TENTH LECTURE 


REGULATION AND CONTROL 

A. Direct Current Systems. 

In direct current three-wire 220 volt distribution systems 
several outside bus bars are used and, with change of load, the 
feeders are changed from one bus bar to another. 

The different bus bars are connected to different machines, 
to the storage battery or to boosters. 

The lighting boosters are low voltage machines separ- 
ately excited from the bus bars. The main generators are 
shunt machines or rather are excited from the bus bars, or ro- 
tary converters, and are usually of 250 volts, that is, the neutral 
brought out by collector rings and compensator. 

In railway circuits, in addition to trolley wire and rail 
return, trolley feeders and ground feeders, or plus and minus 
feeders are sufficient for converter substations, and where the 
distance gets too great for feeders, another substation is 
installed. 

When using direct current generators, series boosters are 
used to feed very long feeders which otherwise would have 
an excessive drop of voltage. In this way feeder drops of 200 
to 300 volts are taken care of by the railway booster. Such a 
large voltage drop is uneconomical and railway boosters are 
therefore used only for small sections for which it does not 
pay to install a separate station, especially where the load is 
very temporary, as for instance, heavy Sunday load, etc. 

Railway boosters are series machines, that is, the series 
field and the machine voltage therefore are proportional to the 
current. In such railway boosters it is necessary to take care in 
the booster design that it does not build up as series generator 


128 GENERAL LECTURES * 

feeding a current through the local circuit between a short 
feeder and a long feeder, as shown in Fig. 25. 


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Fl$. 25. 

A series machine excites if the resistance of its circuit is 
less than a certain critical value. To avoid such local circuit, 
either the trolley circuit is cut between the feeders, or the 
boosting kept below the critical value. 

If the distances are too great for boosters, inverted con- 
verters in the generating station are used to change from 
direct current to alternating current; the alternating current is 
sent by step-up and step-down transformers to the substation 
and changed to direct current by rotary converters. 

If a considerable amount of power is required at a dis- 
tance, it is more convenient at the generating station to use, 
instead of inverted converters, double current generators, that 
is, generators having commutator and collector rings. 

If most of the power is used at a distance, alternating 
current generators are used with rotary converters and fre- 
quently one converter substation is located in the generating 
station. 

Inverted converters and double current generators are 
now used less, since usually the systems are now so large as to 


REGULATION AND CONTROL 129 

require most of the power at a distance, and therefore alter- 
nating current generators are used. 

Many big systems have advanced from direct current gen- 
erators, through inverted converters and double current gen- 
erators, to the present alternators feeding converter substa- 
tions. 

B. A1.TERNAT1NG Current Systems. 

Generator Regulation. 

1st. Close inherent regulation. 

This is secured by low armature reaction and high 
saturation so that the voltage does not vary much with the 
load. 

Advantages — 

Simple, requiring no additional apparatus, etc. 

Instanteous. 

Disadvantages — 

Larger and more expensive generators and when of very 
close regulation, more difficult to run in parallel. 

2nd. Rectifying Commutator. 

The main current goes over a commutator, is rectified, 
and the rectified current sent through a series field. This ar- 
rangement is not used any more. 

Advantage — 

Permits compounding and over-compounding without 
any elaborate apparatus. 

Disadvantages — 

Only limited power can be rectified, therefore suitable 
only for smaller machines. 

Compounds correctly only for constant power factoi ; that 
is, if compounded for non-inductive load, the voltage drops 
on inductive load, since inductive load requires a greater field 
excitation than non-inductive load. 


130 GENERAL LECTURES 

Brushes have to be shifted with change of power factor, 
that is, change from motor load to lighting load, etc. ; other- 
wise commutator sparks badly. 

These machines therefore were good in the early days 
when all the load was lighting load, but are unsuited at present 
for mixed load. 

3rd. Form D alternator or compensated alternator with 
compensating exciter. 

Exciter is connected direct and has the same number of 
poles as the alternator so as to be in synchronism. 

The main current passes by collector rings through the 
exciter armature, usually with interposition of a transformer 
to keep the high voltage away from the exciter. 

The main current is sent through the exciter in such direc- 
tion that with non-inductive load its armature reaction slightly 
magnetizes and so raises the exciter voltage. Then with 
lagging current it magnetizes much more, raises the exciter 
voltage more, and with leading current demagnetizes or lowers 
the exciter voltage. 

