A dc motor is driving a load that requires constant output power the pu value of torque with its field current reduce to half would be (considered rated quantities as 1.0pu)

180 viewed last edited 5 years ago

Anonymous

a) 0.5pu b)1.0pu c)2.0pu d) n1.5pu

rkrishna.dignity (last edited 5 years ago) (Parent, 1^st Grade)

For a DC Motor...……..

Output power P=K(TxN) where T is the Torque and N is the speed of the motor and K is the constant of the motor which is the property of the motor.

This K depends upon various factors like number of poles, length of the rotor etc.

Again torque T is proportional to Field(F) x Armature Current(I)

Therefore P=K(FxIxN).

Hence for constant power, if field current is reduced to half ( that means field is reduced to half), speed remaining the same, armature current 'I' should double to make the torque 'T' double(2.0pu). But this is not possible unless the external applied voltage to the motor is doubled. If the applied voltage is not doubled, in such a situation, the speed of the motor will reduce to N/2.

Mahesh Godavarti (last edited 5 years ago) (Expert, Founder @Qalaxia)

I am trying to figure out the answer using.

http://www.farnell.com/datasheets/62656.pdf

I think the answer is still 1.0 pu as the output power is constant. Am I correct?

tanuja goud (last edited 5 years ago)

thanks for considering my request,i too got 1 p.u but answer is 2p..u.

Mahesh Godavarti (last edited 5 years ago)

Do you have the source? Can you share the original source of the question?