Characteristics of DC Series Motor

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Characteristics of DC Series Motor

In this article, we will study about the characteristics of dc series motor.
Characteristics of dc series motor are:

1. Speed – Current characteristics

2. Torque – Current characteristics ( Also called electrical characteristics)

3. Speed- Torque Characteristics ( Also called mechanical characteristics ).

1.SPEED – CURRENT CHARACTERISTICS.

Since the speed of dc motor is given by;

N= k(E/Φ) , where k= proportionality constant.

N = k{(V – IR)/Φ}

The resistance of armature is very small so voltage drop (IR) can be neglected and supply voltage is constant. So speed is inversely proportional to flux.

Now, in dc series motor the flux is produced by the armature current flowing in field winding. So flux produced is directly proportional to the armature current. Hence speed of series motor becomes inversely proportional to the armature current.
The curve between them is rectangular hyperbola.

         Fig. Speed – current characteristics 

Thus ,when armature current is low then speed will be very large. Therefore, at no load or at light load there is a possibility of dangerously high speed, which may damage the rotor. Hence , dc series motor should never be run at no load.

2. TORQUE- CURRENT CHARACTERISTICS

Torque in case of dc motor is directly proportional to the product of flux and armature current.

But in dc series motor, flux is directly proportional to the armature current upto saturation. Hence, torque become directly proportional to the square of armature current upto saturation.

Hence, the curve is parabola upto saturation. After saturation flux becomes independent of armature current and hence torque become proportional to armature current only.

                                Fig. Torque – Current characteristics

Thus, dc series motor has very high starting torque.

3. SPEED – TORQUE CHARACTERISTICS

The speed-torque characteristics of series motor can be derived from speed-current and torque-current characteristics.

                                 Fig. Speed – Torque Characteristics

Thus, it shows that dc series motor has high torque at low speed and vice – versa.
This property makes the dc series motor useful for traction purpose.

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