1. What happens if the field winding of a running shunt motor suddenly breaks open?
A. Its speed slows down
B. Its speed becomes dangerously high
Q It gives out sparks
D. It stops at once
Answer: B. Its speed becomes dangerously high
Explanation:
- If the field winding of a running shunt motor suddenly breaks open. So, Flux is reduced and speed increase dangerously as flux is inversely proportional to speed.
2. Condition for maximum power output for a motor is
A. Eb=V
B. Eb =V/2
C. Eb=IaRa
D. Eb=0.5IaRa
Answer: B. Eb =V/2
Explanation:
The condition for maximum power output for a motor is
The mechanical developed by a DC motor is maximum when the back EMF is equal to half of the applied voltage.
3. The speed of a DC motor is
A. directly proportional to back emf and inversely proportional to flux
B. inversely proportional to back emf and directly proportional to flux
C. directly proportional to emf as well as flux
D. inversely proportional to emf as well as flux
Answer: A. directly proportional to back emf and inversely proportional to flux
Explanation:
N∝Ebφ
Hence, the speed of a DC motor is directly proportional to back emf and is inversely proportional to flux per pole.
4. The armature resistance of a 200 V shunt motor is 0.4 ohm, and the no-load armature current is 2 A. With load, its armature current is 50 A and its speed is 1200 rpm. The no-load speed would be
A. 1460 rpm
B. 1500 rpm
C.1330 rpm
D. 1200 rpm
Answer: C.1330 rpm
Explanation:
Given: Ra = 0.4-ohm, V = 200 V, N1 = 1200 rpm,
Ia0 = 2A, Ia1 = 50 A
E0 = V – Ia0Ra = 200 – 2 x 0.4 = 199.2 V
E1 = V- Ia1Ra = 200 – 50 x 0.4 = 180 V
N α E/Φ
For shunt motor Φ is constant
N1/N0 = E1/E0 = 180/199.2 = 0.9036
N0= N1/0.9009= 1200/0.9036 = 1328.0212 rpm
5. Why is the air gap between the yoke and armature of an electric motor kept smaller?
A. To achieve a stronger magnetic field
B. To avoid overheating of the machine
C. To make the station easier
D. None of these
Answer: A. To achieve a stronger magnetic field
Explanation:
- The magnetic field always chooses the low reluctance path hence to achieve a strong magnetic field we need to maintain a low reluctance path. It can be achieved by keeping a smaller air gap between the yoke and armature of an electric motor.
- The air gap between the pole pieces and the armature is kept very small to keep the field strength at its highest value.
- This is due to the reason the flux lines can pass through iron very easily than the air gap.
- The small air gap generates high voltage.
- The larger the air gap means stronger must be the magnetizing force to produce the required voltage hence the air gap is kept very small.
6. The rotating part of a DC motor is known as
A. stator
B. pole
C. carbon brush
D. armature
Answer: D. armature
Explanation:
The armature of a DC motor is a cylinder of magnetic laminations that are insulated from one another. The armature is perpendicular to the axis of the cylinder. The armature is a rotating part that rotates on its axis and is separated from the field coil by an air gap.
A DC machine consists of two basic parts: stator and rotor.
Basic constructional parts of a DC machine:
Yoke:
- The outer frame of a dc machine is called as the yoke.
- It is made up of cast iron or steel.
- It not only provides mechanical strength to the whole assembly but also carries the magnetic flux produced by the field winding.
Poles and pole shoes:
- Poles are joined to the yoke with the help of bolts or welding.
- They carry field winding and pole shoes are fastened to them.
- Pole shoes serve two purposes: (i) they support field coils and (ii) spread out the flux in the air gap uniformly.
Field winding:
- They are usually made of copper.
- Field coils are a former wound and placed on each pole and are connected in series.
- They are wound in such a way that, when energized, they form alternate North and South poles.
Armature core:
- The armature core is the rotor of a dc machine. It is cylindrical in shape with slots to carry armature winding.
- The armature is built up of thin laminated circular steel disks for reducing eddy current losses.
- The armature core is made of silicon steel laminations that are insulated from each other by an insulating varnish coating. These laminations are used to reduce eddy current losses.
- It may be provided with air ducts for the axial airflow for cooling purposes.
