21. In a D. C. machine, the number of commutator segments is equal to ________
Number of conductors
Twice the number of poles
Number of coils
None of the above
Answer:3. Number of coils
Explanation:
Commutator segments:
The purpose of the commutator is to rectify the ac waveform.
The segments of the commutator are insulated from each other and the armature shaft.
Wires from the armature are connected to the armature segments.
Armature current is conducted from the commutator to the load by carbon brushes.
To help produce a smoother dc output, more coils and commutator segments are used.
The coils are insulated from each other, the same as the commutator segments.
The number of commutator segments is equal to the number of slots or coils
22. An electric motor is having constant output power. So, motor will have a torque-speed characteristic __________
Circle about the origin.
Straight-line parallel to the speed axis.
Straight-line through the origin.
Rectangular hyperbola
Answer:4. Rectangular hyperbola
Explanation:
The torque-speed characteristics of an electric motor having constant output power will be a rectangular hyperbola which is represented as xy= constant. N ∝ 1/Ia is the equation of rectangular hyperbola of the form y ∝ 1/x or xy =C
23. If a DC motor designed for 45°C ambient temperature is to be used for 55°C ambient temperature, then the motor ______
Of lower HP should be selected
Of higher HP should be selected
Can be used for 50°C ambient temperature also
Is to be de-rated by a factor recommended by the manufacturer
Answer:4. Is to be de-rated by a factor recommended by the manufacturer
Explanation:
Ambient temperature is the temperature of the air surrounding the motor. This is the threshold point or temperature the motor assumes when shut off and completely cool. Temperature rise is the change within a motor when operating at full load.
The ambient temperature is one of a factor in derating. If the ambient temperature given is high, the insulation may reach its maximum temperature limit quickly. Hence, the motor may have to be de-rated, and for application, the next motor must be used.
24. Which of the following quantity will decrease if the supply voltage is increased?
Starting torque
Operating speed
Full-load current
Cannot be determined
Answer:3. Full-load current
Explanation:
The rated power supplied to the DC motor is constant in wattage. So if the supply voltage increases then automatically the current drawn by the supply terminal decreases as well as the electromagnetic torque on the rotor shaft decreases but the motor speed increases. The motor speed is directly proportional to the voltage applied to it.
The current drawn by the motor is directly proportional to the load current (DC series motor). So if the load increase the current drawn by the motor from supply also increases and vice versa.
When the supply voltage is increased full load current will decrease in order to keep output power constant, which will decrease torque at that moment, while starting torque will remain as it is, irrespective of any change in supply voltage.
25. Torque developed by a DC motor depends upon _______
Magnetic field
Active length of the conductor
Current flow through the conductors
Current, active length, no. of conductors, magnetic field all
Answer:4. Current, active length, no. of conductors, magnetic field all
Explanation:
For DC Machine, torque is directly proportional to armature current and flux.
T ∝ ϕ Ia
In dc shunt motors, flux is constant (not dependent on armature current). Hence torque is proportional to armature current.
T ∝ Ia
In dc series motors, flux is directly proportional to armature current. Hence torque is directly proportional to the square of the armature current.
Ta ∝ Ia2
In dc series motor at saturation condition, flux is constant. Hence torque is proportional to armature current.
T ∝ Ia
Where, T = torque, Ia = armature current, ϕ = field flux
Note:- Torque in DC machine also depends on some constant values like active length, no. od armature conductors and magnetic field which is constant for a given machine.
26. The armature shaft of a DC motor must be able to withstand _________
Bending moment due to weight of the armature.
Any unbalanced magnetic pull on the armature core.
Twisting stains due to transmission of torque.
Bending moment, unbalanced magnetic pull and twisting stains
Answer:4. Bending moment, unbalanced magnetic pull and twisting strains
Explanation:
The motor shaft is the central section of the electric motor. The term shaft usually refers to a component of the circular cross-section that rotates and transmits power from a driving device, such as a motor or engine, through a machine.
The armature shaft of a DC motor must be able to withstand bending moment due to the weight of the armature and commutator, any unbalanced magnetic pull on the armature core, twisting stains due to transmission of torque, for a good and long run application of the motor.
27. For very sensitive and wide speed control, which of the following method is preferred?
Armature control
Ward-Leonard control
Multiple voltage control
Field control
Answer:2. Ward-Leonard control
Explanation:
The motor used for high starting torque and wide speed range control requirement is DC motor. In dc motor wide speed range control by using the Ward Leonard speed control method.
