(b) Derive the transformations for currents between a rotating balanced two phase (a,ẞ) winding and a pseudo stationary two phase (d,q) winding. (assume equal turns on all coils). From power invariance, show that the voltage transformations are identical to those for the currents. Describe, from physical concepts, how the transformation may be used to represent a three phase synchronous machine. Explain how torque is developed.

Calculate the flux per pole required on full-load for a 50 kW, 400 V, 8-pole, 600 r.p.m. d.c. shunt generator with 256 conductors arranged in a lap-connected winding. The armature winding resistances is 0.1 Q, the shunt field resistance is 200 Q and there is a brush contact voltage drop of 1 V at each brush on full- [0.162 Wb] 5. load. Calculate the flux in a 4-pole dynamo with 722 armature conductors generating 500 V when running at 1000 r.p.m. when the armature is (a) lap connected (b) wave connected. [(a) 41.56 mWb (b) 20.78 mWb] (City & Guilds, London) 7. A 4-pole machine running at 1500 r.p.m. has an armature with 90 slots and 6 conductors per slot. The flux per pole is 10 mWb. Determine the terminal e.m.f. as d.c. Generator if the coils are lap-connected. If the current per conductor is 100 A, determine the electrical power. [810 V, 324 kW] (London Univ.) 8. An 8-pole lap-wound d.c. generator has 120 slots having 4 conductors per slot. If each conductor can carry 250 A and if…

Question 43

4. In a transformer, which options are correct: A. Works on Faraday’ law and Ampere's law B. DC current in the primary winding will produce voltage output in the secondary winding C. Copper losses are caused by reluctance in magnetic circuit D. Frequency of input power source will not govern the voltage induced in secondary circuit. 5. Voltage ratio of input voltage and output voltage in IDEAL transformer depends on: A. Leakage flux B. Magnetization current C. Copper loss current D. Turns ratio 6. The core of transformer must have A. Low reluctance C. High reluctance B. High resistance D. Low resistance 7. An overexcited synchronous motor behaves like: B. Resistive load D. DC generator A. Inductive load C. Capacitive load 8. The amount of real power delivered by a synchronous generator connected to an infinite bus can be controlled by adjusting A. System frequency C. Full load frequency B. No load frequency D. Terminal voltage

Example : In a 110 V, d.c. compound generator, the resistance of the armature, shunt field and series field are 0.06 , 25 and 0.04 respectively. The load consists of 200 lamps each rated at 55 W, 110 V. Find the total e.m.f. generated and the armature current when the machine is connected in (a) long shun

A 4 pole, lap wound, d.c. generators has 42 coils with 8 turns per coils. It is driven at 1120 r.p.m. If useful flux per pole is 21 mWb, calculate the generated e.m.f. Find the speed at which it is to be driven to generate the same e.m.f. as calculated above, with wave wound armature.

For 6 polar D.C machine armature has 36 number of slots and the type of winding is a double layer simplex lap winding. a. How many coils are present? b. What is the coil span in terms of number of slots? c. If each coil has 4 turns, then what is the total number of armature conductors presents? d. How many parallel paths will be their in the armature? e. Altogether how many brushes will be their?

A d.c.series generator has armature resistance of 0.5Q and series field resistance of 0.032.It drives a load of 50 A.If it has 6 turns/coil and total 540 coils on the armature and is driven at 1500 r.p.m.,calculate the terminal voltage at the load.Assume 4 poles, lap type winding, flux per pole as 2 mWb and total brush drop as 2 V.

Two shunt generators A and B operates in parallel and their load characteristicsmay be taken us straight lines. The voltage of the generator A falls from 240 V atno-load to 220 V at 200 A while that of B falls from 245 V at no load to 220 V at150 A. Determine the current which each machine supplies to a common load of300 A and the bus-bar voltage at this load.

An electromagnet has a movable iron part and dimensions of magnetic circuit with uniform cross section as shown in figure. The core is made of ferromagnetic material with relative permeability of 3000: a) b) c) Calculate the magnetic reluctance of the circuit and the inductance of the main winding. Determine the air-gap flux, the flux density and the field intensity of the core and gap. Because of a fault in the power supply, the exciting current is increased to 8 A causing the mobile element of the iron core to move to a zero gap position as a result of the excessive electromagnetic force action. If this movement occurs in 0.01 s, find the transient voltage induced in the test coil having two turns. 0.5 mm air-gap 2 cm 2 A 3 cm 2 cm 60 turns 18 cm test coil 24 cm 0.5 mm air-gap