Name: Angelo E.
Dante BSEE 5-A Date: December 14, 2018
Reaction Paper
(Reversing of AC motors)
A world without electric motors is difficult to imagine. From the tiniest motor found in a
quartz watch to a million-plus horsepower motor powering a ship, motors are used in
many diverse applications. There are a multitude of motor types to choose from. Each
has its own unique characteristics, making one motor type a better choice for an
application than another. This provides an overview of the major types of electric motors
available today, including advanced motor technologies. It is not intended to turn the
reader into a motor expert, but rather to serve as a concise stand-alone reference with
emphasis on energy efficiency. The characteristics of each are discussed along with
typical applications. Some motors are noted as poor choices for certain applications.
Proper motor maintenance is essential in ensuring long term reliability and performance.
Motor efficiency is very important and is emphasized in this guide. Lifetime energy costs
for high usage motors will exceed the original purchase price many times over. For
some motors, the small incremental cost to buy a more energy efficient motor is easily
justified. Using simple decision tools, the reader will learn how to decide whether it is
better to replace rather than repair a failed motor. ‘‘Rules of thumb,’’ examples and
other anecdotal commentary are sprinkled throughout the text. These commentaries are
formatted in an italic font, as are the equations. An electric motor is a device which
converts electrical energy into kinetic energy. An AC motor is an electric motor driven
by an alternating current (AC). The AC motor commonly consists of two basic parts, an
outside stator having coils supplied with alternating current to produce a rotating
magnetic field, and an inside rotor attached to the output shaft producing a second
rotating magnetic field. The rotor magnetic field may be produced by permanent
magnets, reluctance saliency, or DC or AC electrical windings. The two main types of
AC motors are induction motors and synchronous motors. The induction motor (or
asynchronous motor) always relies on a small difference in speed between the stator
rotating magnetic field and the rotor shaft speed called slip to induce rotor current in the
rotor AC winding. As a result, the induction motor cannot produce torque near
synchronous speed where induction (or slip) is irrelevant or ceases to exist. In contrast,
the synchronous motor does not rely on slip-induction for operation and uses either
permanent magnets, salient poles (having projecting magnetic poles), or an
independently excited rotor winding. The synchronous motor produces its rated torque
at exactly synchronous speed. The brushless wound-rotor doubly fed synchronous
motor system has an independently excited rotor winding that does not rely on the
principles of slip-induction of current. The brushless wound-rotor doubly fed motor is
a synchronous motor that can function exactly at the supply frequency or sub to super
multiple of the supply frequency. The other types of motors include eddy current motors,
and AC and DC mechanically commutated machines in which speed is dependent on
voltage and winding connection. Alternating current technology was rooted in Michael
Faraday's and Joseph Henry's discovery that a changing magnetic field can induce
an electric current in a circuit. Faraday is usually given credit for this discovery since he
published his findings first. With the almost universal adoption of a.c. system of
distribution of electric energy for light and power, the field of application of a.c. motors
has widened considerably during recent years. As a result, motor manufactures have
tried, over the last few decades, to perfect various types of a.c. motors suitable for all
classes of industrial drives and for both single and three phase a.c. supply.
Conversation of electric power into mechanical power takes place in the rotating part of
an electric motor. In d.c. motors, the electric power is conducted directly to the armature
through brushes and commutator. In a.c. motors, the rotor does not receive electric
power by conduction but by induction in exactly the same way as the secondary of two
winding transformer receives its power from the primary that’s why such motors are
known as induction motors. An induction motor also can be treated as a rotating
transformer one in which primary winding is stationary but the secondary is free to
rotate. Of all the a.c motors, the polyphase induction motor is the one which is
extensively used for various kinds of industrial drives. The main advantages of this
motor is it has very simple and extremely rugged, almost unbreakable construction
especially squirrel cage type, its cost is low and it is very reliable, it has sufficiently high
efficiency, in normal running condition, no brushes are needed, frictional losses are
reduced, it has a reasonably good power factor, it requires minimum of maintenance,
and it starts up from rest and needs no extra starting motor and has not be
synchronised. Its starting arrangement is simple especially for squirrel cage type motor.
