Electrical Machines

Vector group of Transformer

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Introduction

In today’s world we are using single phase transformers as well as three phase transformers, In three phase transformer there are three sets of primary winding as well as three sets of secondary windings one set for each phase. Also we can use three number separate single phase transformers and then interconnect the three single phase transformers externally to work as 3-phase transformer. Before interconnecting two transformers externally one should well aware about vector group of transformer otherwise there are chances of short circuit when transformers gets energized.

There are several ways of connecting primary windings, Most common configurations are follows:

  1. Delta Connections :- When the Polarity end of one winding is connected to the non-polarity end of the next winding then that connections are delta connections.

2. Star Connections :- When all the three polarity ends connected or all three non polarity ends connected together then that configuration is called star connections.

Now we can connect primary winding and secondary winding either in a similar fashion or in different manner i.e. delta-delta, or star-star or delta-star or star-delta.

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When primary and secondary winding connected in a similar fashion i.e. delta-delta or star-star then it is required that secondary voltage waveforms are in phase with the secondary waveforms and this condition is called ” no phase shift “.

But when primary and secondary winding connected in a different manner i.e. delta-star or star-delta, secondary waveforms will differ from corresponding primary voltage waveforms by 30 electrical degrees. This is called 30 degree phase shift.

Winding connection designations

First symbol of vector group indicates High Voltage and always used in capital letters shown :- D= Delta, Y=Star, Z=Interconnected star, N=Neutral

Second symbol of vector group indicates Low Voltage and always used in small letters shown:- d=delta, y=star, z=interconnected star, n=Neutral

Third symbol indicates Phase displacement expressed as the clock hour number (1,6,11)

Let’s take an Example – Dyn11

Here Dyn11 indicates:-

  • First letter (D) indicates that primary winding i.e high voltage of transformer is delta connected.
  • Second letter (y) indicates that secondary of transformer is star connected and (n) indicates the star point brought out neutral
  • Third letter (11) indicates that there is a phase shift of 30 degree leading, As 11 shows position of clock when time on clock is 11’o clock.

How to buy Right Air Conditioner in 2023?

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The summer of 2023 is here and getting hot day by day, Air cooler is good to lower down the room temperature but it can’t remove moisture content from air as it can’t dehumidify the air as a result the room environment is clammy so Air conditioner is best suited for summer.Now the question arises which is right air conditioner to be installed a per your room specifications.

As different sizes of Air conditioner available in market like 0.8 Ton, 1 Ton, 1.5 Ton, 2 Ton, 2.5 Ton, 3 Ton, 3.5 Ton. so we have to choose one particular Ton of AC as per specifications of rooms.

How to calculate AC tonnage for room

The actual method of finding AC tonnage for room is very confusing as it consists of British thermal unit (BTU) 1 Ton equals 12000 BTU. but here we tell you the simple method to find out the AC tonnage for room, firstly we have to find out the volume of room now the question arises how to find out the volume of room which is very simple.

Volume = (Length * Breadth * Height) in cubic feet

Room Design

As shown above this is design showing the room, here l indicates the length of room now you have to simple measure the same length of your room with the help of measuring tape in feets. b donates the breadth and h donates the height of room. so you have to measure all the three dimensions of room and calculate volume in cubic feet.

Once you calculate the volume of room Divide the volume of room by 1000 and you will get AC tonnage required for room size.

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This is not the exact AC tonnage required for room as there are also various other factors affecting the ac capacity required for your room listed below:-

  • Whether Direct sunlight falling on your ceiling (because if you are living on top floor then direct sunlight strikes on your ceiling on the other hand if you are living in ground floor and its a double story house i.e. there are floor above that room then sunlight don’t strikes directly over the ceiling.
  • The place you live is very hot or moderate type climate.
  • The Positioning of windows of room.
  • Number of people residing in the room

The table below shows the right AC tonnage required for your room. While preparing this table we have assumed-

  • 3-4 people residing in the room
  • Average height of room from floor to ceiling is 10 feet.
Area of RoomCooling RequirementRecommended AC Tonnage
100 Sq ft0.9 tons1 ton ac
120 Sq ft1.0 tons1 ton ac
150 Sq ft1.3 tons1.5 ton ac
160 Sq ft1.4 tons1.5 ton ac
170 Sq ft1.5 tons1.5 ton ac
200 Sq ft1.7 tons2 ton ac
250 Sq ft2.1 tons2 ton ac
300 Sq ft2.6 tons2.5 ton ac
400 Sq ft3.4 tons3.5 ton ac

Here we didn’t consider direct exposure of sunlight to ceiling and room walls from south west, so Recommended AC Tonnage can change with these variations.