By adjusting the machine it can be made to compound 
perfectly for non-inductive, inductive, and leading current 
load and also can be made to over -compound at constant speed ; 
so that with the engine dropping in speed by the load, it keeps 
constant voltage, or raises the voltage as desired. 

Advantages — 

Very quick and very correct compensation, and if properly 
adjusted, the most perfect arrangement and not liable to get 
out of adjustment 

Disadvantages — 

More expensive machine and the average engineer not 
skillful enough to adjust it. 


REGULATION AND CONTROL 131 

4th. Compensating Commutator (Alexanderson). 

Commutator built multi-segmental so that it does not 
spark with fixed position of brushes. 

Brushes set to rectify completely at inductive load; at 
non-inductive load then rectify incompletely, and so the series 
field gets less current. 

At leading current load the rectified current is reversed 
and so the series field demagnetizes. 

Advantages — 

Takes care of lag and lead. 

Disadvantages — 

Special generator and suitable only for small and moder- 
ate sizes. 

5th. PoTENTiAi. Regulator. 

Tirrill Regulator 

Rheostat in exciter field so large that when in circuit the 
excitation is the lowest, and that when short circuited the exci- 
tation is the highest ever required. 

A potential magnet in the alternator circuit operates a 
contact maker which continuously cuts the resistance in and 
out again, so that the contact maker is never at rest, but always 
cuts in and out, and the average field excitation of the exciter 
is between maximum and minimum. 

If the voltage tends to drop, the contact remains a shorter 
time on the low than on the high position, and so raises exci- 
tation ; if the voltage tends to rise, the contact maker remains 
a shorter time on the high than on the low position, and so 
lowers excitation. 

Advantages — 

Very simple. 


132 GENERAL LECTURES 

Can be applied to any alternator and requires no special 
adjustment. 

Disadvantages — 

Limited in power by the sparking of the contact maker, 
and so can be used only if the regtilation of the alternator is 
not too bad or the machine too large. 

In very large machines usually no regulating device is 
used but hand control of the field rheostat, since in such large 
machines the load only varies slowly and never changes much, 
as for reasons of economy the machines are run near full load ; 
with the change of load, machines are shut down or started up. 

Synchronous Motors and Converters. 

In an alternating current system or part of the system 
containing large synchronous motors or converters the voltage 
can be controlled by varying the motor or converter field in the 
same way as with alternators, that is, by Tirrill Regulator or 
commutator and series field, etc. 

POTENTIAI. REGUIvATORS. 

I. Compensator regulator. 

With step-up or step-down transformers the voltage can 
be regulated by having different taps brought out of the trans- 
former winding and so get different voltages by means of a dial 
switch. Where no transformers are used a compensator with 
different voltage taps gives the same results. 

The taps can be brought out in the primary or in the 
secondary, whichever is the most convenient: in the second- 
ary, if the primary is of very high voltage ; in the primary, if 
the secondary is of very low voltage and large current. 

Advantages — 

Simplest, cheapest and most efficient. 


REGULATION AND CONTROL 133 

D isadvan tages — 

Step by step variation and sparking at the dial switch. 

2. Induction Regulators. 

Built like induction motors with stationary primary in 
shunt and movable secondary in series to the line. 

By moving the secondary the voltage varies from lower- 
ing to raising. 

Induction regulators are usually three-phase and of larger 
sizes for rotary converters in lighting systems. 

When single-phase, the stationary member contains a 
short circuited coil at right angles to the primary. In the 
neutral position this coil acts as short circuited secondary to 
the secondary coil, and so reduces its self-induction. 

Advantages — 

Perfectly uniform variation and considerable inductance 
which is of advantage for rotary converters. 

Disadvantages — 

High cost. 

3. Magneto Regulators. 

A stationary primary coil is in shunt and a stationary 
secondary coil is in series and at right angles to the primary ; an 
iron shuttle moves inside of the coils and so turns the mag- 
netism of the primary coil into the secondary coil either one 
way or the other. 

On the dotted position the primary sends the magnetism 
through the secondary in opposite direction as in the drawn 
position, in Fig. 26. 


134 


GENERAL LECTURES 


Fife. 26 


Advantage — 

Uniform variation. 

Disadvantage — 

More expensive than compensator regulator.