electrical circuit

Objective Questions-Transformer

1.A Transformer will work on......
   Ans-a.c only

2.The primary and secondary of a transformer ............coupled
   Ans-magnetically

3.A transformer is an efficient device because it......
   Ans-is a static device

4.A transformer transfers electrical energy from primary to secondary usually with a change in ......
   Ans-voltage

5.The voltage per turn of the primary of a transformer is .........the voltage per turn of the secondary
   Ans-the same as

6.The winding of the transformer with greater number of turns will be .........
   Ans-high-voltage winding

7.A transformer dose not posses.........changing property
   Ans-power

8.The iron-core is used to...........of the transformer
   Ans-provide tight magnetic coupling

9.If a transformer core has air gaps,then .......
   Ans-magnetising  current is greatly increased    

10.The maximum flux produced in the core of a transformer is...........
   Ans-inversely proportional supply frequency

                                                                                                 to be continue..........

Motor | Induction Motor

Motor-INDUCTION MOTORS

INDUCTION MOTORS AND SPECIAL MACHINES

 Describe the main parts of a single phase induction motor-

The single phase induction motor has two main parts.  They are the stator and the rotor.  The stator is made up of laminated sheets and has slots on the inner periphery to carry the stator winding or the main winding.  The main winding is excited by a single phase ac supply.  The rotor is of squirrel cage type which has number of conducting bars short circuited at the two ends using end rings.

What is synchronous speed?-

The speed at which the rotating magnetic field rotates is called the synchronous speed Ns, which is given by,

                              

      where f is the frequency of ac supply and P is the number of poles on stator.

 What is slip of an induction motor?  Give its importance-

The ratio of the difference between the synchronous speed and the normal running speed of the induction motor to the synchronous speed is called the slip of an induction motor.

                              

State double field revolving theory-

The double field revolving theory states that, any alternating quantity can be resolved into two rotating components, which rotate in opposite directions and each having a magnitude as half of the maximum magnitude of the alternating quantity.

What is the direction in which the rotor of single phase induction motor rotates?  Give reason-

The rotor of a single phase induction motor rotates in the direction of the rotating magnetic field. 

The cause producing the rotation in the rotor is the relative speed between the rotating magnetic field and the rotor.  According to Lenz’s law the rotor rotation will oppose the cause, hence it rotates in the same direction as the rotating magnetic field so that the relative speed can be reduced.  But the rotor can never rotate at synchronous speed because if the rotor rotates at synchronous speed the relative speed which is the cause producing the rotation will become zero and the motor will come to halt.

What is the necessary condition to obtain a rotating magnetic field?

To obtain the rotating magnetic field there should be at-least two alternating fluxes present.  The resultant of two or more alternating fluxes will be a rotating magnetic field.

How can we obtain rotating magnetic field in a single phase induction motor?

We can obtain rotating magnetic field in a single phase induction motor by introducing an auxiliary winding called the starting winding which produces additional alternating flux at some phase angle.

Why is the single phase induction motor not self starting?

The single phase induction motor is not self starting because of the absence of a rotating magnetic field.  Since it is operated on single phase, there exists only one flux which is alternating in nature.  But for the motor to start there should be a rotating magnetic field.

What are the types of single phase induction motors?

There are four main types of single phase induction motors.  They are,

a)    Split phase induction motor

b)    Capacitor start induction motor

c)    Capacitor start capacitor run induction motor

d)    Shaded pole induction motor

Why is the starting torque of a capacitor start induction motor high, when compared to that of a split phase induction motor?

The starting torque of the single phase induction motor is directly proportional to the phase angle between the currents carried by the two windings, namely the main winding and the starting winding.  Split phase induction motor uses a highly resistive starting coil, which gives the phase angle nearly 90°.  But in capacitor start induction motor, the phase angle obtained is greater than 90°.  Hence the starting torque is still higher in this type.

What is a stepper motor?

Stepper motor is an electromechanical device which actuates a train of step movements of shaft in response to train of input pulse.  The step movement may be angular or linear.

What is a step angle?  How is it calculated in a variable reluctance stepper motor?

The angle through which the motor turns or shaft moves for each pulse is known as the step angle, expressed in degrees.

The step can also be defined as the angular displacement of the rotor in response to each input pulse.  The step angle in variable reluctance stepper motor is given by,

                              

where, q is the number of phases and N_r is the number of rotor poles.

What are the types of stepper motors?

 The various types of stepper motors are,

a)    Variable reluctance stepper motors

b)    Permanent magnet stepper motors

c)    Hybrid stepper motors

How will you calculate the number of poles required in the rotor of a variable reluctance stepper motor?

The number of poles required in a rotor is,

                              

      where, N_s is the number of poles in the stator and q is the number of phases.

