This document discusses different types of electric motors: 1. Permanent magnet synchronous motors have constant torque output but are expensive and available only in small sizes. They are used in precision equipment. 2. Stepped motors move in discrete steps and have multiple coil phases. Variable-reluctance stepped motors work by aligning the rotor with the stator's magnetic field. Permanent magnet stepped motors produce more torque but require reversing current to change direction. 3. Brushless DC motors have electronically controlled commutation without brushes, making them more efficient than brushed DC motors. They are used in computer hard drives and other applications.

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 It has poor power output
 Low efficiency and power factor
 Low torque output.
 Only available in small size.
 It is expensive compared to the reluctance motor of the same size. [3] [1]
Applications.
 Sound producing and recording equipment
 Navigation Equipment
 Precision record players
 Timing devices.
 Hard disks
 Tele-printers
b) Stepped Motors
A Stepped motor is a DC motor that moves in discrete steps. It converts digital pulses into analog
shaft motion. It has multiple coils organized in groups called phases. Energizing each phase in
sequence causes the motor to rotate step by step. It rotates by a specific number of degrees when an
electrical pulse is applied.
Typical commercial motors range to up to 400 steps per revolution. There are two types of stepped
motor i.e. The variable-reluctance and permanent magnet type. [3]
Mode of Operation
The variable-reluctance type:
Single-Stack Stepper Motor
When the stator phases are supplied with a dc current, the resultant air gap field steps around
and the rotor follows the axis of the air gap field by virtue of reluctance torque.
The windings are energized in a sequence and the sequence is repeated. For three phase windings,
when winding A is excited , the rotor aligns with the axis of phase A .Next windings B and C are
excited and the resultant mmf axis moves 45° in the clockwise direction .The rotor aligns with the
resulting mmf axis. Thus, at each transition the rotor moves through 45° as the resultant field is
switched around. The direction of rotation is changed by reversing the pattern of switching the
windings. [3]
Multi-stack Stepper Motor
Both the stator and the rotor have the same number of teeth. When a phase is excited, the
positon of the rotor relative to the stator in that stack are aligned, whereas the stator teeth have

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different alignment between stacks.
Taking an example of stacks, A, B and C, when stack A is supplied with power, the rotor and stator
teeth in stack A are aligned unlike for B and C. When the excitation moves to stack B, the stator and
rotor teeth in stack B are aligned by movement of the rotor in the clockwise direction thus the motor
moves one step and the procedure continues. [3]
Permanent magnet type
If phase A windings are excited, the rotor aligns with the two stator poles according to the
winding excitation. When excitation is switched to phase B windings ,the rotor moves but a step of
90°.The follow of current determines the direction of rotation of the motor. [3]
Advantages of Permanent Magnet Stepped Motors
 It produces and hold more torque.
Disadvantages of Permanent Magnet Stepped Motors
 To change the direction of rotation, current in all windings has to be reversed.
 Have a high starting inertia hence a slow acceleration than variable-reluctance step motors.
 It has a lower step rate compared to variable-reluctance step motors.
 Expensive compared to variable-reluctance step motors.
Applications of stepped motors
 Used in interfacing with computers and direct computer control.
 Used in industrial control.
 Used in printers.
 Machine tools.
 Process control system.
 X-Y recorders.
 Robotics. [3]
c) Brushless DC motors
These are electronically commuted motor without brushes. They are DC powered synchronous
motors. [4] [3]

