01․ The direction of rotation of a D.C. series motor can be changed by

By changing the field terminal of D.C motor the polarity of the field electromagnet is reversed and consequently the direction of flux produced changes thus resulting in reversal of direction of rotation of dc motor.

02․ DC Motor

In shunt motor, I

_{sh}= V / R_{sh}. If V is constant I_{sh}will also be a constant. Hence the flux is constant.03․ In DC series motor the speed is ______________ proportional to the armature current.

In dc series motor,the flux φ is produced by the armature current flowing in the field winding and so φ is directly proportional to I

_{a}. Now since φ is inversely proportional to speed. ∴ I_{a}is also inversely proportional to speed .04․ A compound motor has

In compound motor having both series and shunt field winding that is why it is called compound motor. It may be a short shunt compound or long shunt compound motor.

05․ A dc motor with armature resistance of 0.5 ohms connected directly to a 230 Volt supply. If the armature is stationary then the current flowing through armature is

As the armature is stationary so the back e.m.f E is zero. The initial armature current I

_{as}is given by I_{as}= ( V - 0 ) / R_{a}i.e. V / R_{a}. Armature resistance is 0.5 ohm and voltage is 230 V therefore I_{as}= V / R_{a}= 230 / 0.5 = 460 A.06․ The armature of a DC motor is laminated to reduce

Thinner the lamination, greater is the resistance offered to the induced emf smaller the current and hence lesser the I

^{2}R loss.07․ DC Motor

Separately excited motor always obtains accurate speed and most suitable for applications requiring speed variation from very low value to high value, like its required in a paper mill.

08․ In electric locomotive which of the following motor are used ?

In DC series motor starting torque is very high up to 500 % resulting maximum momentary operation of 400 % thus it is employed in locomotive.

09․ On what factors the speed of dc motor depends?

Speed N proportional to E

_{b}/ φ N proportional to V - I_{a}R_{a}/ φ.10․ The speed of a motor falls from 1100 r.p.m at no-load to 1050 r.p.m at rated load. The speed regulation of motor is

(No load sped - Rated speed) / Full load speed
No load speed = 1100 rpm
Full load speed = 1000 rpm
∴ % of Regulation = (1100 - 1050) / 1050 = 4.76%.

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