1.1 GENERAL
By far the greatest consumers of
electric power on an offshore or onshore installation are electric motors. In providing mechanical power to their driven
loads they draw electric power from the system - and this means active power (kilowatts). However, in addition to the mechanical power
converted from the electrical, the motor, like any other machine, suffers
‘losses’: that is to say, some power is consumed within the motor by friction,
windage and internal heating. Such
losses are not passed on to the mechanical drive, but the energy for them must
nevertheless be drawn from the electrical system. Therefore a motor will always give out less
mechanical power than the electrical power which it draws; its output must then
be less than its input. The ratio is called the
‘efficiency’ and is therefore always less than
one. It is usually expressed as a
percentage.
Mechanical output was formerly (and
often still is) expressed in horsepower, but with SI units it should be
expressed in kilowatts (mechanical). One
horsepower equals 0.746kW, or approximately ¾kW (a useful rule-of-thumb). Many nameplates and tags, however, will still
be found to be marked in horsepower.
Electrical power is always expressed in watts or
kilowatts (electrical). If it is
necessary to distinguish between mechanical and electrical kilowatts, a suffix
‘m’ or ‘e’ should be used. Thus:
Efficiency is not constant but varies with the
mechanical loading on the motor. It is
normally highest at full load, falling off rapidly as loading is reduced.
Apart from the active power drawn by the motor to
convert to mechanical power, most motors also draw reactive power (kilovars) to
magnetise themselves. This results in a
mixture of active and reactive power entering the motor, showing as a power
factor less than unity. The matter of
reactive power and power factor of a motor is dealt with in Chapter 4.
1.2 SYNCHRONOUS MOTOR
The synchronous motor is an important type because of
its unique performance. It is relatively
costly, and its design is complex. For
that reason it is not used on offshore installations, and even onshore only in
special applications.
In construction it is identical with the synchronous
generator which is described in the manual ‘Electrical Generation Equipment’
and used to generate power on all platforms.
A synchronous generator is
driven by a prime mover (steam or gas-turbine or diesel). It converts the mechanical power received
into electrical power which it pumps into the network. But that same machine, if uncoupled from its
prime mover and coupled to a mechanical load (such as a brake, pump or compressor as shown in Figure 1.1) will,
without switching, continue to run in synchronism with the a.c. electrical
supply. It will then behave as a motor
however, drawing power from the mains and converting it to mechanical power
which it delivers to the load. Moreover
since it runs in synchronism with the a.c. mains, it must run at constant
speed, whatever the loading. It is this constant
speed facility which singles out the synchronous motor from all other types and
makes it suitable, despite the cost, for drives which demand exact constant
speed.
FIGURE 1.1
SYNCHRONOUS MACHINE AS GENERATOR OR MOTOR
Another advantage is that, since
the motor draws its excitation from a separate exciter and not from the mains,
it is possible, by controlling the excitation, to run a synchronous motor at
unity power factor so that it draws no reactive power, and all its current
contributes to useful work. This can be
a great advantage to a network system where there are other large induction
motor drives with their heavy reactive power demands. Indeed, it is not uncommon to run a large
synchronous motor at a small leading power factor by over-exciting it, in order
to correct for other lagging loads and to maintain system voltage.
It is also possible to use such a
motor as a ‘synchronous condenser’. In
this application it drives no mechanical load, but it is over-excited so that
it generates only reactive power. Its
use in this mode for system power factor correction and maintaining system
voltage is explained in the manual ‘Electrical System Control’.
A synchronous motor must, like the
generator, have independent excitation, which may be brushless. It must also be provided with starting
arrangements which will run it up to speed so that it may be synchronised to
the system before driving its load.
Since there is no difference in
construction between a synchronous generator, a synchronous motor and a
synchronous condenser, all three are often referred to simply as a ‘synchronous
machine’. Indeed in some installations a
single synchronous machine may be used in any of the three modes as desired.
The chief disadvantages of a
synchronous motor, as compared with an induction motor of the same power, are
increased size and complexities, and more particularly the difficulty of
starting it. Without special additional
features it is not self-starting, and these add materially to the cost.
1.3 INDUCTION MOTOR (SQUIRREL CAGE)
There is another type of motor which works on an entirely
different principle: it is the ‘induction motor’. It is very widely - one might say almost
exclusively - used throughout offshore and onshore installations for industrial
drives. In this application it is always
used on 3-phase supply systems.
The principle of operation is described in detail in the
next chapter.
1.4 INDUCTION MOTOR (SPLIT PHASE)
This is a modification of the normal 3-phase induction
motor to enable it to be run on a single-phase supply. Its principal use is with small domestic equipment
where normally only single-phase supplies would be available. This too is described in the next chapter.
1.5 OTHERS
There are many other types of a.c. motors which have
been developed for special applications.
They include:
·
Commutator
motor
·
Repulsion
motor
·
Reluctance
motor
·
Synchronous-induction
motor
·
Schrage
motor.
These will not be met with on offshore installations and
are unlikely to be found in onshore installations. No further descriptions are therefore given.
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