Electrical Machines-I

The property of a material which opposes the creation of magnetic flux in it is known as

Those magnetic materials are best suited for making armature transform cores which have ______ permeability and ______ hysteresis loss.

The current drawn by a 220 V d.c. motor of armature resistance 0.5 $ \Omega $ and back e.m.f. of 200 V is

A d.c. series motor is accidentally connected to single-phase a.c. supply. The torque produced will be

The dummy coils in d.e. machines are useful to

A d.c. shunt generator is supplying a load of 1.8 kW at 200 V. Its armature and field resistances are 0.4 ohm and 200 ohm respectively. What is the generated e.m.f.?

Two transformers when operating in parallel will share the load depending upon which of the following?

When are eddy-current losses in a transformer reduced?

In an auto-transformer, power is transferred through

Open delta transformer can be obtained from

Draw and explain the no-load phasor diagram of a 1-phase transformer. Discuss how primary leakage flux is accounted for in the phasor diagram.

Give some transformer applications in electronic and control circuits.

Define MMF and flux.

Write the basic difference between the magnetizing current of a large capacity transformer and the magnetizing inrush current.

What are the various conditions which have to be fulfilled before a shunt-excited d.c. generator will generate rated voltage across the armature terminals?

In open-circuit test on a 1-phase transformer, the ohmic losses are usually neglected in comparison with core loss. Justify.

Draw the speed vs. armature current, torque vs. armature current and torque vs. speed characteristics of d.c. series motor. Also write the applications of d.c. series motor.

Explain briefly the four bad effects of armature reaction.

Describe, with a neat diagram, the working of a three-point starter used for a d.c. shunt motor.

Which losses of a d.c. shunt motor are constant?

A 6-pole, 148 A d.c. shunt generator has 480 conductors and is wave-wound. Its field current is 2 A. Find the demagnetizing and cross-magnetizing ampere turns per pole at full load, if (i) brushes are on GNA; (ii) brushes are shifted from GNA by $ 5^\circ $ electrical; (iii) brushes are shifted from GNA by $ 5^\circ $ mechanical.

It is found that the voltage of a d.c. shunt generator does not build up. Explain the various possible causes of this failure.

A 250 V d.c. shunt motor has an armature resistance of 0.5 $ \Omega $ and a field resistance of 250 $ \Omega $. When driving a constant torque load at 600 r.p.m., the motor draws 21 A. What will be the new speed of the motor if an additional 250 $ \Omega $ resistance is inserted in the field circuit?

For a dc motor, the field flux speed control method is called a constant power drive method. Explain.

The maximum efficiency of a 50 kVA transformer is 97.4% and occurs at 90% of full load, at unity power factor. Calculate the efficiency at (i) full load at 0.8 power factor; (ii) half the full load at 0.9 power factor.

What are the two functions of a commutator in d.c. machines?

Derive an expression for the e.m.f. induced in a transformer winding. Show that e.m.f. per turn in primary is equal to e.m.f. per turn in secondary.

Why is it preferable to install two or more transformers in parallel than one large unit?

What do you understand by leakage flux in a transformer?

The property of a material which opposes the creation of magnetic flux in it is known as

Those magnetic materials are best suited for making armature transform cores which have ______ permeability and ______ hysteresis loss.

The current drawn by a 220 V d.c. motor of armature resistance 0.5 $ \Omega $ and back e.m.f. of 200 V is

A d.c. series motor is accidentally connected to single-phase a.c. supply. The torque produced will be

The dummy coils in d.e. machines are useful to

A d.c. shunt generator is supplying a load of 1.8 kW at 200 V. Its armature and field resistances are 0.4 ohm and 200 ohm respectively. What is the generated e.m.f.?

Two transformers when operating in parallel will share the load depending upon which of the following?

When are eddy-current losses in a transformer reduced?

In an auto-transformer, power is transferred through

Open delta transformer can be obtained from

Draw and explain the no-load phasor diagram of a 1-phase transformer. Discuss how primary leakage flux is accounted for in the phasor diagram.

Give some transformer applications in electronic and control circuits.

Define MMF and flux.

Write the basic difference between the magnetizing current of a large capacity transformer and the magnetizing inrush current.

