MCQs on Electrical Machines

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01․ Which type of single phase induction motor is having the highest power factor at full load ?
Shaded pole type.
Split phase type.
Capacitor start.
Capacitor run type.

If we want to improve the power factor, we have to connect the capacitor across the power supply at all time. We know that, in the capacitor run type single phase induction motor, the capacitor is permanently connected with the auxiliary winding. But, this is not happening in any other above mentioned motors. So, the capacitor run type single phase induction motors have highest power factor at full load.

02․ The inertia constant H of a synchronous condenser is
greater than that of turbo alternator.
greater than that of hydro generator.
equal to that of the effective length of the line.
none of these.

The inertial constant (H) of a synchronous condenser if the ratio of the stored kinetic energy at rated speed to the rated apparent power of the synchronous condenser.

03․ If the excitation of the synchronous generator fails, it acts as a/an
synchronous generator.
synchronous motor.
induction motor.
induction generator.

Normally the alternatives are connected to the system to which number of others alternator are also connected and hence the system behaves as an infinite bus. Due to influence of this in finite bus, there will be a rotating magnetic field in the stator windings of all alternators and these fields are all rotated in synchronous read. The field created by the rotor windings gets locked with this rotating magnetic field of the stator and also rotates in shame same speed. If the excitation of the generator fails, suddenly there will be no more magnetically locking between rotor and rotating magnetic field of stator. But still the governor will supply same mechanical power to the rotor due to this sudden magnetic unlocking; the rotor will be accelerated beyond the synchronous speed. Hence there will be a negative slip between rotor and rotating magnetic field which creates large slip frequency currents in the rotor circuit to maintain the power output of the machine as an induction generator.

04․ Hopkinss test in DC motor is conducted at ?
No load.
Low load.
Full load.
None of these.

In Hopkinsons test the DC motor is connected to a DC generator. The DC generator supplies required power for driving the DC motor and the DC motor coupled with generator shaft supplies the required mechanical power to rotate the DC generator. In this way both of the machines are made to be operated at their full load capacity. If there is no loss in the system, there is no need of external power supply but practically due to the drop in the generator output voltage and extra voltage source is required to supply proper voltage of the motor.

05․ An unexcited single phase synchronous motor is
AC series motor.
universal motor.
reluctance motor.
repulsion motor.

Usually, unexcited single phase synchronous motor runs at constant speed equal to synchronous speed of revolving flux. They do not need a dc excitation for their rotors. Thats why they are called as unexcited single phase synchronous motors. These motors are divided in two types as, one is reluctance motor and another one is hysteresis motor.

06․ Select the motor that should not be started on no load?
Series motor.
Shunt motor.
Compound motor.
Brushless DC motor.

In a DC series motor, at starting time there is only residual flux in the series field winding because the current is zero. We know that the speed is inversely proportional to flux. Hence, if the residual flux is very low then the speed is enormously high. It damages the internal parts of the motor. So, the DC series motor should not be started on no load.

07․ If the source voltage of a DC motor is increased, which of the following will decrease?
Full load current.
Starting torque.
Starting torque, operating speed, full load current.
Operating speed.

We know that the source voltage (V) is the summation of the back EMF voltage and the armature voltage drop. Hence, the armature load current is, IL= (V-EB)/Ra . So, whenever the source voltage is increased, then the full load current will decrease .

08․ Which of the following statement is true for an AC synchronous motor?
It is a constant speed motor.
It is a variable speed motor.
It is a step angled motor.
It is a high reluctance motor.

The following statements are true in the AC synchronous motor. 1. It is not a self-starting motor. So, it needs external mechanical input or some other method to start the motor. 2. It is a constant speed motor because of the rotating magnetic field of the stator power supply and the magnetic field of the rotor magnets are magnetically coupled. 3. It is doubly excited motor that means both the stator and rotor are excited by external power voice of availability supply. In a synchronous motor, there is a rotating magnetic field in the stator and rotor is a pure electromagnet which magnetically locked with the rotating magnetic field and rotates at the same speed of rotating magnetic field. The rotating magnetic field rotates at synchronous speed depending upon the frequency of applied voltage. As the frequency of applied voltage is normally fixed, the speed of the rotating magnetic field and the rotor of the machine are also fixed. That is why synchronous motor is referred as constant speed motor.

09․ In DC generator, full form of MNA is what?
Magnetic no-load axis.
Magnetic neutral axis.
Magnetic neutral armature co-ordinate.
Magnetically no-load armature co-ordinate.

In a DC machine, the MNA stands for Magnetic Neutral Axis. Basically, in order to achieve the sparkless commutation, the brushes on the commutator should be placed at the points known as neutral point where no voltage exists between adjacent segments. The conductors connected to these segments lie between the poles in position of zero magnetic flux which is termed as magnetic neutral axis (MNA).

10․ The output voltage of a DC generator is
E = V + IaRa.
E = V – IaRa.
E = V + Ia.
E = V + Ra.

We know that the internal armature voltage drop (IaRa) is happened due to the generated EMF (E) in a DC generator. So, we cannot get the total generated EMF as the terminal voltage (V) because the internal armature resistance drops some voltage on it and it delivers rest of the voltage to the terminal side. Hence, the generated EMF is the summation of the voltage drop in the armature circuit and the terminal voltage. In this case, we have neglected the voltage loss due to armature reaction.