The Thumbrule for calculating these factors also are as follows-

  1. Add 0.10 ton to cooling requirement, if you are living in hot place.
  2. Add 0.20 ton to cooling requirement, if there is direct sunlight exposure to ceiling of room.
  3. Add 0.10 ton to cooling requirement, if walls of the room are exposed to sunlight from south-west direction.

So If area of your room is 120sq.ft then as per above table cooling requirement is 1.0 ton, but now if there is direct exposure of sunlight to ceiling and wall exposed to south-west direction and also place you live is hot then cooling requirement for room increases from 1.0 ton to 1.4 ton and hence you have to invest in a 1.5 Ton AC.

Now we have explain each aspect of finding out the AC tonnage required for room Now if again you find it difficult to calculate the AC Tonnage then you can use below Calculator to find out the AC Tonnage, you have to just fill the necessary details asked by the calculator and then click on calculate and you will get cooling requirement along with AC Tonnage required for room.

Calculator

I hope you like this article and get complete information regarding AC Tonnage requirement for room, if any queries you can feel free to contact us.

What is the meaning of DYN11 in a transformer?

Electrical Machines / By

Dyn11 is vector group notation of tansformer.It means LV winding,which is star connected (written in small letters means LV side and vice versa)is 30 degrees lagging by HV winding which is delta connected.In India we go for +-30 deg connection of transformer.

D = Delta connection at primary,y = Star connection at secondary and n = neutral point connected at secondary.
Dyn 11 means that the voltage of the secondary star winding lead the primary phase voltage by 30 degree and it corresponds to 11 o’clock.

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The primary is connected in Delta form the secondary in Y or star form the neutral is grounded and the 11 means that the phase angle between the primary and the secondary winding is 30×11=330=-30 degrees. which means that the secondary voltage leads the primary voltage to 30 degrees.

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transformer mcq

Transformer MCQ

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Previous
1. No –load test on a transformer is carried out to determine
  1. Copper loss
  2. Magnetizing current
  3. Magnetizing current and no-load loss
  4. Efficiency of the transformer

Answer: 3

2. The main purpose of performing open –circuit test on a transformer is to measure its
  1. Cu loss
  2. Core loss
  3. Total loss
  4. Insulation resistance

Answer: 2

3. During short –circuit test, the iron lose of a transformer is negligible becauseer is to measure its
  1. The entire input is just sufficient to meet Cu losses only
  2. Flux produced is a small fraction of the normal flux
  3. Iron core becomes fully saturated
  4. Supply frequency is held constanton resistance

Answer: 2

4. In operating a 400 Hz transformer at 50 Hz
  1. Only voltage is reduced in the same proportion as the frequency
  2. Only KVA rating is reduced in the same proportion as the frequency
  3. Both voltage and KVA rating are reduced in the same proportion as the frequency
  4. None of the above

Answer: 2

5.Transformer are rated in KVA instead of KW because
  1. Load power factor is often not known
  2. KVA is fixed whereas KW depends on load p.f.
  3. Total transformer loss depends on volt-ampere
  4. It has become customary

Answer: 3

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Resistance in series and parallel connection

Power System MCQ

6. What is Transformer?
  1. Transformer is a device used to convert low alternating voltage to a high alternating voltage
  2. Transformer is a device used to convert alternating current to direct current
  3. Transformer is a device used to convert low alternating current to a high alternating current
  4. Transformers are used only for low alternating voltage

Answer: 1

7. Transformer ratings are given in _____________
  1. kVA
  2. HP
  3. kVAR
  4. kW

Answer: 1

8. What is the current transformer?
  1. transformer used with an A.C. voltmeter
  2. transformer used with an A.C. ammeter
  3. transformer used with an D.C. voltmeter
  4. transformer used with an D.C. ammeter

Answer: 2

9. Current transformers are _______________
  1. parallel connected type of instrument transformers
  2. series connected type of instrument transformers
  3. parallel connected normal transformers
  4. series-parallel connected type of instrument transformers

Answer: 2

10. Which of the following is not a part of transformer installation?
  1. Breather
  2. Conservator
  3. Exciter
  4. Buchholz relay

Answer: 3

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Transformer MCQ

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Previous
Next
1. A transformer having 1000 primary turns is connected to a 250-V A.C. supply. For a secondary voltage of 400 V, the number of secondary voltage of 400V, the number of secondary turns should be
  1. 1600
  2. 250
  3. 400
  4. 1250

