JNTU
JNTU B.Tech II ELECTRICAL MACHINES II Previous Question Paper (November/December - 2020 - R13)
Answer any five questions. All questions carry equal marks.
Main
Derive the EMF equation of transformer? Hence derive the voltage ratio.
Answer
Consider a transformer with a primary winding of N1 turns and a secondary winding of N2 turns. When an alternating voltage V1 is applied to the primary, it produces a sinusoidal flux in the core given by Φ = Φm sin(ωt), where Φm is the maximum flux and ω = 2πf is the angular frequency.
According to Faraday's law of electromagnetic induction, the induced EMF in a coil is e = -N(dΦ/dt). For the primary winding:
The maximum value of the induced EMF (E1m) is N1Φmω. Since ω = 2πf, this becomes E1m = 2πfN1Φm. The RMS value of the induced EMF (E1) is the maximum value divided by √2:
Similarly, for the secondary winding:
To derive the voltage ratio, we take the ratio of the two EMF equations:
In an ideal transformer, the induced EMFs are equal to the terminal voltages (E1 ≈ V1 and E2 ≈ V2). Therefore, the voltage transformation ratio (K) is defined as:
This ratio determines whether the transformer is step-up (K > 1) or step-down (K < 1).
Draw the exact equivalent circuit of a transformer and describe briefly the various parameters involved in it?
Answer
The exact equivalent circuit of a transformer represents all physical phenomena occurring within the device. It consists of the primary winding resistance (R1) and leakage reactance (X1), the secondary winding resistance (R2) and leakage reactance (X2), and the magnetizing branch connected in parallel across the primary side. The magnetizing branch comprises the core loss component (Rc) representing hysteresis and eddy current losses, and the magnetizing reactance (Xm) representing the flux required to magnetize the core.
Parameters involved:
- R1, R2: Resistances of the primary and secondary windings representing copper losses.
- X1, X2: Leakage reactances representing flux that does not link both windings.
- Rc: Shunt resistance representing core losses (constant regardless of load).
- Xm: Magnetizing reactance representing the current required to establish flux in the core.
The EMF per turn of a 1-φ, 2200/220 V, 50 Hz transformer is approximately 12V. Calculate: i) The number of primary and secondary turns and ii) The net cross-sectional area of core for a maximum flux density of 1.5 T.
Answer
Sign in to view the complete answer paper.
With neat diagram, discuss the various tests to be conducted on transformer to obtain its equivalent circuit. Derive all related equations.
Answer
Sign in to view the complete answer paper.
A 300 KVA, single-phase transformer is designed to have a resistance of 1.5% and maximum efficiency occurs at a load of 173.2 kVA. Find its efficiency when supplying full-load at 0.8 p.f lagging, normal voltage and frequency?
Answer
Sign in to view the complete answer paper.
Explain with necessary diagrams how two, 3-phase transformers can be used to convert a 3-phase supply to a 2-phase supply.
Answer
Sign in to view the complete answer paper.
Show that an auto-transformer will result in saving copper in place of two winding transformer.
Answer
Sign in to view the complete answer paper.
Two transformers are connected in open-delta and supply a balanced 3 phase load of 240 KW at 400V and a p.f of 0.866. Determine: (i) Secondary line current (ii) the kVA load on each transformer.
Answer
Sign in to view the complete answer paper.
Why the rotor of a poly phase induction motor can never attain synchronous speed? Discuss.
Answer
Sign in to view the complete answer paper.
The power input to a 500V, 50Hz, 6-pole, 3-phase induction motor running at 975 rpm is 40 KW. The stator losses are 1KW and the friction and windage losses total to 2KW, Calculate: i) The slip ii) Rotor copper loss iii) Shaft power.
Answer
Sign in to view the complete answer paper.
With neat diagram discuss the various tests to be conducted on 3phase IM to plot the circle diagram.
Answer
Sign in to view the complete answer paper.