DC Generator Losses MCQ || DC Generator Losses Questions and Answers

1.  DC generator Losses occur while converting ______

1. Electrical energy to Mechanical Energy
2. Mechanical energy to Electrical Energy
3. Electrical Energy to Heat Energy
4. Heat Energy to Mechanical Energy

Answer: 2. Mechanical energy to Electrical Energy

Explanation:

While conversion of mechanical energy into electrical energy, a part of energy dissipated in the form of heat in the surrounding air is called losses in the generator. These losses affect the efficiency of the generator. A reduction in these losses leads to higher efficiency. Thus, the major objective in the design of a DC machine is the reduction of these losses.

2. Which of the following could be lamina-proximately the thickness of laminations of a D. C. machine?

1. 0.005 mm
2. 0.05 mm
3. 0.5 mm
4. 5 mm

Explanation:

• Eddy current loss is basically I2 R loss present in the core due to the production of eddy currents in the core, because of its conductivity.
• Eddy’s current losses are directly proportional to the conductivity of the core.
• Eddy’s current losses can be reduced either by adding silica content (4% – 5 %) to steel or by using a laminated core instead of a solid core.
• In order to reduce the eddy current losses, we use laminations
• In a DC machine, laminations are used to reduce eddy current losses and for insulation purposes. The approximate thickness of laminations is 0.5 mm.
• The stator frame consists of laminations of silicon steel, usually with a thickness of about 0.5 millimeters.

Eddy’s current loss is given by We = KB2m f2t2

Where,

K = π2/6ρ

Bm = maximum flux density,

f = supply frequency,

t = thickness of the laminations

If maximum flux density is constant, and thickness also constant,

In that case, eddy current losses are directly proportional to the square of the frequency.

W∝ f2

2. The power losses in DC Generator is also called as _______

1. Core Losses
2. Copper Losses
3. Iron Losses
4. Core Losses

Explanation:

The various windings of the DC machine, made of copper, have some resistance. When current flows through them, there is power loss proportional to the square of their respective currents. These power losses are called copper losses.

3. _____ loss occurs when the magnetic material is subjected to reversal of magnetic flux.

1. Eddy current
2. Hysteresis loss
3. Core Loss
4. Copper Loss

Explanation:

Whenever a magnetic material is subjected to reversal of magnetic flux, Hysteresis loss occurs. It is due to the retentivity property of the material. Whenever a magnetic material is subjected to reversal of magnetic flux, this loss occurs. It is due to the retentivity property of the material.

4. In order to reduce hysteresis losses, the cores of DC Generator are generally made of:

1. Tungsten steel
2. Cast iron
3. High carbon steel
4. Silicon steel

Explanation:

Hysteresis losses occur in the armature winding due to the reversal of magnetization of the core. When the core of the armature is exposed to the magnetic field, it undergoes one complete rotation of magnetic reversal.

Hysteresis losses can be minimized by using a material having the least hysteresis loop area. Hence silicon steel or high-grade steel is used for the manufacturing of core.

5. The copper losses of a DC shunt generator amount ________ to the following percentage of full-load losses:

1. 10 to 20%
2. 20 to 25%
3. 30 to 40%
4. None of these

Explanation:

• When current flows through them, there is power loss proportional to the square of their respective currents. These power losses are called copper losses.
• This loss contributes about 30 to 40% to full load losses. The armature copper loss is variable and depends upon the amount of loading of the machine.

6. The eddy current loss in a dc generator is 400 W at 40 Hz frequency of reversal and constant flux density. When the frequency is increased to 50 Hz, eddy current loss is

1. 256 W
2. 320 W
3. 500 W
4. 625 W

Explanation:

Given that,

Eddy current loss at 40 Hz frequency (f1) is We1 = 400 W

Maximum flux density Bm is constant

Let’s consider eddy current loss at 50 Hz frequency (f2) as We2

We know that for constant flux density We ∝ f2

We1/We2 = f21/f22

400/We2 = 402/502

We2 = 625 W

7. The hysteresis and eddy current loss in dc machines running at 1000rpm is 500W and 300W respectively. At what speed will the total iron loss be one-fourth keeping the flux constant.

1. 222.24 rpm
2. 300 rpm
3. 333.34 rpm
4. 400 rpm

Explanation:

Hystresis Losses

Ph ∝ K1n

Eddy current Losses

Pe ∝ K1n2

Total loss = 500 + 300 = 800W

One fourth of the total losses = 800/4 = 200w

$200 = \left( {\frac{{500}}{{1000}}} \right)n + \left( {\frac{{300}}{{{{1000}^2}}}} \right){n^2}$

n = 333.34 rpm

7. The Mechanical losses of a DC generator amount ________ to the following percentage of full-load losses:

1. 10 to 20%
2. 20 to 25%
3. 30 to 40%
4. None of these

Explanation:

Mechanical losses: As the armature of a DC machine is a rotating part, some power is required to overcome:

(i) Air friction of rotating armature (windage loss)

(ii) Friction at the bearing and friction between brushes and commutator (friction loss).

These losses are known as mechanical losses. To reduce these losses proper lubrication is done at the bearings. These losses are about 10 to 20% of full load losses.

8. Stray losses in a DC generator are the same as

1. Magnetic losses
2. Mechanical losses
3. Either (1) or (2)
4. Sum of magnetic and mechanical losses

Answer: 4. Sum of magnetic and mechanical losses

Explanation:

The sum of the iron losses and mechanical losses in a DC machine is known as stray losses i.e.,

Stray losses = Iron losses + Mechanical losses.

These losses are difficult to account for. They are usually due to inaccuracies in the designing and modeling of the machine. Most of the time, stray losses are assumed to be 1% of the full load.

9. In a dc generator, the eddy current power loss is

1. Directly proportional to thickness of each lamination
2. Inversely proportional to thickness of each lamination
3. Directly proportional to square of thickness of each lamination
4. None of the above

Answer: 1. Directly proportional to thickness of each lamination

Explanation:

When flux linking with the magnetic material changes (or flux is cut by the magnetic material) an emf is induced in it which circulates eddy currents through it. These eddy currents produce eddy current loss in the form of heat.

Eddy’s current loss is given by

Pe = KB2m f2t2

Where,

Bm = maximum flux density,

f = supply frequency,

t = thickness of the laminations

Therefore Eddy current loss is directly proportional to the thickness of each lamination

10.  In DC generator the air friction losses are a part of ______

1. Electrical Losses
2. Mechanical Losses
3. Both Electrical and Mechanical Losses
4. None of the above

Explanation:

Rotational or mechanical losses can be caused by bearing friction, brush friction on the commutator, or air friction (called windage), which is caused by the air turbulence due to armature rotation. Careful maintenance can be instrumental in keeping bearing friction to a minimum. Clean bearings and proper lubrication are essential to the reduction of bearing friction.

Brush friction is reduced by assuring proper brush seating, using proper brushes, and maintaining proper brush tension. A smooth and clean commutator also aids in the reduction of brush friction.

11. Which of the following loss in a D. C. generator varies significantly with the load current?

1. Field copper loss
2. Windage loss
3. Armature copper loss
4. None of the above