Answer: 1

2. A step –up transformer increases
  1. Voltage
  2. Current
  3. Power
  4. frequency

Answer: 1

3. The primary and secondary windings of an ordinary2-winding transformer always have
  1. Different number of turns
  2. Same size of copper wire
  3. A common magnetic circuit
  4. Separate magnetic circuit

Answer: 3

4. In a transformer, the leakage flux of each winding is proportional to the current in that winding becauseer always have
  1. Ohm’s law applies to magnetic circuits
  2. Leakage paths do not saturate
  3. The two windings are electrically isolated
  4. Mutual flux is confined to the core

Answer: 2

5. In a two-winding transformer, e.m.f. per turn secondary winding is always the induced e.m.f. per turn in primary
  1. Equal to k times
  2. Equal to 1/k times
  3. Equal to
  4. Greater than

Answer: 3

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Why we need Transformer

6. In relation to a transformer, the ratio 20:1 indicates that
  1. There are 20 turns on primary one turn on secondary
  2. Secondary voltage is 1/20thof primary voltage
  3. Primary current is 20 times greater than the secondary current
  4. For every 20 turns on primary, there is one turn on secondary

Answer: 4

7. In performing the short circuit test of a transformer
  1. High voltage side is usually short circuited
  2. Low voltage side is usually short circuited
  3. Any side is short circuited with preference
  4. None of the above

Answer: 2

8. The equivalent resistance of the primary of a transformer having k=5 and R1=0.1 ohm when referred to secondary becomes __ohm.
  1. 0.5
  2. 0.02
  3. 0.004
  4. 2.5

Answer: 4

9. A transformer has negative voltage regulation when its load power factor is
  1. Zero
  2. Unity
  3. Leading
  4. Lagging

Answer: 3

10. The primary reason why open circuit test is performed on the low-voltage winding of the transformer is that it.
  1. Draw sufficiently large no-load current for convenient
  2. Requires least voltage to perform the test
  3. Needs minimum power input
  4. Involves less core less

Answer: 1

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Electrical Machines MCQ

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Previous
1. The normal value of the armature resistance of a D.C motor is
  1. 0.005
  2. 0.5
  3. 10
  4. 100

Answer: 2

2. The Eb/V Ratio of a D.C motor is an indication of its
  1. Efficiency
  2. Speed regulation
  3. Starting torque
  4. Running torque

Answer: 1

3. The mechanical power developed by the armature of a D.C. motor is equal to
  1. Armature current multiplied by back e.m.f.
  2. Power input minus losses
  3. Power output multiplied by efficiency
  4. Power output plus iron losses

Answer: 1

Transformer Installation

4. The induced e.m.f .in the armature conductors of a D.C. motor is
  1. Sinusoidal
  2. Trapezoidal
  3. Rectangular
  4. alternating

Answer: 1

5. D.C motor can be looked upon as D.C. generator with the power flow
  1. Reduced
  2. Reversed
  3. Increased  
  4. Modified

Answer: 2

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Why we need Transformer

6. In a D.C. motor, the mechanical output power actually comes from.
  1. Field system
  2. Air- gap flux
  3. Back e.m.f.
  4. Electrical input power

Answer: 4

7. The maximum torque of D.C. motor is limited by
  1. Commutation
  2. Heating
  3. Speed
  4. Armature current

Answer: 1

8. which of the following quantity maintains the same direction whether D.C. machine runs as a generator or as a motor?
  1. Induced e.m.f
  2. Armature current
  3. Field current         
  4. supply current

Answer: 1

9. Under constant load conditions, the speed of a D.C. motor is affected by
  1. Field flux               
  2. Armature current
  3. Back e.m.f.                     
  4. Both (b) and(c)

Answer: 1

10. It is possible to increases the field flux and, at the same time, increase the speed of a D.C. Motor provided its- is held constant
  1. Applied voltage
  2. Torque
  3. Armature circuit resistance                  
  4. Armature current

Answer: 4

11. The current drawn by a 120-v D.C Motor of armature resistance 0.5 ohm and back e.m.f 110 V is ________ amperer
  1. 20
  2. 240
  3. 220                  
  4. 5

Answer: 1

12. The shaft torque of D.C. motor is less than its armature torque because of _________________ losses
  1. Copper 
  2. Mechanical
  3. Iron                
  4. rotational

