MACHINE DESIGNING - QUESTIONS FOR PRACTICES

1.      Obtain expression for output equation of a rotating machine. Apply this for a synchronous, induction and dc machine.

2.      Discuss factors affecting size of rotating machine and how the separation of main dimensions is done.

3.      Explain SCR in synchronous machines.

4.      Find the main dimensions of a 20 MVA, 11kV, 50Hz, 150r.p.m., 3-phase water wheel generator. The average gap density is 0.6 Wb /m² and ampere conductors per meter are 35,000. The peripheral speed should not exceed 65 m/s at normal running speed in order to limit the run-away speed.

5.      Give procedure for designing the field system of a three-phase turbo generator from the given design data. Also indicate how much excitation power may be required for such a generator of about 100 MW capacity.

6.      A 1500 kVA, 3-phase, star-connected, 3300 V, 250 rpm water wheel generator has the following design data:

Effective gap length                           = 7mm

Effective gap area/pole                       = 0.075 m²

Winding factor                                    = 0.955

Field mmf per pole under  rated condition                 = 7850 A

No. of field turns/pole                                    = 180

The peak value of fundamental flux density distribution B…= 0.94 T and peak value of the actual flux density distribution B…=0.9T.

The permissible value of current density in field winding is 3 A/mm². Determine:

(i)     Turns  / phase

(ii)   MMF for air gap

(iii) Armature MMF /pole

(iv) Field current

(v)   Sectional area of field conductor.  

7.      Design the suitable values of diameter and length of a 75 MVA, 11 KV, 50 Hz, 3000 rpm, 3-phase star connected alternator. Also determine the value of flux, conductor per slot, no of turns per phase, and size of armature conductor. Given

Average gap density               = 0.6    T

Amp conductor per m             = 50,000

Peripheral speed                      = 180 m/s

Current density                       = 6 A/m²

8.      Calculate the diameter, core length, number of conductors of the stator, size of conductor, number of stator slots of a 30 MVA, 11KV, 3000 r.p.m. 50Hz star connected turbo alternator. Assume suitable data:

Bav =0.55 Wb/m²,         ac =55000 A/m,          Kw = 0.955,

Peripheral velocity = 160 m/s.

9.      If two synchronous machines running at the same speed and having the same number of poles, the physical dimensions are in the ratio 3 : 2. Compare the outputs, armature copper losses and iron losses in the two machines. Assume specific magnetic loading and current density to be same for both the machines. 

Read More

ELECTRICAL MACHINES DESIGN QUESTIONS FOR PRACTICES

10. Find the amount of cooling air required in m³ per second at the inlet temperature of 25^OC for a 25000KVA alternator working at full load. The efficiency is 96% and the power-factor is 0.87. The temperature of air coming out of machine is 50^OC. Determine also the amount of hydrogen required with the same data.

11. Discuss, briefly, about the “Frame Size” and “Standard Frames”.

12. Explain heating and cooling process of electrical machines also define heating & cooling time constants.

13. What do you understand by the term “Impregnation of winding”?

14. Does the selection of current density Amp /mm² in the design of electrical machines, depend on duty-cycle of the respective machine? Explain in brief.

15. Justify the statement, “By exceeding the rate of cooling, the overload capacity of an electrical machine can be enhanced.”

Read More

ELECTRICAL MACHINE DESIGNING QUESTIONS ANSWERS - PRACTICES

1. Discuss basic concepts of design giving limitations in design.

2. What is understood by the terms, Standards and Standardization as applied to the design of electrical machines? Explain their importance.

3. What is understood by the term, “Specifications? Write down specifications for transformers and induction motors.

4. What are the modern trends in design and manufacturing techniques giving their relative advantages.

5. Give a classification of insulating materials used in electrical machines in context of permissible temperature rise.

6. Name a few insulating materials used in a transformer and give their temperature limits.

7. Give modes of heat dissipation in electrical machines and obtain temperature rise-time curves.

8. Describe the types of enclosures used for induction motors and their effect on cooling.

9. Explain radial, axial, induced and forced cooling as applied to electrical machines. What is direct cooling and how the quantity of cooling medium estimated.

Read More

INDUCTION MOTORS QUESTIONS ANSWERS

1.                  Explain construction and working principle of a 3-phase indcuiton motor.

2.                  Explain the terms slip, slip frequency, wound rotor and cage rotor.

3.                  Draw and explain the torque-slip & torque-speed charactoristics of a typical 3-phase induction motor. Mark the starting torque and maximum troque on the daigram so drawn.

4.                  Explain  the terms air-fap power P_g, inernal mechanical power developed and shaft power P_sh. How  are these terms related with each others. Hence show that :

P_g :Rotor ohmic loss : P_m =1 :s (1-s)

5.                  A 3-phase induction motor runs at 1140 rpm at full load when supplied with power from a 60 Hz, 3-phase line calculate                                       

                        (i)         No of poles.

