Electrical Machines – APSEEE

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Introduction to Electrical Machines

Electrical Machines 

Electrical Machines are the machines that required electrical energy for their operation. These machines may be ac or dc (means what kind of supply requires for their operation). The ac machines are two types static and rotating machines. On the other hand dc machines are only rotating one. In this article we will study about the different kind of machines.

AC Machines

First of all we will talk about the AC Machines. AC Machines are two types static and rotating.

Transformers falls into the category of static machine it is used in generating station for raising the voltage level, used in substation for lowering the voltage to a suitable level. it plays an  important role in electrical engineering.

Three Phase Induction Machines are rotating machines, the application of these machines found in industries. these machines are cheap in cost and robust in construction. These motors are singly excited motors. Induction motor are two types squirrel cage induction motor and slip ring or wound rotor induction motor.

Synchronous Machines  are divided into two types synchronous motors and synchronous generators or alternators. Synchronous  motors are used to improve the power factor and synchronous generators are installed at power generating station  to generate electrical energy. Synchronous machines are divided into two types salient pole and non-salient pole type.

Single Phase Induction Machines are used for domestic purposes. In mixer grinder, fans etc. single phase induction motors are used.

DC Machines

These machine are doubly excited machines. DC machines are divided into two types DC Motor and DC Generator.

DC Motors are of three types:-

DC Shunt Motors are used where constant speed is required for all load. The characteristics of these motors are similar to three phase induction motors.

DC Series motors are used in traction system. These motors have starting torque. These motors have poor speed regulation. DC series motors can not run at no-load and with belted load.

DC Compound Motors

 

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EMF Equation for Transformer

EMF Equation for Transformer

For drive an emf equation for transformer we will consider the case of an ideal transformer. An ideal transformer one which have no copper loss(I2R losses) and magnetic leakage flux. In other words, we can say that an ideal transformer consists of windings which have zero ohmic resistance and loss-free core.



In previous article, we have studied about basics of transformer such as working principle of transformer and necessity or need of transformer at generating power station, substations etc. In this article we will review the last article, by considering ideal case of transformer.

Consider an ideal transformer, whose secondary is open and primary is connected to alternating supply source. The alternating current flows in the primary winding. Since coils of primary winding is purely inductive and it draws magnetizing current only which is necessary to set up magnetic flux only. This magnetizing current is small in magnitude and lags primary voltage V1 by 900. The magnetic flux Φ sets up in primary circuit links with secondary circuit. According to faraday’s laws of electromagnetic induction induced emf is produced in secondary winding. If the secondary winding is closed current start flowing through the load.

Let the sinusoidal variation of flux Φ be expressed as

Φ = Φmax sinωt

emf equation for transformer

Φmax = maximum value of flux in webers

ω = angular frequency in rad/sec

The emf e1 induced in primary winding by the alternating flux Φ is given by

E1max  = N ω Φmax

Primary induced emf

E1max  = N1 ω Φmax

E1max = 2⊓ fN1 Φmax Volts.

According to above equation

I. Induced emf is directly proportional to no. of primary or secondary turns.

II. Induced emf is directly proportional to rate of change of flux linkage.

III. Induced emf is depends upon supply frequency.

Voltage Transformation Ration (K)

It is the ration of secondary voltage to the primary voltage.

  Or

Secondary induced emf to the primary induced emf.

Or

Secondary number of turns to the primary number of turns.

Or

Primary current to the secondary current





K is called transformation ratio of the transformer.

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Transformer on Load

Transformer on Load

When some load is connected across the secondary of the transformer, then it is said to be transformer on load. The current I2 flows through load and secondary winding. The magnitude of current I2 depends upon the terminal voltage V2 and impedance of load. The angle between current I2 and voltage V2 is depends upon the nature of load. Whether it resistive, inductive and capacitive. In this article, we will study about the transformer when it is loaded or certain load is connected to the secondary side. We will explain it step by step.

When certain load is connected to secondary side of the transformer. It draws no load current I0. The no load current I0 produces and mmf N1I0 which sets up magnetic flux in the core, shown in the figure.

transformer on load

The secondary current I2 sets up it own mmf (N2I2) and hence flux Φ2. This set up flux oppose the main primary flux Φ which is setup by no load current I0.

As secondary flux Φ2 oppose the primary flux, therefore the resultant flux also decreases and cause in reduction in self induced emf E1. This results the transformer draws additional current I1′  from supply main and flux in the core restored to its original value. So that V1 becomes equals to E1. The additional current draws by the primary winding is called counter balancing current. The additional current I1 produces and mmf N1I1 which sets up the flux Φ which is same in the direction of primary current and cancels the flux Φ2.

