Electrical Machines : Special Machines

By Mohd. Irshad|Updated : June 28th, 2021

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BYJU'S Exam Prep Brings you 60 Days Study Plan for the preparation of DFCCIL Electricals & Electronics Engineering. This Study Plan will be free and will be very beneficial for the students preparing and targeting the DFCCIL Exam. Save this article as it will get updated on a daily basis as scheduled. 

Construction of Rotor of Hysteresis Motor

The rotor of the hysteresis motor consists of the core of aluminium or some other non-magnetic material which carries a layer of special magnetic material. The figure below shows the rotor of the hysteresis motor.


Construction of Hysteresis rotor

The outer layer has several thin rings forming a laminated rotor. The rotor of the motor is a smooth cylinder, and it does not carry any windings. The ring is made of hard chrome or cobalt steel having a large hysteresis loop as shown in the figure below.


 Hysteresis loops

 Operation of a Hysteresis Motor

The following illustration shows the basic functioning of a hysteresis motor.


 Working principal of hysteresis motor

When supply is given applied to the stator, a rotating magnetic field is produced. This magnetic field magnetises the rotor ring and induces pole within it. Due to the hysteresis loss in the rotor, the induced rotor flux lags the rotating stator flux. The angle δ between the stator magnetic field BS and the rotor magnetic field BR is responsible for the production of the torque. The angle δ depends on the shape of the hysteresis loop and not on the frequency.

Thus, the value of Coercive force and residual flux density of the magnetic material should be large. The ideal material would have a rectangular hysteresis loop as shown by loop 1 in the hysteresis loop figure. The stator magnetic field produces Eddy currents in the rotor. As a result, they produce their own magnetic field.

 As the electromagnet torque is developed by the motor is because of the hysteresis loss and remains constant at all rotor speed until the breakdown torque. At the synchronous speed, the eddy current torque is zero and only torque due to hysteresis loss is present.

Torque Speed characteristic of Hysteresis Motor

The speed torque curve of the motor is shown below:


Torque-Speed Characteristics

Curve 1 is the ideal curve, and the curve 2 is the practical hysteresis motor curve. The torque-speed characteristic of the hysteresis motor is different from an induction motor. Since, at the synchronous speed, the torque developed by an induction motor becomes zero, whereas in the hysteresis motor the torque is constant at all the speed even at the synchronous speed. Thus, from the curve, it is seen that the locked rotor, starting and pull out torque is equal.

The noise level of the hysteresis motor is very low as compared to the induction motor because it operates at a constant speed and its rotor is smooth. This type of motor is smoothest running, quietest single-phase motor and is used for quality sound reproduction equipment like record players, tape recorders, etc. It is also employed in electric clocks and other timing devices.


A single-phase synchronous Reluctance Motor is basically the same as the single cage type induction motor. The stator of the motor has the main and auxiliary winding. The stator of the single phase reluctance and induction motor are same. The rotor of a reluctance motor is a squirrel cage with some rotor teeth removed in the certain places to provide the desired number of salient rotor poles.

The figure below shows the 4-pole reluctance type synchronous motor.


Four-pole reluctance type synchronous motor

In the above figure the teeth have been removed in four locations to produce a 4-pole structure. The two end rings are short circuited. When the stator is connected to a single-phase supply, the motor starts as a single-phase induction motor. A centrifugal switch disconnects the auxiliary winding as soon as the speed of the motor reaches about 75% of the synchronous speed. The motor continues to speed up as a single-phase motor with the main winding in operation.

A reluctance motor torque is produced due to the tendency of the rotor to align itself in the minimum reluctance position, when the speed of the motor is close to the synchronous speed. Thus, the rotor pulls in synchronism. The load inertia should be within the limits, for proper effectiveness. At synchronism, the induction torque disappears, but the rotor remains in synchronism due to synchronous reluctance torque.

 Torque Speed Characteristic

The Torque Speed Characteristic of a single-phase Reluctance Motor is shown below.


 Torque-speed characteristics

The starting torque depends upon the rotor position. The value of the starting torque varies between 300 to 400 % of its full load torque. As we know that as motor attains speed nearly of synchronous speed the auxiliary winding is disconnected, and the rotor continues to rotate at the synchronous speed.

The motor operates at a constant speed up to a little over than 200% of its full load torque. If the loading of the motor is increased above the value of the pull-out torque, the motor loose synchronism but continues to run as a single-phase induction motor up to over 500% of its rated torque. At the starting the motor is subjected to Cogging. This can be reduced by skewing the rotor bars and by having the rotor slots not exact multiples of the number of poles.

The rotor of a Reluctance Motor is unexcited; therefore, the power factor is low as compared to the induction motor. As the motor has no DC field excitation so the output of a reluctance motor is reduced. Hence, the size of the motor is large as compared to synchronous motor.

Applications of a Reluctance Motor

The various applications of the Reluctance Motor are as follows:

  • Simple construction as there is no slip rings, no brushes and no DC field windings).
  • Low cost
  • Maintenance is easy.
  • It is used for many constant speed applications such as electric clock timer, signalling devices, recording instruments etc.


