SAFETY
PRECAUTIONS
1. Must come prepared for the lab;
study the related chapter of the experiments before coming to the lab.
2. Proper care must be observed when moving equipments and parts.
3. Lab demonstration has to be attended prior to starting your lab work.
4. Power off prior to making any wire changes.
5. Do not touch any wire while the power is on inform the lab
instructors immediately if a wire come out or is found hanging.
6. All lab partners must be at the workstation at all times before the
power is turned on.
7. The motor should be started
and stopped with load
8. Brake drum should be cooled
with water when it is under load.
THEORY
A DC series
motor converts electrical energy to mechanical energy. Its principle of
operation is based on a simple electromagnetic law that states that when a
magnetic field is created around current carrying conductor and interacts with
an external field, rotational motion is generated.
The key
components of a DC series motor are the armature (rotor), stator, commutator,
field windings, axle, and brushes. The stationary part of the DC Seies Motor
motor, the stator is made up of two or more electromagnet pole pieces, and the
rotor is comprised of the armature, with windings on the core connected to the
commutator. The output power source is connected to the armature windings
through a brush arrangement connected to the commutator. The rotor has a
central axle about which the rotor rotates.
The field
winding should be able to support high current because the greater the amount
of current through the winding, the greater will be the torque generated by the
motor. So the winding of the motor is made up of thick heavy gauge wire. Heavy
gauge wire does not allow a large number of turns. The winding is made up of
thick copper bars as it helps in easy and efficient dissipation of heat
generated as a result of flow of large amount of current through winding.
Principle of
Operation
An external
voltage source is applied across the series configuration of field winding and
armature. So one end of the voltage source is connected to the winding and the
other end is connected to the armature through the brushes.
Initially at
the motor start up, with the voltage source connected to the motor, it draws a
huge amount of current because both the winding and the armature of the motor,
both made up of large conductors, offer minimum resistance to the current path.
The large current through the winding yields a strong magnetic field.
This strong
magnetic field provides high torque to the armature shaft, thus invoking the
spinning action of the armature. Thus the motor starts rotating at its maximum
speed in the beginning. The rotating armature in the presence of the magnetic
field results in counter EMF, which limits the current build up in the series
combination of armature and winding.
Thus series
motors once started will offer maximum speed and torque but gradually, with an
increase in speed, its torque will come down because of its reduced current.
Practically this is what required from the motors. Due to the high torque
provided by the armature, the load on the shaft is set to rotate initially.
Subsequently lesser torque will keep the load on the move. This further helps
in increasing the heat dissipation of the motor. However, the amount of torque
generated by motor is directly proportional to the winding current. The higher
current demands a higher power supply, too.
Motor Speed
In DC series
motors, a linear relationship exists between the amount of torque produced and
the current flowing through the field windings. The speed of the motor can be
controlled by varying the voltage across the motor, which further controls the
torque of motor.
To increase
the speed of the motor, decrease the field current by placing a small
resistance in parallel to the winding and armature. The decrease in current
will result in lowering of magnetic flux and counter EMF, which further hastens
the motor’s speed.
To decrease
the speed, use an external series resistance along with the field winding and
armature. This will reduce the voltage across the armature with the same
counter EMF, thus resulting in a lower speed of motor.
Unlike DC
shunt motors, series motor does not operate at the constant speed. The speed of
the motor varies with change in the shaft load, so speed control of the motor
is not easy to put into practice.
Applications,
Advantages and Precautions
Series
motors can produce large turning effect, or torque, from a stand still. These
motors have found application in small electrical appliances where high torque
is necessary at start up. DC series motors are used mainly for industrial
applications, e.g. elevator and pulley and winches systems for carrying heavy
loads. Heavy and magnificent cranes drawing thousands of amperes are driven by
this motor. An automobile engine can be started by this motor which draws
around 500A of current. However, these motors are not suitable where constant
speed is required as the speed of series motors is dependent (varies with load)
on load unlike DC shunt motors (see link below for an article similar to this
one that covers DC shunt motors) whose speed is independent of load.
The
construction, designing, and maintenance of these motors is very easy. Series
motors are cost effective as well. A final advantage of series motors is that
they can be used by providing either an Alternating Current (AC) or Direct
Current (DC) power source.
Proper care
should be taken that a series motor is not operated without any load as they
are totally dependent on shaft loads. As the armature speed increases, the
current through the winding decreases which further helps in reducing the
counter EMF. This reduction fastens the speed of the armature. As this process
continues, the motor speed increases beyond the limit thus causing devastation
to the motor.
EXPERIMENT No. : 1
AIM : To conduct load test on DC Series Motor and
to find efficiency.
CIRCUIT DIAGRAM :
PROCEDURE
1.
Connections are made as per the
circuit diagram.
After
checking the load condition, mcb is closed and starter resistance is gradually
removed.
3.
For various loads, Voltmeter,
Ammeter readings, speed and spring balance readings are noted.
4.
After bringing the load to initial
position of 1kg , bring the two pointer to clockwise direction to start the motor
.
5. Note down the corresponding
Voltage,Current and Load on the spring load.
6. To take the
Observation:-
S. No.
|
Supply Voltage
|
Current
I
(Amps)
|
Load In Kg (From Spring Balance)
|
To see the complete working video of the
above panel please visit this link
https://www.youtube.com/watch?v=VGPpUozWIjg
EXPERIMENT No. : 2
AIM: To conduct Test on DC Series Motor to
identify the Core Loss
CIRCUIT DIAGRAM :
PROCEDURE
1.Connections are made as per the
circuit diagram.
2.
After checking the load condition,
mcb is closed and starter resistance is gradually removed.
3.
Apply the maximum load to the
motor for maximum rated current.
4.
Note down the reading of Supply
Voltage and Armature Current.
5.
Calculate the Armature Resistance
using the Formulae
V=IR
6. Calculate the loss using the formulae
CL=I*I*R
S.
No.
|
Supply Voltage
|
Current
I
(Amps)
|
Armature Resistance
|
Core Loss =I*I*R
|
MODEL GRAPH
List of Accessories
1.
Patch cord
............................................................................................................... 11
Nos.
2.
Operating
Manual...................................................................................................... 01
No.
3.
Digital
Tachometer...................................................................................................... 01No.
4.
DC
Motor with Stand................................................................................................. 01
No.