The flux produce secondary current is neutralized by flux produce by current I1. The flux produce secondary current is neutralized by flux produce by current I1. The total primary current I1 is the vector sum of current I0 and I1.

I1 = I0 + I1

 

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EMF Equation for DC Machine

EMF Equation for DC Machine

DC machine may be either works as a dc motor or dc generator. EMF equation is important for both. In case of DC generator an induced emf is called generated emf and in case of DC motors generated emf is called Back or Counter emf  The DC generator is rotate with the help of prime mover. Prime mover is directly coupled to the generator. When armature of dc generator rotates its conductor cuts by the magnetic flux that is produce in field winding and this results of an emf is induced in it. The induced emf depends upon the type of winding of dc generator whether it is wave or lap. In this article, we will drive the emf equation for dc machine.





Let,

Φ = flux per pole in weber

Z = total number of armature conductor

= number of slots * number of conductors per slot

P = number of parallel paths in armature

N = rotation speed of the armature in revolution per minute (r.p.m)

E = emf induced in any parallel path in armature

Eg = Generated emf

Flux cut by one conductor in one revolution = PΦ wb

Time to complete one revolution, t = 60/N seconds.

Average induced emf in one conductor,

emf equation for dc machine

The number of conductors connected in series in each parallel path = Z/A

The emf generated across terminals,

E = Average emf induced in one conductor * the number of conductors connected in series in each parallel path

Number of parallel paths in wave winding, A = 2

Number of parallel paths in lap winding, A = P

Conclusions from EMF Equation for DC Machine

  • In above equation, poles remain constant.
  • The emf induced in the armature is directly proportional to the flux per pole and speed.
  • The polarity of induced emf in armature is depends upon the connections of field winding and the direction of rotation. If reversed the connection of field winding and direction of rotation, the polarity induced emf will change.
  • The induced emf is fundamental phenomenon to all dc machines either it is operated as generator or motor.
  • When the machine is operating as a generator, this induced emf is called the generated emf Eg.
  • When the machine is operating as a motor, this induced emf is called back or counter emf, Eb. This emf plays an important role in dc motor. By using this equation we can find back emf.
  • Wave winding is used for high  voltage and low current applications and where as Lap winding used where large current at low voltage is generated.




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Parallel Operation of Alternators

Parallel Operation of Alternators

When the number of smaller units is connected in parallel instead of installing a large unit is called parallel operation of alternators. There are numbers of reasons, connecting alternator in parallel such as cost become less, maintenance and repair, efficiency and reliability of the power system. All the alternator of the system, work in parallel form a large capacity alternator. In this article, we will discuss about need for parallel operation of alternators

Need for parallel operation

In modern power systems alternators are operated in parallel to supply a common total load. Due to following reasons the alternators are connected in parallel.

  1. The demand of electrical power is huge, and it cannot be met by a single unit and it is difficult to build a large alternator, therefore to meet the demand of electrical power several alternators are connected in parallel.
  2. The parallel operation increases the reliability of the electric supply. If we use single large alternator in the event of fault on alternator or turbine whole the system is paralyzed. But with several alternators work in parallel maintain the continuity of supply rather than breakdown of one unit.
  3. Maintenance and repair of the alternator is more convenient if more number of small capacity alternators are installed at the power station. For repairing of one alternator there is no need to shut down the whole power plant.
  4. With increase the demand of electrical energy, we can install a alternator with existing plant.
  5. Transportation problems are faced with single large unit. But this problem can be eliminated by using small units.
  6. The load on power plant varies, usually having its peak value during the day and its minimum value during the night time.

Thus the number of units operating at a particular time can be varied depending upon the load at that time. If the alternators are connected in parallel the less efficient alternator can be shut down when the load requirement is less.

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DC Motor

DC Motor

DC motor is a electrical machine which converts electrical energy into mechanical. There are three types of dc motors which are employed at various industrial applications. The dc motors are classified on the basis of connection of field winding with respect to armature. Most important application of dc motor found in traction system. Dc series motors are most suitable for traction system.


Working Principle of DC Motor

When a current carrying conductor placed in a magnetic field a mechanical force experienced by the conductor. The twisting movement of shaft about its axis due to force produced by interaction of field produced by current carrying conductor in armature and field produced by field winding is called torque. This type of toque is called electromagnetic torque.

Dc motor, opeartion, types of dc motors, characteristis of dc motors, speed control of dc motors

Pole-N and S are field magnets. It may be electromagnet or permanent magnet. The armature conductor is carrying current and placed in the magnetic field. Interaction between field magnet and field produce by armature conductor producing a driving torque.

Back E.M.F

The e.m.f induced in armature conductor due to flow of direct current which opposes the applied voltage is called back e.m.f or counter e.m.f.

