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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

           Ra = 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%

       IL = 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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