Month: January 2022

electrical machines mcq

Electrical Machines MCQ

Electrical Machines / By
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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

Also Read

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Transformer MCQ

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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electrical machines mcq

DC Machines MCQ

Electrical Machines / By
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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.

Also Read

Resistance in series/parallel connection

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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Resistance in Series/Parallel Connection

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Resistance in Series:-

More than one resistances can be connected either in series or in parallel connections. Now if we connect two resistances end on end then they are said to be connected in series.  Let’s take an example: Consider three resistances R1 and R2 be connected in series as shown below:

Series Connection

Now Equivalent resistance of this circuit equals to the sum of individual resistances,let’s prove it now:
In series circuit
(1) Same current flows through each resistance.
(2) Voltage drop across each resistance is different.
(3) Net voltage drop applied across circuit equals the sum of all voltage drops across each resistance

Therefore Vs= V1 + V2 = IR1 + IR2

Since Vs= IR 
(R is the equivalent resistance of circuit)

Then      IR = IR1 + IR2
              IR = I (R1+R2)
Cancelling out I from both sides we get

                  R = R1 + R2

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Series & Parallel connections of resistor

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Resistances in Parallel:-

As shown In below figure Resistances are said to be connected in Parallel connection.

In parallel circuit 
(1) Potential difference across all resistances is same.
(2) Current flowing through each resistance is different.
(3) Net current flowing through circuit is sum of three currents.


             I = t1 + t2 + t3  = V/r1 + V/r2 + V/r3

             Also I = V/R
where V is the voltage applied and R is the equivalent resistance of circuit

              So V/R =  V/r1 + V/r2 + V/r3

                    1/R = 1/r1 + 1/r2 + 1/r3

Also read:

Transformer MCQ

Why we need Transformer

What is Electrical Resistance? Definition and Units.

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Resistance is the property of a substance due to which it opposes the flow of electrons (current) through it.

It varies from substance to substance, some substance have low resistance like class A Metals like Silver, Copper and Aluminium. These metals offers very less resistance to flow of electricity through them when we applied potential difference(voltage) across them, so these metals are called good conductors of Electricity.

                                    Those substances which offer relatively greater resistance or hindrance to the flow of electrons through them are called poor conductor or one can say Insulators of electricity like bakelite, Mica, glass, rubber P.V.C. (polyvinyl chloride) and wood etc. These substances are bad conductor of electricity and does allow electric charge to flow through them.

Resistance is donated by symbol R.

Units of Resistance   

SI Unit of Resistance is ohm.

A conductor contain one ohm resistance if it permits one ampere current to flow through it when one volt of potential difference is applied across it.

Bigger units of Resistance are Mega-ohm = 10⁶ ohm , Kilo-ohm = 10³ohm used for insulators having high resistance.

Smaller Units are Milli-ohm = 10ˉ³ohm and Micro-ohm=10ˉ⁶ ohm used for good conductors having low resistance.

Symbol used to donate ohm is Ω.

Also Read:

Electrical Energy

Resistance Laws:-

Resistance R offered by a conductor depends on:

  1. R is directly proportional to length L of conductor.
  2. R is inversely proportional to cross sectional area A of the conductor.
  3. R depends on the nature of material.
  4. R depends on the nature of material.
  5. R also depends on the temperature of conductor.

Neglecting last two factor we get:-

R = ρΙ/A

Where ρ is constant called specific resistance or resistivity.

l is length of conductor.

A is cross sectional area of conductor.

If length l =1metre and area A= 1 metre² then R =ρ

Units of Resistivity

Since R = ρΙ/A

Then from above equation

ρ=AR/l = A metre² * R ohm/ l metre

ρ = ohm-metre

Hence Unit of resistivity is ohm-metre (Ω-m)

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Electrical Energy

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What is Electric Energy? Definition & Example

Definition

Electric Energy is the product of Electric Power taken by the device and the time period.

Equation

Electric Energy = Electric Power(Watt) * Time(Hour)

Units

It is measured in Watt-Hour. As Electric Power is measured in Watt and Time is measured in Hour, So Product of Power(Watt) and Time(Hour) is Watt-Hour.

1 Watt-Hour equals to 1 Watt of Power released for 1 Hour. In terms of Joules 1 Watt-Hour equals to 3600 Joules (3.6*10³ Joules) of Energy. Mostly Joule is used as unit of Energy Except Electrical Energy.