Consider a shunt wound motor connected to the dc supply. A dc current start flowing through the armature conductor. Field magnets are also energized by dc supply which produces a magnetic field. When the interact armature field and main field driving torque is produced. As the armature rotates, back e.m.f is produced which opposes the applied voltage V.

Net voltage across armature circuit = V – Eb

Armature current Ia = V-Eb/Ra

Significance or Importance of Back EMF

The presence of back e.m.f makes the dc machine self regulating machine. The armature resistance is very small. If it carries the large current it will burn out. So the back e.m.f makes the motor to draw as much armature current as it just adequate to develop the torque required by the load

Armature current, Ia = V-Eb/Ra

Types of DC Motors

The DC motors are divided into three types according to their connections of field winding in relation to the armature.

  • DC series motor
  • DC shunt motor
  • DC compound motor

DC compound motor further into two types

  • Cumulative compound motor
  • Differential compound motor


DC Series Motor OR Series Wound Motor

In DC series motor the armature and field winding connected in series. The current flowing through the field winding is the same as that in the armature, if the mechanical load on the shaft increase the armature current also increases. The resultant increase in magnetic flux causes reduction in speed.

dc series motor

DC Shunt Motor or Shunt Wound Motor

DC shunt motor in which the field winding is connected in parallel with the armature. The shunt winding has fine and large number of coil. The shunt winding carries the constant current from the supply main.

types of dc motor shunt motor

Armature current, Ia = Ia – Ish

Shunt current, Ish = V/Rsh

DC Compound Motor or Compound Wound Motor

There are two types of DC compound motor or compound wound motors. Compound motors are those motors which has two field winding. One winding is connected in series with the armature and the other winding is connected in parallel with the armature series winding has less number of turns having large cross sectional area. Where as the shunt winding has more turns having small cross sectional area. There are two types of Compound Wound Motors.

  1. Cumulative compound wound motor
  2. Differential compound wound motor

Commulative Compound Wound Motor

When the shunt field winding directly connected across the armature terminals is called Cumulative Compound Motor.

Differential Compound Wound Motor

When the shunt field winding is connected in such a way that it shunts the series combination of armature and series field, it is called Differential Compound Motor.

dc compound motors

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DC Generator

DC Generator

DC Generator is an electrical machine that converts mechanical energy into dc electrical energy.



It is usually driven by some source of mechanical power which may be a diesel or petrol engine, steam turbine etc.

Working Principle of Generator

The working principle of dc generator based on the dynamically induced emf. It states that whenever there is a relative movement of a conductor with respect to field, emf is induced in the conductor. In simple words, according to faraday’s law of electromagnetic induction we can say that whenever a conductor cuts across the magnetic field, an emf is induced in the conductors.

In case of dc generator, the magnetic field is stationary and the conductors move. The direction of induced emf is given by Fleming’s right hand rule.

For induced emf, the essential component of the generator is

  • Conductor or a group of conductors
  • A magnetic field
  • Motion of the conductor with respect to filed

Types of DC Generators

The magnetic field in a generator is usually produced by electromagnets. So, the generators are generally classified according to their methods of field excitation. On the basis of these criteria, they can be classified as

Separately Excited DC Generator

Self Excited DC Generator

DC generator

Separately Excited DC Generator

A dc generator in which current is supplied to the field winding from an external dc source is known as separately excited  generator. The voltage output depends upon speed and filed current. The separately excited generators are rarely used in practice because additional dc source is required for producing necessary flux.

sepsartely excited dc generator

Self Excited DC Generator

A dc generator whose field winding is supplied current from the output of the generator it self is called a self excited  generator. According to connection of field winding with armature, it is divided into three types.

  • Series wound generator
  • Shunt wound generator
  • Compound wound generator

Series Wound Generator

In series wound generator, the filed winding is connected in series with the armature winding. Therefore, full armature current of load current flows through the filed winding. Field winding has few turns of thick wire having low resistance.

series wound dc generator

Shunt Wound Generator



In a shunt wound generator, the filed winding is connected in parallel with the armature. Full terminal voltage applied across the filed winding. Field winding has many turns of fine wire having very high resistance.

 

shunt wound dc generator

Compound Wound Generator

In compound wound generators, two field windings are used on each pole. One winding is connected in series and other is connected in parallel with armature. The compound wound generators are divided into two types.

  • Long shunt
  • Short shunt

compound wound dc generator

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Characteristics of DC Series Motors

Characteristics of DC Series Motors

The performance of DC motor can observe by its operating characteristics curve is called motor characteristics. In this article we will discuss about characteristics of DC series motors.

characteristics of DC series motors

Speed Armature Current (N-Ia) Characteristics

The speed of a dc series motor given by the relation.