The disadvantages also is its speed cannot be varied without sacrificing some of its
efficiency, just like a d.c. shunt motor its speed decreases with increase in load, its
starting torque is somewhat inferior to that of a d.c. shunt motor. In both induction
and synchronous motors, the AC power supplied to the motor's stator creates
a magnetic field that rotates in synchronism with the AC oscillations. A synchronous
motor's rotor turns at the same rate as the stator field, an induction motor's rotor rotates
at a somewhat slower speed than the stator field. The induction motor stator's magnetic
field is therefore changing or rotating relative to the rotor. This induces an opposing
current in the induction motor's rotor, in effect the motor's secondary winding, when the
latter is short-circuited or closed through an external impedance. The induced currents
in the rotor windings in turn create magnetic fields in the rotor that react against the
stator field. Due to Lenz's Law, the direction of the magnetic field created will be such as
to oppose the change in current through the rotor windings. The cause of induced
current in the rotor windings is the rotating stator magnetic field, so to oppose the
change in rotor-winding currents the rotor will start to rotate in the direction of the
rotating stator magnetic field. The stator of an induction motor consists of poles carrying
supply current to induce a magnetic field that penetrates the rotor. To optimize the
distribution of the magnetic field, windings are distributed in slots around the stator, with
the magnetic field having the same number of north and south poles. Induction motors
are most commonly run on single-phase or three-phase power, but two-phase motors
exist, induction motors can have any number of phases. Many single-phase motors
having two windings can be viewed as two-phase motors, since a capacitor is used to
generate a second power phase 90° from the single-phase supply and feeds it to the
second motor winding. Single-phase motors require some mechanism to produce a
rotating field on start up. Cage induction motor rotor's conductor bars are typically
skewed to avoid magnetic locking. Standardized NEMA & IEC motor frame sizes
throughout the industry result in interchangeable dimensions for shaft, foot mounting,
general aspects as well as certain motor flange aspect. Since an open, drip proof (ODP)
motor design allows a free air exchange from outside to the inner stator windings, this
style of motor tends to be slightly more efficient because the windings are cooler. At a
given power rating, lower speed requires a larger frame. The power factor of induction
motors varies with load, typically from around 0.85 or 0.90 at full load to as low as about
0.20 at no-load, due to stator and rotor leakage and magnetizing reactances. Power
factor can be improved by connecting capacitors either on an individual motor basis or,
by preference, on a common bus covering several motors. For economic and other
considerations, power systems are rarely power factor corrected to unity power
factor. Power capacitor application with harmonic currents requires power system
analysis to avoid harmonic resonance between capacitors and transformer and circuit
reactances. Common bus power factor correction is recommended to minimize
resonant risk and to simplify power system analysis. The stator of an induction motor is
the same as that of a synchronous motor or generator. It is made up of a number of
stampings, which are slotted to receive the windings. The stator carries a three phase
winding and is fed from a three phase supply. It is wound for definite number of poles,
the exact number of poles being determined by the requirements of speed. Greater the
number of poles, lesser the speed and vice versa.
I can say that the method of changing the direction of rotation of an induction motor
depends on whether it is a three-phase or single-phase machine. In the case of three-
phase, reversal is straight forwardly implemented by swapping connection of any two
phase conductors. In a single-phase split-phase motor, reversal is achieved by
changing the connection between the primary winding and the start circuit. Some single-
phase split-phase motors that are designed for specific applications may have the
connection between the primary winding and the start circuit connected internally so
that the rotation cannot be changed. Also, single-phase shaded-pole motors have a
fixed rotation, and the direction cannot be changed except by disassembly of the motor
and reversing the stator to face opposite relative to the original rotor direction.
