Download Hola Agustin - UniMAP Portal

Lesson Objectives :

Describe the construction and properties of various types of alternating current
motor

Explain the principle of operation of various types of a.c motor

Perform a basic connection and test for ac motor
6.1
Introduction
The major parts of all electrical power generated by ac, so most motors are designed for
ac operation. Ac motors can duplicate the operation of dc motors and also are less
troublesome to operate. Dc machines encounter difficulties from the action of
commutation, which involves brushes, brush holders, neutral planes, etc. However, ac
motors usually operate well over a very narrow speed range.
Ac motors are particularly well suited for constant-speed applications, since the speed is
determined by the frequency of the ac applied to the motor terminal. It is also
manufactured with variable speed characteristics within certain limits.
Ac motor can be designed to operate from a single-phase or a multi-phase ac supply.
Whether the motor is single phase or multi phase, it operates on the same principle: that
the ac applied to the motor generates a rotating magnetic field, and this rotating
magnetic field causes the rotor of the motor to turn.
6.2
Types of motor
Generally ac motors are classified into two basic types:
1. The synchronous motor
2. The induction motor
Figure 6.1
Types of ac motor
Electric motors and generators whose speeds are exactly synchronized with the line
frequency are called synchronous machines. The synchronous motor is an ac generator
operated as a motor. In it, ac is applied to the stator and dc is applied to the rotor.
The induction motor differs from the synchronous motor in that its rotor is not connected
to any source of power, but is powered by magnetic induction.
The ac series motor, which is widely used for some appliances and small tools, is a
modified version of the dc series motor. It has the advantage of readily adjustable
speed, and also be used in applications where the dc series motor is used.
Between the two basic types of ac motors mentioned, the induction motor is more
commonly used.
6.3
Concept of rotating field
Figure 6.2
Three-phase stator winding to which three-phase ac is applied
The windings are physically spaced 120o apart and are connected in delta. The two
windings in each phase are wound in the same direction.
At any instant the magnetic field generated by one particular phase depends on the
current through that phase. If the current is zero, the magnetic field is zero. If the
current is a maximum, the magnetic field is a maximum.
Since the currents in the three windings are 120o out of phase, the magnetic fields
generated will also be 120o out of phase. Then, the three magnetic fields that exist at
any instant will combine to produce one field, which acts on the rotor.
Figure 6.3
Waveform of the three-phase ac current applied to the stator windings
These waveforms are 120o out of phase with each other. Actually the waveform can
represent either the three alternating magnetic fields generated by the three phases, or
the currents in the phases.
At point 1:
Waveform C is positive and waveform B is negative. The current flow flows in opposite
direction through phases B and C. The polarity is shown that B1 is a north pole and B is
a south pole, and C is a north pole and C1 is a south pole. No current flowing through
phase A, its magnetic field is zero. The magnetic fields leaving poles B1 and C will move
toward the nearest south poles C1 and B as shown. The magnetic fields of B and C are
equal in amplitude; the resultant magnetic field will lie between two fields and will have
the direction shown.
At point 2:
60o later, the input current waveforms to phases A and B are equal and opposite, and
waveform C is zero. This change in current value per phase causes the flux to shift 60o in
a clockwise direction. The resultant magnetic field has rotate through 60o as well.
At point 3:
Waveform B is zero, and the resultant magnetic field has rotated through another 60o.
Following this procedure for one complete cycle (corresponding from point 1 to 7) will
show that the magnetic field rotates 360o, or one complete revolution, in a clockwise
direction.
The conclusion is that the application of three-phase ac to three winding symmetrically
spaced around a stator causes a rotating magnetic field to be generated.
Two-phase system will also generate a rotating magnetic field. In fact, any number of
phases will generate a rotating field. However, single phase system will not start,
therefore, special arrangements are necessary in single-phase ac motors to make them
operate properly.
6.4
Speed of the rotating magnetic field
The field rotates 1 cycle for 1 current cycle. If the current were supplied from a 60-Hz
source, the field would rotate 60 times per second or 3,600 times per minute. However if
the number of stator coils were doubled (a 4-pole machine), the field would rotate half
as fast. A 2-pole machine operating at 60Hz has speed of 3,600 rpm and a 4-pole
machine has a speed of 1,800 rpm.
For a given frequency output, the prime mover speed went down in proportion to the
number of poles. To obtain lower speeds it is necessary to increase the number of poles
on the stator. This speed is referred to as the synchronous speed of the motor. It can be
determined by using the following formula;
N = 120F/P
6-1
Where
N = synchronous speed (rev/min)
F = Frequency of the supply current (Hz)
P = Number of poles on the stator
The revolving field provides the driving force for most ac motors; hence, within narrow
limits most ac motors are constant-speed devices.
Example 6.1
What is the synchronous speed of a 12-pole, 60 Hz, squirrel cage induction motor?
6.5
A synchronous motor
A synchronous motor is so called because its rotor is synchronized with the rotating field
set up by the stator. The construction of the synchronous motors is essentially the same
as the construction of the salient-pole alternator. In fact, such an alternator may be run
as an ac motor, see figure 6.4.
Synchronous motors have the characteristic of constant speed between no load and full
load. They are capable of correcting the low power factor of an inductive load when they
are operated under certain conditions.
Figure 6.4
A synchronous motor
They are often used to drive dc generators. Synchronous motors are designed in sizes
up to thousands of horsepower. They may be designed as either single-phase or
