# DC Machines

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This is a short quiz on DC machines fundamentals

• 1.

### Winding used for low voltage high current

• A.

Lap winding

• B.

Wave Winding

• C.

Gram ring winding

• D.

Open winding

C. Gram ring winding
Explanation
Gram ring winding is employed for low-voltage, high-current applications, such as in direct current (DC) motors or generators. It involves winding conductors in multiple layers around a cylindrical core, ensuring efficient current distribution and reduced resistance to accommodate high currents while maintaining low voltage operation.

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• 2.

### No. of parallel paths in lap winding=_________

• A.

Number of poles

• B.

2

• C.

4

• D.

6

A. Number of poles
Explanation
The number of parallel paths in a lap winding is equal to the number of poles in the machine. This means that for every pole in the machine, there will be a corresponding parallel path in the lap winding. Therefore, the correct answer is "number of poles".

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• 3.

### In long shunt compound generator Ise = ____________

• A.

Ia-Ish

• B.

IL

• C.

Ia

• D.

Ia+Ish

C. Ia
Explanation
In a long shunt compound generator, the external shunt field current (Ish) and the armature current (Ia) are connected in parallel. Therefore, the total current passing through the external shunt field (Ise) is equal to the armature current (Ia). Hence, the correct answer is Ia.

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• 4.

### Generated emf is __________ speed of prime mover

• A.

Directly proportional to

• B.

Inversely proportional to

• C.

Equal to

• D.

Less than

A. Directly proportional to
Explanation
The correct answer is "Directly proportional to." This means that the generated electromotive force (emf) is directly related to the speed of the prime mover. As the speed of the prime mover increases, the emf also increases, and vice versa. This suggests that there is a linear relationship between the two variables, where a change in one variable will result in a corresponding change in the other variable in the same direction.

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• 5.

### For a 4 pole lap wound DC generator with 32 armature slots, average pitch is

• A.

2

• B.

4

• C.

8

• D.

16

C. 8
Explanation
The average pitch of a lap wound DC generator is calculated by dividing the total number of armature slots by the number of poles. In this case, the generator has 32 armature slots and 4 poles. Therefore, the average pitch would be 32/4 = 8.

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• 6.

### ________ compound generator is used for arc welding

• A.

Long shunt

• B.

Short shunt

• C.

Differential

• D.

Commulative

C. Differential
Explanation
A differential compound generator is used for arc welding because it provides a constant voltage output regardless of the load variations. This is important in arc welding as the arc length and the material being welded can cause significant fluctuations in the load. The differential compound generator has both series and shunt field windings, which work together to regulate the voltage output and ensure a stable welding process.

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• 7.

### % efficiency = ___________

• A.

(Po/Pi)x100

• B.

(Po/Po+Losses)x100

• C.

Both a & b

• D.

None of above

C. Both a & b
Explanation
The correct answer is "Both a & b." This is because the formula for efficiency can be expressed as both (Po/Pi)x100 and (Po/Po+Losses)x100. The first formula calculates efficiency by dividing the output power (Po) by the input power (Pi) and multiplying by 100. The second formula calculates efficiency by dividing the output power (Po) by the sum of the output power and losses (Po+Losses) and multiplying by 100. Therefore, both formulas are valid and can be used to calculate efficiency.

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• 8.

### If VN > VL , Voltage regulation is _________

• A.

Negative

• B.

Positive

• C.

Zero

• D.

1

B. Positive
Explanation
If VN is greater than VL, it means that the actual voltage (VN) is higher than the desired voltage (VL). In this case, the voltage regulation is positive because the actual voltage is above the desired voltage, indicating an overvoltage condition.

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• 9.

### Power P (in watts) = __________

• A.

BHPx746

• B.

BHP/746

• C.

BHPx764

• D.

