‫الدليل االسترشادي لطلبت الدبلىم الفني الصحي لشعبت صيانت االجهزة الطبيت‬
8102/8102 ‫مادة الحساساث و محىالث الطاقت – الفرقت الثانيت – الفصل الدراسي الثاني لعام‬
I) Choose the correct answer:
1) Ferromagnetic materials are …. Of the following :
a) Iron, nickel, and cobalt
b) Copper, lead and silver
c) Aluminum, gold, and tin
2) In electric circuit the current is due to ….
a) Magnetomotive force
b) Electromotive force
c) Electromagnetic force
3) Magnetic field is produced around a conductor when it carries ….
a) An electric flux.
b) An electric current.
c) Both of them.
4) Magnesium, molybdenum, lithium and tantalum are ….
a) Ferromagnetic
b) Diamagnetic
c) Paramagnetic
5) Magnetic flux in magnetic circuit is due to ….
a) Magnetomotive force
b) Electromagnetic force
c) Electromotive force
6) A hysteresis loop shows the relationship between ….
‫الدليل االسترشادي لطلبت الدبلىم الفني الصحي لشعبت صيانت االجهزة الطبيت‬
8102/8102 ‫مادة الحساساث و محىالث الطاقت – الفرقت الثانيت – الفصل الدراسي الثاني لعام‬
a) B and H
b) ɸ and A
c) N and I
7) The EMF is equivalent to ….
a) the flux density
b) the voltage drop
c) the current
8) Tesla is defined as :
a) Wb/(m2)
b) AT/Wb
c) AT
9) Eddy currents and hysteresis can be greatly reduced by :
a) Having a solid iron core
b) Laminating the magnetic path
c) None of them
10) The current can be represented by ….
a) EMF/R
b) E/R
c) Both of them
11) In the core form transformer, the primary and secondary windings are
wound ….
a) Outside and surround the core ring.
b) Pass inside the steel magnetic core.
‫الدليل االسترشادي لطلبت الدبلىم الفني الصحي لشعبت صيانت االجهزة الطبيت‬
8102/8102 ‫مادة الحساساث و محىالث الطاقت – الفرقت الثانيت – الفصل الدراسي الثاني لعام‬
c) Both of them.
12) In the shell form transformer, the primary and secondary windings are
wound ….
a) Outside and surround the core ring.
b) Pass inside the steel magnetic core.
c) Both of them.
13) In both types: core form transformer and shell form transformer.
a) There is no loss of magnetic flux linking the primary and secondary
windings.
b) There are big losses of magnetic flux linking the primary and secondary
windings.
c) There are no flux in the core.
14) If the secondary voltage is less than the primary voltage, we will have ….
a) Step-up transformer.
b) Step-down transformer.
c) None of the previous.
15) Helical windings and screw windings are ….
a) The same as each other.
b) The same as pancake.
c) Different from each other.
16) Dots are used to ….
a) Identify a given end of each winding.
b) Identify a given end for one coil and the start for the other.
‫الدليل االسترشادي لطلبت الدبلىم الفني الصحي لشعبت صيانت االجهزة الطبيت‬
8102/8102 ‫مادة الحساساث و محىالث الطاقت – الفرقت الثانيت – الفصل الدراسي الثاني لعام‬
c) Identify both.
17) Tap changers are used to :
a) Change the turns ratio.
b) Change the voltage ratio.
c) Both of them.
18) The losses of the transformer are :
a) Copper losses and iron loses.
b) Copper losses and hysteresis.
c) Copper losses and eddy currents.
19) The losses of energy due to iron losses :
a) Appear as motion.
b) Appear as heat.
c) Both of them.
20) Copper losses are mainly due to :
a) Electrical resistance of the primary windings.
b) Electrical resistance of the secondary windings.
c) The sum of both of them.
