Lesson Objectives :

Explain the principles upon which dc motor operate

Describe the construction of dc motor

Classify the different types of dc motors and their operating characteristics
4.1
Introduction
The electric motor utilizes electrical energy and magnetic energy to produce mechanical
energy. The purpose of a motor is to produce a rotating force (torque).
The basic construction of a dc motor is very similar to that of a dc generator. Each has a
frame, end bells, field poles, an armature, and a commutator. There is however, a vast
difference in their use and operation.
A generator is driven by some type of mechanical machine (steam or water turbine,
gasoline engine, or electric motor). It requires mechanical energy for its operation. The
goal is to convert mechanical energy into electrical energy.
A motor requires electrical energy for its operation. An electric current flowing through
the motor winding causes the armature to rotate. The goal is to produce mechanical
motion, thus converting electrical energy into mechanical energy.
4.2
Basic motor operation
The operation of a motor depends upon the interaction between two magnetic fields.
One magnetic field is stationary and the other is free to rotate. All current-carrying
conductors produce a magnetic field. This magnetic field is circular in shape, and the
direction of the flux depends upon the direction of current see figure 4-1.
Figure 4-1
Magnetic field around a conductor carrying current
The phenomenon action between two magnetic forces within magnetic field, reach of
each other can be used to demonstrate the theory of motor action. See figure 4-2.
Figure 4-2
Two current-carrying conductors within magnetic reach of each other
In figure 4-2, the crowding of the magnetic line causes a pressure which tends to force
the conductor apart. A similar condition exists in figure 4-3. If a current-carrying
conductor is looped through a magnetic field, it will cause the main flux to become
distorted.
Figure 4-3
Current-carrying conductor looped through a magnetic field
The lines of force are crowded below the conductor on the left and above the conductor
on the right. This produces an upward force on the left-hand conductor, and a downward
force on the right-hand conductor. If the loop is free to rotate, it revolves in a clockwise
direction until it is midway between the two poles. In this position the force are equal
and opposite to each other, figure 4-4.
Figure 4-4
Current-carrying conductor in the neutral plane
If the loop is forced beyond this point, either by centrifugal force or other means, the
magnetic action will be as shown in figure 4-5.
Figure 4-5
Magnetic reaction between two magnetic fields produces an oscillating force
An oscillating force is produced rather than a rotating force. In order to develop a
rotating force, it is necessary to reverse the direction of current through the loop every
time the loop passes the midpoint between the poles.
A single loop motor does not produce a steady torque, nor is it practical. In order to
construct a motor that will develop a reasonably steady and usable torque, it is
necessary to have many coils on the armature. The armature now becomes a practical
electromagnet with a north pole and a south pole. A cutaway end view of the armature is
illustrated in figure 4-6.
Figure 4-6
End view of motor armature, the wire is wound in the slots to form a coil
When a voltage is applied to the armature coils, a current will flow, causing the magnetic
reaction shown in figure 4-7.
Figure 4-7
Magnetic reaction in a dc motor
Notice that the north pole of the armature is attracted to the south pole of the main field
and the south pole of the armature is attracted to the north pole of the main field. When
the opposite poles are aligned, there is no longer a rotating force. In order to have
continuous torque, the current must be reversed in the armature at the instant the
unlike poles are aligned. This reversal of current is accomplished through the action of
the commutator.
When the current flow is reversed, the magnetic polarity of the armature reverses. The
south pole of the armature is near the south pole of the main field; the north pole of the
armature is near the north pole of the main field. Because as poles repel, the armature
will continue to rotate. This action continues as long as the motor is in operation.
The operating principle of a motor depends upon the following laws of magnetism:
1. When a current-carrying conductor is placed in a magnetic field, it will move at
right angles to the field.
2. Like magnetic poles repel each other and unlike magnetic poles attract each other.
4.3
Force exerted on a conductor
The amount of force acting on a current-carrying conductor in a magnetic field is directly
proportional to the following factors:
1. The strength of the main field flux
2. the length of the conductor within the field
3. the amount of current flowing in the conductor
The following equation can be used to calculate this force:
F = (8.85 X B X I X ℓ)/ 108
4-1
Where
F = force, in pounds (lb)
ℓ = length of the conductor within the field measured, in inches (in.)
