Question

A d.c.motor takes large value of correct at the time of starting due to:​

Answer 1

Answer:

Basic operational voltage equation of a DC motor is given as

E = Eb + IaRa   and hence,     Ia = (E - Eb) / Ra

Now, when the motor is at rest, obviously, the back emf Eb = 0. Hence, armature current at the moment of starting can be given as Ia = E / Ra. In practical DC machines, armature resistance is basically very low, generally about 0.5 Ω. Therefore, a large current flows through the armature during starting. This current is large enough to damage the armature circuit.

Due to this excessive starting current -

the fuses may blow out and the armature winding and/or commutator brush arrangement may get damaged.

very high starting torque will be produced (as torque is directly proportional to the armature current), and this high starting torque may cause huge centrifugal force which may throw off the armature winding.

other loads connected to the same source may experience a dip in the terminal voltage.

A large DC motor will pick up speed rather slowly due to its large rotor inertia. Hence, building up the back emf slowly causing the level of high starting current maintained for quite some time. This may cause severe damage. To avoid this, a suitable DC motor starter must be used. Very small dc motors, however, may be started directly by connecting them to the supply with the help of a contactor 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.

DC Motor Starters

To avoid the above dangers while starting a DC motor, it is necessary to limit the starting current. So, a DC motor is started by using a starter. There are various types of dc motor starters, such as 3 point starter, 4 point starter, no-load release coil starter, thyristor controller starter etc.

The basic concept behind every DC motor starter is adding external resistance to the armature winding during starting.

From the followings, 3 point starters and 4 point starters are used for starting shunt wound motors and compound wound motors.

Explanation: