# ELECTRICAL MACHINES: PART 1 MCQ and Notes

1. For a D.C Motor : :

V = E + Ia . Ra    . . . . (i)

T = ka . Φ . Ia    . . . . . (ii)

E = kn . Φ . N    . . . .(iii)

where V is the supply voltage to the motor.

E is the  Back e.m.f of the motor.

Ia is the Armature current.

Ra is the Armature resistance.

T is the torque produced in the motor.

ka and kare the proportionality constant for Torque and Back e.m.f. respectively.

Φ is the flux.

N is the rotation speed (in rpm i.e. revolutions per minute).

Based on these formulae , a lot of numerical can be framed where the values of few parameters will be given and the other(s) needs to be found out.

using (ii) and (iii) we can have an auxiliary formula as:

T1 . N1 = T2 . N2

where  Tis the torque at  Nrpm and  T2 is the torque at  N2 rpm.

• 2. Series D.C. Generator cannot build up on Open-circuit .

• 3. Series Generator has the poorest Voltage Regulation.

• 4. D.C. Generator Ideal has ZERO(0) Voltage Regulation.

• 5. D.C. Generator having negative Voltage Regulation is for over compound type.

• 6. Transformer Core decreases the reluctance of the common magnetic circuits.

• 7. DC Series motor : Φ ∝ Ia , which means that as the armature current increases so does the flux.

Hence, T = ka . Ia2  which is derived from point 1 equation (ii)    [look above]

• 8. Transformer No Load test helps us to find out the No load losses and magnetizing current in the transformer

• 9. Transformer has lagging power factor because of drawing magnetizing current for its working.

• 10. Transformer cores are laminated in order to reduce eddy current losses.

Eddy current loss ∝ f2 . kf2 . Bm2 . t2 . V

where,  f is the frequency of reversal of magnetic field ( Hz )

kis the form constant.

Bis the maximum value of flux density ( wb/m)

t is the thickness of lamination ( meter )

V is the volume of magnetic material ( m)