- The armature is keyed to the shaft.
Armature winding:
- It is usually a former wound copper coil that rests in armature slots.
- The armature conductors are insulated from each other and also from the armature core.
- Armature winding can be wound by one of two methods: lap winding or wave winding.
- Double-layer lap or wave windings are generally used. A double-layer winding means that each armature slot will carry two different coils.
Brushes:
- Brushes are usually made from carbon or graphite.
- They rest on commutator segments and slide on the segments when the commutator rotates keeping physical contact to collect or supply the current.
8. Voltage equation for the DC motor is
A. V=Eb-IaRa
B. V=Eb+IaRa
C. Eb=0.5IaRa
D. Eb=V+IaRa
Answer: A. V=Eb-IaRa
Explanation:
V = Eb + IaRa
The above relation is known as the “Voltage Equation of the DC Motor”.
9. The function of the commutator in a DC machine is
A. to change alternating current to direct current
B. to improve commutation
C. for easy speed control
D. to change the alternating voltage to direct voltage
Answer: D. to change the alternating voltage to direct voltage
Explanation:
- In the case of the DC generator, the commutator is used to convert generated AC in the armature into DC.
- In the case of the DC motor, the commutator is used to convert DC to A.C.
- Due to the limitation of the commutator dc generators are not usually designed beyond 650 V
- The physical connection to the armature winding is made through a commutator-brush arrangement.
- The function of a commutator in a dc generator is to collect the current generated in armature conductors.
- While in the case of a dc motor, the commutator helps in providing current to the armature conductors.
- A commutator consists of a set of copper segments that are insulated from each other.
- The number of segments is equal to the number of armature coils. Each segment is connected to an armature coil and the commutator is keyed to the shaft.
10. In a DC motor, the iron losses occur in the
A. armature
B. yoke
C. none of these
D. field
Answer: A. armature
Explanation:
- In the DC motor, iron losses occur in the 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.
- These eddy current losses and hysteresis losses are collectively known as core losses or iron losses.
11. Carbon brushes are used in electric motors to
A. brush off carbon deposits on the commutator
B. provide a path for the flow of current
C. prevent overheating of armature windings
D. prevent sparking during commutation
Answer: B. provide a path for the flow of current
Explanation:
- Brushes are used to serve electrical contact between the stationary points and the moving surfaces. They are mainly used to transfer power from rotatory armature coils to stationary wires.
- They are used to reduce damage of the motors. Split rings are used to supply unidirectional current to rotor coils to generate torque in the same direction.
12. DC shunt generator has
A. slightly drooping characteristics
B. appreciably rising characteristics
C. constant voltage characteristics
D. appreciably falling characteristics
Answer: A. slightly drooping characteristics
Explanation:
- For stable parallel operation, the most suitable type of DC generator is a shunt generator as it has slightly drooping characteristics.
- If there is any tendency for a generator to supply more or less than its proper share of load it changes system voltage which certainly opposes this tendency.
- This restores the original division of load. Thus the shunt generators automatically remain in parallel, once they are paralleled.
13. Which one of the following generators is used for charging batteries?
A. Compound generator
B. Shunt generator
C. Series generator
D. Tacho
Answer: B. Shunt generator
Explanation:
Shunt generators are used where the main requirement is a constant voltage over a narrow load range.
Applications of shunt generator:
- Used for general lighting
- Used to charge the battery because they can be made to give constant output voltage
- Used for giving excitation to the alternators
- Used for small power supply (such as a portable generator)
Important Points
Applications of series generator:
- Used for supplying field excitation current in DC locomotives for regenerative breaking
- Used as boosters to compensate the voltage drop in the feeder in various types of distribution systems such as railway service
- Used in series arc lightening
Applications of compound generator:
- Cumulative compound generators (over compounded) are generally used for lighting, power supply purpose and for heavy power services because of their constant voltage property; these are also used for driving a motor
- The flat compounded generators are generally used for small distance operation, such as power supply for hotels, offices, homes, and lodges
- The differential compound wound generators are used for arc welding where huge voltage drop and constant current is required because of their large demagnetization armature reaction
14. What is the effect produced by the electric current in an electric motor?
A. Magnetic effect only
B. Magnetic as well as the heating effect
C. Heating effect only
D. Heating as well as chemical effect
Answer: B. Magnetic as well as heating effect
Explanation:
- When an electric current is supplied to the coil of the electric motor, it gets deflected because of the magnetic field.