What are the various conditions which have to be fulfilled before a shunt-excited d.c. generator will generate rated voltage across the armature terminals?

In open-circuit test on a 1-phase transformer, the ohmic losses are usually neglected in comparison with core loss. Justify.

Draw the speed vs. armature current, torque vs. armature current and torque vs. speed characteristics of d.c. series motor. Also write the applications of d.c. series motor.

Explain briefly the four bad effects of armature reaction.

Describe, with a neat diagram, the working of a three-point starter used for a d.c. shunt motor.

Which losses of a d.c. shunt motor are constant?

A 6-pole, 148 A d.c. shunt generator has 480 conductors and is wave-wound. Its field current is 2 A. Find the demagnetizing and cross-magnetizing ampere turns per pole at full load, if (i) brushes are on GNA; (ii) brushes are shifted from GNA by $ 5^\circ $ electrical; (iii) brushes are shifted from GNA by $ 5^\circ $ mechanical.

It is found that the voltage of a d.c. shunt generator does not build up. Explain the various possible causes of this failure.

A 250 V d.c. shunt motor has an armature resistance of 0.5 $ \Omega $ and a field resistance of 250 $ \Omega $. When driving a constant torque load at 600 r.p.m., the motor draws 21 A. What will be the new speed of the motor if an additional 250 $ \Omega $ resistance is inserted in the field circuit?

For a dc motor, the field flux speed control method is called a constant power drive method. Explain.

The maximum efficiency of a 50 kVA transformer is 97.4% and occurs at 90% of full load, at unity power factor. Calculate the efficiency at (i) full load at 0.8 power factor; (ii) half the full load at 0.9 power factor.

What are the two functions of a commutator in d.c. machines?

Derive an expression for the e.m.f. induced in a transformer winding. Show that e.m.f. per turn in primary is equal to e.m.f. per turn in secondary.

Why is it preferable to install two or more transformers in parallel than one large unit?

What do you understand by leakage flux in a transformer?

What interpoles are used for in a DC motor?

Define commutation in DC machine.

Draw sample waveform of wave winding and lap winding.

Why is laminated core used in transformers?

What is the order of the efficiency of a general purpose transformer? State reason.

State Biot-Savart law.

State Ampere law.

Draw magnetic lines of a magnate showing magnetic flux lines clearly.

On the same platform, draw the B-H curve for soft as well as hard magnetic material.

With the help of neat and labeled diagram, show energy and co-energy.

Draw and discuss general representation of an electromechanical energy conversion device. Further, explain lossless electromechanical energy conversion.

The self and mutual inductances of a doubly excited magnetic system are as follows:
$ L_s=0.65+0.18\cos 2\theta_r $ henry
$ L_r=0.78+0.29\cos 2\theta_r $ henry
$ M_s=0.86\cos\theta_r $ henry
Considering the winding resistance as zero, find the magnitude and direction of torque when the currents are $ i_s=15 $ A DC, and $ i_r=12 $ A DC. When stationary rotor has an angular position of $ \theta_r=60^{\circ} $, consider all notations in their standard representation.

Derive the expression for the maximum efficiency of a single-phase transformer.

Draw and explain the exact and approximate equivalent diagram of a single-phase transformer. Give the mathematical formulations to represent the impedances, voltage and current of a side, when referred to another side.

Following test results are obtained from a 10 kVA, 2500/250 1- transformer.
OC test: 250 V, 0.8 A, 50 W
SC test: 60 V, 3.0 A, 45 W
Calculate the efficiency at the load at 0.8 p.f.

Why are OC/SC tests performed on a transformer? Discuss what side they are performed and why. Elaborate the procedure to perform the tests.

Draw and discuss the important constructional details of a DC motor. List two applications of DC series motor depicting its characteristic.

Differentiate lap and wave windings along with their roles in a DC motor. Given the number of conductors in the machine as 16, calculate the average pitch of a 4-pole DC machine for (i) lap and (ii) wave windings.

What do you understand by armature reaction? Elaborate its effect on the working of a DC generator and suggest the remedies to remove it.

What are the problems faced with regard to commutation in a DC machine? Suggest the constructional changes need to be performed in order to protect the machine from commutation failure.

A 4-pole, 32-conductor, DC generator is lap wound and is running at a speed of 1500 r.p.m. Assuming per pole to be 10 Wb, find the EMF generated by the DC machine.

