Skip to main content

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 .

Labels:

Comments

Post a Comment

Comment Policy
We’re eager to see your comment. However, Please Keep in mind that all comments are moderated manually by our human reviewers according to our comment policy, and all the links are nofollow. Using Keywords in the name field area is forbidden. Let’s enjoy a personal and evocative conversation.

Popular posts from this blog

Practical Commutation

The e.m.f. induced in each coil of armature is alternating in nature. If load is connected, the current flowing will also be alternating. But the flow of current in a d.c. generator must be undirectional. This can be achieved by the use of commutator. When the armature conductors are under the influence of one pole they carry current in one direction whereas the current is reversed when the conductors are under the influence of other pole. This reversal of current takes place along the magnetic neutral axis. Fig. 1 Note : The reversal of current is likely to take place in short interval when a coil is short circuited by a brush so that transfer of current from one direction to other is carried out without any sparking. This process is called commutation.        Thus a process by which current in the short circuited coil is reversed while it crosses the MNA is called commutation. The time during which the coil remains short circuited is known as...
1 comment
Read more

Demagnetising and Cross Magnetizing Conductors

The conductors which are responsible for producing demagnetizing and distortion effects are shown in the Fig.1. Fig. 1        The brushes are lying along the new position of MNA which is at angle θ  from GNA. The conductors in the region AOC = BOD = 2θ  at the top and bottom of the armature are carrying current in such a direction as to send the flux in armature from right to left. Thus these conductors are in direct opposition to main field and called demagnetizing armature conductors.         The remaining armature conductors which are lying in the region AOD and BOC carry current in such a direction as to send the flux pointing vertically downwards i.e. at right angles to the main field flux. Hence these conductors are called cross magnetizing armature conductors which will cause distortion in main field flux.        These conductors are shown in the Fig. 2 Fig. 2  ...
Post a Comment
Read more

Armature Voltage Control Method or Rheostatic Control of dc motor

Speed Control of D.C. Shunt Motor (Part2)  2. Armature Voltage Control Method or Rheostatic Control        The speed is directly proportional to the voltage applied across the armature. As the supply voltage is normally constant, the voltage across the armature can be controlled by adding a variable resistance in series with the armature as shown in the Fig. 1. Fig. 1 Rheostat control of shunt motor        The field winding is excited by the normal voltage hence I sh is rated and constant in this method. Initially the reheostat position is minimum and rated voltage gets applied across the armature. So speed is also rated. For a given load, armature current is fixed. So when extra resistance is added in the armature circuit, I a remains same and there is voltage drop across the resistance added ( I a R). Hence voltage across the armature decreases, decreasing the speed below normal value. By varyi...
Post a Comment
Read more