JNTU
JNTU B.Tech II ELECTRICAL MACHINES I Previous Question Paper (March - 2022 - R15)
Answer any five questions
All questions carry equal marks
Main
Show that, the torque developed in a doubly excited magnetic system is equal to the rate of increase of the field energy with respect to displacement at constant currents?
Answer
In a doubly excited magnetic system, the energy balance equation is given by dWe = dWf + dWm, where We is the electrical input, Wf is the stored field energy, and Wm is the mechanical work done. The electrical input is dWe = i1dλ1 + i2dλ2. The field energy Wf is a function of λ1, λ2, and displacement θ. The total differential of field energy is dWf = (∂Wf/∂λ1)dλ1 + (∂Wf/∂λ2)dλ2 + (∂Wf/∂θ)dθ. Substituting these into the energy balance equation, we get dWm = [i1 - (∂Wf/∂λ1)]dλ1 + [i2 - (∂Wf/∂λ2)]dλ2 - (∂Wf/∂θ)dθ. Since i = ∂Wf/∂λ at constant θ, the terms in brackets vanish. The mechanical work done is dWm = Tedθ, where Te is the electromagnetic torque. Thus, Te = - (∂Wf/∂θ)|i1, i2=const. For a linear system, Wf = 1/2 L11i12 + 1/2 L22i22 + L12i1i2. The torque is Te = 1/2 i12(dL11/dθ) + 1/2 i22(dL22/dθ) + i1i2(dL12/dθ). This shows the torque is the rate of increase of field energy with respect to displacement at constant currents.
For a singly excited magnetic system, derive the relation for the magnetic stored energy in terms of reluctance?
Answer
For a singly excited magnetic system, the magnetic stored energy Wf is defined as the area under the magnetization curve, Wf = ∫ i dλ. In a linear magnetic circuit, the flux linkage is λ = Nφ and the magnetomotive force is F = Ni = φR, where R is the reluctance. Substituting i = φR/N and dλ = Ndφ, the energy becomes Wf = ∫ (φR/N) * (Ndφ) = R ∫ φ dφ. Integrating this expression yields Wf = 1/2 R φ2. Alternatively, since φ = λ/N, we can express this in terms of flux linkage as Wf = 1/2 R (λ/N)2 = 1/2 (R/N2) λ2. Given that the inductance L = N2/R, this simplifies to Wf = 1/2 λ2/L, which is consistent with the standard energy formula.
Describe the principle of energy conversion. From a consideration of the various energies involved, develop the model of an electromechanical conversion device.
Answer
Sign in to view the complete answer paper.
Derive the expression for generated e.m.f in a DC generator. Discuss the factors affecting the generated e.m.f?
Answer
Sign in to view the complete answer paper.
A separately excited dc generator has armature circuit resistance of 0.22 Ω and a total drop at brushes is 2.5 V. When running at 1200 rpm, it delivers a current of 120 A at 220 V to a load of constant resistance If the generator speed drops to 750 rpm, with field current unaltered, find the current delivered to load.
Answer
Sign in to view the complete answer paper.
How demagnetizing and cross magnetizing ampere turns per pole are calculated in a DC machine?
Answer
Sign in to view the complete answer paper.
The lap wound armature of a 4 - pole generator armature has 51 slots. Each slot contains 20 conductors. What will be the e.m.f generated in machine when driven at 1500 r.p.m. If useful flux per pole is 0.01Wb?
Answer
Sign in to view the complete answer paper.
Explain the voltage build up process in d.c. shunt generator.
Answer
Sign in to view the complete answer paper.
A d.c. shunt generator is supplying load connected to a bus - bar voltage of 220 V. It has an armature resistance of 0.025 Ω and field resistance of 110 Ω. Calculate the value of load current and load power when it generates an e.m.f of 230 V. Neglect the effect of armature reaction. Draw circuit diagram.
Answer
Sign in to view the complete answer paper.
The open circuit characteristic for a d.c. shunt generator at 800 r.p.m is given by the following data.
| If(A) | 0 | 0.2 | 0.4 | 0.65 | 1.02 | 1.75 | 3.15 | 5.0 |
| E(V) | 10 | 40 | 80 | 120 | 160 | 200 | 240 | 260 |
Determine the critical filed resistance at 900 r.p.m.
Answer
Sign in to view the complete answer paper.
What are the different speed control methods of DC shunt motor? Explain each method and enumerate advantages and disadvantages.
Answer
Sign in to view the complete answer paper.
Determine (i) the total torque developed (ii) the useful torque of a 250 V, 4pole d.c series motor with 782 wave connected conductors developing 8 kW and taking 40 A with a flux per pole of 25 mWb. The armature resistance of the motor is 0.75 ohms.
Answer
Sign in to view the complete answer paper.
Explain the significance of Swinburne’s test conducted on DC machine.
Answer
Sign in to view the complete answer paper.
The Hopkinson test on two similar dc shunt machines gave the following results: Line voltage: 220 V; Line current excluding filed current: 40 A; Armature current of motoring machine: 200 A; field currents are 6 A and 7 A. Calculate the efficiency of each of the machine at the given load conduction. The armature resistance of each machine is 0.05 Ω.
Answer
Sign in to view the complete answer paper.