Page 168 - Industrial Power Engineering and Applications Handbook

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Table 6.5 Comparative study of performance of an a.c. drive, variable speed fluid coupling and a d.c. drive

Features A.C. drive Variable speed.fluid coupling D.C. drive
~
1 Manufacturing range Any range 40 to IO 000 kW by GE, USA 0.1 to IO 000 kW
In India up to 3000 kW by
Pembril Engineering and
Fluidomat
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2 Starting acceleration Soft and stepless start Soft and stepless start Soft and stepless start
3 Starting current (I,J Can be controlled to any desired level Cannot be controlled. Although Can be controlled to any
subject to the minimum T,,required duration of thc starting inrush desired level. Normally up to
is very short because of no-load 1.5 times the rated current
start
~.
4 Starting torque (TJ As desired As desired As desired up to 5-6 times the
T,, through field forcing (i.e. by
raising the field voltage during
the start up period)
~
5 Variation in torque with Through Vlfcontrol, the torque T - N' Characteristics are similar to an
speed can be keut constant induction machine as shown in
Figure 6.51:
(i) Up to the base speed N, torque
up to a desired speed, N. Since can be kept constant through
h.p. = T.N., h.p. varies with armature voltage control,
speed V = Id, field current (I, = @),
Beyond N, the h.p can be kept remaining constant
constant by keeping the voltage
fixed and raising f (h.p. = T.N). (I-..-- @"a - constant, I~,
N
The speed-torque characteristic is being the' armature current)
similar to a d.c. machine as shown (ii) Beyond the base speed N, h.p.
in Figure 6.51 can be kept constant (h.p. =
T.N.) by reducing the field
current (I, = $) and keeping
the armature voltage as
constant (I, = constant).
Torque will now diminish
exponentially, since @
diminishes and N rises, N
being in denominator, has the
same effect as @ (Fig. 6.51)
6 Magnetising losses of Vary with f Remain at 1004% Remain constant for speed
the motor variations within the required
speed N, as the field current is
kept fixed and only the
armature voltage is varied. For
speed variations beyond N,
however, when the armature
voltage is kept constant and the
field current is varied, the
magnetizing losses also vary
7 Copper (PR) losses Low at reduced speeds, due to low No reduction because of same At lower speeds more than the
magnetizing current (I,) and magnetizing losses and ax. drives as I, remains the
correspondingly lower I, therefore relatively higher I, same and a relatively higher I,

8 Power factor Although I, is low, overall p.f. may Low as I, remains the same Lower than the a.c. drive
be slightly lower on the line side, because of same field current
because of harmonic contents and
inclusion of L. L is introduced to
(i) Limit the current harmonics and
(ii) Limit the rate of rise of current,
i.e. ripples (dildr)
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9 Combined efkiency of At rated speed 90% and more. (i) At rated speed = 87-90% At rated speed up to 80-90%.
the motor and the drive Reduces slightly at lower speeds (coupling efficiency as At lower speeds reduces more
because of poor efficiency of the high as 97-98%) than the a.c. drives because of