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SSC JE Electrical Engineering 2017 Set 3 Paper

Question : 129 of 200

Marks: +1, -0

The speed of a series motor at no load is ____

Solution:

Speed of a series motor at no load is infinite.

E_b = V - I_a R_a

K \phi w = V - I_a R_a

In DC series motors Armature current (

I_a

) = Field current (

I_f

) because both armature and field are connected in series then

\phi \propto I_f

I_a \propto \phi

K_1 I_a w = V - I_a R_a

∴

w = \frac{V}{K_1 I_a} - \frac{R_a}{K_1}

w \propto \frac{1}{I_a}

at no load

I_a

= 0, So speed is infinite

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