                        (ii)        Slip at full load.

(iii)             Frequency of rotor voltage.

(iv)             Speed of rotor field with respect to rotor.

(v)               Speed of rotor field with respect to stator and stator field.

(vi)             Speed of rotor at a slip of 10% and rotor frequency at this speed.

(vii)           If the applied voltage per phase is 230 V, find th0 rotor induced emf at stand still and at 10% slip, with stator to rotor turn ration of 1: 0.5

(Ans: - (i) 1200 rpm, (ii) 5% (iii) 3 Hz (iv) 60 rpm (v) 0 rpm (vi) 1080 rpm, (vii) 11.5 V)

6.                  A 6-pole, 3-phase indcution motor develops 30 hp including 2 hp mechanical losses at a speed of 950 rpm on 550 V, 50 Hz mains. The power factor is 0.88 lagging. Calculate (i) slip (ii) torot copper loss (iii) total input if stator losses are 2 kW (iv) efficiecny (v) line current. (Ans : - (i) 0.05, (ii) 1161 Watts, (iii) 20594 Watts, (iv) 81.64% (v) 30 A)

7.                  Explain construction and principle of operation of 1-phase induction motor.

8.                  Why single phase induciton moto is not self starting ?

9.                  What are the different methods of statring a single phase induction motors and what are the diffenet types of 1-phase induction motors ? 

Read More

DC MACHINES AND SYNCHRONOUS MACHINES - QUESTIONS FOR PRACTICES

1. Explain construction and working principle of dc machines with the help of a neat diagram. Drive an emf equation for the same. What is back emf?
2. What are the different efficiency associated with dc generator and dc motors explain?
3. Drive an expression for torque in case of dc machine.
4. A d.c. generator has an armature emf of 100 V when the useful flux per pole is 20 mWb, and the speed is 800 rpm. Calculate the generated emf (i) with the same flux and a speed of 1000 rpm, (ii) with a flux per pole of 24 mWb and a speed of 90 rpm. (Ans.:- (i) 125 V, (ii) 135 V)
5. An 8 pole dc generator has 500 armature conductors and a useful flux of 0.05 Wb. What will be the emf generated, if it lap connected and runs at 1200 rpm? What must be the speed at which it is to be driven to produce the same emf, if it is wave wound? (Ans.:- 500V, 300 rpm)
6. A 4 pole dc shunt generator with lap connected armature has field and armature resistance of 80 Ω and 0.1 Ω respectively. It supplies power to 50 lamps rated for 100 volts, 60 watts each. Calculate the total armature current and the generated emf by allowing a contact drop of 1 V per brush. (Ans.:- 26 A, 104.6 V)
7.  A d.c. shunt machine connected to 230 V supply has resistance of armature as 0.115 Ω and of field winding as 115 Ω. Find the ratio of the speed as a generator to the speed as a motor with the line current in each case being 100 A. (Ans.:- 1.1052:1)
8. A d.c. shunt motor draws 10 A at no load from 230 V mains and runs at 1500 rpm. At full load, armature current is 100 A and speed is 1470 rpm. Armature resistance of the motor is 0.1 Ω and field current negligible. Find (a) back emf at no – load and full load (b) speed at which armature should be run to make it deliver 100 A at 220 V as a generator. Assume same flux as with motor operation at full load of 10 A. (Ans.:- (a) 229 V , 220 V (b) 1536.8 rpm)
9. A 200 V dc series motor runs at 500 rpm when taking a current of 25 A. The resistance of the armature is 0.5 Ω and that of the field is 0.3 Ω. If the current remains constant, calculate the resistance necessary to reduce the speed to 250 rpm. (Ans.:- 3.6 Ω)
10. A d.c. series motor has an armature resistance of 0.12 Ω and field resistance of 0.08 Ω. The supply voltage across the motor terminals is 230 V. Determine the back emf and power developed by the motor when line current drawn by motor is 30 A. (Ans.:- 224 V, 6.72 kW)

11. What are the different types of synchornous machines ? Explain the principle of operation of a 3-phase synchronus motor. Why it is not self starting.