B= flux density, in lines per square inch
I = amount of current flowing in the conductor, in amperes (A)
Example 4.1
Twelve in. of conductor are in a magnetic field. The current in the conductor is 50A. If
the main field flux has a density of 70,000 lines per square inch, how much force acts on
the conductor?
Solution:
F = (8.85 X B X I X ℓ)/ 108
= (8.85 X 7 X 104 X 50 X 12)/ 108
= 3.717 lb
4.4
Torque and power
Because the armature of a dc motor is wound much the same as a dc generator, the
current from the supply must divide and flow through two or more paths. The current
through the conductors under a specific pole are in the same direction, see figures 4.8
and 4.9
Figure 4.8
Two-pole motor
Figure 4.9
Four-pole motor
The force acting on the conductors causes the armature to rotate in the direction
indicated by the arrows. As the conductors move from the influence of one pole into the
influence of the next, the current flow in that conductor is reversed. This reversal of
current flow maintains a constant torque in one direction.
The force acting on each conductor adds to produce a total force. The effect of these
forces in the armature depends upon their magnitude (total pounds exerted). The effect
also depends upon the radial distance (the distance from the center of the rotor to the
center of the conductors) through which they act. This effect is usually expressed as the
product of the force and the radial distance. The unit of measurement is pound-feet
(lb.ft). The SI unit is Newton-meters or N.m.) The equation for torque is
T = FR
Where
4-2
T = torque, in pound-feet (lb.ft)
F = force, in pounds (lb)
R = radial distance, in feet (ft)
Example 4.2
The motor in figure 4.8 and 4.9 has 20 conductors within the magnetic field. The length
of each conductor within the field is 10 in. (25.4cm) and the radial distance of the
armature is 3 in. (7.62cm). The current through each conductor is 20A and the flux
density is 60,000 lines per square inch. (1 in.2 = 6.4516 cm2). Find the torque developed
by the motor, in newton-meters. (1lb.ft = 1.3588 N.m).
Solution:
An instrument designed to measure torque is called a prony brake. There are several
types available to industry. The most common types are the mechnanical, the eddy
current, and the electrodynamometer. Other prony brakes available are the hysteresis,
fluidic, and the hydraulic types.
4.5
Generator action in a motor
In the motor, as in the generator, conductor rotates through a magnetic field. If an emf
is induced into the armature conductor of a generator because they are cutting flux, the
same must be true of motors.
Consider the motor in figure 4-8, if the armature is rotating in the direction indicated by
the arrow, the emf is in a direction opposite to the current flow. This can be verified by
using left-hand rule for a generator.
From this information it can be seen that there are two voltages at work in a motor
armature:
1. the applied voltage from the source which causes the current to flow
2. the induced emf which opposes the current flow
The induced emf is called counter electromotive force (cemf) because it acts against the
applied voltage. The cemf serves a very useful purpose. Because it opposes the current
flow, the armature circuit resistance may be kept to a minimum. The lower the
resistance of the armature, the fewer the I2R losses and the more efficient the motor.
Because the induced voltage (cemf) opposes the applied voltage, the formula for current
is:
Ia = (Va – Vg)/Ra
4-3
Where
Ia = armature current
Va = voltage applied to the armature
Vg = voltage induced into the armature (cemf)
Ra = resistance of the armature windings
Example 4.3
A 5-horsepower, 230V with resistance of the armature winding of the motor is about
0.1Ω and voltage induced into the armature is 228V. Calculate the current flowing in the
motor armature
Solution:
4.6
Armature reaction in a motor
The armature current must periodically reverse its direction of flow in order to produce a
constant torque. This is accomplished by the commutator, which is a type of rotating
switch. The commutator must change the connections to the armature coils at the
instant the coils pass through the neutral plane. At the same instant, the induced
voltage in the armature also reverses the direction.