multiphase machines. The discussion that follows is based on a three-phase design.
To understand how the synchronous motor works, assume that the application of threephase ac power to the stator causes a rotating magnetic field to be set up around the
rotor. The rotor is energized with dc (it acts like a bar magnet). The strong rotating
magnetic field attracts the strong rotor field activated by the dc. This results in a strong
turning force on the rotor shaft. The rotor is therefore able to turn a load as it rotates in
step with the rotating magnetic field.
It works this way once it's started. However, one of the disadvantages of a synchronous
motor is that it cannot be started from a standstill by applying three-phase ac power to
the stator. When ac is applied to the stator, a high-speed rotating magnetic field appears
immediately. This rotating field rushes past the rotor poles so quickly that the rotor does
not have a chance to get started. In effect, the rotor is repelled first in one direction and
then the other. A synchronous motor in its purest form has no starting torque. It has
torque only when it is running at synchronous speed.
A squirrel-cage type of winding is added to the rotor of a synchronous motor to cause it
to start. The squirrel cage is shown as the outer part of the rotor in figure 6.5.
Figure 6.5
The squirrel cage rotor winding
It is so named because it is shaped and looks something like a turnable squirrel cage.
Simply, the windings are heavy copper bars shorted together by copper rings. A low
voltage is induced in these shorted windings by the rotating three-phase stator field.
Because of the short circuit, a relatively large current flows in the squirrel cage.
This causes a magnetic field that interacts with the rotating field of the stator. Because
of the interaction, the rotor begins to turn, following the stator field; the motor starts.
To start a practical synchronous motor, the stator is energized, but the dc supply to the
rotor field is not energized. The squirrel-cage windings bring the rotor to near
synchronous speed. At that point, the dc field is energized. This locks the rotor in step
with the rotating stator field. Full torque is developed, and the load is driven. A
mechanical switching device that operates on centrifugal force is often used to apply dc
to the rotor as synchronous speed is reached.
The practical synchronous motor has the disadvantage of requiring a dc exciter voltage
for the rotor. This voltage may be obtained either externally or internally, depending on
the design of the motor.
6.6
Induction Motor
The induction motor is the most commonly used ac motor because of its simplicity, its
robust construction, and its low manufacturing cost. These characteristics of the
induction motor are due to the fact that the rotor is self-contained and usually not
connected physically to the external source of voltage. The induction motor derives its
name from the fact that ac currents are induced in the rotor circuit by the rotating
magnetic field in the stator.
The stator constructions of the induction motor of the synchronous motor are almost
identical, but their rotors are different. The rotor of the induction motor is a laminated
cylinder with slots in its surface. The windings in these slots are of two types.
The most common is called a squirrel cage winding, which is made up of heavy copper
bars connected together at either end by a metal ring made of copper or brass, see
figure 6.6. No insulation is required between the core and the bars because of the very
low voltages generated in these rotor bars. The air gap between the rotor and the stator
is kept very small so as to obtain maximum field strength.
The other type of winding contains coils placed in the rotor slots. This type of rotor is
called a wound rotor as shown in figure 6.6.
Figure 6.6
The rotors used in the induction motor
Regardless of the type of rotor used, the basic principle of operation is the same. The
rotating magnetic field generated in the stator induces an emf in the rotor. The induce
current in the rotor circuit sets up a magnetic field. The two fields interact and cause the
rotor to turn.
Wound rotor motors often have slip rings connecting the windings to external
resistances. The variable resistances provide a means for increasing rotor resistance
during starting to give better starting characteristics. When the motor is up to speed, the
windings are shorted and the operation is like a squirrel cage rotor.
When ac is applied to the stator winding, a rotating magnetic field is generated. This
rotating field cuts the bars of the rotor and induces a current in rotor, see figure 6.7
Figure 6.7
Inducing a field in a rotor
The induced current will generate a magnetic field around the conductors of the rotor,
which will try to line up with the stator field. However, since the stator field is rotating
continuously, the rotor must always follow along behind it.
From Lenz’s law, any induced current tries to oppose the changing field that induces it.
The force exerted on the rotor by the reaction between the rotor and the stator fields will
set about trying to cancel out the continuous motion of the stator field. That is to say,
the rotor will move in the same direction as the stator field, and try to line up with it.
In practice, it gets as close to the moving stator field as its weight and its load will allow.
Dc motor and synchronous motor gets their armature current by means of conduction.
The induction motor receives its rotor current by induction.
6.7
Induction motors slip
The difference between the synchronous speed and the rotor speed is called the slip of
the rotor and stated in revolutions per minute or percent. The percentage of slip
calculated as follows:
%S = (n1 – n2)/n1 X 100
6-2
Where:
%S = percentage of slip
n1 = synchronous speed, in revolutions per minute (rev/min)
n2 = rotor speed, in revolutions per minute (rev/min)
Slip in revolutions per minute is calculated as follows:
S = n1 – n2
6-3
Where:
S = slip, in revolution per minute (rev/min)
n1 = synchronous speed, in revolutions per minute (rev/min)
n2 = rotor speed, in revolutions per minute (rev/min)
The rotor frequency is directly proportional to the slip, therefore,
fr = Sfs
6-4
Where
fr = rotor frequency
S = slip percent
fs = slip frequency
Example 6.2
Calculate the percentage of slip and the rotor frequency of a 50 Hz, 8 pole motor
operating at 840rev/min.