Electrical Engineering Answers

a)

Transistors of a certain type are specified to have β values in the range 50

to 150. Find the range of their α values.

b)

Measurement of

a

npn BJT in a particular circuit shows the base current to

be 14.46 μA, the emitter current to be 1.460 mA, an

d the base

–

emitter voltage

to be 0.7 V. For these conditions, calculate α, β, and

𝐼

𝑆

.

c)

Consider

a

npn transistor with

𝑉

𝐵𝐸

= 0.7 V at

𝐼

𝐶

= 1 mA. Find

𝑉

𝐵𝐸

at

𝐼

𝐶

=

0.1 mA

.

Graphically represent the following two signals.  f₁(t) = 3δ(k) + 2δ(k-1) + δ(k-2), f₂(t) = ε(t-1) - ε(t-2)

What is the unit sequence response?

 Graphically represent the following two signals. 

( ) 3 ( ) 2 ( 1) ( 2) f1 t   k   k    k  , ( ) ( 1) ( 2)

 A 2,500 kVA, three – phase, 60 Hz, 6.6 kV wye connected alternator has a field resistance of 0.45 Ω and an armature resistance of 0.05 Ω per phase. The field current at full load 0.85 pf, is 200 A. The stray power losses amount to 82 kW. Calculate the efficiency of the alternator at full load, 0.85 pf lagging.

A 2 MVA, 2.3 – kV three – phase, Y – connected alternator operates at rated kVA at a power factor of 80%. The dc armature winding resistance between terminals is 0.08 Ω. The field takes 70 A at 125 V from an exciter equipment. Friction and windage loss is 20 kW, iron losses is 36 kW and stray load losses are 2.0 kW. Calculate the efficiency of the alternator. Assume the effective armature – winding resistance is 120% of the DC resistance.

To get the armature resistance of a 100 MW, 13.8 kV and 90% power factor generator, two terminals are connected to a DC source. The measured current and voltage are 87.6 amperes and 6 volts respectively. What is the DC resistance per phase ?

A 2500 kVA, 6600 V synchronous turbo alternator is operating at full load and 0.8 pf lagging. This machine has a synchronous reactance per phase is 10.4 ohms and a negligible resistance. Calculate the percentage voltage regulation?

A 6 pole, 3 – phase, 60 – cycle alternator has 12 slots per pole and four conductor per slot. The winding is 5/6 pitch. There are 2, 500, 000 maxwells (0. 025 weber) entering the armature from each north pole and this flux is sinusoidally distributed along the air gap. The armature coils are all connected in series. The winding is wye – connected. Determine the open circuit emf of the alternator. 

A two pole three phase 50 Hz, Y – conncted AC generator has 9 slots per pole and 6 conductors per slot. What is the flux per pole if the voltage on open circuit is 1.1kV. Assume a coil span of unity.