In a generator, the armature current flows in the same direction as the induced emf. In
a motor, the armature current flows in the direction opposite from the induced emf
(cemf). Therefore, for the same direction of armature rotation, the magnetic polarity of
the armature in a motor is the reverse of that in a generator. As a result, their neutral
planes shift in opposite directions.
In a generator, the neutral plane shifts in the direction of the armature rotation. In a
motor, the neutral plane shifts in the direction opposite to the direction of armature
rotation. Figure 4.10 shows the field distortion and shifting of the neutral plane for a
motor.
Figure 4-10
Armature reaction in a motor. The result is a distorted field flux and a shifting of neutral plane
It should be noted that the directions of rotation and field polarity are the same for both
generator and motor. The armature current, however, is in opposite directions. The
direction of the current through the armature coils determines the pattern of the main
flux. As a result, in a motor the brushes must be shifted opposite to the direction of
rotation.
Self induction take place in a motor just as it does in a generator. It can be said that a
motor also has three neutral planes:
1. The no-load neutral plane
2. The magnetic neutral plane
3. The commutating plane.
4.7
Motor interpoles
Interpoles are as important in motor as they are in generators. Is a motor were not
equipped with some means of compensating for armature reaction, the brushes would
have to be shifted with every change in load.
Interpoles serve the same purpose in both motors and generators. The difference is their
polarity. For motors the interpoles must have the same polarity as the main poles
directly behind them. For generators the polarity of the interpoles must be the same as
the main poles just ahead of them. Figures 4-11 and 4-12 illustrate the correct polarity.
Figure 4-11
Interpoles in a two-pole generator and four-pole generator
Figure 4-12
Interpoles in a two-pole motor and four-pole motor
4.8
Motor speed
The speed of a dc motor is proportional to the cemf. A weakening of the main field flux
reduces the cemf. The lower cemf allows more current to flow in the armature circuit.
This increase in the armature current provides a stronger magnetic field in the armature,
which causes an increase in the armature speed. The speed increases until the cemf can
limit the armature current to a new value. This value is determined by the main field
strength. At this point the motor drives the load at a constant speed.
Decreasing the armature current also affects the motor speed. Assume that the motor is
supplying a constant load. A decrease in the armature current results in a decrease in
armature reaction. The decrease in armature reaction allows the main field to increase,
and the armature slows down.
A motor is rated at its maximum horsepower and speed at a constant load. In other
words, a 10-horsepower motor rated at 1200 revolutions per minute can deliver a
maximum of 10 horsepower at a speed of 1200 rev per minute.
From equation, hp = TN/5252, it can be seen that at a specific horsepower, an increase
in speed requires a similar decrease in torque in order to maintain the same horsepower.
Therefore, to operate over a wide range of speeds a motor does not generally drive its
maximum load. It should operate as closely as possible to its maximum load. Motors are
most efficient when operating at full load. Efficiency, as well as speed should be of major
concern when selecting motors, particularly for those requiring wide variations in speed.
4.9
Speed regulation
Speed regulation refers to the manner in which a motor adjusts its speed to changes in
load. It is the ratio of the loss in speed, between no load and full load, to the full load
speed. The formula for the percent of speed regulation is:
%Reg = ((N1 – N2)/ N2) X 100
4-4
where
%Reg = percent of regulation
N1 = no-load speed of the motor
N2 = full-load speed of the motor
Example 4.4
A motor is rated at 1725 r/min. what is the percent of speed regulation if the no-load
speed is 1775 r/min?
Solution:
ASSIGNMENT 1: WORK GROUP
1.
Explain the development of torque by stating the definition of torque, the
measurement and calculation of torque in dc motor. Explain right hand rules for
motor
2.
Explain the operating characteristic of series dc motor ( Torque and speed or load
characteristics)
3.
Explain the operating characteristic of shunt dc motor ( Torque and speed
characteristics )
4.
Explain the operating characteristic of compound dc motor ( Torque and speed
characteristics )
Activities:
1. Students will be divided by 4 groups. Each group will have min 4 or max 5
members
2. Do research and discussion among group members
3. Presentation of findings – (each group one topic)
4. Submission of findings
Resources:
Student note, book, and internet