12. Why the field winding of synchronus generators is placed on rotor.

13. Draw and explain V-curve for synchronous motor

Read More

NON LINEAR SYSTEM AND LINEARIZATION - MODERN CONTROL SYSTEM - ELECTRICAL CONTROLLING ENGINEERING

1. Describe the process of linearization by small signal analysis.

2. Linearize following non-linear equation with equilibrium state x0=0

                     

                             𝑥̇=𝑥2 𝑥̇2=𝜇(1−𝑥2)𝑥2−𝑥1 𝜇>0

     Also check its stability

3. The state equation of a nonlinear system are given below. Determine all points of equilibrium and investigate the stability in the neighbourhood of these points

                                            𝑥1̇=𝑥2 𝑥̇2=−𝑥12−𝑥22−2𝑥1−2𝑥2

4. Derive the describing function of following

    a. Ideal relay

    b. Practical relay

    c. Combination of dead zone and saturation

    d. Backlash

5. How stability is decided by describing function method?

6. Determine stability of following system by describing function method. The describing function of nonlinearity is given by

Read More

QUESTIONS ABOUT MODERN CONTROL SYSTEM - ELECTRICAL CONTROL ENGINEERING

 Modern Control System Discreet Data System

1. What is sampling? Write expression for the output of an ideal sampler.

2. What is z-transformation and how it is calculated from Impulse Train Laplace Transformation?

3. Write down some properties of z-transformation.

4. Calculate z-transformation of f(t) = cos ωt.

5. Find z-transformation of f(t) = 1/(s2+2s+2) using sampling time T= 1 second.

6. Determine the z-transform of the finite duration signals

a. 𝑥(𝑛)={0,⃗⃗ 0,1,2,6,−2,3} (Ans= z-2+2z-3+6z-4-2z-5+3z-6)

b. 𝑥(𝑛)={1,2,6,⃗⃗ −2,0,3}

(Hint: Use two sided z-Transform 𝐹(𝑧)=Σ𝑓(𝑘)𝑍−𝑘∞𝑘=−∞

Ans= z2+2z+6+-2z-1+3z-3)

7. Find inverse z-transformation of the function given below given that sampling time T= 1 second by power series and partial fraction method.

𝐹(𝑧)=0.632𝑍𝑍2−1.368𝑍+0.368

8. Solve the difference equation c(k+2)-5c(k+1)+6c(k)=u(k). Given that c(0)=0 and c(1)=1.

9. Solve the difference equation c(k+1)+c(k)=r(k); c(0)=0 by using Z-transform method.

Ans: c(k) = (-2)k-1, k≥1.

10. Derive an expression for Pulse Transfer Function of two cascaded blocks each using a sampler in the input. 

11. Derive an expression for Pulse Transfer Function when only error signal is sampled.

12. Show that the stable region in sampled data control system is inside the unit circle of z-plane. What does outside region of z-plane represent?

13. Explain how Routh-Hurwitz criterion can be applied to sampled data control system?

14. Determine the pulse transfer function and stability for the following system using Bilinear transformation.

15. Determine the pulse transfer function and stability for the following system using Bilinear transformation. (Given transfer function of ZOH=(1-e-Ts)/s)

Use comment box below for your answers and more clearification.

Read More

THREE PHASE AND SINGLE PHASE INDUCTION MOTOR QUESTIONS FOR PRACTICE

1.                  Explain construction and working principle of a 3-phase indcuiton motor.

2.                  Explain the terms slip, slip frequency, wound rotor and cage rotor.

3.                  Draw and explain the torque-slip & torque-speed charactoristics of a typical 3-phase induction motor. Mark the starting torque and maximum troque on the daigram so drawn.

4.                  Explain  the terms air-fap power P_g, inernal mechanical power developed and shaft power P_sh. How  are these terms related with each others. Hence show that :

P_g :Rotor ohmic loss : P_m =1 :s (1-s)

5.                  A 3-phase induction motor runs at 1140 rpm at full load when supplied with power from a 60 Hz, 3-phase line calculate                                       

                        (i)         No of poles.

                        (ii)        Slip at full load.

(iii)             Frequency of rotor voltage.

(iv)             Speed of rotor field with respect to rotor.

(v)               Speed of rotor field with respect to stator and stator field.

(vi)             Speed of rotor at a slip of 10% and rotor frequency at this speed.

(vii)           If the applied voltage per phase is 230 V, find th0 rotor induced emf at stand still and at 10% slip, with stator to rotor turn ration of 1: 0.5

(Ans: - (i) 1200 rpm, (ii) 5% (iii) 3 Hz (iv) 60 rpm (v) 0 rpm (vi) 1080 rpm, (vii) 11.5 V)

6.                  A 6-pole, 3-phase indcution motor develops 30 hp including 2 hp mechanical losses at a speed of 950 rpm on 550 V, 50 Hz mains. The power factor is 0.88 lagging. Calculate (i) slip (ii) torot copper loss (iii) total input if stator losses are 2 kW (iv) efficiecny (v) line current. (Ans : - (i) 0.05, (ii) 1161 Watts, (iii) 20594 Watts, (iv) 81.64% (v) 30 A)

7.                  Explain construction and principle of operation of 1-phase induction motor.

8.                  Why single phase induciton moto is not self starting ?

      9.          What are the different methods of statring a single phase induction motors and what are                        the different types of 1-phase induction motors ?

Read More