23/09/2017

Function of starter in the DC Motor


Necessity of Starter

The motor armature current is given by

 Ia = ( V – Eb ) / Ra

 Where V = Supply voltage

           Eb = Back emf and

           R= Armature resistance

  • When the motor is at rest, back emf developed across the armature winding is equal to zero. 
  • Let us consider a case of 230 V, 5 kW DC motor having armature resistance of 0.5 W and full load current of 27.0 A. 
  • If this DC motor is directly connected to supply mains, it will draw a starting current of 17 times its full load current.

       ( IfL = 5000 / ( 230 × 0.8 )

              = 27.17 Amp

Assume efficiency = 80%

       I= 230 / 0.4

           = 460.0 Amp

Starting current drawn by motor

           = 460 / 27.17

           = 17 times full load current )

  • This excessive current ( I ) Blow out the fuses ( II ) Damage the commutator, brushes and also armature winding and ( III )  Produces large voltage drops in the supply voltage line. 
  • Therefore the motor must be protected against the flow of excessive current during starting period ( say 5 to 10 seconds ). 
  • A high resistance is connected in series with the armature winding in order to protect the motor which limit the starting current to a safe value. 
  • The starting resistance is gradually cut out as the motor gains speed and develops the back emf. As the motor attains its normal speed, additional resistance from the armature circuit is totally disconnected.

What is happened when the additional external resistance does not disconnect from the armature circuit?

  • Large loss of power resulting in reduction efficiency
  • Reduces the speed of the motor

Why Fractional kW ( Small ) motors do not require any starter at starting ?

  • The fraction HP motor does not require starter during starting because
  • The resistance and inductance of small motors are sufficiently large to
  • Limit the excessive current during starting
  • Due to low moment of inertia it speeds up quickly.

Three Point Starter

  • The internal wiring diagram for such a starter is shown in the Figure A. 
  • It consists of starting resistance divided into several sections, holding coil ( or NO volt release ), overload release, brass arc and soft iron piece attached to starter arm. 
  • As there are three terminals ( L – Supply line, F – Field, A – armature ) available in the starter, it is called as three point starter. 
  • The positive supply terminal is connected to starter arm through path line terminal ( L ) and overload release. 
  • The negative supply terminal is connected to one end of the field winding and armature winding
  • One end of the field winding is connected to brass arc through holding coil. One end of the armature winding is connected to starting resistance's last stud.  
  • A spring is placed over the lever of the starter arm.


three-point-starter.png



Operation
  • The main switch is closed to start the DC motor and this will result in starting arm moves from left side to the right side. 
  • As soon as the starting arm makes contact with first stud, the field winding is directly connected to supply line through brass arc and whole of the starting resistance ( R ) is placed in series with the armature. 
  • As the starting arm is further moved to right side, the starting resistance is cut out in steps………and finally entire starting resistance is cut out.

Hold on coil or NO volt release
  • As soon as the starter arm reaches the last stud, it is held against spring tension by attraction force between holding coil magnet and soft iron piece attached to the starter arm. 
  • The coil of NO volt release is connected in series with the field winding
  • When the supply fails / gets disconnected or break in the field winding, the holding coil is de-energized and so releases the starting arm which go back to OFF position ( or first stud ) due to spring tension. 
  • As a result motor gets disconnected from the supply mains.

Overload Release coil
  • It is connected in series with the supply line to protect the motor against overload. 
  • When the motor is overload, overload release coil is magnetize and it lifts the armature to the upward and short circuit the No volt release coil as shown in the figure A.  
  • As soon as the NO volt coil is short circuited, it demagnetized and releases the starting arm from 'ON' position. 
  • Therefore the motor is disconnected from the supply and protected against overload. 
  • The speed control of the DC shunt motor is achieved by connecting a variable resistance is in series with the field winding as shown in the Figure B. 
  • The speed of the DC shunt motor can be increased by weakening the field flux. 
  • The variation in the field flux is achieved by variable field resistance ( R ). 
  • There is limitation of field rheostat in order to achieve high speed if the field rheostat cuts too much rheostat. 
  • Too much weaken of field flux demagnetize the NO volt coil and thus starter arm moves from ON to OFF position consequently motor is shut down. Therefore the three point starter is not suitable for speed control.
Four Point Starter
  • Figure B shows a four point starter with internal wiring diagram of a long shunt compound motor 
  • It will be noticed that NO volt coil has been taken out of the shunt field winding and has been connected directly across the supply line through a protecting resistance ( R ). 
  • When the starting arm touches first stud, the line current divides into three parts.

( I ) One parts through starting resistance, series field and armature winding.
( II ) Second part passes through field rheostat and field winding.
( III ) Third part passes through no volt coil and protecting resistance.
  • It should be noted that any change of current in shunt field does not affect the function of the holding coil or NO volt coil.

four-point-starter.png




SERIES MOTOR STARTER
  • Figure C shows a two point starter for DC series motor
  • The positive terminal is connected to starter arm through overload release and negative terminal is connected to armature of the DC series motor
  • The function of the NO volt coil and overload release is similar to that of we discussed in three point starter. 
  • The Operation of a two point starter is similar to that of three point starter but there is one important difference that the flux does not remain constant in the DC series motor but varies with the armature current. 
  • Therefore the back emf at any given speed depends on the current variation between upper limit and lower limits. 
  • As a result a series motor starter has a smaller numbers of steps than that of a shunt motor of the same rating with same upper and lower current limits.

series-motor-starter.png

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20/09/2017

Characteristics of DC Shunt, DC Series and DC Compound Generator

The most important characteristics of DC generators are given below.
Magnetizing or open circuit characteristics ( O.C.C ) ( Eo  If )
  • It shows the relation between no load emf in armature for different value of field current. 
  • The magnetizing characteristic is practically same for all types of generators.