BHP/764

A. BHPx746
Explanation
The correct answer is BHPx746. This formula is used to calculate power in watts. BHP stands for Brake Horsepower, which is a unit of measurement for engine power. By multiplying BHP by 746, we can convert it into watts, as 1 horsepower is equal to 746 watts. Therefore, the correct formula to calculate power in watts is BHP multiplied by 746.

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• 10.

### Series generators are used for

• A.

Series arc lighting

• B.

Voltage booster

• C.

Regenerative breaking

• D.

All of above

D. All of above
Explanation
Series generators are used for all of the above purposes. Series arc lighting refers to the use of series generators to produce high-intensity light in arc lamps. Voltage boosters are devices that increase the voltage output of a generator, and series generators are commonly used for this purpose. Regenerative braking is a technique used in electric and hybrid vehicles to recover energy during braking, and series generators play a crucial role in this process by converting the kinetic energy of the vehicle into electrical energy. Therefore, series generators are used for all of the mentioned purposes.

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• 11.

### A 110V, 1000 rpm generator will generate ____ V at 1100 rpm

• A.

220

• B.

200

• C.

150

• D.

120

D. 120
Explanation
A 110V, 1000 rpm generator will generate 120V at 1100 rpm because the voltage output of a generator is directly proportional to its speed. Therefore, when the speed increases by 10%, the voltage output will also increase by 10%. Hence, at 1100 rpm, the generator will produce 120V.

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• 12.

### Cu-Losses are also known as

• A.

I2R

• B.

IR2

• C.

V2/R

• D.

VI

A. I2R
Explanation
Cu-Losses refers to the power losses that occur in a conductor due to its resistance (R) when current (I) flows through it. These losses are proportional to the square of the current and the resistance (I2R), which explains why the correct answer is I2R.

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• 13.

### _________ field winding has small number of turns of thick copper wire.

• A.

Shunt

• B.

Series

• C.

Compund

• D.

Separately excited

B. Series
Explanation
The field winding in a series motor has a small number of turns of thick copper wire. In a series motor, the field winding is connected in series with the armature, which causes the current flowing through the field winding to be the same as the current flowing through the armature. This configuration allows for a high magnetic field strength, which is necessary for the motor to produce a high torque. Therefore, the series field winding is designed with a small number of turns of thick copper wire to handle the high current.

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• 14.

### Carbon brushes are preferred on copper brushes because of

• A.

Armature reaction

• B.

Reduced reactance voltage

• C.

Hardness of material

• D.

Low resistance

B. Reduced reactance voltage
Explanation
Carbon brushes are preferred over copper brushes because they have a lower reactance voltage. Reactance voltage is the voltage drop that occurs when current flows through a brush. By using carbon brushes, the reactance voltage is reduced, resulting in less power loss and more efficient operation of the electrical system. Additionally, carbon brushes have good electrical conductivity and are able to withstand high temperatures, making them a suitable choice for various applications.

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• 15.

### In Fleming's right-hand rule, the thumb point towards

• A.

Direction of flux

• B.

Direction of induced e.m.f.

• C.

Direction of motion of conductor

• D.

Any of above

C. Direction of motion of conductor
Explanation
Fleming's right-hand rule is used to determine the direction of the induced current or the motion of a conductor in a magnetic field. According to this rule, if the thumb of the right hand is pointed in the direction of the motion of the conductor, then the curled fingers will represent the direction of the induced current or the magnetic field lines. Therefore, the correct answer is "direction of motion of conductor."

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• 16.

### The output voltage of a simple d.c. generator is

• A.

A.c. square wave.

• B.

A.c. sinusoidal wave

• C.

Pure d.c

• D.

Pulsating d.c

D. Pulsating d.c
Explanation
The output voltage of a simple d.c. generator is pulsating d.c. This is because a d.c. generator produces an unidirectional current, but due to the nature of the commutator and brushes, the output voltage will have some fluctuations or ripples. These ripples cause the voltage to pulsate rather than remain constant. Therefore, the correct answer is pulsating d.c.