Used to Measure Electricity Bill 

Since In Asia Electricity Bill is generated by computing No. of Units consumed by the consumer, then multiplying the same with the tariff or rate of one Unit. Unit is calculated with the help of Electric Energy Meter installed at the premises of Consumer.

Let’s take an Example

Consider A consumer consumes 100 Units of Energy. The tariff rate at their area is 5 rupees per unit. What is their Electricity Bill?

Solution : Here No. of Units = 100

                 Rate of one Unit   = 5

                 Electricity Bill = 100*5 = 500 rupees Answer

Now the Question rises How much one Unit Equals to?

So The answer is One Unit equals to 1 KWH  which is unit of Electric Energy, 

One KWH equals to 1 KW of Power drawn by the device during one hour.

Also Read:

Basic Electrical MCQ

Why we need Transformer?


Example

If A 100 W Bulb glows for 10 Hours then How much Units does it Consume?

Solution: Here Power = 100 W

                         Time  = 10 H

So Electrical Energy  = 100*10 = 1000 W-H

1000W-H = 1 KWH = 1 Unit

So If A 100 W Bulb glows or 10 Hours then it consumes 1 Unit, Similarly if it glows for 20 Hours then it consumes 2 Unit.

In a similar fashion we can Calculate Units consumed by different devices used at home and then calculate our Electricity Bill. 

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power system mcq

Electric Power

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Electric power is basically amount of Electric Energy used in a particular span of time. It is measured in watts.

Electric Power (watts) = Electrical Energy (Joule)/Time (Second)

Ø  If a device consume 1 Joule of energy in one second then power is 1 Watt.

As Power = 1 (joule)/1 (second) =1 Watt

So it’s clear Power is measure of energy per unit time like a second, it’s better to say Watt rather than joule/second.

Another definition:

Electric Power is product of Voltage (V) supplied to a device and Current(A) drawn by the device.

Electric Power (watts) = Voltage * Current

P = V*I

Let’s take an Example:-

·         Consider a Motor is feed with 220V supply and it draws 10A of load then what is Power of Motor?

Solution: Here Voltage (V) = 220V

                            Current (I) = 10A

                So Power = V*I

                                   = 220*10   = 2200 Watt

Since 1 KW = 1000W

Then 2200 Watt = 2200/1000 = 2.2KW

So Motor Power= 2.2KW Answer

Also Read

Why we need Transformer?

Transformer MCQ

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transformer mcq

Why we need Transformer?

Electrical Machines / By

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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Transformer MCQ

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

Electrical Machines / By
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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

Transformer MCQ

Electrical Machines / By
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1. In a transformer zero voltage regulation is achieved at a load power factor which is
  1. leading
  2. lagging
  3. unity
  4. zero

Answer: 1
Explanation : At leading power factor the voltage regulation can be negative or zero. This can be found from this equation % regulation = εxcosθ – εrsinθ

2. What is the magnitude of mutually induced emf, E2 in a transformer?
  1. directly proportional to rate of change of flux and number of secondary turns
  2. inversely proportional to rate of change of flux and number of secondary turns
  3. proportional to rate of change of flux and inversely proportional to number of secondary turns
  4. inversely proportional to rate of change of flux and proportional to number of secondary turns

Answer: 1
Explanation: Due to the supply voltage, a secondary induced emf is produced. V1 is in antiphase with E2 and the magnitude of E2 is proportional to rate of change of flux and number of secondary turns.

3. Which of these will happen in a transformer when the number of secondary turns is less than the number of primary turns?
  1. The voltage gets stepped up
  2. The voltage gets stepped down
  3. The power gets stepped up
  4. The power gets stepped down

Answer: 2
Explanation: According to the fundamentals of electrical engineering, the voltage transformation ratio is the ratio of the number of secondary turns to that of the number of primary turns. When the transformation ratio is less than 1, the step-down operation occurs.

4. Which of the following Transformer can have tap changer?
  1. Power transformer.
  2. Earthing transformer.
  3. Current transformer
  4. Potential transformer

Answer: 1

5. The constant losses in transformer is/are
  1. eddy current loss.
  2. hysteresis loss.
  3. copper loss.
  4. both 1 & 2

Answer: 4
Explanation: Eddy current loss and hysteresis loss are almost independent of load, significantly depending on supply voltage and frequency. As the flux density or flux is constant for a given voltage and frequency, eddy current loss and hysteresis loss remain constant at any load. Therefore, these losses are called constant losses. Copper loss varies as the square of load current and called variable loss.