19/09/2017

Commutation in the DC Machines

  • The function of the commutator is to make alternating current into unidirectional. 
  • The direction of armature conductors are in one direction when conductors pass through N – poles and in opposite direction when they are under S – poles and vice versa.  
  • The reversal of current takes place when conductors pass out under influence of the N – poles and enter that of S – poles. 

Losses in the DC Machines


The various losses occurring in DC machines can be given as follows.

  • Iron loss (Magnetic or Core Loss) : Hysteresis Loss and Eddy Current Loss
  • Copper Losses : Armature Copper Loss, Shunt Field copper loss and Series field copper loss

17/09/2017

Armature Reaction in the DC Generator

  • The flux which is produced by field winding is called as main flux or field flux.
  •  The armature conductors produce armature flux. 
  • The effect of armature flux on main flux or field flux is known as armature reaction. 

16/09/2017

Types of DC Generator


The DC Generators are classified according to the way their field windings are connected and excited.

(A) Separately excited Generator

(B) Self excited Generator : There are two types of self excited generator

15/09/2017

Equalization Ring or Equalization connection

  • All the Conductors lay in any parallel path under one pair of poles in the Lap winding. 
  • If the flux from all the poles is exactly same, the induced elf and hence current carried by each path will be equal in the ideal condition. 

Dummy coils

  • The wave winding is possible only with particular number of conductor or coils.
  • But sometimes the armature slots available in the armature winding do not meet the requirement of the winding. ( It means that the available number of slots is more than the required number of conductors ). 

14/09/2017

EMF Equation of the DC Generator

Let P = Numbers of poles
     Ф = Flux / pole in weber
      Z = Total numbers of armature conductors
         = Numbers of slots × Numbers of Conductors / Slot

13/09/2017

Parts of DC Machines


The following are main parts of DC machine. Figure A shows the construction of the 2 – Pole DC machine.

  •  Yoke or Frame
  •  Pole cores and pole – shoes

11/09/2017

Practical DC Generator

  • There are basic following elements of an electrical generator
Stationary part
  • Field system which produce magnetic field
Rotating part

09/09/2017

Principle of DC Generator


Principle of DC Generator

  • It is an electrical machine which converts mechanical energy into electrical energy. 
  • The principle of operation of the dc generator is based on dynamically induced emf.  

Singly Excited - Doubly Excited System

  • The electromechanical energy conversion is done through medium of magnetic field. 

  • It is assumed that there is no loss of energy in the magnetic field. There are two types of the excitation systems.

07/09/2017

Electro - Mechanical Energy Conversion

  • The electrical energy plays vital role in different energy conversion process. 
  • The following are the energy conversion process in which electrical energy is common in all.

06/09/2017

Different Methods of Study of Electrical Machines

The following are main methods for the study of electrical machines.
Conventional method
  • It is assumed that working principles of induction machines, DC machines and synchronous machines are different in this method of study of electrical machine.

Energy - Balance Theory

Energy Balance Theory

In this article, the energy balance theory is given which is given to the most of rotating machines. The electric motor receives the energy at the input terminal and delivers it at the mechanical output terminals. The mechanical energy is given at the input terminal whereas the electrical energy produces at the output terminals in the case of generator.

Energy Balance Theory: Motor

  • Electrical Input energy = Mechanical Output energy + Change in stored energy

Energy Balance Theory: Motor

  • Mechanical Input energy = Electrical Output energy + Change in stored energy

Energy Balance Theory Equation

The energy balance equation can also be written in following way

dWelect = ( dWmech + dWf + dWloss  ) ..... Motor

dWmech = ( dWelect + dWf + dWloss ) ..... Generator

Where

dWelect = Change in electrical energy

dWmech = Change in mechanical energy

dWf = Change in field energy

dWloss = Change in energy losses

  • The transferred energy are always taken as positive ( + Ve ) but change in stored energy in the coupling field may be either positive or negative in the energy balance equation. 
  • Its value can be taken as positive for increase in the stored energy and negative for decrease in the stored energy. 
  • If the losses in the electro – mechanical systems are neglected, the energy balance equation can be written in general form
  • Input energy = Output energy + Change in stored energy

You may also like to read these articles:

Law of conservation of Energy

Single Excited System – Doubly Excited System

How voltage is built up in Generator?

Armature Reaction in the DC Generator

Current harmonic – Voltage harmonic

 

Law of Conservation of Energy

The energy in a system may take on various forms, such as light, sound, thermal, kinetic, potential, heat. 
  • The law of energy of conservation states that “The energy cannot be created nor be destroyed but the only thing that can happen with energy is that it can change