Which of the following statements is/are correct regarding the generation of EMF in rotating electrical machines in the armature winding? EMF is generated ____________
Any of the above mentioned methods will produce EMF in the armature windings.
The EMF equation e = NωrΦsinωrt is applicable to ____________
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.
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
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.
In AC rotating machines, the generated or speed EMF
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.
In a short pitched coil, the coil pitch factor kp, is given by ____________
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.
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 ____________
EMF induced = NΦωrkpsinωrt,
Emax = NΦωrkp when sinωrt = 1
RMS value, E = Emax/√2 = √2πfrkpNΦ
The effect of short pitched coil on the generated EMF is _____________
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.
A winding is distributed in the slots along the air gap periphery
(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
From these, the correct answer is
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.
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 ____________
E = √2πkpkdfrNΦ, kd is the distribution factor.
A polyphase induction motor of the slip ring or wound rotor type can be used ____________
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.
The equation for slip speed is ____________
The relative speed between rotor and rotating flux wave, i.e, (ω-ωr) is referred to as the slip speed in rad/sec
slip speed = (ω-ωr) rad/sec
If the rotor of an induction motor is made to revolve in a direction opposite to the rotating flux wave, then RMS value of EMF induced in one phase of rotor E is proportional to ____________
Relative velocity between rotor winding and rotating flux wave becomes
(ω+ωr) = ω+ω(1-s) = ω(2-s) and
E = 2πf(2-s)NphrkwΦ
In an alternator, frequency per revolution is equal to
For a P-pole machine, in n rev/sec, P/2n cycles/second are generated and cycles per second is referred to as frequency f of the EMF wave. Here in this question n=1
⇒ frequency per revolution = P/2
⇒ number of pole pairs.
A 6 pole machine is rotating at a speed of 1200rpm. This speed in mechanical rad/sec and electrical radians per second is respectively
N = 1200rpm,P = 6 and f = PN/120 = 60 cycles per sec.
1cycle = 360° electrical degrees = 2π electrical radians
f in electrical radians per sec = 60∗2π electrical radians/sec = 120π elect radians/sec
speed in mechanical radians/sec = 2/P(speed in electrical rad/sec) = 2/P(120π)=40π mechanical rad/sec.
The DC machines are designed with flat topped flux density waves because
For the same value of peak flux density Bp, it is easy to see that average value of brush voltage could be more for a flat topped B-wave than for a sinusoidal flux density wave.