The main advantages of the Ward Leonard drive are as follows:-
Smooth speed control of DC motor over a wide range in both the direction is possible
It has an inherent braking capacity
The lagging reactive volt-amperes are compensated by using an overexcited synchronous motor as the drive and thus, the overall power factor improves
When the load is intermittent as in rolling mills, the drive motor is an induction motor with a flywheel mounted to smooth out the intermittent loading to a low value
28. In DC machines the residual magnetism is present. The order of residual magnetism is _______
2 to 3 percent
10 to 15 percent
20 to 25 percent
50 to 75 percent
Answer:1. 2 to 3 percent
Explanation:
Residual magnetism is the small magnetic field left in the iron cores of the shunt fields when the DC machine is at rest. . When the current is reduced to zero, there is still magnetic power left in those coil’s core. This phenomenon is called residual magnetism.
The core of a DC machine is made of ferromagnetic material. The order of residual magnetism is about 2 to 3%.
29. Field weakening control method is used for the speed _________
Above rated speed
Below rated speed
Can be used anywhere
None of the above
Answer:1. Above rated speed
Explanation:
In the field weakening method, we are reducing the working flux to increase the speed, by reducing the field current. Therefore, the effect of armature flux on main field flux will increase in the case of the field weakening method.
By reducing of field current, the flux will also reduce, because the flux is directly proportional to the field current passing through the winding. As speed is inversely proportional to flux, this method can be used for only above-rated speed.
30. Armature resistance control method is used for the speed ________
Above rated speed
Below rated speed
Can be used anywhere
None of the above
Answer:2. Below rated speed
Explanation:
In the armature circuit resistance method armature resistance is increased. As a result, the back emf decreases. As the speed is directly proportional to back emf speed will also decrease. Therefore this method is only used for below-rated speed.
The armature resistance control method involves additional resistance in series with armature resistance, the power losses are high. Thus, efficiency is minimum in this case.
31. Tapped-field control method is used for the speed ______
Above rated speed
Below rated speed
Can be used anywhere
None of the above
Answer:1. Above rated speed
Explanation:
Tapped Field Control
This is another method of increasing the speed by reducing the flux and it is done by lowering the number of turns of the field winding through which current flows.
In this method, a number of tapping from field winding are brought outside. This method is employed in electric traction.
In the tapped-field control method, the number of turns is reduced in the field winding thus reducing the field flux, which in turn increases the speed of the DC series motor.
Similar to the field diverter method, this method can be used to control the speed above the rated speed of the DC series motor.
32. Why field winding of a DC series motor is provided with thick wire?
As it carries large load current
To provide large flux
In order to reduce eddy current to provide large flux
To reduce the of insulting materials
Answer:1. As it carries large load current
Explanation:
Field control is a method of controlling the speed of a DC shunt motor above normal by regulating the field flux by means of a field regulating variable resistor connected in series with the field winding.
A resistance of low ohmic value and high current-carrying capacity is employed in the armature circuit, whereas the resistance of high ohmic value of thick wire and low current-carrying capacity is employed in the field circuit because it carries a large load current.
33. Which function is performed by counter emf of a DC motor?
Exceeds supply voltage
Aids applied voltage
Helps in energy conversion
Regulates its armature voltage
Answer:3. Helps in energy conversion
Explanation:
When the armature of a DC motor rotates under the influence of the driving torque, the armature conductors move through the magnetic field and hence emf is induced in them as in a generator. The induced emf acts in opposite direction to the applied voltage V (Lenz’s law) and is known as Back EMF or Counter EMF (E. It acts against the applied voltage that’s causing the motor to spin in the first place and reduces the current flowing through the coils of the motor.
We know that the back emf opposes the supply voltage.
The supply voltage induces the current in the coil which rotates the armature.
The electrical work required by the motor for causing the current against the back emf is converted into mechanical energy and that energy is induced in the armature of the motor.
Thus, we can say that energy conversion in DC motor is possible only because of the back emf.
34. The output power of any electrical (AC or DC motor is taken from the _________
Field
Coupling mounted on the shaft
Armature
Motor frame
Answer:2. Coupling mounted on the shaft
Explanation:
A shaft coupling is a mechanical component that connects the driveshaft and driven shaft of a motor, etc. in order to transmit power. Shaft couplings introduce mechanical flexibility, providing tolerance for shaft misalignment.
The output power of any electrical motor is always taken from the coupling mounted on the shaft because the motor converts input electrical power into output mechanical power. So, we get mechanical power at shaft coupling.
35. Insulating material, cotton or oiled cambric tapes, are used for which part of machines?
Rectangular conductors such as used in large D.C. machines
Taping armature and field coils of traction motor stator coils of high-voltage alternators
Insulation of commutators
For insulating terminals of high-voltage machines
Answer:1. Rectangular conductors such as used in large D.C. machines
Explanation:
Insulation materials used in different parts of Electrical Machine:
Cellulosic backing paper, mica splittings, shellac varnish, and industrial methylated spirit. insulation of coils up to high operating voltages such as alternator armature winding, DC series motor (traction motor), tapping, etc.
Mica separation insulation is commonly used as commutator insulation.
Insulating material, cotton or oiled cambric tapes, are used for the rectangular conductors or strip conductors in DC machines.