The motors which can be used with a single-phase AC source as well as a DC source of supply and voltages are called as Universal Motor. It is also known as Single Phase Series Motor. A universal motor is a commutation type motor. If the polarity of the line terminals of a DC Series Motor is reversed, the motor will continue to run in the same direction.

The direction is determined by both field polarity and the direction of current through the armature. As torque is proportional to the flux and the armature current. Let the DC series motor be connected across a single-phase AC supply. Since the same current flows through the field winding and the armature winding. The AC reversal from positive to negative or vice-versa will affect the field flux polarity and the current direction through the armature.

The direction of the developed torque will remain positive, and direction of the rotation will be as it was before. The nature of the torque will be pulsating, and the frequency will be twice that of line frequency as shown in the waveform below.


 Characteristics of universal motor

Thus, a Universal motor can work on both AC and DC. However, a series motor which is mainly designed for DC operation if works on single phase AC supply suffers from the following drawbacks.

  • The efficiency becomes low because of hysteresis and eddy current losses.
  • The power factor is low due to the large reactance of the field and the armature windings.
  • The sparking at the brushes is in excess.

In order to overcome the above following drawbacks, certain modifications are made in a DC series motor so that it can work even on the AC current. They are as follows:

  • The field core is made up of the material having a low hysteresis loss. It is laminated to reduce the eddy current loss.
  • The area of the field poles is increased to reduce the flux density. As a result, the iron loss and the reactive voltage drop are reduced.
  • To get the required torque the number of conductors in the armature is increased.

The construction of the universal motor is same as that of the series motor. In order to minimize the problem of commutation, high resistance brushes with increased brush area are used. To reduce Eddy current losses the stator core and yoke are laminated. The Universal motor is simple and less costly. It is used usually for rating not greater than 750 W.

The characteristic of Universal motor is similar to that of the DC series motor. When operating from an AC supply, the series motor develops less torque. By interchanging connections of the fields with respect to the armature, the direction of rotation can be altered.

Speed control of the universal motors is obtained by solid state devices. This motor is most suitable for applications requiring high speeds. Since the speed of these motors is not limited by the supply frequency and is as high as 20000 rpm.

 Applications of Universal Motor

The Universal motor is used for the purposes where speed control and high values of the speed are necessary. The various applications of the Universal Motor are as follows:

  • Portable drill machine.
  • Used in hair dryers, grinders and table fans.
  • A universal motor is also used in blowers, polishers and kitchen appliances.


A DC Motor whose poles are made of Permanent Magnets is known as Permanent Magnet DC (PMDC) Motor. The magnets are radially magnetized and are mounted on the inner periphery of the cylindrical steel stator. The stator of the motor serves as a return path for the magnetic flux. The rotor has a DC armature, with commutator segments and brushes.

The cross-sectional view of the 2 pole PMDC motor is shown in the figure below.


The cross-sectional view of the 2-pole PMDC motor

The Permanent Magnet DC motor generally operates on 6V, 12V or 24 Volts DC supply obtained from the batteries or rectifiers. The interaction between the axial current carrying rotor conductors and the magnetic flux produced by the permanent magnet results in the generation of the torque.

The circuit diagram of the PMDC is shown below.

In conventional DC motor, the generated or back EMF is given by the equation shown below.

F = kϕN …  …  … (1)

The electromagnetic torque is given as

Te = kϕ Ia … … … (2)

In Permanent Magnet DC motor, the value of flux ϕ is constant. Therefore, the above equation (1) and (2) becomes

E = K1 N  … … … (3)

Te = K1 Ia … … … (4)

Considering the above circuit diagram, the following equations are expressed.

V = E + IaRa … … … (5)

Putting the value of E from the equation (3) in equation (5) we get

V = K1N + IaRa or

Where k1 = k ϕ and is known as speed-voltage constant or torque constant. Its value depends upon the number of field poles and armature conductors.

The speed control of the PMDC motor cannot be controlled by using flux control method as the flux remains constant in this type of motor. Both speed and torque can be controlled by armature voltage control, armature rheostat control, and chopper control methods. These motors are used where the motor speed below the base speed is required as they cannot be operated above the base speed.

 Applications of the Permanent Magnet DC Motor

The PMDC motors are used in various applications ranging from fractions to several horsepower.  They are developed up to about 200 kW for use in various industries. The following applications are given below.

  • PMDC motors are mainly used in automobiles to operate windshield wipers and washers, to raise the lower windows, to drive blowers for heaters and air conditioners etc.
  • They are also used in computer drives.
  • These types of motors are also used in toy industries.
  • PMDC motors are used in electric toothbrushes, portable vacuum cleaners, food mixers.
  • Used in a portable electric tool such as drilling machines, hedge trimmers etc.



Servo Motor are also called Control motors. They are used in feedback control systems as output actuators and does not use for continuous energy conversion. The principle of the Servomotor is similar to that of the other electromagnetic motor, but the construction and the operation are different. Their power rating varies from a fraction of a watt to a few hundred watts.

