Question

*Please be clear in conceot and handwriting, thank
you.*

A 20 kVA three-phase synchronous machine is connected to a 480 V, 60 Hz electrical service.

The per-phase armature reactance is 80%. Armature resistance is negligible, as are all other sources of power loss.

**Part A: Motor Configuration**

The machine is configured to run as a motor. Its rated mechanical
power is 20 HP and it is 94% efficient.

The motor provides rated output mechanical power.

The excitation is set such that the motor presents a 0.85 lagging power factor.

Excitation is then changed such that the power factor is unity.

Assume the point of common connection is an infinite bus.

a. Calculate the phasors V?, EA and IA for both cases.

b. Draw phasor diagrams for both cases, showing the above phasors
as well as ? and ?

c. Explain why the torque angle changes the way it does.

**Part B: Generator Configuration**

The machine is configured to run as a generator to provide 15 kW and maximum leading reactive power.

Again, assume the point of common connection is an infinite bus.

a. Determine this maximum reactive power.

b. Calculate the phasors V?, EA and IA.

c. Draw a phasor diagram, showing the above phasors as well as ?
and ?.

Answer #1

Please be clear in concept and handwriting.
A 20HP, three-phase, Wye-connected synchronous motor is
connected to a 277VAC, 60Hz
power supply. At rated conditions, the motor runs at 3600 rpm and
presents a 0.85 lagging
power factor to the power supply. The per-phase armature reactance
is 350m?. Armature
resistance is negligible, as are all other sources of power
loss.
Considering rated conditions,
a. draw a single-phase equivalent circuit showing V?, Ea and IA,
with values.
b. draw a phasor diagram...

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