Download SEE 3433 MESIN ELEKTRIK LECTURERS: NIK RUMZI NIK IDRIS

SEE 3433 MESIN ELEKTRIK
LECTURERS:
NIK RUMZI NIK IDRIS
NIK DIN MUHAMAD
TIME:
2 hours
ANSWER 4 QUESTIONS
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SEE 3433
Question 1
(a)
With the help of a simple electromechanical system, explain briefly how energy conversion
process occurs in the system. Assume the system is lossless.
(b)
[6 marks]
Consider a cross sectional view of a cylindrical rotating machine which has both stator and
rotor windings as shown on Fig. Q1. The mutual inductance between the stator and rotor is
given by 0.9 cos θ, where θ = [ωmt + δ], is the angular rotor position, with initial position δ.
The stator current is given by is(t) = 10cos(2π50)t. It can be assumed that the self inductance of
the rotor and stator are independent of θ.
(i) The rotor is initially in stationary (ωm = 0) and the rotor circuit is unexcited.
- Obtain the expression for the induced emf on the rotor circuit
[4 marks]
- If the rotor is open circuit, will the rotor be able to rotate continuously? Why?
[3 marks]
- If the rotor is short circuit, will the rotor be able to rotate continuously? Why?
[3 marks]
(ii) Now assume that the rotor current is given by ir(t) = 5 cos[(2π10)t + α) in which it can be
shown that the torque produced is given by:
=
+
+
+
{ω
{ω
{ω
{ω
+ π
− π
+
+ π
−
− π
−
+
+ α + δ}
− α + δ}
− α + δ}
+ α + δ}
If δ = 15o and α = 30o, give two different rotor speeds (ωm) at which a non-zero average
torque can be developed, and hence calculate these two non-zero average torques
[6 marks]
If δ = 30o and α = 30o and rotor is rotating at (2π60) rad/s, calculate the average torque
developed?
[3 marks]
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SEE 3433
stator
rotor
θ
X
X
Fig. Q1
Question 2
(a) Briefly explain what do you understand by an armature reaction in a DC machine
What effect does the armature reaction has on:
(i)
the flux per pole?
(ii)
generated back emf (Ea)?
(iii) commutation action?
Give two methods to reduce armature reaction
[6 marks]
(b) A 6kW, 120V, 1200 rpm DC machine has an armature resistance Ra = 0.2Ω and field winding
resistance Rfw = 100Ω. An external resistance Rfc, which can have variable value between 0 to
200Ω is connected to the field circuit. The Attachment Q2 shows the magnetization curve of the
DC machine at 1200 rpm. The machine is connected as a self-excited shunt generator.
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SEE 3433
Clearly draw the schematic diagram of a self excited DC shunt machine.
[3 marks]
Use the given magnetization (Attachment Q2) curve to answer the following questions (clearly
mark your work on the sheet) :
(i)
Find the possible minimum and maximum values of no load terminal voltage by adjusting
the field control resistance, Rfc
(ii)
Determine the value of external Rfc such that the no load terminal voltage is 120V.
Neglecting the armature reaction, determine the terminal voltage at rated armature current.
(iii) With the same Rfc as above and considering the equivalent field current caused by armature
reaction as If(AR) = 0.1A, determine the terminal voltage when a rated armature current
flows.
(iv) What will happen when the load resistance is reduced to zero? What will be the value of
the armature current?
.[16 marks]
Question 3
(a)
With the help of a simple example, explain the differences between lap-winding and wave
winding in a DC machine.
(b)
[4 marks]
A speed of a separately excited DC motor is to be controlled by varying its armature voltage for
speeds below its rated speed and by controlling its field current (field weakening) for speeds
above its rated speed.
Draw a schematic diagram to show how this scheme can be achieved using two three-phase
controlled rectifiers.
[5 marks]
Suppose the motor has the following specifications:
Rated power : 90kW,
Ra = 0.08Ω
Rated voltage: 600V
kΦa = 3.15 V/rads
Rated speed: 1800 rpm
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SEE 3433
With the motor unloaded, the armature current is found to be 10% of its rated value. It can be
assumed that the armature current is continuous at all time. The 3-phase input voltage to the
rectifiers is 600V(line-line).
(i)
Find the firing angle of the armature controlled rectifier at full load and at rated speed.
Calculate the speed regulation of the DC motor.
(ii)
.
[7 marks]
Referring to (i), what would be the firing angle of the armature controlled rectifier if the
speed is to be maintained at its rated value after the full load is completely removed?
[5 marks]
(iii) Suggest a way to improve the speed regulation of the DC motor
[4 marks]
Question 4
(a)
Draw the per-phase equivalent circuit of an induction motor recommended by the IEEE. Briefly
explain the meaning of all the parameters in the circuit.
[4 marks]
Show how an increase in rotor circuit resistance will increase the starting torque of an induction
motor. What should be the maximum limit of such increase of rotor resistance?
(b)
[3 marks]
The parameters for a 3-phase, 18.6kW, 460V, 50Hz, 1760 rpm induction machine are as
follows:
R1 = 0.25Ω
R’2 = 0.2Ω
X1 = 1.2Ω
X2’=1.1Ω
Xm = 35Ω
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SEE 3433
(i) Derive the Thevenin’s equivalent circuit for the machine. Show that the per phase torque
expression is given by:
=
ω
(
) +(
+
)
+
,
where Vth, Rth and Xth are the Thevnin’s equivalent voltage, resistance and reactance
respectively. Calculate the starting torque of the motor.
[10 marks]
(ii) It can be shown that the maximum torque occurs at a slip given by:
=
[
+
+
]
Calculate the maximum torque of the motor.
Assume the winding ratio of stator to rotor is 1. If 3 external resistors are to be connected
to each of the phase of the rotor circuit, what would be the value of each resistor so that
maximum torque occurs at start-up?
[8 marks]
Question 5
(a)
Explain the need for damper winding in a synchronous machine.
(b)
Draw the open circuit and short circuit characteristics of a synchronous generator. Explain the
shape of the characteristics.
(c)
[4 marks]
[4 marks]
A star-connected 3-phase, 20kVA, 415V, 4-pole synchronous machine has a synchronous
reactance Xs = 1.5Ω. The per phase resistance can be neglected. The machine is connected to
an infinite bus bar of 415V.
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SEE 3433
(i) The mechanical power and field current excitation are adjusted such that the machine is
delivering 10kW at a power factor of 0.8 lagging. Determine the excitation voltage Ef and
the power angle δ. Draw the phasor diagram for this condition.
[10 marks]
(ii) With the mechanical power unchanged and the field current adjusted to give a unity power
factor, determine the percentage change in the field current compared to (i).
[7 marks]
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SEE 3433
Attachment Q2
Name:
Seksyen:
140
130
120
110
100
90
Ea (V)
80
70
60
50
40
30
20
10
0
0
0.1 0.2 0.3 0.4 0.5
0.6 0.7 0.8 0.9
If (A)
1
1.1 1.2 1.3
1.4
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