Answer: 4

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Electrical Machines MCQ

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Previous
Next
1. The external characteristic of a shunt generator can be obtained directly from its _____________ characterstics.
  1. internal
  2. open-circuit
  3. load saturation
  4. performance

Answer: 2

2. The slight curvature at the lower end of the O.C.C. of a self-excited dc generator is due to
  1. V and Ia
  2. E and Ia
  3. Eo and If
  4. V and If

Answer: 3

3. The voltage build up process of a d.c. generator is
  1. difficult
  2. delayed
  3. cumulative
  4. infinite

Answer: 3

4. For the voltage built up of a self excited d.c. generator, which of the following is not an essential condition?
  1. there must be some residual flux
  2. field winding mmf must aid the residual flux
  3. total field circuit resistance must be less than the critical value
  4. armature speed must be very high

Answer: 4

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5. Which of the following d.c. generator cannot build up on open-circuit?
  1. shunt
  2. series
  3. short shunt
  4. long shunt

Answer: 2

6. If a self excited d.c. generator after being installed, fails to build up on its first trial run, the first thing to do is to
  1. increase the field resistance
  2. check armature insulation
  3. reverse field connections
  4. increase the speed of prime mover

Answer: 3

7. If residual magnetism of a shunt generator is destroyed accidentally, it may be restored by connecting its shunt field
  1. to earth
  2. to an a.c. source
  3. in reverse
  4. to a d.c. source

Answer: 4

8. An ideal d.c. generator is one that has _____________ voltage regulation.
  1. low
  2. zero
  3. positive
  4. negative

Answer: 2

9. The ____________ generator has poorest voltage regulation
  1. series
  2. shunt
  3. compound
  4. over compound

Answer: 1

10.The voltage regulation of an over compound d.c. generator is always ________
  1. positive
  2. negative
  3. zero
  4. high

Answer: 2

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DC Machines MCQ

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Next
1. The armature of DC machine is laminated in order to reduce
  1. hysteresis losses
  2. copper losses
  3. frictional losses
  4. eddy current losses

Answer: 4

2. The yoke of a DC machine is generally made of
  1. silicon steel
  2. cast steel
  3. soft iron
  4. cast steel

Answer: 4

3. If field current is decreased in shunt dc motor, the speed of the motor
  1. increases
  2. decreases
  3. remain same
  4. none of the above

Answer: 1
As a shunt field current If decreases, φ also decreases and the speed rises as speed is inversely proportional to flux

4. In Ward-Leonard system, the lower limit of the speed imposed by
  1. armature resistance
  2. field resistance
  3. residual magnetism of the generator
  4. none of the above

Answer: 3
In Ward-Leonard method of speed control, the lower limit of speed is imposed by residual magnetism of the generator.

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Why we need Transformer

5. Ward-Leonard control is basically a _________________________ control method.
  1. armature resistance control
  2. field control
  3. armature voltage control
  4. field diverter control

Answer: 3
Ward-Leonard speed control method consists a motor with a constant excitation and applying a variable voltage to its armature to provide the required speed. Hence it is armature voltage control method.

6. In a DC Machine, the current flow in a armature circuit is
  1. pulsating DC
  2. pure DC
  3. AC
  4. none of the above

Answer: 3

7. As the armature reaction effect increases, the main flux of DC machine is
  1. increases
  2. decreases
  3. remain same
  4. none of the above

Answer: 2

8. In DC machines, slot wedges are generally made of
  1. cotton
  2. fibre
  3. impregnated paper
  4. silicon steel

Answer: 2

9. Eddy current loss will depends on
  1. flux density
  2. frequency
  3. thickness
  4. all of the above

Answer: 4

10. Hysteresis loss will depends on
  1. f
  2. f²
  3. f³
  4. f1.6

Answer: 1

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Why we need Transformer?

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Since Transformer converts electricity from one voltage level to another voltage level as per our requirement, So Transformer is used for electricity transmission purpose.

Hydro Power Plant

Electricity is generated at Power stations either Hydro Power stations or Thermal Power stations by moving Turbine with water and steam respectively.

