OBJECTIVES:
1. Study
the performance characteristics of a DC series motor.
2. Get
familiarized with the components involved.
APPARATUS: Dismantled DC machine
DC series motor
Ammeter 0-30A
Voltmeter 0-300V
Rheostat 20A/6Ω
Tachometer
Absorption Dynamo meter
PROCEDURE:
The machine was examined and the terminals were identified.
Then the circuit was connected with the rheostat, ammeter and the dc series
motor connected in series. Then the selected voltmeter was connected in
parallel to the dc series motor. The whole system was then supplied with a 220V
dc supply.
Then the motor was started with a sufficient load on the
pan. The values of the load (W), spring load (w), voltage (V), armature current
(A) and the speed of the motor (Nr) were noted down. Then the load on the pan
was decreased in suitable amounts and the above quantities were noted down at
these different loads.
In order to avoid excessive heating the readings were taken
as fast as possible and once they were taken the machine was switched off and
motor wheel was cooled with water in order to avoid damage to the machine.
Thereafter, the resistance of the armature and field winding
was taken down. Further thereafter the circumference value of the motor wheel
was measured and noted down.
THEORY:
Direct current (DC) motors
operate on a magnetic field produced by the field winding in the stator
(stationary part of the motor) interacting with the field produced by the
armature winding in the rotor (rotating part). The basic constructional
features of a typical two pole DC motor and the circuit model are shown in the
figure.
The field is
produced by direct current in field coils or by permanent magnets on the
stator. The output, or armature, windings are placed in slots in the
cylindrical iron rotor. In a direct-current generator—a simplified machine with
only one rotor coil—the rotor is fitted with a mechanical rotating switch, or commutator, that connects the rotor coil to the stationary
output terminals. This commutator reverses the connections at the two instants
in each rotation when the rate of change of flux in the coil is zero, i.e. when the enclosed flux is maximum (positive)
or minimum (negative). The output voltage is then unidirectional but is
pulsating for the single case of one rotor coil. In practical machines, the
rotor contains many coils symmetrically arranged in slots around the periphery
and all connected in series. Each coil is connected to a segment on a multi-bar
commutator. In this way, the output voltage consists of the sum of the induced
voltages.
The DC
motors needs slip rings or split rings (commutator) on the rotor shaft and a
set of brushes positioned over them to supply the armature winding. DC motors
can be categorized into four basic types depending on the method used for
connecting the winding.
CALCULATIONS:
The first set of reading of the observed values are used
below to calculate the Electrical Input, Torque, Mechanical Output, Efficiency,
Copper loss, Mechanical loss.
W (weight) = 50 x 0.4536
=
22.68 kg
w (weight) = 5
x 0.4536
=
2.27 kg
Radius
(r) = circumference/ 2л
=
0.72 x 7/44
=
0.1146m
Speed (rad s-1) =
2 x л x Nr/60
=
2 x л x 1400/60
=
146.6 rad/s
Electrical Input = V x
I
=
200 x 19.2
=
3840 W
Torque =
(W-w) x g x r
=
(22.68 – 2.27) x 9.8 x 0.1146
=
22.92 Nm.
Mechanical
Output = τ x ω
= 22.92 x 146.6
= 3600.0 W
Efficiency = Mech. Output / Elec. Input
=3600.0 /3840
= 87.5 %
Copper loss =
I2R
= 19.22 x (0.6+1.4)
= 737.26 W
Mechanical loss = Electrical Input – Mechanical output
– Copper loss
= 3840 – 3600.0 – 737.26
= - 497.28 W
RESULTS:
|
Set
|
Angular
Velocity
(rad/s)
|
Electrical
Input
(W)
|
Torque
(Nm)
|
Mechanical
Output
(W)
|
Efficiency
(%)
|
Copper
loss
(W)
|
Mechanical
Loss
(W)
|
|
1
|
146.6
|
3840
|
22.92
|
3600.0
|
87.5
|
737.26
|
-
497.28
|
|
2
|
150.77
|
3760
|
21.90
|
3301.8
|
87.8
|
706.88
|
-248.68
|
|
3
|
152.86
|
3720
|
21.39
|
3269.7
|
87.8
|
691.92
|
-241.62
|
|
4
|
154.96
|
3640
|
20.38
|
3158.1
|
86.7
|
662.48
|
-180.58
|
|
5
|
157.05
|
3560
|
18.34
|
2880.3
|
80.9
|
633.68
|
46.02
|
|
6
|
159.15
|
3480
|
16.81
|
2675.3
|
76.7
|
605.52
|
199.18
|
|
7
|
165.42
|
3280
|
15.28
|
2527.6
|
77.0
|
537.92
|
214.48
|
|
8
|
173.80
|
3160
|
13.25
|
2302.9
|
72.8
|
499.28
|
319.18
|
DISCUSSION:
Direct current (DC) motors operate on a magnetic field
produced by the field winding in the stator (stationary part of the motor)
interacting with the field produced by the armature winding in the rotor
(rotating part). The field is produced by direct current in field coils or by
permanent magnets on the stator. The output, or armature, windings are placed
in slots in the cylindrical iron rotor. In a direct-current generator—a
simplified machine with only one rotor coil—the rotor is fitted with a
mechanical rotating switch, or commutator, that connects the rotor coil to the
stationary output terminals. This commutator reverses the connections at the
two instants in each rotation when the rate of change of flux in the coil is
zero—i.e., when the enclosed flux is maximum (positive) or minimum (negative).
