CHAPTER 1 TYPES OF A.C. ELECTRIC MOTORS

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.