Draw the open circuit and load characteristics of a separately excited DC generator. Also explain the curvature that is seen in an open circuit characteristic of a self-excited DC generator.

Classify various types of braking used in a DC machine. Explain the type of braking demonstrating a train runs down a hill top. Support your answer with mathematical expressions.

"A DC shunt motor can provide a wide range of speed control." Justify the statement theoretically and mathematically. Additionally, draw neat and labeled diagram of Ward Leonard method to control the speed of a DC shunt motor.

Differentiate between a 1- transformer and an autotransformer. How is copper saving done in an autotransformer? What are additive and subtractive polarities in an autotransformer?

Why is back-to-back test performed on a transformer? Write the steps to conduct the test. In back-to-back test, the wattmeter readings for primary and secondary are 25 W and 50 W, respectively. Calculate the copper loss of a transformer at half the rated load, if both the transformers used are identical.

What interpoles are used for in a DC motor?

Define commutation in DC machine.

Draw sample waveform of wave winding and lap winding.

Why is laminated core used in transformers?

What is the order of the efficiency of a general purpose transformer? State reason.

State Biot-Savart law.

State Ampere law.

Draw magnetic lines of a magnate showing magnetic flux lines clearly.

On the same platform, draw the B-H curve for soft as well as hard magnetic material.

With the help of neat and labeled diagram, show energy and co-energy.

Draw and discuss general representation of an electromechanical energy conversion device. Further, explain lossless electromechanical energy conversion.

The self and mutual inductances of a doubly excited magnetic system are as follows:
$ L_s=0.65+0.18\cos 2\theta_r $ henry
$ L_r=0.78+0.29\cos 2\theta_r $ henry
$ M_s=0.86\cos\theta_r $ henry
Considering the winding resistance as zero, find the magnitude and direction of torque when the currents are $ i_s=15 $ A DC, and $ i_r=12 $ A DC. When stationary rotor has an angular position of $ \theta_r=60^{\circ} $, consider all notations in their standard representation.

Derive the expression for the maximum efficiency of a single-phase transformer.

Draw and explain the exact and approximate equivalent diagram of a single-phase transformer. Give the mathematical formulations to represent the impedances, voltage and current of a side, when referred to another side.

Following test results are obtained from a 10 kVA, 2500/250 1- transformer.
OC test: 250 V, 0.8 A, 50 W
SC test: 60 V, 3.0 A, 45 W
Calculate the efficiency at the load at 0.8 p.f.

Why are OC/SC tests performed on a transformer? Discuss what side they are performed and why. Elaborate the procedure to perform the tests.

Draw and discuss the important constructional details of a DC motor. List two applications of DC series motor depicting its characteristic.

Differentiate lap and wave windings along with their roles in a DC motor. Given the number of conductors in the machine as 16, calculate the average pitch of a 4-pole DC machine for (i) lap and (ii) wave windings.

What do you understand by armature reaction? Elaborate its effect on the working of a DC generator and suggest the remedies to remove it.

What are the problems faced with regard to commutation in a DC machine? Suggest the constructional changes need to be performed in order to protect the machine from commutation failure.

A 4-pole, 32-conductor, DC generator is lap wound and is running at a speed of 1500 r.p.m. Assuming per pole to be 10 Wb, find the EMF generated by the DC machine.

Draw the open circuit and load characteristics of a separately excited DC generator. Also explain the curvature that is seen in an open circuit characteristic of a self-excited DC generator.

Classify various types of braking used in a DC machine. Explain the type of braking demonstrating a train runs down a hill top. Support your answer with mathematical expressions.

"A DC shunt motor can provide a wide range of speed control." Justify the statement theoretically and mathematically. Additionally, draw neat and labeled diagram of Ward Leonard method to control the speed of a DC shunt motor.

Differentiate between a 1- transformer and an autotransformer. How is copper saving done in an autotransformer? What are additive and subtractive polarities in an autotransformer?

Why is back-to-back test performed on a transformer? Write the steps to conduct the test. In back-to-back test, the wattmeter readings for primary and secondary are 25 W and 50 W, respectively. Calculate the copper loss of a transformer at half the rated load, if both the transformers used are identical.