Name: Angelo E. Dante BSEE 5-A Date: December 14, 2018
Reaction Paper
(Reversing of DC motors)
One of the fundamental elements of an electrical engineer is knowing about the different
types of electric motors. Without electric motors, we would not have power tools,
household appliances, or even cars. Those who are looking to enter an online or
traditional electrical engineering program, or currently enrolled students, can use this
post as a handy reference guide for studying the basics of DC and AC electric motors.
Although ac motors are used in most of the cases, DC motors have many applications
and used for multi-purpose applications. To truly understand the operating principles of
an electronic drive, it is first necessary to understand basic direct current (DC) and
alternating current (AC) motor theory. As covered previously, the drive is the device that
controls the motor. The drive and motor interact to provide the torque, speed, and
horsepower necessary to operate the application. Slight differences occur between
manufacturers when it comes to motor design, but the basic characteristics apply, no
matter what motor is being controlled. Direct current motors have been the backbone of
industrial applications, ever since the Industrial Revolution. This is due to the motor's
high starting torque capability and smooth speed control, and its ability to quickly
accelerate to speed in the opposite direction. A machine that converts dc power into
mechanical energy is known as dc motor. Its operation is based on the principle that
when a current carrying conductor is placed in a magnetic field, the conductor
experiences a mechanical force. The direction of the force is given by Fleming’s left-
hand rule. DC stands for “direct current,” and they were the first type of motors that
were commonly used because they could be powered from a pre-existing power
distribution system. The speed in a DC motor can be adjusted through the intensity of
its current. The current is carried through an armature or stator. The armature contains
the coiled or aluminum wiring or commonly referred to as “windings”, and they both
provide points for the wire to connect to the rest of the motor so that the electricity can
continue to flow properly. There are a few different types of DC motors, one is Brushed
(BDC) Motors have a brush on the inside that alternates the electrical current or
commutates through the armature at the same speed as the rotation of the motor. There
are three main variations of BDC motors, and the difference between them lies in how
speed and voltage are controlled and pass through the motor’s windings. First is Shunt
Wound motor, second is Series Wound motor and the last is Compound Wound motor.
The second type of motor is Brushless (BLDC) Motors as the name implies, do not have
internal brushes. Instead, the current in the motor is commutated electronically through
the stator and the magnetic field. A rotor or a circular piece of the motor designed to
rotate on its axis to create torque rotates the stator and the magnets (producing the
magnetic field) at the same frequency to continue the current’s flow. DC motors are
rarely utilized in normal applications as a result of all electrical supply firms furnish
electrical energy but, for special applications like in steel mills, mines and electric
traction, it's advantageous to convert AC into DC so as to use DC motors. The rationale
is that speed or torque characteristics of DC motors are much more superior thereto of
AC motors. Therefore, it's not stunning to notice that for industrial drives, DC motors are
as common as 3-phase induction motors. Similar to DC generators, DC motors are also
classified into 3 kinds; they are series-wound, shunt-wound and compound- wound. The
employment of a specific motor depends upon the mechanical load it's to drive. A
machine which transforms the DC power into mechanical power is called as a DC
motor. Its operation relies on the principle that once a current carrying conductor is
placed in a very magnetic field, the conductor experiences a mechanical force. The
direction of this force is given by Fleming’s left hand rule.
In a shunt wound motor the field winding is connected in parallel with the armature. The
current through the shunt field winding is not the same as the armature current. Shunt
field windings are designed to produce the necessary m.m.f. by means of a relatively
large number of turns of wire having high resistance. Therefore, shunt field current is
relatively small compared with the armature current. A shunt DC motor connects the
armature and field windings in parallel or shunt with a common D.C. power source. This
type of motor has good speed regulation even as the load varies but does not have the
starting torque of a series DC motor. It is typically used for industrial, adjustable speed
applications, such as machine tools, winding or unwinding machines and tensioners. In
a series-wound motor the field winding is connected in series with the armature.
Therefore, series field winding carries the armature current. Since the current passing
through a series field winding is the same as the armature current, series field windings
must be designed with much fewer turns than shunt field windings for the same mmf.
