Handbook of Electrical Installation Practice - BeKnowledge

408 Handbook of Electrical Installation Practice
In the example given in Fig. 15.14 a pump running at 100% speed is delivering
40m 3 /min and operates at point A where its head/volume characteristic, 1,
intercepts the system characteristic, 3, and is driven by a 400kW motor. The pump
flow is to be reduced to 30m 3 /min. Two possibilities exist:
(1) The pump flow may be restricted or throttled which produces a new system
characteristic, 4, and the pump then runs at intercept B.
(2) The pump speed may be reduced by 16% which gives a new pump head/volume
characteristic, 2, and the pump then operates at intercept C.
The power absorbed by the pump at B is approximately 330kW and at point C,
220kW. Thus in this example the power saving possible by pump speed adjustment
rather than by throttling is 110kW.
For process and process line control, for example in continuous production
processes such as rubber, plastics, paper and metals, variable speed drives are
essential. The ability to control motor speeds means that other process variables
can be accommodated and that product quality and production rates can be
optimised.
Alternating current variable speed drives
There are a number of a.c. variable speed drives available, the choice often being
dictated by the requirements of the application. Gaining in popularity are variable
speed inverter-controlled cage motors, especially for single motor pumps and fan
drives having centrifugal load characteristics.
Variable frequency inverters
Apart from single motor drives, variable frequency inverters have for some time
been employed on multiple motor drives such as steel mill run-out table applications.
The advantage here is that several usually identical motors can be supplied in
parallel from a single inverter in order that they all run at the same speed at any
time and the speed of the group can be controlled as a whole.
Thyristor regulators are presently in use which provide controlled adjustable frequency
outputs up to 15MW. Depending on rating, the regulators may comprise one
of two predominant systems:
(1) Fixed voltage d.c. link The link is supplied from a free running diode bridge
which in turn feeds a thyristor or transistor chopper unit. The chopper output
is a series of pulses which build up an alternating current (Fig. 15.15). This is
known as a pulse width modulated (pwm) system in which the voltage output
is controlled normally in proportion to frequency output so that the motors they
supply receive constant flux conditions.
(2) Variable voltage d.c. link These are current or voltage fed inverters which
have a conventional thyristor regulator as the input. The chopper unit is more
simple, its output frequency being determined by the d.c. link voltage. Known as
quasi square-wave inverters they also maintain constant output voltage to frequency
ratio and commutate or switch only at output frequency rate (Fig. 15.16).