy on the union
to bind them together.
Wrap one layer of electrical tape around each half of the iron nail, then wind four
layers of 26 gauge enameled magnet wire and tape onto the iron nail, making sure
to always wind in the same direction. Simply cross over the 12 gauge wire shaft in
the center and continue each layer on the other side, as if the shaft were not there.
Home Made DC
Motor
The commutator in my motor is made from a section cut out of a broken shovel handle.
Drill a hole in the center into which the wire shaft will fit fairly snugly, and cut a groove
into each of it's sides. Slide this onto the short end of the shaft. The next step is to
fashion the slip rings. I used a tuna fish can, and cut it into 2 strips the width of the
commutator using tin snips. The ends of the strips should be folded down into the
grooves in each side of the commutator. Use a small screwdriver to fold them neatly into
the grooves in the wooden piece. It is important that the slip rings are as round as
possible when the commutator is assembled, and that none of the metal extends past the
edges of the wooden part or your motor will not function properly.
Home Made
DC Motor
Cut a small notch in the folded part of the slip rings so that you have something to
which you can attach the wires from the armature. Using a small butane lighter, burn
the insulation from the ends of the armature wires and clean with a piece of steel wool.
You can solder the wires in place if you wish, but I simply used some miniature
alligator clips to hold it in place. Snap the slip rings onto the wooden block, and wrap
half of them tightly with electrical tape to hold them in place. Make sure that they are
as round as possible, and that they do not touch each other in the notches.
Make sure that the 12 gauge wire shaft is straight and even, and that the slip rings on
the commutator are as round as possible. The gap between the slip rings should be at
about a 90 degree angle to the armature assembly. Try spinning the shaft in your
fingers to be sure that the assembly is fairly well balanced.
Home
Made DC
Motor
Slide the armature assembly into the
front bearing (a.k.a. a screw eye) from
the center of the wooden frame until the
armature is against the frame. If the
other end of the wire shaft is too long to
fit in the rear screw eye, trim it off a bit.
Insert the back end of the shaft into the
rear bearing, and slide the whole
assembly back until the slip rings line
up with the brushes.
Home Made DC Motor
You should have to bend the brushes slightly outward to get the commutator
between them. If they don't touch the slip rings when you are done, slide the
assembly forward enough to bend them in toward the shaft, then gently slide the
commutator back between them.
There is not much to the electrical wiring of the motor - I did not even use an
on/off switch. The use of the terminal strip in the back of the motor is optional,
but does make life a lot easier. Apply power to the motor by connecting a 12
volt lantern battery, and it should spin merrily away. If the armature wants to
lock in position, then you have the wires to the commutator reversed, causing
an opposite magnetic field. Even if you have the magnetic poles in the correct
orientation, to get the motor to run properly you may have to disconnect the
battery and adjust the position and tension of the brushes. You can also slightly
adjust the speed of the motor by slightly rotating the commutator on the shaft so
that you change the angle between the armature and the field coils.
Home Made DC Motor
If your motor still does not work properly, connect a couple of
"D" cell batteries (3 volts dc instead of 12) and manually turn
the shaft. You should be able to feel the magnetic fields as
resistance or attraction at certain points in the rotation and you
should then be able to figure out where the problem lies. Do not
leave the motor connected to the batter for very long or the coils
will get very hot, and the battery will get drained quickly, due to
the extremely poor efficiency of this design.
If you want your motor to work better than mine, and possibly
at a lower voltage, figure out how to reduce the friction of the
brushes, and use a more rigid shaft mounted in bearings for the
armature.
ELEC 3105 Basic EM and Power
Engineering
Rotating DC Motor Links
Making DC Motors Click picture
to link
ELEC 3105 Basic EM and Power
Engineering
Rotating DC Motor Brushless
Brushless DC Motors
Brushless DC motors are referred to by many aliases: Brushless
permanent magnet, permanent magnet ac motors, permanent
magnet synchronous motors ect. The confusion arises because a
Brushless dc motor does not directly operate off a dc voltage
source. However, as we shall see, the basic principle of operation
is similar to a dc motor. A Brushless dc motor has a rotor with
permanent magnets and a stator with windings. It is essentially a
dc motor turned inside out. The brushes and commutator have
been eliminated and the windings are connected to the control
electronics. The control electronics replace the function of the
commutator and energize the proper winding.
As shown in the animation the winding are energized in a pattern which rotates around the stator. The
energized stator winding leads the rotor magnet, and switches just as the rotor aligns with the stator.
There are no sparks, which is one advantage of the bldc motor. The brushes of a dc motor have several
limitations; brush life, brush residue, maximum speed, and electrical noise. BLDC motors are
potentially cleaner, faster, more efficient, less noisy and more reliable. However, BLDC motors
require electronic control.
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