Where is back emf and its value is equal to (V-Ia (Ra+Rse) and Φ is flux.

At starting time, the value of current is small and flux produce also small. We know that flux is proportional to the current.

According to above relation the speed is inversely proportional to the flux. The speed will be high at starting time.

 

Torque-Armature Current (T-Ia) Characteristics

We know that, torque dc motor is directly proportional to product of flux and armature current.

Tα Φ Ia

The current passes through the field winding is same as that in the armature. In case of dc shunt motor, the flux is constant but in case of dc series motor, flux depends upon the current.

Upto magnetic saturation, field flux is directly proportional to armature current.

Φ α Ia

Hence, the torque of dc series becomes proportional to square of armature current.

T α Ia Ia

T α Ia2

After magnetic saturation, the flux become constant and armature torque become proportional to the armature current.

T α Ia

Torque current characteristics of dc series motor is also called electrical characteristics of the motor.

The torque and armature current reveals that the dc series motor gives high starting torque. That is why applications of dc series motor found where high starting torque is required at the starting time such as in Electric Locomotives.

Speed and Torque (N-T) Characteristics

When mechanical load on a dc series motor increases, torque also increases which reduces the speed of a dc series motor.

 

 

 

 

So that torque is inversely proportional to the speed.

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Characteristics of DC Shunt Motors

Characteristics of DC Shunt Motors

In case of dc shunt motors, for constant supply voltage, the field current is constant. Hence, the flux in dc shunt motors is practically constant. In this article,we will discuss about various characteristics of dc shunt motor.

characteristics of dc shunt motor

Speed Current (N-Ia) Characteristics

The speed of dc motor is directly proportional to back emf and inversely proportional to flux.

Since flux of dc shunt motor is constant. In this way speed only depends upon back emf Eb.

N α Eb

N α V – IaRa

If the armature drop (IaRa) is negligible, the speed of the motor will remain constant for all values of load. As the armature current increases due to increase of load, armature drop IaRa increases and speed of the motor decreases slightly. Hence, actual speed curve is slightly drooping. But for all practical purposes, dc shunt motor is taken as a constant speed motor.

speed current characteristics

AB line shows constant speed.

AC line represents slight decreases in speed due to (I2aR) losses.

Because there is no change in the speed of a dc shunt motor from no load to full load. Therefore, shunt motors can be used for the loads which are totally and suddenly thrown off with out resulting in excessive speed. So shunt motor are suitable for driving shafting, machine tools and lathes and for all other purposes, where constant speed is required.

Torque-Armature Current(T-Ia) Characteristics

The torque of dc motors is directly proportional to the flux Φ.

Ta α Φ Ia

Ta α Ia

Since in case of dc shunt motor the flux per pole Φ is considered to be constant. Hence, the current increase in torque.

characteristics of dc shunt motor speed

This characteristic is also known as electrical characteristics. Electrical characteristics are straight line passing through the origin.

Speed-Torque (N-T) Characteristics

The speed torque characteristics are derived from the first two characteristics. When torque increases, armature current increases, the speed decreases slightly. Thus with increase in load or torque, the speed decreases.

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Applications of Synchronous Motor

Applications of Synchronous Motor

Synchronous motor is constant speed motor. This motor is not used for drive heavy load. In this topic we will discuss about various applications of synchronous motor.

The Synchronous motors are used for various purposes which are given below:-

  1. The synchronous motors are used in power houses and electrical substations in order to improve power factor. For this purpose the synchronous motors are operated on no load and overexcited.
  2. In large industries the power is lagging since there are large number of induction motors are installed. These motors are run at lagging power factor. To improve this lagging power factor there is over excited synchronous motors are used in the industries.
  3. because of high efficiency and high speed of the synchronous motors, these motors are used for those applications where constant speed is required. Speed above the 500rpm, synchronous motors are used for drives, such as centrifugal pumps, compressors, rubber and paper mills, line shafts, blowers etc. Speed below the 500rpm synchronous motors are used for drives such as ball and tube mills, electroplating, generators, vacuum pumps, metal rolling mills, centrifugal and screw-type pumps etc.
  4. Synchronous motors are used to regulate voltage at the end of the transmission line. The voltage at the end of the long transmission lines changes appreciably due to presence of the large inductive load. When this load is disconnected from the lines suddenly, voltage tends to rise because of capacitance of line is increased. To keep this voltage at normal level synchronous motors are installed. This voltage can be controlled by vary the excitation of the synchronous motors. When load decreased due to inductive load, the motor excitation is increased. If the line voltage is increased due to capacitive effect, then motor excitation is decreased to maintain the line voltage within normal level.

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