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Sunday, 19 January 2020

Characteristics of DC Shunt Motor


The characteristics curves of d.c. motor are curves drawn to show the relation between armature current, speed and torque. The following are the general characteristics of interest in a d.c. motor.

a. Electrical characteristics : This shows a relation between torque developed and armature current in a d.c. motor.
b. Speed and armature current characteristics : This shows the relation as to how the speed varies with armature current.
c. Speed-Torque characteristics : This is also mechanical characteristics. It gives the relation between speed and torque in a d.c. motor.

Characteristics of D.C. Shunt Motor

1. Electrical Characteristics :

In a d.c. shunt motor, shunt field flux is dependent on terminal voltage V and the shunt field resistance. Assuming that the terminal voltage is constant, the shunt field flux is also constant i.e., Φ is constant, then

Torque developed is given by = EbIa/ω
Torque developed in armature = 0.159 [ΦZla] P/A = IaK1

Where K1 = 0.159 x ZΦP/A is a constant.

The torque developed in armature (Ta) Ia.
It is a linear relation and practically a straight line front the origin. Since shaft torque is equal to the armature torque minus the rotational losses, the shaft torque line also follows the armature torque line as shown in dotted in Fig.

2. Speed and Armature Current Characteristics:

For a dc shunt motor assuming that flux remaining constant

Eb = V – IaRa = ZΦNP/60A
Or V – IaRa = N x ZΦP/60A = K2N
Where, K2 = ZΦP/60A
Or V – IaRa = K2N

The terminal voltage V and armature resistance are constant. If Ia = 0, then N is maximum and as Ia increases N decreases. As la increases, load on the shaft also increases. Invariably the speed of the shunt motor decreases with the increase of load as shown in Fig.

3. Speed-Torque Characteristics :

In a d.c. shunt motor Ta Ia and V - laRa = K2N

From these two equations, it is evident that as torque increases the armature current also increases proportionately and the net value of the back e.m.f. decreases. Decrease in back will result in decrease in speed. Hence, when speed is zero, back e.m.f. is zero, armature current is also zero and torque is maximum. The curve is shown in Fig. As speed increases armature current decreases to minimum but does not become zero and torque decrease. Therefore the characteristics line does not follow the speed axis.

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