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• 17.

### When tow generators are running in parallel and field of one of them is weakened too much then it will

• A.

Deliver large share of the total load

• B.

Deliver small share of the total load

• C.

Run in the opposite direction

• D.

Run as a motor in the same direction

D. Run as a motor in the same direction
Explanation
When two generators are running in parallel and the field of one of them is weakened too much, it will run as a motor in the same direction. This means that instead of generating electricity, it will consume electricity and act as a motor, causing the generator to rotate in the same direction as the other generator. As a result, the generator with the weakened field will not contribute to the total load and will instead rely on the other generator for power.

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• 18.

### In shunt generator interpole winding carries ____ current

• A.

Armature

• B.

Shunt field

• C.

• D.

None of these

A. Armature
Explanation
The interpole winding in a shunt generator carries armature current. This is because the interpole winding is connected in series with the armature winding, allowing it to carry the same current as the armature. The purpose of the interpole winding is to create a magnetic field that opposes the armature reaction and reduces the distortion of the main magnetic field, thereby improving the performance and voltage regulation of the generator.

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• 19.

### Due to which of the following reasons a separately excited generator is better

• A.

Low price

• B.

Exciting current is independent of load current

• C.

Low armature resistance

• D.

All of above

B. Exciting current is independent of load current
Explanation
A separately excited generator is better because the exciting current is independent of the load current. This means that the amount of current needed to excite the generator's field winding remains constant regardless of the load being supplied by the generator. This allows for more stable and efficient operation, as the generator can maintain a consistent level of excitation regardless of the varying load conditions. This is especially important in applications where a stable and reliable power supply is required.

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• 20.

### Why is the armature of a d.c. machine made of silicon steel

• A.

To reduce eddy current loss

• B.

To reduce hysteresis loss

• C.

Ease of creating slots

• D.

To achieve high permeability

A. To reduce eddy current loss
Explanation
The armature of a d.c. machine is made of silicon steel to reduce eddy current loss. Silicon steel has high electrical resistivity, which limits the flow of eddy currents. By reducing eddy current loss, the efficiency of the machine is improved, resulting in less energy wastage and better performance.

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• 21.

### compound generator is said to be flat compounded if VFL

• A.

> VNL

• B.

< VNL

• C.

= VNL

• D.

= 2VNL

C. = VNL
Explanation
A compound generator is said to be flat compounded if the terminal voltage (V) is equal to the no-load voltage (VNL).

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• 22.

### The commutator segments of a d.c. machine are made of

• A.

Carbon

• B.

Copper

• C.

Mica

• D.

Ceramic

B. Copper
Explanation
The commutator segments of a d.c. machine are made of copper because copper is a good conductor of electricity and has excellent thermal conductivity. It also has high mechanical strength and is resistant to wear and tear. These properties make copper an ideal material for the commutator segments, which are responsible for transferring electrical current between the stationary and rotating parts of the machine.

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• 23.

### If the number of poles in a lap wound generator be doubled, then the generated e.m.f .will

• A.

Become half

• B.

Become double

• C.

Increase to four times

• D.

Remain constant

D. Remain constant
Explanation
The number of poles in a lap wound generator refers to the number of magnetic poles on the rotor. Doubling the number of poles does not directly affect the generated electromotive force (e.m.f.). The e.m.f. is determined by factors such as the speed of rotation, the strength of the magnetic field, and the number of turns in the coil. Therefore, the generated e.m.f. will remain constant.

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• 24.

### Which of the following d.c. generator can build up without any residual magnetism

• A.

Compound generator

• B.

Self-excited generator

• C.

Series generator

• D.

Shunt generator

B. Self-excited generator
Explanation
A self-excited generator is able to build up without any residual magnetism because it has a separate field winding that provides the initial magnetic field. This field winding is connected in series with the armature, and when the generator starts rotating, the armature current induces a magnetic field in the field winding. This self-induced magnetic field then reinforces the initial magnetic field, allowing the generator to build up its voltage without relying on any residual magnetism.

