An external resistance is added in the motor circuit during starting condition. The main purpose of this is to _______.

Need of Stater in DC Motor:

• Armature current (Ia) in DC motor is given by,
  • \(I_a=\frac{V-E_b}{R}\)
  • And, Back EMF (Eb) ∝ N
• At starting the speed of DC Motor is very less and hence the armature current will be approximately equal to (V/R).
• Hence, the starting current will become 7 to 8 times of rated current which further causes heating, loss, etc.
• To overcome that there is a need for external resistance insert in series with the DC Motor circuit to maintain starting current.
• There are mainly 3 types of DC motor starters, as 2 point starter, 3 point starter, and 4 point starter.
• 3 point starters and 4 point starters are used for starting dc shunt-wound motors and dc compound wound motors.
• 2 point starter is used for starting dc series motors.
• However, very small dc motors may be started directly by connecting them to the supply with the help of a contractor or a switch.
• It does not result in any harm because they gather speed quickly due to small rotor inertia.
• In this case, the large starting current will die down quickly because of the fast rise in the back emf.