POWER ELECTRONICS
ASSIGNMENT
2 2010
Due date:
Wednesday
,
27
October 20
10
,
9
.30 a.m
.
Question 1
An overhead crane shown in figure 1 employs a d.c. motor as a drive for the main hoist.
Figure 1
The motor is known to have the following character
istics
Nameplate Ratings:
Voltage
3
6
0V d.c.
Power
1
0
kW
Speed
1500 rpm
Measurements show that the machine has an armature resistance (R
a
) of 0.4 ohms and an
armature inductance (
L
a
)
of
0.002 H. An open circuit test using rated field excitat
ion
gave an armature constant (
K
a
)
of
2.0V sec rad

1
.
The motor is supplied through a three phase, 415 volt, fully controlled converter as
shown in figure 2. The motor is mechanically coupled to the winding drum through a
gearbox of ratio 50:1. The windi
ng drum has a diameter of 0.4 meters and the maximum
load to be hoisted is 1 tonne.
If t
he velocity of the hoist is to be 0.5 m/s.
, find:
a.
i.
The maximum acceleration of the load in m. sec

2
without exceeding rated
current.
Is this acceleration reason
able?
ii.
The speed of rotation of the motor.
iii.
Torque developed by the motor during the constant velocity mode of
operation.
iv.
The armature current.
v.
The trigger angle of the converter.
vi.
Estimate the losses in the converter.
vii.
The power delivered
by the supply includ
ing the estimated converter losses
and the power factor presented to the supply
.
viii.
Draw a graph of the terminal voltage of the motor, the armature current and
the line current in the red phase of the supply.
The drive is required to lower the load in a re
generative mode. If the load is lowered at
the same speed, find:
b.
i.
The trigger angle of the converter.
ii.
The power returned to the supply taking into account the estimated losses in
the converter.
iii.
Draw a graph
of the waveforms indicated in a.
viii.
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