In this post we will discuss about the Speed Current Characteristics of DC Series Motor. After going through this post ask yourself the answer of the question “Why DC Motor should not be Started at No Load?” I am sure that you will be able to answer.

In DC Series Motor the field winding remain in series with the Armature winding as shown in figure below.

Therefore the current flowing through the Field coil and Armature will be same. So If Ia is the Armature current then Ia will also be the Field current. If saturation and Armature reaction is neglected then Magnetic Flux produced by the Field winding of DC Series Motor will be proportional to the Armature current Ia.

So, for DC Series Motor we can write,

Ø = Main Flux = KIa  …………………..where K is some constant.

Suppose DC Voltage V is applied across the terminals of DC Series Motor, then

V = Ea +Ia(Ra+Rs)      ……………………………..(1)

Where Ea = Back emf of Armature

            Ra = Armature winding Resistance

            Rs = Resistance of Series Field Winding of DC Series Motor

From equation (1),

Ea = V- Ia(Ra+Rs)   

But Ea = Ka Øωm   where ωm = Rotational speed of DC Series Motor and Ka = constant.

Thus, Ka Øωm   = V- Ia(Ra+Rs 

ωm = [V- Ia(Ra+Rs)] / Ka Ø

      = [V- Ia(Ra+Rs)] / KaxKIa        (As Ø = KIa)

      = V/KaKIa – (Ra+Rs)/KaK    …………………………………(2)

Thus from equation (2), it is clear that speed of DC Series Motor is inversely proportional to Armature Current Ia. If we plot the curve of speed and Armature current Ia then it shall be a Hyperbola as shown in figure below.

Now suppose the DC Series Motor is started at no load then DC Series Motor will take very small current from the supply mains as the input power needed is to only overcome the no load losses. Therefore the no load current of DC Series Motor can be assumed approximately zero i.e. Ia~0

Therefore from equation (2),

Speed of DC Series Motor ωm = Very high, which is also clear from the Speed Current Characteristics of DC Series Motor as shown in figure above.

Therefore if the DC Series Motor is started at No Load then the speed of DC Series Motor will become dangerously high which will definitely damage the DC Series Motor. In view of this, DC Series Motor shall always be started and operated under load mechanically coupled with it.

If any query, please write in comment box. It will be my pleasure to answer. Thank you!

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