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. 

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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.”

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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.

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WHAT IS THE FREQUENCY OF DC VOLTAGE.

What is the frequency of DC voltage?
   > Due to the graphically straight line, DC voltage has 'NO' frequency but if this voltage is pure DC.
      Unpure frequency has 1.2Hz frequency.

What we do to do DC voltage pure?
   >We use Capacitor for filter the dc voltages for pure dc voltage with 'NO' frequency.

Your answer will be below. Comment below what you think about this.

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ELECTRICAL SWITCHBOARDS STANDARD TESTING PROCEDURE

Electrical Switchgear or Switchboard standard testing procedures step by step guide. These standard testing procedures are applying at Siemens Pakistan Co. as our knowledge (not confirm) also these are according to International Electromechanical Commission IEC and ISO standard too.

All are not in details here but just names were given, for hints just. After this, you have to research it on the Internet for more details and for your knowledge on YouTube or Google. You will find more details easily when you search it by names.
Find the testing procedure names from above header pages of How Electrical blog where new beginners learn easily. Or you can direct go to Electrical switchboard testing page where you will get all these testing procedures easily by Click here and Watch this video #Siemens Switchgear.

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