# MCQs on Electrical Machines

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01․ A belt-driven cumulatively-compound dc generator is delivering power to the dc mains. If the Belt snaps then the machine will run as a
cumulative compounded motor in the same direction
differentially compounded motor in the same direction
cumulative compounded motor in opposite direction
differentially compounded motor in the opposite direction

When suddenly belt snaps current direction in the series field and armature will be reversed but current through shunt field will be same. As current direction in the series field is reversed cumulative machine will act as a differential machine and by flemming’s left hand rule we get motor runs in the same direction as dc generator.

02․ In a dc machine, 1. physically, the brush axis is along the direct axis 2. physically, the brush axis is along the interpolar axis 3. schematically, the brush axis is along the direct axis 4. schematically, the brush axis is along the quadrature axis 5. armaturemmf is directed along the brush axis From these, the correct statements are
1, 4,5
1 , 3, 5
2, 3, 5
2, 4, 5

Physically the brush axis is along the direct axis and schematically the brush axis is along quadrature axis. Armature mmf is directed along the brush axis.

03․ The direction of generated emf in the armature coil of a DC machine is 1. the same as that of the current in a generator 2. the same as that of the current in a motor 3. opposite to that of the current in a generator 4. opposite to that of the current in a motor From these, the correct answer is
1, 3
2, 3
2, 4
1, 4

Conventionally we take the direction of emf from + ve terminal to – ve terminal and current direction is also from + ve terminal to – ve terminal. So, in case of generator the direction of armature current is nothing but direction of generated emf .In case of a motor a generator action also takes place and the direction of generated emf can be derived by flemming’s right hand rule which is exactly opposite to the supply current in the armature i.e. we can say the direction of generated emf in the armature coil of a dc machine is the same as that of the current in a generator and opposite to that of the current in a motor.

04․ The counter (or back) emf in a dc machine is 1. the same as generated emf in a dc motor 2. proportional to field flux and armature speed 3. not the same as generated emf in a dc motor 4.inversely proportional to field flux 5. proportional to number of armature conductors 6. constant from no load to full load From these, the correct answer is
1, 2, 5
2, 4, 5, 6
2, 3, 6
3, 4, 5

The counter or back emf Ebα NΦZ Where, N = speed of the motor Φ = Field flux Z = Number of armature conductors

05․ A 220 V dc machine has an armature resistance of 1Ω.If the full-load current is 20 A, the differenc in the induced voltages when the machine is running as a motor and as a generator is
20 V
zero
40 V
50 V

In case of dc-generator, Induced voltage In case of dc-motor, Induced voltage So, the difference between induced voltages is 220 -180 = 40 V

06․ A separately-excited dc motor has an armature resistance of 0.5 Ω. It runs off a 250 V dc supply Drawing an armature current of 20 A at 1500 rpm. The torque developed for an armature current Of 10 A, for the same field current, will be
15.28 Nm
15.92 Nm
15.6 Nm
16.55 Nm

We know, We also know TαIa Hence,

07․ A bipolar dc machine with interpoles has a main-pole flux of ф per pole and an interpole flux of Ф per pole. The yoke of the machine is devide into four quadrants by the main-pole axis and The commutation axis. The flux-distribution in the quadrants will be
½ ( ф + фᵢ ) in all the four quadrants
½ ( ф - фᵢ ) in all the four quadrants
½ ( ф + фᵢ ) in two diametrically opposite quadrants and ½ ( ф - фᵢ ) in the remaining two Quadrants
½ ( ф + фᵢ ) in two adjacent quadrants and ½ ( ф - фᵢ ) in the remaining two quadrants

If we treat flux as currents then we can easily get distribution by kcl ½ ( ? + ?? ) in two diametrically opposite quadrants and ½ ( ? - ?? ) in the remaining two quadrants.

08․ The residual magnetism of a self-excited dc generator is lost. To build up its emf again
the field winding must be replaced
the armature connection must be reversed
the field winding connection must be reversed
field winding must be excited by low voltage dc supply

If residual magnetism of a self-excited dc generator is lost then to build up its emf again field winding must be excited by low voltage dc supply.

09․ The armature mmf of a DC machine has
triangular space distribution and rotates at the speed of armature
trapezoidal space distribution and is stationary in space
stepped distribution and rotates at the speed of armature
triangular space distribution and is stationary in space

The armature mmf of a DC machine has triangular space distribution and is stationary in space. Armature mmf varies from brush to middle of the pole and middle of the pole to the brush. Plotting of these variations of armature mmf gives a triangular shape. Under a pole current direction through the armature remains always same i.e. mmf also remains same. So when we see the mmf with respect to space it remains stationary.

10․ The flux is maximum in the following part of a dc motor :
pole core
under the interpole
under leading pole tip
under trailing pole tip

The flux is maximum under leading pole tip of a dc motor.

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