The commutator is a mechanical rectifier, so the commutator collects induced EMF or current developed in the armature.
In a motor, the commutator applies electric current to the windings. By reversing the current direction in the rotating windings each half turn, a steady rotating force (torque) is produced.
Therefore commutator work as the mechanical rectifier and current controller.
Ques 91. The purpose of using a dummy coil in a dc machine is to
Eliminate harmonics developed in the machine
Eliminate armature reaction
Bring mechanical balance to the armature
Bring mechanical balance to the body of the motor
Answer.3. Bring mechanical balance of the armature
Explanation:
A Dummy coil is used with wave winding when the requirement of the winding is not met by the standard armature.
The Dummy coil is not connected to the commutator so they do not influence the electrical characteristics of the winding
Dummy coil ends are cut short and taped.
Their main use is to provide mechanical balance for the rotor because the rotor having some slots without winding would be out of balance mechanically.
Ques 92. An inductor with ferromagnetic core supplied from a sinusoidal voltage source with frequency ‘f’. The current drawn by the inductor will be
Sinusoidal with frequency ‘f’
Sinusoidal with frequency ‘2f’
A sawtooth wave
Non-sinusoidal with frequency ‘f’
Answer.4. Non-sinusoidal with frequency “f”
Explanation:
Inductors with ferromagnetic cores have additional energy losses due to hysteresis and eddy currents in the core, which increase with frequency. At high currents, iron core inductors also show a gradual departure from ideal behavior due to nonlinearity caused by magnetic saturation of the core
If the current through a ferromagnetic core coil is high enough that the magnetic core saturates, the inductance will not remain constant but will change with the current during each half of the period to provide the maximum flux density, therefore, the inductor with a ferromagnetic core end up having non-sinusoidal excitation current.
Ques 93. For a 6-pole dc machine with wave wound armature, the number of brushes required is
2
4
6
12
Answer.1. 2
Explanation:
In wave winding the number of brushes is always 2 irrespective of the number of poles.
Ques 94. The function of interpoles in a dc machine are to
Reduce field winding heating
Improve commutation
Compensate for air-gap variation
Reduce losses
Answer.2. Improve Commutation
Explanation:
Interpoles are small poles compared to the main poles and are placed in the Interpolar region between the main poles and they are connected in series with the armature winding.
Their purpose is to counterbalance the armature reaction due to the flow of current in the armature winding.
Interpole’s primary functions are
It produces a counter flux on the coil which is undergoing commutation to nullify the REACTANCE VOLTAGE.
It also nullifies the armature flux in the inter-polar region automatically.
Ques 95. The commutator segments of the dc machine are made of
Tungsten
Hard-drawn copper
Soft copper
Electrolytic copper
Answer.2. Hard-Drawn copper
Explanation:
Commutators require periodic maintenance such as brush replacement, therefore, to reduce wear and tear commutator segment s of dc machines are made up of hard-drawn copper.
Ques 96. Which of the following is a speed control method for a three-phase squirrel cage induction motor?
Plugging method
Star-delta switch method
Pole-changing method
Centrifugal clutch method
Answer.3. Pole-changing method
Explanation:
The pole changing method is used in squirrel cage IM because like slip ring IM we can’t add external resistance in the rotor to reduce speed.
In this method, it is possible to have one or two speeds by changing the number of poles. This is possible by changing the connection of the stator winding with the help of simple switching.
Ques 97. In the circuit, as shown, the voltage measured between A, B is found to be 70 V. Value of M is
Ques 98. Two coupled coils, connected in series, have an equivalent inductance of 16 mH or 8 mH depending on the connection. The mutual inductance between the coils is
12 mH
8 √2 mH
4 mH
2 mH
Answer.4. 2mH
Explanation:
The overall inductance of 2 coil L1 and L2 connected in series with mutual inductance aiding self-inductance L1 with mutual inductance opposing self-inductance L2 then the mutual inductance M is given as
1/2 (L1 – L2)
1/2 (16 – 8)
= 2mH
Ques 99. Tesla is the unit of
Electric flux density
Magnetic flux intensity
Electric field intensity
Magnetic flux density
Answer.4. Magnetic Flux Density
Explanation:
The tesla (symbol T) is a derived unit of magnetic flux density (informally, magnetic field strength).
One tesla is equal to one weber per square meter.
Ques 100. Which one of the following is a valid value of the coefficient of coupling between two inductors?
1.414
0.9
1.732
17.32
Answer.2. 0.9
Explanation:
The value of the coefficient of coupling is always greater than 0 and less than 1, or 0% and 100% respectively. A coefficient of coupling of 0 would represent no coupling, and 1 would represent perfect coupling.