The rotor inertia of the motors is low and have a high speed of response. The rotor of the Motor has the long length and smaller diameter. They operate at very low speed and sometimes even at the zero speed. The servo motor is widely used in radar and computers, robot, machine tool, tracking and guidance systems, processing controlling, etc.

  Applications of the Servo Motor

The power rating of the servo motor may vary from the fraction of watts to few hundreds of watts. The rotor of servo motor has low inertia strength, and therefore they have a high speed of inertia. The Applications of the Servomotor are as follows:

  • They are used in Radar system and process controller.
  • Servomotors are used in computers and robotics.
  • They are also used in machine tools.
  • Tracking and guidance systems.

 Classification of Servo Motor

They are classified as AC and DC Servo Motor. The AC servomotor is further divided into two types.


 Classification of Servo motor

DC Servo Motor

DC Servo Motors are separately excited DC motor or permanent magnet DC motors. The figure (a) shows the connection of Separately Excited DC Servo motor and the figure (b) shows the armature MMF and the excitation field MMF in quadrature in a DC machine.


This provides a fast torque response because torque and flux are decoupled. Therefore, a small change in the armature voltage or current brings a significant shift in the position or speed of the rotor. Most of the high-power servo motors are mainly DC.

The Torque-Speed Characteristics of the Motor is shown below.


 Torque-speed Characteristics

As from the above characteristics, it is seen that the slope is negative. Thus, a negative slope provides viscous damping for the servo drive system.

AC Servo Motor

The AC Servo Motors are divided into two types 2 and 3 Phase AC servomotor. Most of the AC servomotor are of the two-phase squirrel cage induction motor type. They are used for low power applications. The three-phase squirrel cage induction motor is now utilised for the applications where high-power system is required.


The name Stepper Motor itself shows that the rotor movement is in the form of various steps or discrete steps. It is also known as Stepping Motor. The number of pulses fed into the controller circuit determines the angular rotation of the motor. Each input pulse produces one step of the angular movement. The drive is considered as an analog to digital converter. It has an inbuilt logic, which causes appropriate windings to be energised and de-energized by the solid-state switches in the required sequence.


Step Angle in Stepper Motor

Step angle is defined as the angle which the rotor of a stepper motor moves when one pulse is applied to the input of the stator.

The positioning of a motor is decided by the step angle and is expressed in degrees. The resolution or the step number of a motor is the number of steps it makes in one revolution of the rotor. Smaller the step angle higher the resolution of the positioning of the stepper motor.

The accuracy of positioning of the objects by the motor depends on the resolution. Higher the resolution greater will be the accuracy. Some precision motors can make 1000 steps in one revolution with a step angle of 0.36 degrees. A standard motor will have a step angle of 1.8 degrees with 200 steps per revolution. The various step angles like 90, 45 and 15 degrees are common in simple motors.

 Advantages of Stepper Motor

The various benefits of the Stepping Motor are as follows:

  • The motor is simple in construction, reliable.
  • At the standstill condition, the motor has full torque.
  • The motors are less costly.
  • They require little maintenance.
  • The stepper motor has an excellent and accurate starting, stopping and reversing response.

 Disadvantages of Stepper Motor

The various disadvantages of the stepping motor are as follows:

  • The motor uses more current as compared to the DC motor.
  • At the higher speed, the value of torque reduces.
  • Lower efficiency.
  • The Resonance condition arises and requires micro stepping.
  • At the high speed, the control is not possible.


Repulsion Motor is a special kind of single-phase AC motor which works due to the repulsion of similar poles. The stator of this motor is supplied with 1 phase AC supply and rotor circuit is shorted through carbon brush.

 Construction of Repulsion Motor

The main components of repulsion motor are stator, rotor and commutator brush assembly. The stator carries a single-phase exciting winding similar to the main winding of single-phase induction motor. The rotor has distributed DC winding connected to the commutator at one end just like in DC motor. The carbon brushes are short circuited on themselves.


 Working of Repulsion Motors

In the above figure, the stator winding has single phase AC winding which produces the working MMF in the air gap. The brushes on rotor are shown to be shorted. As the rotor circuit is shorted, the rotor receives power from stator by transformer action. 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.

Advantages of Repulsion Motor

The advantages of repulsion motor include the following:

  • Starting torque is high.
  • Good speed regulation.
  • For sudden heavy loads, the torque can be developed.
  • Starting current will be reduced.

Disadvantages of Repulsion Motor

The disadvantages of repulsion motor include the following:

  • Sparks will occur at brushes
  • The power factor is very less at less speed.
  • The speed at no-load condition is extremely high & unsafe.
  • Brushes & commutator exhaust quickly due to heat generation & arcing at the assembly of the brush.

Applications of Repulsion Motor

The applications of repulsion motor include the following:

  • Hoists
  • Machines in Textile
  • Printing presses
  • Air compressors
  • Pumps & Fans
  • Mixing machines
  • Machine tools
  • Air pump
  • Mining tools
  • Petrol pumps
  • Drive compressors

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