Interview Question Of Dc Motor Part 2

Q1.What is the principle of the DC Motor ?

A. If the current carrying conductor is placed in the maganetic field, mechanical forced is experienced on the conductor, and the direction of the force is given by the Fleming’s left hand rule and hence conductor moves in the direction of force.     F  = B I L

where   F = force
B = magnetic field strength W/m2
I  = Current in Amp.
L = length of conductor in meter

Q2. What is the back emf ?

A. When the motor armature continues to rotate due to motot action the armature conductor cuts te magentic flux, and therefore emf are induced in them the direction of this induced emf is such that it oppose the applied voltage therefore this is called Back emf. .

Q3. Why the DC motors are mostly gear motor ?

A. Because the low speed motor are much more expensive than high speed motors of the same kW o/p rating.

Q4. What is the suitable dc motor for electric traction ?

A. DC series motor.

Q5.  What is the suitable dc motor for vacuum cleaner ?

A. DC series motor.

Q6.  What is the suitable dc motor for paper making ?

A.  DC cumulative compound motor.

Q7. What is the suitable dc motor for shearing and punching ?

A. DC cumulative compound motor.

Q8. What will be the effect on the direction of rotation of dc motor if the supply terminal are reversed ?

A. No effect because direction of the rotation changes when the direction of magnetic field changes.

Q9. Why series motors are preferred for traction purpose ?

A. due to large starting torque and slow down with the increase the load and automatically relieved from the heavy excessive load.

Q10. How can the direction of the dc motor be determined ?

A. By applying Fleming’s left hand rule .