3. 4
Figure 1 BLDC Motor
Principle of Operation
The rotor is made of a permanent magnet and the stator made of an electromagnetic material. The
power supply is connected to the stator terminals of the motor. Current flows in the stator coils
producing a uniform field in the air gap. Switching from one stator coil to the next produces a
trapezoidal shape AC voltage waveform. Field force interaction causing different switching states
between the rotor and the stator keeps the rotor still rotating. [5] [6]
Advantages of Brushless Motors
 They are more efficient.
 Mechanical energy loss due to friction is less since it doesn’t have brushes.
 They can operate at high-speed under any condition.
 Brushless motors produce no sparking and have a silent operation.
 They can accelerate and decelerate easily due to low inertia.
 Produce a large amount torque.
 They are more reliable and have a high life expectancy since they have no brushes.
 Maintenance free operation. [4] [5]
Disadvantages of Brushless Motors
 They are expensive compared to the brushed DC motors.
 Limited power output.
 Complex drive circuit design.
Applications
 Computer hard drives and DVD/CD players
 Used in electric vehicles
 Robotics design and CNC machine tools.
 Washing machines fans ,blowers and compressors. [5]

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d) Reluctance Motors
These are motors that produce torque due to reluctance. When the motor rotates at synchronous
speed, the saliency causes torque due to reluctance. The torque is due to the rotor aligning itself with
the rotating field. [3]
Principles of Operation
When the stator is connected to a single phase supply, the motor operates as a single phase induction
motor. As it approaches the synchronous speed at approximately 75%, a centrifugal switch
disconnects the auxiliary windings leaving only the main windings.
As it tends further to the synchronous speed, the rotor aligns itself to the rotating forward air gap
field until it reaches synchronous speed. [3]
Advantages of Reluctance Motors
 They provide more power at a low cost.
 Maintenance free operation.
Disadvantages of Reluctance Motors
 High torque ripple when operating at low speed.
 Operation noise due to the torque ripple.
 Low power factor
 Low efficiency
Applications
 Analog electric meters
 Hard disk drives.
 Control in nuclear reactors. [3]
e) Self-synchronous permanent magnet motors
These kinds of synchronous motors have stator windings like induction motors and the rotor is made
up of a permanent magnet. As shown in the figure below, two rectifiers are used at the source and
machine end. “The thyristors in the supply rectifier are commutated by the supply voltage and those
in the machine side by excitation voltage of the motor.” [3]

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Figure 2 Open-loop control of self-synchronous motor
When a power supply is connected to the stator windings, a uniform field is produced in the air-gap.
The interaction of the field from the stator windings and the rotor permanent magnet produces a
torque that spins the rotor at the same speed as the stator magnetic field.
When load changes, the torque angle increases beyond 90° which tends to lead to loss of
synchronism. The rotor position sensor which is mounted on the rotor shaft generates signals for
processing in the control logic circuit. When the load changes, it leads to change of the rotor speed
hence changing the firing frequency of the thyristors instantaneously using the firing circuit as
required and in turn the stator frequency, to keep the motor in synchronism.
Advantages.
 Synchronous operation makes field orientation easy.
 Precise speed monitoring and regulation.
 Reliability due to absence of brushes and commutator.
Disadvantages.
 Rotor position sensing required.
 Position sensor or sensor-less technique is required for motor operation.
Applications.
 Oil and centrifugal pumps.
 Locomotive drives
 Air compressors
2) Brushless DC motors and Self-synchronous permanent magnet motors.
 The Self-synchronous permanent magnet motors have sinusoidal back emfs while as

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Brushless DC motors trapezoidal back emfs.
 Brushless DC motors the stator windings are concentrated and so the stator waveform will be
trapezoidal. Whereas in Self-synchronous permanent magnet motors, the stator winding is
distributed winding and so the stator waveform is sinusoidal when both their rotor is a
permanent magnet to produce constant torque. [3]
3) Types of Braking in DC motors and Induction Motors.
a) DC motors
There are three types of braking in DC motors i.e. Regenerative braking, Dynamic braking and
plugging.
 Regenerative Braking
In this type of braking, the kinetic energy of the motor is returned to the power supply. It occurs when
the loaded motor runs at a speed higher than the its no load speed at constant excitation. Back emf is
higher than the supply voltage and the armature current is reversed. [7]
At this point, the DC motor is working as a generator .Regenerative braking can be used to control
the speed of a motor above the no-load speed. [7]
 Dynamic Braking
Is the type of braking where the DC supply is disconnected and replaced with a braking resistance
across the armature immediately .The motor will work as a generator and produces a braking
torque .
The kinetic energy is stored in the movable parts of the motor and is dissipated as heat in the braking
resistance and the armature resistance. It is inefficient since the energy is lost as heat.
 Plugging
It is the type of braking where the armature terminals are reversed while running. The supply voltage
and the induced voltage will act in the same direction.
The armature current is reversed thereby producing a braking torque with a positive field current.
The effective voltage across the armature will be + .
A High braking torque is produced. It’s not efficient since power is lost in resistances. [7]
b) Induction Motors
 Regenerative Braking in Induction Motor
If the speed of the motor is more than the synchronous speed, the relative speed between the motor
conductors and the air-gap rotating field reverses and regenerative braking takes place and the motor
works as a generator. With a variable frequency regenerative braking can occur for sub-synchronous
speed.