Thermal Power Plant
Turbine Generator Mechanism

Both types of Power stations are situated very far away around 1000/1200 KM from residential area because Hydro Power stations are situated at hilly areas where dams constructed over river and water stored in the reservoir for turbine running purpose. Since Dams are constructed in-between mountains which are far away from cities, Also Thermal Power stations are situated near the coal mines where coal is easily available and less transportation of coal required so cost reduces since coal mines are available far from cities

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Electric Power

Coal Mines for Thermal Power Plant Steam Generation

 So Thermal Power stations are also situated far away from cities. Now Electricity is generated in Power stations but we have to use this electricity in houses, shops, Industries which is around 1000/1200 KM away from Power stations, So for that we have to transmit electricity through wires. Since Generating Voltage is 11KV

and if we transmit 11KV from Power stations then due to large current heavy transmission losses takes place and very small amount of electricity reaches at consumers end and maximum part get wasted in form of transmission loss by joules law of heating i.e I²R losses. Now only path to transmit electricity is by lower down the current which is only possible by raising the voltage level, since Voltage is inversely proportional to Current i.e. if Voltage is high then current is low and vice-versa. So we have to raise the voltage levels to higher voltages like 132/220/400/765 KV which is only possible with the help of Transformer.

Transmission Towers

So at Power stations Power Transformer/Step UP Transformer are used which converts low voltage i.e 11 KV into high voltages i.e. 132/220/400/765 KV as per requirement. Then Electricity is transmitted at these voltage levels from Power stations to Sub-stations with the help of large Transmission Towers,which is situated in the vicinity of cities. Due to high voltages transmission losses are almost negligible and maximum electricity reaches at substation. 

Step Down Transformer at Substation

After that at Substation Voltage levels are lower down with the help of again Transformer’s but at sub-stations Step Down Transformers are used where Higher voltages 132/220/400/765 KV gets converted to 33 KV which is also high voltage but used for 30-40 km transmission purpose.

Then this 33 KV Voltage is transmitted to 33 KV Sub-station which is situated in between the city or residential area.Now at 33 KV Sub-station Again Step-Down Transformer is used to convert 33 KV into 11 KV and this voltage is transmitted to Distribution Transformer’s which is installed on H pole structure’s you have already seen near by your houses.

Distribution Transformer

Distribution Transformers are also step down transformers which converts 11 KV into 440 V for three phase and 220 V for single phase which is also called LT supply. Then this voltage is transferred to houses by LT lines and service wires, since Voltage requirement of devices used at our houses is 220 V so LT supply i.e. 220 V is supplied to houses.

So Transformer plays a major role in Electricity transmission and we can’t imagine Electricity transmission without Transformer’s. Also Transformer is very rugged and less maintenance required equipment as there is no moving part in it. It is also called static device.

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Transformer MCQ

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Previous
Next
1. When the iron losses of a distribution transformer decreases, then its all day efficiency
  1. increases
  2. decreases
  3. unaffected
  4. none of the above

Answer: 1

2. The main purpose of using core in a transformer is to
  1. decrease iron losses.
  2. decrease reluctance of the common magnetic circuit.  
  3. eliminate magnetic hysteresis
  4. prevent eddy current loss.

Answer: 2

3. A transformer has hysteresis loss of 30 W, at 240 V, 60 Hz. The hysteresis loss at 200 V, 50 Hz will be
  1. 28 W
  2. 25 W
  3. 30 W
  4. 36 W

Answer: 2

4. The functions of using stepped core in transformer is to reduce
  1. eddy current losses
  2. hysteresis losses
  3. volume of copper
  4. reluctance of core

Answer: 3

5. Which part of the transformer is most affected by the overheating?
  1. winding insulation
  2. winding of transformer
  3. transformer core
  4.  transformer tank

Answer: 1

6. When the transformer is loaded then the secondary terminal voltage will fall for
  1. leading power factor.
  2. lagging power factor.
  3. unity power factor.
  4.  lagging and leading power factor.

Answer: 2

7. The full load copper loss and iron loss of transformer are 6400W and 5000W respectively. The copper loss and iron loss at half load will be respectively?
  1. 3200 W and 2500 W
  2. 3200 W and 5200 W
  3. 1600 W and 1250 W
  4. 1600 W and 5000 W

Answer: 4
Explanation : Iron losses do not depend on the load, iron losses remain constant for any load. Therefore iron losses are considered as constant losses. Copper losses vary as square of load current and these are considered as variable losses.

8. In a star-delta transformer, the delta side phase voltage leads the star sides phase voltage by an angle
  1. + 30°.
  2. + 45°.
  3. – 30°.
  4. – 45°.

Answer: 1

9. The chemical used in breather of a transformer should have property of
  1. absorbing moisture.
  2. ionizing air.
  3. absorbing heat.
  4. cleaning oil.

Answer: 1

10. Minimum voltage regulation occurs when the power factor of the load is
  1. lagging
  2. leading
  3. unity
  4. zero

Answer: 2