6. The efficiency of two identical transformers under loaded conditions can be determined by
  1.  open circuit test.
  2. short-circuit test.
  3. back-to-back test.
  4. any of the above.

Answer: 3

7. What is the number of primary turns in a 200/1000 V transformer if the emf per turn is 10V?
  1.  5
  2. 10
  3. 20
  4. 40

Answer: 3
Explanation: According to the electrical engineering fundamentals, the number of primary turns is the ratio of induced emf in the primary to the emf induced per turn. Thus, the number of primary turns in a 200/1000 V transformer with emf per turn of 10V is 200/10 = 20.

8. What is the equivalent resistance of the primary of a transformer with a voltage transformation ratio of 10 and primary resistance of 0.05Ω when referred to secondary?
  1.  0.005
  2. 0.5
  3. 5
  4. 50

Answer: 3
Explanation: According to fundamentals of electrical engineering, the equivalent resistance of the primary of a transformer with a voltage transformation ratio of K and primary resistance of RΩ when referred to secondary is equal to K2*R. Thus, the answer will be 102*0.05 = 5.

9. In a transformer, hysteresis and eddy current losses depend upon
  1.  load current
  2. maximum flux density
  3. supply frequency
  4. both 2 and 3

Answer: 4

10. In a transformer operating at constant voltage if the input frequency increases, the core
  1.  increase
  2. decrease
  3. remains constant
  4. none of the above

Answer: 2

Basic Electrical MCQ

Basic Electrical, Uncategorized / By
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1. An electric current is the

  1. storage of charge
  2. flow of electrons
  3. ionization of atom
  4. opposition to electrons

Answer: 2

2. The dielectric material used in variable capacitor is generally

  1. air
  2. mica
  3. ceramic
  4. electrolyte

Answer: 1

3. How many electrons will constitute 2 Coulombs of charge?

  1. 6.24 * 1018 electrons
  2. 12.48 * 1018 electrons
  3. 1.602 * 1019 electrons
  4. 3.204 * 1019 electrons

Answer: 2
Explanation: One Coulomb of charge consists of 1/(1.602*10-19) electron that is 6.24 * 1018 electrons. A coulomb is a unit for the charge. Thus, for 2 Coulombs of charge will have 6.24 * 1018 * 2 = 12.48 * 1018 electrons.

4. What is responsible for the current to flow?

  1. Protons
  2. Electrons
  3. Nucleus
  4. Protons and Electrons

Answer: 2
Explanation: For the current to flow in a circuit electrons are required. Electrons are negatively charged and when the potential difference is applied these electrons flow to constitute a current. The current direction is opposite to the electron flow.

5. Which of the following type of circuits in electrical engineering cannot be analyzed using Ohm’s law?

  1. Unilateral
  2. Bilateral
  3. Linear
  4. Conductors

Answer: 1
Explanation: Ohm’s law cannot be used for analyzing unilateral networks as such networks only allow current flow in one direction. A unilateral network can consist diode, transistor, etc.

6. Which of the following is correct about direct current?

  1. Frequency is zero
  2. Can be transported to larger distances with less loss in power
  3. Flows in one direction
  4. Magnitude is constan

Answer: 1
Explanation: A direct current has a fixed value and does not change with time. The frequency of the direct current is equal to zero as it does not change with time.

7. A capacitor consists of two

  1. insulation separated by a dielectric
  2. ceramic plates and one mica disc
  3. silver-coated insulators
  4. conductors separated by an insulator

Answer: 4

8. In a cable capacitor, voltage gradient is maximum at the surface       of the

  1. sheath
  2. earth
  3. conductor
  4. insulator

Answer: 3

9. The capacitance of a cable capacitor depends on

  1. core diameter
  2. ratio of cylinder radii
  3. potential difference
  4. insulation thickness

Answer: 2

10. The time constant of an R-C circuit is defined as the time during     which capacitor charging current becomes ——— percent of its        —— value.

  1. 37, initial
  2. 37, final
  3. 63, initial
  4. 63, final

Answer: 1