21. γ is given as 1.585. Resistance at maximum allowable current is given equal to 4 Ω, what is the 5th step resistance?
0.235
0.370
1.476
2
Answer:1. 0.235
Explanation:
γ = 1.585.
So γ-1 = 1/1.585 = 0.631. R1
0.631. R1 is provided and it is equal to 4 Ω.
r1= (1 − 0.631) × 4 = 1.476 Ω
r2 = 1.476 × 0.631= 0.931 Ω
similarly r5= 0.235 Ω.
22. Statement (I): A Direct-On-Line (DOL) starter for starting DC motor is used for reasons of economy
Statement (II): DOL starter limits the starting current to a safe limit
Both Statement I and Statement II are individually true and Statement II is the correct explanation of Statement I
Both Statement I and Statement II are individually true but Statement II is not the correct explanation of Statement I
Statement I is true but Statement II is false
Statement I is false but Statement II is true
Answer: 3. Statement I is true but Statement II is false
Explanation:
A DOL starter (or Direct On Line starter or across the line starter) is connected directly across motor terminals.
As the DOL starter connects the motor directly to the main supply line, the motor draws a very high current compared to the full load current of the motor (up to 5-8 times higher). The value of this large current decreases as the motor reaches its rated speed.
The main advantages of DOL starter are it is simple and most economical. It is more comfortable to design, operate and control.
23. Four-point starter is used when _______
Motor field current is varied in a narrow range
Motor speed is varied in small range
Motor field current is varied over wide range
Can be used anywhere
Answer: 3. Motor field current is varied over wide range
Explanation:
A 4-point starter protects the armature of a DC shunt motor or compound wound DC motor against the initially high starting current of the DC motor.
The most significant difference between the three-point and the four-point starter is that in a three-point starter the no voltage coil (NVC is connected in series with the field winding whereas in a four-point starter the NVC is directly connected to the supply voltage.
In a four-point starter, the hold-on coil is connected across the line instead of in series with the shunt field circuit. This makes a wide range of field adjustments possible.
24. The stator of a DC machine comprises of:
Main poles
Inter poles
Frame
All of these
Answer: 4. All of these
Explanation:
A DC machine consists of two basic parts; stator and rotor.
The stator consists of Yoke, Frame, Compole or Interpol, Field while rotor consists of armature winding.
Basic constructional parts of a DC machine are described below.
Yoke:
The outer frame of a dc machine is called a 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 former wounds 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 which 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 the 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.
Commutator and brushes:
In the case of the DC generator, the commutator is used to convert generated AC in armature into DC
In the case of a DC motor, the commutator is used to convert DC to A.C
25. Which of the following are protective devices in DC motor Starter?
No volt coil
Overvolt coil
Both 1 and 2
None of the above
Answer: 1. No volt coil
Explanation:
Normally a dc motor starter is provided with two protective devices namely
(i) no-volt release
(ii) overload release.
NVC (no-volt coil): In case of failure of field current (due to accidental or otherwise open circuiting), this coil releases the handle (held electromagnetically), which goes back to the OFF position under the spring action.
OL (overload release): The contact of this relay at armature current above a certain value (overload/short circuit) closes the NVC ends, again bringing the handle to OFF position.
26. Statement (I): The direct-on-line (DOL) starter is used to start a small dc motor.
Statement (II): DOL starter limits the initial current drawn by the armature circuit.
Both Statement (I) and Statement (II) are individually true and Statement (II) is the correct explanation of Statement (I)
Both Statement (I) and Statement (II) are individually true but Statement (II) is NOT the correct explanation of Statement (I)
Statement (I) is true but Statement (II) is false
Statement (I) is false but Statement (II) is true
Answer: 3. Statement (I) is true but Statement (II) is false
Explanation:
A DOL starter (or Direct On Line starter or across the line starter) is connected directly across motor terminals.
As the DOL starter connects the motor directly to the main supply line, the motor draws a very high current compared to the full load current of the motor (up to 5-8 times higher). The value of this large current decreases as the motor reaches its rated speed.
By this method, small motors up to 5 kW rating are started to avoid the supply voltage fluctuation.
The main advantages of a DOL starter are it is simple and most economical. It is more comfortable to design, operate and control.
Therefore, Statement I is true but Statement II is false.
Note:
An extra resistance must be included along the DOL starter to limit the starting current.
27. Which of the following are the disadvantages of a three-point starter?
Field Current Become weak
Disconnect the motor from the supply
The flow of current through the shunt field is decreased.
All of the above
Answer: 4. All of the above
Explanation:
Limitation of three-point starter:
In this starter, the NVC and the field winding are in series. So while controlling the speed of the motor above the rated speed, the field current is reduced by adding an extra resistance in series with the field winding. Therefore current through the shunt field is reduced. Due to this, the current through NVC also reduces.
Due to this, magnetism produced by NVC also reduces and NVC may not be able to keep the starter arm in the ON position. This may disconnect the motor from the supply when it is not desired.
When NVC (no volt trip coil) is associated in series by shunt field, then the minute current will decrease the power of the electromagnet.
This magnet may liberate the arm of the H-handle through the usual motor operation as well as detach it from the power supply.
Therefore, the 4 Point Starter can be used, where the no-volt trip coil is allied in the parallel field.
28. The four-point starter provides the motor with ______
No Voltage Protection
Thermal Protection
No current Protection
Any of the above
Answer: 1. No Voltage Protection
Explanation:
The four-point starter provides the motor with no voltage protection. Should the power fail, the motor must be disconnected from the line. If not, full line voltage will be applied ‘ to the armature without the benefit of starting resistors when power is restored. The holding magnet, being connected across the line, releases the arm when the voltage drops below a specific value, thus protecting the motor when the power is restored.
29. The main disadvantage of a Four-point starter in DC Machine is
Unable to provide high-speed motor Protection
Field Current Become weak
Disconnect the motor from the supply
The flow of current through the shunt field is decreased.
Answer:1. Unable to provide high-speed motor Protection
Explanation:
Disadvantage of Four-point starter
The only limitation of the four-point starter is, it does not provide high-speed protection to the motor.
If under the running condition, the field gets opened, the field current reduces to zero. But there is some residual flux present and N∝ 1/φ the motor tries to run at a dangerously high speed. This is called the high speeding action of the motor.
In a three-point starter as NVC is in series with the field, under such field failure, NVC releases the handle to the OFF position. But in the four-point starter, NVC is connected directly across the supply and its current is maintained irrespective of the current through the field winding.
Hence it always maintains the handle in the RUN position, as long as the supply is there and thus it does not protect the motor from field failure conditions which results in the high speeding of the motor.
30. In a four-point starter NVC is connected _______ across the supply.
Series
Parallel
Independently
Series Parallel
Answer: 3. Independently
Explanation:
The basic difference between three-point and four-point starters is the connection of NVC. In three-point, NVC is in series with the field winding while in four-point starter NVC is connected independently across the supply through the fourth terminal.
Hence any change in the field current does not affect the performance of the NVC. Thus it is ensured that NVC always produces a force that is enough to hold the handle in the ‘RUN’ position, against the force of the spring, under all the operating conditions.