7. 8
 Plugging Braking.
When the phase sequence of the motor is reversed, by interchanging the terminals for single phase
and any two phases of the stator for 3 phase during operation to change the direction of rotation of
the motor. The motor decelerates to zero and gradually accelerates in the opposite direction.
 Dynamic Braking
Dynamic braking is classified into:
1. AC dynamic braking.
The supply to one of the stator phases is cut off and the motor runs as a single phase. Negative
sequence components are induced in the supply and then the motor stops.
2. DC injection braking.
The AC supply to the stator is disconnected and replaced with a DC supply on two of the phases. The
motor generates a constant magnetic field which induces a voltage in the rotor windings and
dissipating energy as losses hence braking motor
3. Capacitive braking.
The power supply is disconnected from the stator terminals and replaced with capacitors. The motor
sets up a magnetic field which cuts the rotor windings generating a voltage in the rotor windings and
dissipates energy as losses hence braking the motor. [8]
4)
i) Sinusoidal wave-form motors
The block diagram of a synchronous servomotor drive with sinusoidal wave forms
Figure 3 Synchronous servomotor drive block diagram
An absolute position sensor which is normally pre-aligned to measure the rotor field position with
respect to the known axis. determines the absolute rotor field position. Using the pre-stored cosine
table in the ROM. Cosine functions are generated for two of the three phase currents.
Considering a two pole motor we have;

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( ) = cos[ ( )] … 4.1
For a p-pole motor if is the mechanical angle measured, then the electrical angle is calculated as
( ) = cos[ ( )] … 4.2
( ) = cos[ ( ) − 120°] … 4.3
( ) = cos[ ( ) − 240°] … 4.4
The Stator current amplitude is determined by the torque-speed loop using the equation;
= … 4.5
Once the reference currents ∗ ∗
are defined for phases
∗
= − ∗
− ∗
… 4.6
A current-regulated voltage source inverter is used to force the motor currents to equal the reference
currents. The air gap flux density distribution and the induced excitation voltages in the stator phase
windings are sinusoidal and the torque angle is maintained at 90°. With the frequency of the stator
currents synchronized to the rotor position, and the position continuously measured, there is no
possibility of losing synchronism. If a holding torque is required at zero speed to overcome the load
torque, a synchronous servomotor drive provides the torque by applying dc currents to the stator as
given in the equations 4.2-4.4. [9]
ii) Trapezoidal wave form
The motors are designed with concentrated coils and the magnetic structure is shaped such that the
flux density of the field have trapezoidal waveforms because of the permanent magnets and the
induced excitation voltages.
Considering a servomotor with the rotor rotating in a counterclockwise direction at a speed of
radians per second and is measured with respect to the stator.
The electrical angle for a p pole motor is obtained from
( ) = ( )
The amplitude is proportional to the rotor speed such that
=
Where is the motor voltage constant.
A voltage waveform is produced in phase as shown below.

