Necessity of DC Motor Starters

     When a full-voltage is applied across the dc motor during its starting period it draws a much more current than its rated current. When such a heavy currents passes through its armature windings it may cause damage to the winding by overheating. Hence to reduce this heavy currents a resistance must connected in series with armature winding. Therefore a starter which consists of set resistance in connected to limit this current.

Starting of DC Motors :

     At starting when a supply is given to the terminals of the dc motor it draws a heavy currents more than its rated current. Let us see the equation for current of a dc motor is given by      

Ia  =  V - E / Ra

     Initially when a dc motor is started the emf induced in the armature will be zero as the speed N will be zero. From the above equation we can see the ratio of voltage V to armature resistance Ra with the armature current Ia. The armature resistance for dc motor will be very low, and therefore when a voltage is connected across the armature terminals armature will draw a heavy currents ( hundreds of amperes ). The value of this currents are more than rated value of current of the motor, when this heavy currents will flow in armature winding it may cause damage to the winding.

     However as the motor starts rotating the emf induced will also increases, thus it gradually decreases the starting current as the speed goes on increasing. At a moment the value of this current becomes less than the rated value. Hence the current should be limited at starting period only in order to prevention of damage to the winding. 

     To limit this starting currents to flow in  armature winding, a resistance is introduced in series with armature winding a shown in figure. As the motor gains the speed the value of this resistance gradually cut off. Therefore the expression for armature current when extra resistance R is connected is shown below.  

Ia  =  V - E / Ra + R

     We can see that extra resistance R is added to the armature resistance which in turn reduces the armature current Ia. As the speed of the motor increases the value of induced emf also increases which turns to increases the value of V - E value so that current can be limited. From here we can say that value of extra resistance can be reduced with respective to speed, and finally removed from the armature circuit. Otherwise energy will be lost in the from of Ia2 R loss in the resistance.

Necessity of DC Motor Starters

     If the extra resistance is not removed from the armature circuit. From the diagram we can see that field winding is connected in parallel with armature armature winding but not in series, current drawn by the field winding will be reduced ( due to heavy IRa drop in resistance R, the voltage available across the armature terminals will be small which in turn reduces the field current ). Therefore the torque developed by the motor will be reduced. 

     In practice a device is used to bring this starting resistance with the armature winding, know as starter. It consists of set of resistor placed in a enclosure. This value of this resistance can be reduced by reducing the number of resistors connected, which can made by moving handle ( starter arm ). The starter is provided with protective devices to connected the supply when failure occurs or in overload condition.

     When supply failure and if starter fails to disconnect the armature from the supply, the armature will get directly connected across the supply when it is restored. An excessive currents will flow in the armature winding when the motor is overloaded, therefore overloaded protection will be equipped in the starter.

There are mainly two types of starters for dc motor,

They are : 

    (i) Three point starter

    (ii) Four point starter.

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