- When an electric current flows through the conductor some losses produced because of armature resistance i.e. I2R losses. These losses produce heating and hence machine will get heat.
- So, both magnetic as well as the heating effect produced by the electric current in an electric motor.
15. The highest speed attained by a DC shunt motor is
A. equal to infinity at rated flux
B. higher than no-load speed at rated flux
C. equal to no-load speed at rated flux
D. lower than no-load speed at rated flux
Answer: C. equal to no-load speed at rated flux
Explanation:
- In case of DC shunt motor, flux is practically constant as the field is parallel to the armature.
- If the load is increased, then the speed of the motor will remain almost constant because the field current remains almost constant.
- So that it is also known as constant speed motor.
- Hence, the highest speed attained by the DC shunt motor at rated flux is equal to no-load speed.
- DC shunt motor is used to drive the constant speed line shafting, lathes, blower, and fan, etc.
16. The dummy coil in DC machines is used to
A. eliminate reactance voltage
B. eliminate armature reaction
C. bring about mechanical balance of armature
D, eliminate harmonics developed in the machine
Answer: C. bring about mechanical balance of armature
Explanation:
- The wave winding is possible only with number of conductor or coils.
- But sometimes the armature slots available in the armature winding do not meet the requirement of the winding. (It means that the available number of slots is more than the required number of conductors).
- Some slots are kept without armature winding in that case and dummy coil or coils are employed in that slots.
- The dummy coils are similar to other coils except their ends are cut, short and taped. They do not connect with the commutator bars.
- The dummy coils are simply to provide mechanical balance for the armature.
- As they do not connect with commutator bars, they do not affect the electrical characteristics of the winding.
17. A simple method of increasing the voltage of a DC generator is to
A. decrease the air gap flux density
B. increase the speed of rotation
C. decrease the speed of rotation
D. increase the length of the armature
Answer: B. increase the speed of rotation
Explanation:
E α w
The induced voltage is directly proportional to the speed.
So, on increasing the speed of the DC generator, the induced voltage also increases.
18. The function of the starter in a DC machine is to
A. avoid the excessive current at starting
B. control the speed
C. avoid armature reaction
D. avoid excess heating
Answer: A. avoid the excessive current at starting
Explanation:
At starting, when the motor is stationary, there is no back EMF in the armature because the speed is zero.
Consequently, if the motor is directly switched on to the mains, the armature will draw a heavy current because of small armature resistance.
This heavy starting current may result in:
(i) burning of armature due to excessive heating effect,
(ii) damaging the commutator and brushes due to heavy sparking,
(iii) excessive voltage drops in the line to which the motor is connected.
- In order to avoid excessive current at starting, a variable resistance (known as starting resistance) is inserted in series with the armature circuit.
- This resistance is gradually reduced as the motor gains speed (and hence Eb increases) and eventually, it is cut out completely when the motor has attained full speed.
- The value of starting resistance is generally such that starting current is limited to 1.25 to 2 times the full-load current.
19. The emf of a DC generator depends on
A. commutation
B. speed
D. brush contact drop
C. frequency
Answer: B. speed
Explanation:
the generated emf depends on the number of conductors, speed, number of poles, and the number of parallel paths.
20. An external resistance is added in the series with the field of a DC shunt motor. When the motor runs, the effect of resistance is to
A. reduce the speed of the motor
B. increase the speed of the motor
C. reduce the armature current drawn by the motor
D. reduce the losses
Answer: B. increase the speed of the motor
Explanation:
- By adding external resistance in the field circuit, we can control the speed of DC shunt motor above the normal speed.
- In this method, speed variation is accomplished by means of a variable resistance inserted in series with the shunt field.
- An increase in controlling resistances reduces the field current with a reduction in flux and an increase in speed.
- This method of speed control is independent of load on the motor. Power wasted in controlling resistance is very less as field current is a small value.
- By varying flux, we can increase the speed more than its base speed. This method is a constant power and variable torque drive.