1.Which of the following statements is/are correct regarding the generation of EMF in rotating electrical machines in the armature winding? EMF is generated ____________
a) by rotating armature windings through a magnetic field
b) by rotating magnetic field with respect to the armature windings
c) by designing the magnetic circuit to have variable reluctance with rotor rotation
d) all of the mentioned
Explanation: All of the above mentioned methods will produce EMF in the armature windings.
2. The EMF equation e = NωrΦsinωrt is applicable to ____________
a) AC systems with time variant field flux
b) DC systems with time variant field flux
c) Both AC and DC systems with time invariant field flux
d) Both AC and DC systems with time variant field flux
Explanation: e = NωrΦsinωrt-NdΦ/dtcosωrt, for time invariant field flux dΦ/dt = 0 ⇒ e = NωrΦsinωrt, is the general equation, and is applicable to both AC and DC systems.
3. In the equation for RMS value of the generated EMF in a full pitched coil of an AC machine, E = Emax/√2 = √2πfrNΦ, fr depends on ____________
a) rotating speed of the armature coil
b) rotating speed of the flux density wave
c) relative velocity between the flux density wave and armature coil
d) all of the mentioned
Explanation: fr is called the rotational or speed frequency, since its value depends upon the relative velocity between the flux density wave and the armature coil.
4. In AC rotating machines, the generated or speed EMF ___________
a) leads Φ by 90°
b) lags Φ by 90°
c) is in phase with working flux
d) lags Φ by 180°
Explanation: e = Emaxcos(ωrt-π/2)
ψ = NΦcosωrt
It reveals that the speed or generated EMF lags by 90° the flux that generates it, and is true when flux is time invariant and is sine distributed in space.
5. In a short pitched coil, the coil pitch factor kp, is given by ____________
a) kp = cosε
b) kp = cosε/2
c) kp = √2πfrNΦcosε
d) kp = cosε/2cosωrt
Explanation: The effect of short pitched coil is to reduce the generated EMF, and this reduction factor is cosε/2, and is referred to as coil pitch factor.
6. Which of the following equations represents the RMS value of the generated EMF in a short-pitched N-turn armature coil of an AC machine ____________
a) E = √2πkpfrNΦ
b) E = 2πkpfrNΦ
c) E = NΦωrkpsinωrt
d) None of the mentioned
Explanation: EMF induced = NΦωrkpsinωrt,
Emax = NΦωrkp when sinωrt = 1
RMS value, E = Emax/√2 = √2πfrkpNΦ
7. The effect of short pitched coil on the generated EMF is _____________
a) increasing
b) decreasing
c) either increasing or decreasing
d) none of the mentioned
Explanation: The effect of short pitched coil is to reduce the generated EMF and this reduction factor is cosε/2 and is referred to as coil pitch factor.
8. A winding is distributed in the slots along the air gap periphery. Which of the following statements are true?
(i) to add mechanical strength to the winding
(ii) to reduce the amount of conductor material required
(iii) to reduce the harmonics in generated EMF
(iv) to reduce the size of the machine
(v) for full utilization of iron and conductor materials
a) (i), (iii), (iv), (v)
b) (i), (ii), (iv), (v)
c) (i), (ii), (iii), (v)
d) (i), (iii), (v)
Explanation: In rotating electrical machines, the armature turns are usually distributed in slots rather than concentrated in single slot. This is essential from the view point of utilizing the armature periphery completely, and add mechanical strength.
9. In an AC machine, the effect of distributing the turns in different slots, results in a further reduction of generated EMF by the factor kd. This factor is called ____________
a) distribution/speed factor
b) coil pitch factor
c) winding factor
d) all of the mentioned
Explanation: E = √2πkpkdfrNΦ, kd is the distribution factor.
10. A polyphase induction motor of the slip ring or wound rotor type can be used ____________
a) for high start-up torque applications
b) as a frequency converter
c) any of the mentioned
d) none of the mentioned
Explanation: A polyphase induction motor can be used as a frequency converter (or changer) for changing the supply frequency f to other frequencies sf and (2-s)f at the slip ring terminals.