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Figure 4 Waveform for phase a
Similar waveforms are induced in the b and c with a displacement of 120° − 120° respectively.
A current regulated voltage source inverter as in sinusoidal wave-form motors is used where the
sinusoidal reference currents are replaced by rectangular current references. To obtain these current
references the rotor position is usually measured by Hall Effect sensors that indicate the six current
communication instants per electrical cycle of waveforms separated by 60 electrical degrees. [9]
Differences
The trapezoidal waveforms servomotors are designed with concentrated coils and the magnetic
structure is shaped such that the flux density of the field have trapezoidal waveforms because of the
permanent magnets and the induced excitation voltages while as the sinusoidal waveforms motors
are designed with distributed coils such that induced excitation emfs in the stator due to the field flux
are sinusoidal and the stator produces a sinusoidal field [9].
5)
i)
PWM inverters help control frequency, magnitude of the voltage and current applied to a motor while
the 6-step inverter only controls the frequency.
PWM inverters controls the speed of the motor by driving the motor with a series of “ON-OFF”
pulses and varying the duty cycle of the pulses while keeping the frequency constant.
The power supply to the motor is controlled by changing the width of the applied pulses and in turn
the average DC voltage at the terminals. There’s a small power loss in the switching transistor since
the transistor is either fully “ON” or fully “OFF” giving it a linear type of control which results in
better speed stability at low speed and efficiency.
Also the amplitude of the motor voltage remains constant so the motor is always at full strength.
Hence the motor can rotate at a much slower rate without it stalling. [3] [10]
ii)
The Cyclo-converters produce an acceptable output waveform only at frequencies below the supply
frequency usually one third of the input frequency of the supply. Since frequency is proportional to

10. 11
speed, low frequencies will give low motor speeds and better efficiency. [11] [12]
iii)
Cyclo-converters vary a constant voltage and the frequency waveform of a supply to a different
desired value load frequency usually lower without using an intermediate DC link.
A Cyclo-converter waveforms are much distorted for higher frequencies which in turn distorts the
sinusoidal waveform. For frequencies higher than one third of the supply input frequency, the
waveform is completely deformed. And since frequency is proportional to the speed, high speeds
will give a poor output waveform. [12]
6) Minimum and maximum currents for a class A chopper operated dc motor with back emf [13]
When the chopper is on ( ≤ ≤ )
Figure 5 Chopper ON
From the circuit above
= + +
Taking the Laplace transform
= ( ) + [ ( ) − (0 )] +
At = 0, initial current (0 ) =
( ) =
−
+
+
+
… .1
Taking the inverse Laplace transform
( ) =
−
1 − + 0 ≤ ≤ … 2
Load current at the instant the chopper is turned off:
( ) =
When the chopper is turned off , 0 ≤ ≤
V
R
L
E

11. 12
Figure 6 Chopper OFF
From the circuit above
0 = + +
Taking the Laplace transform
0 = ( ) + [ ( ) − (0 )] + … 3
At = 0, initial current (0 ) =
( ) =
+
−
+
… .4
Taking the inverse Laplace transform
( ) = − 1 − 0 ≤ ≤ … 5
Load current at the instant the chopper is turned ON:
( ) =
From equations 2 and 5
At = = ( ) =
Therefore
=
−
1 − + … 6
At = = − , ( ) = = = −
therefore
= − 1 − … 7
Substituting for in equation 6
=
1 −
1 −
−
Substituting for in equation 7
R
L
E

12. 13
=
− 1
− 1
−
7) Advantages offered by dc chopper drives over line-commutated converter controlled dc drives.
Chopper controlled DC drives have a high efficiency, better control, less bulky, small size, quick
response and regeneration down to very low speed compared to line commutated converters.
Ripple content in the output is small.
DC-chopper can operate in 4 quadrants while as line commutated converters operated only in 2-
quadrant.
Current drawn by the chopper is smaller than in line commutated controlled dc converters.
Chopper circuit is simple.
The chopper is supplied from a constant dc voltage hence a good power factor while as line
commuted has a low power factor since the angle is delayed.

13. 14
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