Therefore, a series field winding has a relatively small number of turns of thick wire and,
therefore, will possess a low resistance. A series DC motor connects
the armature and field windings in series with a common D.C. power source. The motor
speed varies as a non-linear function of load torque and armature current, current is
common to both the stator and rotor yielding current squared (I^2) behavior. A series
motor has very high starting torque and is commonly used for starting high inertia loads,
such as trains, elevators or hoists. This speed or torque characteristic is useful in
applications such as dragline excavators, where the digging tool moves rapidly when
unloaded but slowly when carrying a heavy load. A series motor should never be started
at no load. With no mechanical load on the series motor, the current is low, the counter-
Electro motive force produced by the field winding is weak, and so the armature must
turn faster to produce enough counter-EMF to balance the supply voltage. The motor
can be damaged by overspeed. This is called a runaway condition. Series motors
called universal motors can be used on alternating current. Since the armature voltage
and the field direction reverse at the same time, torque continues to be produced in the
same direction. However they run at a lower speed with lower torque on AC supply
when compared to DC due to reactance voltage drop in AC which is not present in DC.
Since the speed is not related to the line frequency, universal motors can develop
higher-than-synchronous speeds, making them lighter than induction motors of the
same rated mechanical output. This is a valuable characteristic for hand-held power
tools. Universal motors for commercial utility are usually of small capacity, not more
than about 1 kW output. However, much larger universal motors were used for electric
locomotives, fed by special low-frequency traction power networks to avoid problems
with commutation under heavy and varying loads. Compound wound motor has two
field windings; one is connected in parallel with the armature and the other is in series
with it. There are two types of compound motor connections, one is Short-shunt
connection and Long shunt connection. When the shunt field winding is directly
connected across the armature terminals it is called short-shunt connection. When the
shunt winding is so connected that it shunts the series combination of armature and
series field it is called long-shunt connection. A compound DC motor connects the
armature and fields windings in a shunt and a series combination to give it
characteristics of both a shunt and a series DC motor. This motor is used when both a
high starting torque and good speed regulation is needed. The motor can be connected
in two arrangements: cumulatively or differentially. Cumulative compound motors
connect the series field to aid the shunt field, which provides higher starting torque but
less speed regulation. Differential compound DC motors have good speed regulation
and are typically operated at constant speed.
In dc series motors the field winding is connected to armature in series but in shunt
motor the field winding is connected in parallel to the armature so if we want to change
the direction of rotation, we have to change the direction of current in either field winding
or armature winding but not both. But is case of series motor if we change direction of
current in one winding other winding direction also changes. So to overcome this we
have separate the both winding and making it to run as separately excited dc motor
while changing the rotation. To reverse the direction of a separately excited DC motor
there are two methods are using for it one is reverse the terminal of the field supply
voltage and reverse the terminal of the armature supply voltage. One of the above
procedure is can be applied to make the rotation reverse. If you applied both of the
above procedure simultaneously then you can't make the rotation in opposite direction.
But you can't reverse the direction of the DC series motor. If you change the terminal
supply voltage the current direction will change but the applied torque on the rotor shaft
become positive. So the rotation can't be made reverse in direction. In reversing also
will depends on the motor construction. For a permanent magnet DC motor just reverse
the polarity of the supply voltage. For a “standard” brushed DC motor, it is easiest to do
on a separately excited motor, where the field winding has separate terminals to the
main armature winding. This is typically done on a motor designed for speed control. To
reverse the motor, you need to change the polarity of the supply voltage to either the
field winding or the armature winding, but not both. Generally it is better to reverse the
field voltage because the field current is less than the armature current, so your
reversing switchgear is more lightweight. If you cannot separate the field terminals and
armature terminals, then reversing cannot be done. For a brushless DC motor,
reversing the supply polarity won’t work and will possibly damage the motor. The
electronic commutation needs to be reversible, which has to be part of the motor
design.