Give the expression for Torque developed in a variable reluctance stepper motor-

                  The torque developed in a variable reluctance stepper motor is given by,

                             

               

where L is the inductance of the relevant phase at an angle q. 

Define rotor tooth pitch in a variable reluctance stepper motor-

The distance through which the rotor moves when all three phases are excited once is called one rotor tooth pitch.(RTP)

                               

Give the advantages of a variable reluctance stepper motor-

 The advantages of a variable reluctance stepper motor are,

a)    High torque to inertia ratio

b)    High rates of acceleration

c)    Simple and low cost machine

d)    Fast dynamic response

e)    Rotor construction is robust

List out the applications of stepper motors-

The applications of stepper motor are,

a)    Compatible to digital circuit, hence widely used in computer peripherals such as printers, Floppy disc drives, etc.

b)    Numeric control machine tools

c)    Robotic control systems

d)    Watches

Define Holding torque, Critical torque and limiting torque-

The holding torque is defined as the maximum static torque that can be applied to the shaft of an excited motor without causing a continuous rotation.

The critical torque is defined as the maximum load torque at which rotor does not move when an exciting winding is energized.  This is also called pullout torque.

The limiting torque is defined as the maximum load torque at which motor follows the control pulse without missing any step.  This is also called pull in torque.

What is slewing rate?

The slewing rate is defined as the maximum rate at which the motor can step uni-directionally.

Motor-Basic Principle of DC Motor

DC Motor Principle

A machine that converts dc power into mechanical energy is known as dc motor. Its operation is based on the principle that when a current carrying conductor is placed in a magnetic field, the conductor experiences a mechanical force. The direction of the force is given by Fleming’s left hand rule.

Fleming Left Hand Rule:

Back or Counter EMF :

When the armature of a d.c. motor rotates under the influence of the driving torque, the armature 

conductors move through the magnetic field and hence an e.m.f. is induced in them. The induced e.m.f. acts in opposite direction to the applied voltage V(Lenz’s law) and is known as back or 

counter e.m.f. Eb

.

Significance of Back E.M.F :

The presence of back e.m.f. makes the d.c. motor a self-regulating machine i.e., it makes the motor to draw as much armature current as is just sufficient to develop the torque required by the load.Back e.m.f. in a d.c. motor regulates the flow of armature current i.e., it automatically changes the armature current to meet the load requirement.

DC Motor Types :

• Shunt Wound

• Series Wound

• Compound wound

Shunt Wound Motor :

In shunt wound motor the field winding is connected in parallel with armature. The current through the shunt field winding is not the same as the armature current. Shunt field winding's are designed to produce the necessary m.m.f. by means of a relatively large number of turns of wire having high resistance. Therefore,shunt field current is relatively small compared with the armature current

Series Wound Motor :

In series wound motor the field winding is connected in series with the armature. Therefore, series field winding carries the armature current. Since the current passing through a series field winding is the same as the armature current, series field winding's must be designed with much fewer turns than 

shunt field winding's for the same mmf. Therefore, a series field winding has a relatively small number of turns of thick wire and, therefore, will possess a low resistance.

Compound wound motor :

Compound wound motor has  two field winding's; one connected in parallel with the armature and the other in series with it. There are two types of compound motor connections 

1) Short-shunt connection

2) Long shunt connection

When the shunt field winding is directly connected across the armature terminals it is called short-shunt connection. 

Compound Wound Motor :

When the shunt winding is so connected that it shunts the series combination of armature and series field it is called long-shunt connection. 

Parts of a DC Motor :

A simple motor has six parts :

• Armature/ Rotor 

• Commutator

• Brushes

• Axle

• Permanent Magnet

• DC Power supply

Commutation in DC Motor :

In order to produce unidirectional force (or torque) on the armature conductors of a motor, the conductors under any pole must carry the current in the same direction at all times.The function of commutator and brush gear in a dc motor is to cause the reversal of current in a conductor as it moves from one side of a brush to the other.

Voltage Equation of Motors :

Let in a d.c. motor 

V = applied voltage

Eb= back e.m.f.

Ra= armature resistance

Ia= armature current

Since back e.m.f. Eb

acts in opposition to the applied voltage V, the net voltage across the armature circuit is V-Eb. The armature current Ia is given by 

 

This is known as voltage equation of the d.c. motor

Motor-(DC Motor)

DC Motor:

D.C. Motor are seldom used in ordinary application because all electric supply companies furnish alternating current. However,for special application such as in steel mills mines and electric trains it is advantageous to convert alternating current into direct current in order to use d.c. motors. The reason in that speed/torque characteristics of d.c. motors are much more superior to that of a.c. motors. Like generators  d.c. motors are also of three types series-wound, shunt wound and compound-wound.The usu of a particular d.c. motor depends upon the type of mechanical load it has to drive