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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GENERATION OF HIGH VOLTAGES | AC AND DC VOLTAGE GENERATION

HVDC is used for testing HVAC cables of long lengths as these have very large capacitance and would  require very large values of currents if tested on HVAC voltages. Even though D.C. tests on A.C cables is convenient and economical, these suffer from the fact that the stress distribution within the insulating material is different from the normal operating condition.

In industry it is being used for  electrostatic precipitation of ashing in thermal power plants, electrostatic painting, cement industry, communication systems etc. HVDC is also being used extensively in physics for particle acceleration  and in medical equipment's (X-Rays).

The most efficient method of generating high D.C. voltages is through the process of rectification employing voltage multiplier circuits. Electrostatic generators have also been used for generating

high D.C. voltages.

According to IEEE standards 4-1978, the value of a direct test voltage is defined by its arithmetic mean value   and is expressed mathematically as 

where T  is the  time period  of the voltage wave having a frequency f = 1/ T.  Test voltages generated using rectifiers are never constant in magnitude. These deviate from the mean value periodically and this  deviation is known as  ripple . The magnitude of the ripple voltage denoted by  δV  is defined as half the difference between the maximum and minimum values of voltage  i.e.,

and ripple factor is defined as the ratio of ripple magnitude to the mean value  V d

  i.e.,   δV/V d .  The  test voltages should not have ripple factor more than 5% or as specified in a specific standard for a particu-lar equipment as the requirement on voltage shape may differ for different applications.

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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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