1. DC generator Losses occur while converting ______

Electrical energy to Mechanical Energy

Mechanical energy to Electrical Energy

Electrical Energy to Heat Energy

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?

0.005 mm

0.05 mm

0.5 mm

5 mm

Answer: 3. 0.5 mm

Explanation:

Eddy current loss is basically I^{2} 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 W_{e} = KB^{2}_{m} f^{2}t^{2}

Where,

K = π^{2}/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_{e }∝ f^{2}

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

Core Losses

Copper Losses

Iron Losses

Core Losses

Answer: 2. Copper 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.

Eddy current

Hysteresis loss

Core Loss

Copper Loss

Answer: 2. Hysteresis 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:

Tungsten steel

Cast iron

High carbon steel

Silicon steel

Answer: 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:

10 to 20%

20 to 25%

30 to 40%

None of these

Answer: 3. 30 to 40%

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

256 W

320 W

500 W

625 W

Answer: 4. 625 W

Explanation:

Given that,

Eddy current loss at 40 Hz frequency (f_{1}) is W_{e1} = 400 W

Maximum flux density B_{m} is constant

Let’s consider eddy current loss at 50 Hz frequency (f_{2}) as W_{e2}

We know that for constant flux density W_{e} ∝ f^{2}

W_{e1}/W_{e2} = f^{2}_{1}/f^{2}_{2}

400/W_{e2} = 40^{2}/50^{2}

W_{e2} = 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.

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

10 to 20%

20 to 25%

30 to 40%

None of these

Answer: 1. 10 to 20%

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

Magnetic losses

Mechanical losses

Either (1) or (2)

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

Directly proportional to thickness of each lamination

Inversely proportional to thickness of each lamination

Directly proportional to square of thickness of each lamination

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 = KB^{2}m f^{2}t^{2}

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 ______

Electrical Losses

Mechanical Losses

Both Electrical and Mechanical Losses

None of the above

Answer:2. Mechanical Losses

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?

Field copper loss

Windage loss

Armature copper loss

None of the above

Answer:3. Armature copper loss

Explanation:

Armature Loss:- When current flows in the armature then this loss is equal to the square of armature current armature resistance.

Armature copper loss = I_{a}^{2}R_{a}

These losses are about 30% of the total full load losses.

Armature copper losses in a DC generator vary significantly with the load current.