21) For ideal transformer :
a) V/N, NI are constant, and ɸ1 = ɸ2
b) V1 = V2, I1 = I2
c) ɸ1 > ɸ2
22) Copper losses are proportional to :
‫الدليل االسترشادي لطلبت الدبلىم الفني الصحي لشعبت صيانت االجهزة الطبيت‬
8102/8102 ‫مادة الحساساث و محىالث الطاقت – الفرقت الثانيت – الفصل الدراسي الثاني لعام‬
a) Voltage squared.
b) Current squared.
c) Iron losses.
23) Iron losses are :
a) Proportional to voltage.
b) Proportional to copper losses.
c) Constant.
24) When the transformer is loaded :
a) V2 = E2
b) V2 > E2
c) V2 < E2
25) For parallel connected secondary transformer, the secondary current is the
….
a) Sum of the secondary windings currents.
b) Difference of the secondary windings currents.
c) Same of the secondary windings currents.
26) The phase time between balanced three-phase transformer:
a) 120 o
b) 180 o
c) 360 o
27) The heat rise in transformers is caused by losses :
a) Causes damages in paper insulation.
b) Causes damages in liquid insulation medium.
‫الدليل االسترشادي لطلبت الدبلىم الفني الصحي لشعبت صيانت االجهزة الطبيت‬
8102/8102 ‫مادة الحساساث و محىالث الطاقت – الفرقت الثانيت – الفصل الدراسي الثاني لعام‬
c) Both of them
28) For oil-immersed transformer of capacity > 30 MVA , we can select the
cooling method:
a) Oil forced – Water forced.
b) Oil natural – Air forced.
c) Both of them
29) For transformers rated more than 3 MVA cooling by natural air method is
….
a) Enough.
b) Inadequate.
c) None of them.
30) Instrument transformers transform currents or voltages from :
a) Low values to higher values
b) High values to lower values
c) High values to 0 (zero)
II) Define
Magnetic flux –Magnetic flux density – MMF – EMF – Reluctance – Laminations –
Lenz’s Law – Diamagnetic – Paramagnetic – The magnetic force – Step up
transformer – Step down -transformer – Closed-core type transformer – Shell form
transformer – Ideality assumption – Regulation – Multiple winding transformer –
Three phase transformer bank – Thermal -conductivity – Convection – Radiation –
Advection – Cooling of transformer – Instrument
‫الدليل االسترشادي لطلبت الدبلىم الفني الصحي لشعبت صيانت االجهزة الطبيت‬
8102/8102 ‫مادة الحساساث و محىالث الطاقت – الفرقت الثانيت – الفصل الدراسي الثاني لعام‬
III) Complete
1) The hysteresis losses are in the form of …. .
2) EMF will force …. to flow in the circuit, MMF will force …. to flow in a magnetic
circuit.
3) If two magnets are arranged so that the like poles are near to each other, then ….
4) Current is measured by …. and magnetic flux is measured by …. .
5) When a conductor is moved across a magnetostatic field …. is produced in the
conductor.
6) A hysteresis loop shows the relationship between …. and …. .
7) Ferromagnetic materials get their magnetic properties because their atoms carry
…. and
the material is made up of …. .
8) Paramagnetic materials …. attracted by a magnetic field.
9) …. materials are large and ….. susceptibility.
10) …. materials are ….. and negative susceptibility.
11) In the step-up transformer the number of turns in the primary is …. than the
number of
turns of the secondary.
12) Transformer windings and coils can be classified into …. and …. .
13) Iron losses are …. and …. .
14) Copper losses are mainly due to the electrical resistance of …. and …. .
‫الدليل االسترشادي لطلبت الدبلىم الفني الصحي لشعبت صيانت االجهزة الطبيت‬
8102/8102 ‫مادة الحساساث و محىالث الطاقت – الفرقت الثانيت – الفصل الدراسي الثاني لعام‬
15) N1 / …. = …. / V2 = …. / I1 , whereas N1 is …. , ….. is ….. , V2 is …. , …. is ….
, I1 is …. , …. is …. .
16) The ideality assumption indicates that :
a- ….. b- ….. c- ….
17) Copper losses at half full load = full load copper losses * …. .
