The efficiency of a DC machine will be maximum when the line current is such that constant loss is equal to the variable loss. i.e armature circuit loss is equal to the sum of no-load rotational loss and field circuit loss.
Variable loss – Constant loss = 0
2. DC generators are normally designed for maximum efficiency at or near _______
Full-load
Minimum load
Rated output
At all loads
Answer: 3. Rated output
Explanation:
The maximum efficiency occurs at the load where the variable losses are equal to the constant losses. The machine is normally designed to have the rated output near about the maximum efficiency load.
3. The conversion of mechanical energy into electrical energy doesn’t occur in-
Transformer
DC Generator
Motor
Both 1 and 3
Answer: 4. Both 1 and 3
Explanation:
The conservation of mechanical energy into electrical energy doesn’t occur in a Transformer, as a transformer only transforms the power from the primary side to the secondary side at the same frequency.
The conservation of mechanical energy into electrical energy doesn’t occur in Motors as the motors are meant for the conversion of electrical energy into mechanical energy.
An AC or DC generator converts mechanical energy into electrical energy by the means of electromagnetic induction.
4. Mechanical efficiency of a diesel engine is defined as _____
BHP ÷ IHP
BHP × IHP
IHP – BHP
IHP ÷ BHP
Answer:1. BHP ÷ IHP
Explanation:
A diesel engine is an internal-combustion engine in which air is compressed to a sufficiently high temperature to ignite diesel fuel injected into the cylinder, where combustion and expansion actuate a piston. It converts the chemical energy stored in the fuel into mechanical energy.
Mechanical efficiency:
It is a performance parameter that gives the effectiveness of an engine in terms of how much input energy is converted into output energy. For an internal combustion engine, it is the ratio between the Brake Work done and Indicated Work done.
Mechanical efficiency = Brake Horse Power (BHP)/Indicated Horsepower (IHP)
5. In a DC generator, the iron losses mainly take place in ________
Yoke
Commutator
Armature conductors
Armature rotor
Answer: 4. Armature rotor
Explanation:
In the DC machine, iron losses occur in armature because the armature core is made of iron and it rotates in a magnetic field. Hence a small current gets induced in the core. Due to this current, eddy current losses and hysteresis losses occurs in the armature iron core.
6. A DC generator delivers a 30-A current to a load at 120 V by consuming 4500 J/s of mechanical energy. Determine the efficiency of the generator.
90%
85%
75%
Answer:4. 80%
Explanation:
Given,
Current delivered to load, I = 30 A
Voltage at the load, V = 120 V
Mechanical power input, Pm = 4500 J/s = 4500 W
Electrical power output, Pe = V × I = 120 × 30 = 3600 W
Efficiency η = Pe/Pm
η = 3600/4500
η = 80%
7. If DC generators are located near load centers, which losses can be minimized?
Iron losses
Eddy’s current losses
Line losses
Corona losses
Answer: 3. Line losses
Explanation:
DC generator are installed near the load centres to reduce the line losses because power loss depends upon the length of the transmission line
The transmission over long distances creates power losses; The major part of the energy losses comes from Joule effect in transformers and power lines
This energy is lost as heat in the conductors
8. A DC shunt generator delivers 45A at a terminal voltage of 220V. The armature and the shunt field resistances are 0.01Ω and 44Ω respectively. The stray losses are 375W. The percentage efficiency of the DC generator is
9. Nature of efficiency curve of DC machine is ______
First decreases then increase
First constant then decreases
First constant then increases
First increases then decrease
Answer:4. First increases then decrease
Explanation:
Efficiency is generally expressed as a percentage. The efficiency of a dc motor depends on the power output.
Initially, with low power outputs, efficiency is poor due to the constant losses. As the output increases, the efficiency increases till it reaches a certain maximum value.
If the output is still further increased, then the efficiency decreases due to the rapid rise in the variable losses.
The motors are normally designed to have maximum efficiency while working at or near full load.
The efficiency of a dc motor at normal operating conditions may be as high as 95%. A typical efficiency curve showing the manner in which the efficiency varies with the output
10. The main drawback of _________ is that the accuracy in determining the mechanical power output of the DC motor is limited.
Hopkinson’s test
Direct method of testing
Indirect method of testing
Swinburne’s test
Answer:2. Direct method of testing
Explanation:
The main drawback of the Direct method of testing is that the accuracy in determining the mechanical power output of the DC motor is limited.
In the direct testing method, the machine is put on full load, and whole developed power is wasted by applying the brake. The direct test method is used, for small machines. Because of in case of large motors, it is difficult to dissipate the large amount of heat generated at the brake.
11. Why retardation test is carried out on the DC machine?
To find stray losses
To find eddy current losses
To find field copper losses
To find windage losses
Answer: 1. To find stray losses
Explanation:
In the retardation test, the dc machine is run as a motor at a speed just above the normal. Then the supply to the armature is cut off while the field is normally excited. The speed is allowed to fall to some value just below normal.
This test is very efficient to find out stray losses in dc machine. In this test, we get total stray losses that are the combination of mechanical (friction & windage) and iron losses of the machine.
12. Separately excited and self-excited are types of which machine?
DC machine
Synchronous machine
Induction machine
None of the above
Answer: 1. DC Machine
Explanation:
Separately excited and self-excited are types of DC machine.
Separately Excited DC Machine: A DC Machine whose field magnet winding is supplied from an independent external DC source (e.g., a battery, etc.) is called a separate DC Machine.
Self-Excited DC Machine: A DC Machine whose field magnet winding is supplied current from the output or input of the DC Machine itself is called a self-excited DC Machine.
13. Which of the following losses is not under constant losses?
Friction and windage losses
No-load core losses
Shunt field losses
Copper Loss
Answer.4. Copper Loss
Explanation:
The losses in a DC machine may also be sub-divided into two categories, i.e.,
Constant losses;
Variable losses.
Constant losses: The losses in a DC machine that remain the same at all loads are called constant losses. The constant losses in a DC machine are: (i) Iron losses; (ii) Mechanical losses; (iii) Shunt field copper losses
Variable losses: The losses in a DC machine that vary with load are called variable losses. The variable losses in a DC machine are:
(i) Armature copper loss; (ii) Series field copper loss; (iii) Interpol winding copper loss.
Copper loss is proportional to the square of the current i.e., copper loss at half full-load is one-fourth of that at full-load. As the current depends upon the load connected to the DC Machine. The copper loss varies with variation in load and is also called Variable Loss.
14. The efficiency of a dc shunt generator is maximum when:
magnetic loss is equal to the mechanical loss
armature circuit loss is equal to the sum of no-load rotational loss and field circuit loss
field ohmic loss is equal to the constant loss
stray load loss is equal to the armature circuit loss
Answer.4. armature circuit loss is equal to the sum of no-load rotational loss and field circuit loss
Explanation:
The efficiency of a DC generator is maximum when losses which are proportional to the square of the load current (variable losses) are equal to the constant losses of the DC generator i.e. armature circuit loss is equal to the sum of no-load rotational loss and field circuit loss.
15. Variable losses are proportional to ________
Armature current
Square of armature current
Inverse of armature current
Inverse of the square of armature current
Answer.2. Square of armature current
Explanation:
Variable losses: The losses in a DC machine that vary with load are called variable losses. The variable losses in a DC machine are:
(i) Armature copper loss; (ii) Series field copper loss; (iii) Interpol winding copper loss.
Variable losses are proportional to the square of the current I2a i.e., copper loss at half full-load is one-fourth of that at full-load. As the current depends upon the load connected to the DC Machine. The copper loss varies with variation in load and is also called Variable Loss.
