En. No. : En. No. : Div. : 3 rd EC Sub. : EM topic : Starter

 Need of starter  Effect of high starting current  Principle of starters  Type of starters  three point starter (for dc shunt motor)  For point starter (for dc shunt motor)  Three point starter (for dc shunt motor)  For point starter (for dc shunt motor)  Disadvantage of Four point starter  Solid state starter

 Starter is an additional external device which is required to be connected along with a dc motor, in order to start the in a safe manner.  That means staring a dc motor without a starter is not safe for the motor.  So we should understand the need of starter.

 We know that, V = E b + I a R a…. for a dc shunt motor And V = E b + I a ( R a + R s ) for a dc series motor  Hence the expression for as follows: I a = V-E b / R a dc shunt motor

 And I a = V-E b / ( R a + R s ) dc series motor  At the time of starting the motor, speed N = 0 and hence the back emf E b = 0.  Hence the armature current at the time of starting is given by, I a (starting) = V / R a dc shunt motor And I a = V / ( R a + R s ) dc series motor

 Since the values of R a and R s are small,the starting current will tremendously large if the rated voltage V is applied at the time of staring.  The staring current of the motor can be 15 to 20 times higher than the full load current.  Such a high starting current has the following objectionable effects.

 Due to very high starting current, the supply voltage affects the other equipments operating on the same supply.  Due to excessive current, the insulation of the armature winding may burn.  The fuse will blow, circuit breakers will trip.  So in order to avoid all these effects we have to keep the starting current of the motor below the safe limits. This is achieved by using a starter.

 Starter is basically a resistance which is connected in series with the armature winding only at the time of starting the motor.  Its job is to limit the starting current. The starter or starter resistance will remain in the circuit only at the time of staring and will go out of circuit or becomes ineffective When the motor speed up to the desired speed.

 The starter resistance is then gradually cutoff. The motor will speed up, back emf will be developed and it will regulate the armature current. The starter is not necessary after this.

 There are two types of starters used for the dc shunt motors :  Three point starters  Four point starter

 The construction details of a three point starter are show in fig. (a)

 This is called as a three starter because three point L, A and F are brought out for the external connections.  Point L : It is the line terminal and should be connected to the positive terminal of the dc supply.  Point A : It is to be connected to the armature winding.

 Point F : It is to be connected to the field winding.  Point L is connected to the overload coil the second end of which is connected to the handle of the starter.  We can move this handle from OFF to RUN positions manually. The spring attached to the handle can bring the handle back to its original position i.e. OFF position.

 The starter is basically a variable resistor which is divided into a number of section.  The contact point 1,2,3…..10 are called as studs. The handle makes contact with these studs when it is moved from points  1 to 10 or in the other direction.  A connection is made from stud 1 to the hold on coil or no volt coil as shown in fig.

 The other end of no volt coil is connected to the terminal F.  The overload coil (OLC) and no volt coil (NVC) are the additional protections Provided in the three point starter.

 The problem discussed for the three point starter can be solved by using the four point starter.  The construction of four point starter is as shown in fig.  An important difference between three point and four point starter becomes evident.

 There are four points brought out for the connection with the outside world namely L +, L - or N, F and A.  Hence the name of this starter is four point starter.  The operating principle of four point starter is same as that of a three point stater.

 The function of hold on coil (NVC) and the overload coil are exactly same as those for the three point starter.  The resistance R connected in series with the hold on coil limits the current through the hold on coil.

 The only disadvantage of four point starter is that it dose not provide any protection against the field failure or high speed protection to the motor.  The field failure can be explained as follows. Under the running condition, if the field current reduces to zero due to field winding getting open circuited or due to some other reason then the condition is called as field failure condition.

 If I f reduce to zero, then the only flux present will be the spelling flux which is very very small.  The three point starter can protect the motor under the field failure condition as follows

 As I f reduces to zero.  The current through the hold on coil reduces to zero since it is connected in series with filed.  So the handle is released and goes to OFF position.

 This will turn off the motor and protect it.  The four point starter will not be able to provide such a protection, since the hold on coil is not connected in series with the field winding.

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