MCQs on Electrical Machines

01․ Two generators G₁ and G₂ are to be operated in parallel to deliver a total current of 60 A. Generator one has a terminal voltage of 280 V on no load and 240 V when supplying 40 A current. Similarly the second generator has a voltage of 300 V when on no load and 240 V when supplying of 50 A of current. The output voltage of each generator will be

In first generator, voltage drop per ampere = ( 280 - 240 ) / 40 = 1 A. Let output current is I_G1, then output voltage will be V_G1 = 280 - 1 X I_G1 In second generator, voltage drop per ampere = ( 300 - 240 ) / 50 = 1.2 A. Let output current is I_G2, then output voltage will be V_G2 = 300 - 1.2 X I_G2. For parallel operation, output of each generator would be same i.e. V_G1 = V_G2 ⇒ - I_G1 + I_G2 = 20 Total current delivered, I_G1 + I_G2 = 60 ⇒ I_G1 = 23.6 A and I_G2 = 36.4 A So, the output voltage V_G1 = V_G2 = 256.4 V

02․ In above question, the output power delivered by generator G₁ and G₂ will be

In from hints number 19, we already get V_G1 = V_G2 = 256.4 V and I_G1 = 23.6 A and I_G2 = 36.4 Amp. So, power delivered P = VI.

03․ Armature reaction in dc motor results

The effect of the magnetic field developed by the current carrying armature conductors on the distribution of flux under the main poles is called armature reaction. Because of armature reaction the flux will reduce then emf will reduce then speed will increase.

04․ In shunt dc motor the direction of rotation of the motor will reversed. When

Whenever field or armature terminals is reversed then the direction of rotation of the motor will reverses.

05․ What is the necessity of copper brushes in a dc motors?

The main function of the brushes is to relay the current from external circuit to the rotating commutator form where it flows into the armature windings. Which is not given in the options.

06․ The armature core of the rotor bars of dc motors are laminated

The armature core of the rotor bar of a dc motor is made of several sheets of laminated steel stacked together, to reduce the path for the flow of eddy current, and thereby reducing eddy current loss in the core.

07․ A lap wound 4 pole dc motor has 460 conductors. Find the number of parallel paths between the conductors?

In case of a lap wound dc motor the number of parallel paths is equal to the number of poles, irrespective of all other parameters. It can be easily remembered by remembering the word LAP ------- L A = P. So here number of poles = 4

08․ The armature resistance of the dc motor is generally of the range

The armature of the dc motor is designed in such a manner so as to keep the armature winding resistance in and around 0.5 ohm. If the resistance is increased beyond that, losses will be too high.

09․ The power flow of a dc motor, with respect to a DC generator is

In case of DC generator mechanical input is supplied by rotating the shaft of the machine with a prime-mover and we get electrical energy as output. Whereas in case of a dc motor, we supply electrical energy to the supply terminals and we get rotational torque or mechanical energy as output. So the power flow in case of a dc motor is just the reverse of power flow in case of a dc generator.

10․ A 4 pole dc motor with armature winding resistance of 0.6 ohm receives a supply of 230 Volt. If the back emf produced during running condition is equal to 210 volt. Calculate the current flowing through the armature at this point.

Given E = 230 volt. E_b = 210 volt. The voltage equation of a dc motor is given by E = E_b + I_aR_a. ⇒ I_a = (E - E_b) / r_a Or I_a = ( 230 - 210 ) / 0.6 = 33.3 Amp.