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• 25.

### The efficiency of a d.c. machine is maxiinum when

• A.

Variable losses = constant losses

• B.

Stray losses are = copper losses

• C.

Field copper losses = armature copper losses

• D.

Magnetic losses - windage loss.

A. Variable losses = constant losses
Explanation
The efficiency of a d.c. machine is maximum when the variable losses (such as iron losses, friction losses, and brush losses) are equal to the constant losses (such as core losses and losses in field winding). This means that the losses in the machine are balanced, resulting in the highest efficiency. When the variable losses exceed the constant losses, the efficiency decreases. Similarly, when the constant losses exceed the variable losses, the efficiency also decreases. Therefore, when the variable losses are equal to the constant losses, the machine operates at its maximum efficiency.

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• 26.

### In a d.c. machine constant losses composed of

• A.

Mechanical losses

• B.

Commutator losses

• C.

Iron losses and mechanical losses

• D.

Total copper losses

C. Iron losses and mechanical losses
Explanation
The correct answer is "iron losses and mechanical losses". In a DC machine, constant losses refer to the losses that occur regardless of the load on the machine. These losses include iron losses, which are caused by hysteresis and eddy currents in the iron core of the machine, and mechanical losses, which are caused by friction and windage. These losses are independent of the current flowing through the machine and are constant regardless of the load.

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• 27.

### Which of the following regulation is preferred for d.c. generator?

• A.

2%

• B.

20%

• C.

60%

• D.

80%

A. 2%
Explanation
A regulation of 2% is preferred for a d.c. generator. This means that the generator can maintain a constant voltage output within a range of plus or minus 2% of the rated voltage. A lower regulation percentage indicates better voltage regulation, which is important for applications that require a stable and reliable power supply.

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• 28.

### During parallel operation of two d.c. generators an equilizer bar is used

• A.

To increase the series flux

• B.

To increase generated e.m.f.

• C.

To reduce the combined effect of armature reaction

• D.

To properly divide current on both genarators

D. To properly divide current on both genarators
Explanation
The equilizer bar is used during parallel operation of two d.c. generators to properly divide the current on both generators. This is necessary because when generators are connected in parallel, they share the load. The equilizer bar ensures that the current is distributed evenly between the generators, preventing one generator from taking on more load than the other. This helps to maintain a balanced operation and prevents overloading of one generator while the other is underutilized.

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• 29.

### External characteristics of DC generator are relationship between

• A.

Eg and Ia

• B.

Eg and IL

• C.

VT and Ia

• D.

VT and IL

D. VT and IL
Explanation
The external characteristics of a DC generator refer to the relationship between the terminal voltage (VT) and the load current (IL). As the load current increases, the terminal voltage decreases due to the internal resistance of the generator. This is because the generator has a limited capacity to supply current to the load. Therefore, the correct answer is VT and IL.

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• 30.

### If commutator pitch Yc=1, winding is said to be

• A.

Preogressive

• B.

Retrogressive

• C.

Simplex

• D.

Duplex

A. Preogressive
Explanation
If the commutator pitch Yc is equal to 1, the winding is said to be progressive. This means that the winding progresses or moves forward by one coil pitch per commutator segment. In a progressive winding, the coil sides are connected to consecutive commutator segments, resulting in a smooth and continuous flow of current in the armature windings. This type of winding is commonly used in DC machines to achieve a unidirectional torque output.

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• 31.

### In simplex lap winding Yb-Yf =

• A.

1

• B.

2

• C.

4

• D.

6

B. 2
Explanation
In simplex lap winding, Yb-Yf represents the number of parallel paths between the brushes. Since the answer is 2, it means that there are two parallel paths between the brushes.

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• Mar 29, 2024
Quiz Edited by
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• Dec 31, 2012
Quiz Created by
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