GATE Electrical Engineering (EE) Test: Direct Current Machine of Machines

For dc shunt machine, field current is very less, therefore for required field AT/Pole, number of turns of field winding is increased. Thus, R should be high for low I_f.
For dc series machine field winding current is equal to armature current which is of larger value. Therefore, number of turns of field winding is reduced to get the required field AT/Pole. As it has to handle large current, therefore resistance of field winding should be less by keeping the winding thick. Statement-3 is wrong.

Given, speed,
N= 1800 rpm 
n = 1800/60 rps = 30 rps
Average emf induced per conductor 

Peripheral velocity of commutator is 

Now, brush width = v x time of commutation

or, time of commutation

We know that,

Now, rated armature current is 

So, the terminal voltage is

The reason of assertion is that dc series motor develop high torque at start due to which these motors are used for hard to start loads. Reason is also correct because these are variable speed motors.

The solution can be summarised as follows:

• Armature resistance control is considered wasteful and inefficient for controlling the speed of DC motors.
• This method results in a large amount of power being wasted in the control resistances.
• This inefficiency is particularly noticeable in shunt motors.

Therefore, both the assertion and the reason are true, and the reason correctly explains the assertion.

Solution:

• Field test: Used to determine the performance of machines regarding commutation and temperature rise.
• Swinburne’s test: Measures the constant losses in a DC machine.
• Hopkinson test: Evaluates the efficiency and performance of two identical machines.
• Retardation test: Determines the stray losses in DC shunt machines.

We know that,

∴ 
or, 
∴ 
Thus, voltage generated is increased by 8%.

For LAP winding, number of parallel path = number of poles.
Thus, resistance is less compared to wave winding. Hence, current rating is more and voltage rating is less compared to wave winding.

In a DC machine, iron losses (also called core losses) occur mainly due to hysteresis and eddy currents in the parts of the machine that carry alternating magnetic flux. These losses are significant only in components where the magnetic field varies with time.

Let’s evaluate each part:

1. Pole Core – Carries DC flux, and since the magnetic field is not alternating, iron losses are negligible.

2. Brushes – These are electrical contacts, not magnetic or iron parts. They don’t carry flux, so no iron loss occurs here.

3. Yoke – Also carries DC flux, and again, since there’s no alternating flux, iron losses are negligible.

4. Armature Core – This rotates in the magnetic field, and the magnetic flux through it alternates due to rotation. Thus, it does experience iron losses.

So, Option C is the correct answer.