EE Department
Instrumentation and Measurement
Experiment No: 14
Open Ended Lab (OEL)
Special Motors (Repulsion Motors, Stepper Motors).
Objective:

To study and understand the construction and working of special motors.
Apparatus and Components:



Repulsion motor
Stepper motor
Connecting wires
Resume of Theory:
1. Repulsion Motor:
A repulsion motor is a single-phase electric motor that operates by providing input AC. The main
application of repulsion motor is electric trains. It starts as a repulsion motor and runs as an induction motor,
where the starting torque should be high for a repulsion motor and very good running characteristics for an
induction motor. In repulsion motor direction of rotation of the motor is the same as that of brush shift.
Figure: 14.1 Repulsion Motor
EE Department
Instrumentation and Measurement
Construction of repulsion motor:
The repulsion motor is equipped with the stator, rotor, and commutator brush assembly. The stator
carries a single-phase exciting winding similar to the main winding of a single-phase induction motor. The
rotor has distributed DC winding connected to the commutator at one end just like in the DC motor. The
carbon brushes are short-circuited on themselves and find a use for conducting current using the armature.
Working principle of repulsion motor:
The basic principle behind the working of repulsion motor is that “similar poles repel each other.”
This means two North poles will repel each other. Similarly, two South poles will repel each other. When
the repulsion motor winding is supplied with single-phase AC, it produces a magnetic flux along the direct
axis. When this magnetic flux links with the rotor winding, it creates an EMF. Due to this EMF, a rotor
current is produced. This rotor current in turn produces a magnetic flux that is directed along the brush axis
due to commutator assembly. Due to the interaction of stator and rotor produced fluxes, an electromagnetic
torque is produced.
Types of Repulsion Motors:
There are three types of repulsion motor they are
i.
ii.
iii.
Compensated repulsion motor
Repulsion-start induction motor
Repulsion induction motor
Applications of Repulsion Motor:
The applications of repulsion motors include the following.





Applications of Farm Motors
Film winding machines
Hoists
Machines in Textile
Machines for floor maintenance
Advantages of a Repulsion Motor:
Excellent beginning torque, low starting current, and a wide range of speed control with smooth
speed fluctuation are all advantages of a repulsion motor. The following are some of the repulsion motor’s
drawbacks. At lower speeds, the power factor is much lower. Brushes and commutators quickly exhaust
owing to heat production and arcing at the brush assembly.
Thus, this is all about the repulsion motor. Most of the commutator motors are restricted to about
1500 V as high voltages provide rise to a threat of arcing across it. These motors find use where high
voltages are required because the circuit of the rotor is not connected electrically to the power supply.
EE Department
Instrumentation and Measurement
2. Stepper Motor:
A stepper motor is an electromechanical device it converts electrical power into mechanical power.
Also, it is a brushless, synchronous electric motor that can divide a full rotation into an expansive number
of steps. The motor’s position can be controlled accurately without any feedback mechanism, as long as the
motor is carefully sized to the application. Stepper motors are similar to switched reluctance motors. The
stepper motor uses the theory of operation for magnets to make the motor shaft turn a precise distance when
a pulse of electricity is provided. The stator has eight poles, and the rotor has six poles. The rotor will
require 24 pulses of electricity to move the 24 steps to make one complete revolution. Another way to say
this is that the rotor will move precisely 15° for each pulse of electricity that the motor receives.
Construction & Working Principle:
The construction of a stepper motor is fairly related to a DC motor. It includes a permanent magnet
like Rotor which is in the middle & it will turn once force acts on it. This rotor is enclosed through a no. of
the stator which is wound through a magnetic coil all over it. The stator is arranged near to rotor so that
magnetic fields within the stators can control the movement of the rotor.
Figure: 14.2 Stepper Motor
The stepper motor can be controlled by energizing every stator one by one. So the stator will
magnetize & works like an electromagnetic pole which uses repulsive energy on the rotor to move forward.
The stator’s alternative magnetizing as well as demagnetizing will shift the rotor gradually &allows it to
turn through great control.
The stepper motor working principle is Electro-Magnetism. It includes a rotor which is made with
a permanent magnet whereas a stator is with electromagnets. Once the supply is provided to the winding of
the stator then the magnetic field will be developed within the stator. Now rotor in the motor will start to
move with the rotating magnetic field of the stator. So this is the fundamental working principle of this
motor.
In this motor, there is a soft iron that is enclosed through the electromagnetic stators. The poles of
the stator as well as the rotor don’t depend on the kind of stepper. Once the stators of this motor are
energized then the rotor will rotate to line up itself with the stator otherwise turns to have the least gap
through the stator. In this way, the stators are activated in a series to revolve the stepper motor.
EE Department
Instrumentation and Measurement
Types of Stepper Motor:
There are three main types of stepper motors, they are:
i.
ii.
iii.
Permanent magnet stepper
Hybrid synchronous stepper
Variable reluctance stepper
Advantages of Stepper Motor:
The advantages of stepper motor include the following.






Ruggedness
Simple construction
Can work in an open-loop control system
Maintenance is low
It works in any situation
Reliability is high
Disadvantages of Stepper Motor:
The disadvantages of stepper motor include the following.
 Efficiency is low
 The Torque of a motor will declines fast with speed
 Accuracy is low
 Feedback is not used for specifying potential missed steps
 Small Torque toward Inertia Ratio
 Extremely Noisy
Applications of Stepper Motor:
The applications of stepper motor include the following.
1. Industrial Machines – Stepper motors are used in automotive gauges and machine tooling automated
production equipment.
2. Security – new surveillance products for the security industry.
3. Medical – Stepper motors are used inside medical scanners, samplers, and also found inside digital
dental photography, fluid pumps, respirators, and blood analysis machinery.
4. Consumer Electronics – Stepper motors in cameras for automatic digital camera focus and zoom
functions.
EE Department
Instrumentation and Measurement
EE-301(L) Electrical Machines lab
Rubric
Lab
Learning
Outcomes
(LLOs)
LLO-1
Data
Acquisition
Program
Learning
Outcome
s
(PLOs)
PLO-5
(Modern
tool
Usage)
Blooms
Taxonomy
4.0
P.3
(Guided
Response)
LLO-2
Safety
LLO-3
(Lab Report)
Excellent
PLO-6
(The
Engineer
and
Society)
A-2
(Responding
)
PLO-10
(Commu
nication)
C-3
(Apply)
Good
3.0
Satisfactory
Poor
2.0
1.0
Data gathered
Data gathered Data gathered
Data gathered independently in independently in independently in
independently least organized organized manner least organized
in organized
manner and
only
manner
manner and evaluate further
evaluate it
further
Adhere to all
Adhere to all Less information No information
safety
safety
to safety
to safety
precautions precautions less
precautions
precautions
while
cautiously while
performing lab performing lab
and acquiring and acquiring
data
data
Complete Lab Complete Lab
Issues in
Fail to submit
report
report with
formatting of
lab report in
according to formatting / data report as well as
given time
required
issues
data missing
format
Total score:12
Score