The output voltage is then unidirectional but is pulsating for the single case
of one rotor coil. In practical machines, the rotor contains many coils
symmetrically arranged in slots around the periphery and all connected in
series. Each coil is connected to a segment on a multi-bar commutator. In this
way, the output voltage consists of the sum of the induced voltages.The DC motors
needs slip rings or split rings (commutator) on the rotor shaft and a set of
brushes positioned over them to supply the armature winding.
A series wound DC motor normally drives loads that require
high torque and do not require precise speed regulation. Series DC motors are
ideal for traction work where the load requires a high breakaway torque. Such
uses include locomotives, hoists, cranes, automobile starters, or oil drilling
rig applications. An increase in load results in an increase in both armature
and field current. As a result, torque increases by the square of a current
increase. Speed regulation in series motors is inherently less precise than in
shunt motors. If motor load diminishes, current flowing in both the armature
field circuits reduces as well. This results in a greater increase in speed
than in shunt motors. Removal of mechanical load from series motors results in
an indefinite speed increase which can destroy the motor or bearings. Small
series motors usually have enough internal friction to prevent high-speed
breakdown, but larger motors require to be controlled.
Components
of a series motor include the armature and the field. The same current is
impressed upon the armature and the series field. The coils in the series field are made of a
few turns of large gauge wire, to facilitate large current flow. This provides
high starting torque, approximately 2 ¼ times the rated load torque. Series
motor armatures are usually lap wound. Lap windings are good for high current,
low voltage applications because they have additional parallel paths for
current flow. Series motors have very poor speed control, running slowly with
heavy loads and quickly with light loads. A series motor should never drive
machines with a belt. If the belt breaks, the load would be removed and cause
the motor to over speed and destroy itself in a matter of seconds. Common uses
of the series motor include crane hoists, where large heavy loads will be
raised and lowered and bridge and trolley drives on large overhead cranes. The
series motor provides the starting torque required for moving large loads.
Traction motors used to drive trains are series motors that provide the
required torque and horsepower to get massive amounts of weight moving. On the coldest days of winter the series
motor that starts your car overcomes the extreme cold temperatures and thick
lubricant to get your car going.
The shunt motor is probably the most common dc motor used in
industry today. Components of the shunt motor are the armature and the field.
The coils in the shunt field are composed of many turns of small wire,
resulting in low shunt field current and moderate armature current. This motor
provides starting torque that varies with the load applied and good speed
regulation by controlling the shunt field voltage. If the shunt motor loses its
field it will accelerate slightly until EMF rises to a value sufficient to shut
off the torque producing current. In
other words, the shunt motor will not destroy itself if it loses its field, but
it won’t have the torque required to do the job it was designed for. Some of
the common uses of the shunt motor are machine shop lathes, and industry
process lines where speed and tension control are critical.
When comparing the advantages of the series and shunt
motors, the series motor has greater torque capabilities while the shunt motor
has more constant and controllable speed over various loads. These two
desirable characteristics can be found in the same motor by placing both a
series field and shunt field winding on the same pole. Thus, we have the
compound motor. The compound motor responds better to heavy load changes than a
shunt motor because of the increased current through the series field
coils. This boosts the field strength,
providing added torque and speed. If a shunt coil is added to a series motor at
light loads (when a series motor tends to over speed) the added shunt field
flux limits the top speed, eliminating self-destruction. Common uses of the
compound motor include elevators, air compressors, conveyors, presses and
shears. Compound motors can be operated as shunt motors by disconnecting the
series field. Many manufacturing process lines are designed this way. The
reason being that, most off the shelf motors are compound motors, and the
series field can always be connected later to provide additional torque, if
needed. Compound motors can be connected two ways, cumulatively and
differentially. When connected cumulatively, the series field is connected to
aid the shunt field, providing faster response than a straight shunt motor.
When connected differentially, the series field opposes the shunt field.
Differentially connected compound motors are sometimes
referred to as “suicide motors,” because of their ability for self-destruction. If perhaps, the shunt field circuit were to
suddenly open during loading, the series field would then assume control and
the polarity of all fields would reverse. This results in the motor stopping,
and then restarting in the opposite direction. It then operates as an unloaded
series motor and will destroy itself. Differentially connected motors can also
start in the opposite direction if the load is too heavy. Therefore, it is
seldom used in industry.
Applications of motors
1.Shunt excited dc motors
These have fairly constant speeds against a varying load or
torque. Therefore applications include situations where a constant speed is
required.
E.g. Lathes, Conveyors, Fans, Machine tool drives
2. Series excited dc motors
These are able to create large torques at low speeds (high
starring torque) it can be used to accelerate very heavy loads from stand
still.
E.g. Driving cranes, Driving electric locomotives, Steel
rolling mills
3. Compound excited dc motors
These have Combine characteristics of both shunt and series
wound motors. The series winding gives good starting torque and shunt winding
ensures a comparatively constant speed.
E.g. Planers, Shears, Guillotines, Printer machines, Power
presses which needs peak loads at certain
instances
4. Separately excited dc motors
These are used in applications where an independent armature
control and a field control are required.
E.g. Steel and
Aluminum rolling mills, Controls motors
5. Permanent magnet motors
These are used for low power applications.
E.g. Automobiles, Starter motors, Wiper motors, Lowering
windows, Toys, Electric tooth brushes
REFERENCE:
Machine Elements
in Mechanical Design by Robert L. Mott.
Electric Machines
by I.J. Nagrath and D.P.Kothari
