Page 151 - Electric Drives and Electromechanical Systems

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144 Electric Drives and Electromechanical Systems

FIG. 5.7 A linear ampliﬁer connected as a voltage ampliﬁer. The gain is set by R1 to R4 with voltage feedback via
pins 1 and 4.
is used (see Fig. 5.7)inwhich alinearampliﬁercan effectivelybeconsideredasan
operational ampliﬁer with a high-power output stage. The application of a linear
ampliﬁer is relatively straightforward, as it can be conﬁgured as either a voltage or a
current ampliﬁer, with adjustable gains. In the selection of the ampliﬁer, considerable
care must be taken to ensure that the maximum power rating of the package is not
exceeded. The worst possible scenario combines a low speed with a high torque,
particularly when the motor is at stall under load. Application of Eq. (5.4) will allow the
power-dissipation requirements to be estimated and will allow comparison with the
manufacturer’s rating curves. Consideration should also be given to when the motor is
decelerating - or plugging - in which case the motor’s voltage is added to the output of
the ampliﬁer, the current being limited only by the armature’s resistance or by the
ampliﬁer’s current limit. The energy dissipated in the system can be determined by
consideration of the kinetic energy stored in the system. Apart from the energy that is
dissipated in the motor’s armature, all the energy is dissipated in the drive; if the motor
is subjected to excessive speed reversals the power rating of the ampliﬁer must be
considered in detail. Therefore, in the selection of a linear ampliﬁer, the thermal-
dissipation problems are of considerable concern to the system designer.
Commercial linear ampliﬁers are available in power ratings up to 1.5 kW and with
output voltages of 60 V; this necessitates forced air cooling and derating of the power
rating at high ambient temperatures. In most cases, a thermal trip circuit is provided to
disable the ampliﬁer if the temperature approaches the rated value.
The use of a linear ampliﬁer gives the system designer considerable beneﬁts over
other forms of drives, which normally are based on a switching principle. In particular a
linear drive may have very high bandwidths, typically greater than 500 Hz; this allows
exceptional performances to be obtained with motors of low inertia and/or inductance,

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