18) For series connected secondary transformer, the secondary terminal voltage is the
…. of
the two individual wind voltages.
19) With the three-phase transformer, the magnetic flux’s differ in phase by …. timedegrees.
20) The current in each phase of a star-connected group of transformers is …. , then
IL …. LS .
21) There are three cooling methods available for transformers …. , …. and …. .
22) The difference between the transformer’s input and output power is converted into
….
by …. losses and …. losses .
23) If the transformer’s heat losses is not dissipated properly the temperature will ….
which
damages …. and …. .
24) For cooling dry-type transformers , we use : …. or …. .
25) The current transformer consists of …. or …. Turns as its primary winding.
‫الدليل االسترشادي لطلبت الدبلىم الفني الصحي لشعبت صيانت االجهزة الطبيت‬
8102/8102 ‫مادة الحساساث و محىالث الطاقت – الفرقت الثانيت – الفصل الدراسي الثاني لعام‬
IV)
A) Draw the hysteresis loop of a ferromagnetic material.
B) A circular wooden ring of mean diameter 20 cm has a coil of 800 turns
uniformly wound around it. If the magnetic field strength is 5000 A/m, calculate
the current in the coil.
C) Draw the step-up transformer and step-down transformer showing the
relative number of each.
D) A 5 kVA single-phase transformer has a turns ration of 10:1 and is fed from a
2 kV supply.
Neglecting losses, determine:
1) The full load secondary current.
2) The minimum load resistance which can be connected across the secondary
winding
to give full load kVA.
3) The primary current of full load kVA.
E) A 400 kVA transformer has a primary winding resistance of 0.5 Ω and a
secondary winding resistance of 0.001 Ω. The iron loss is 2.5 kW and the primary
and secondary voltages are 5 kV, and 320 V respectively. If the power of actor of
the load is 0.85 , determine the efficiency of the transformer :
a) On full load. b) On half load.
F) Draw three phase transformer Δ connection.
G) Draw three phase transformer Y connection.
‫الدليل االسترشادي لطلبت الدبلىم الفني الصحي لشعبت صيانت االجهزة الطبيت‬
8102/8102 ‫مادة الحساساث و محىالث الطاقت – الفرقت الثانيت – الفصل الدراسي الثاني لعام‬
H) The primary winding of a delta-star (Δ-Y) connected 50 VA transformer is
supplied with a 100 volt, 50 Hz three-phase supply. If the transformer has 500
turns on the primary and 100 turns on the secondary winding. Calculate the
secondary side voltages and currents.
V) Truth or false
1) The hysteresis losses will be in the form of energy.
2) Eddy current and hysteresis can be eliminated completely.
3) Total amount of energy will remain constant as energy cannot be created nor
destroyed.
4) Magnetic fields can be static or time varying.
5) Permeability of free space is equal to 4π * (10-7) wb/mAT.
6) An electric field is produced around a conductor carries electric current.
7) The sum of the product of (HL) is numerically equal to the resistance of the
conductor.
8) Diamagnetic materials are large and positive susceptibility.
9) Atoms are composed of protons, neutrons, and electrons.
10) The frequency of a magnetic field is the same of the source current that produced
the field.
11) Dots are used to identify a given end of each winding.
12) Iron losses and copper losses are the same.
13) Cooling of a transformer is important to save the insulating materials life time.
14) The regulation indicates that when the transformer is loaded, E2 ≠ V2.
‫الدليل االسترشادي لطلبت الدبلىم الفني الصحي لشعبت صيانت االجهزة الطبيت‬
8102/8102 ‫مادة الحساساث و محىالث الطاقت – الفرقت الثانيت – الفصل الدراسي الثاني لعام‬
15) For series connected secondary transformer, the secondary current is the same
flowing through the secondary windings.
16) For parallel connected secondary transformer, the secondary voltage is the same
for the secondary windings.
17) In a delta-connected group of transformers VL = VS .
18) The air as a cooland provides better conduction than the oil
19) The transformer’s input power is less than its output power.
20) Instrument transformers transform currents or voltages from high values to lower
values.