TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
TRANSFORMERS AND REGULATORS
7-1. Voltage provisions covered.
primary voltage inputs. The transformer will have a
manual tap changer, which can be operated if the
This chapter provides maintenance and repair re-
transformer is de-energized. However, on substa-
quirements for transformers used in the transmis-
tions which serve varying loads, such as pumping
sion and distribution of electrical energy and for
facilities, or on large installations with long primary
voltage regulators. Requirements apply to units
feeder lines, taps may not provide sufficient voltage
having at least one medium-voltage winding and
regulation and other means are necessary.
generally providing three-phase service, although
(1) Load-tap-changing (LTC). This feature in-
single-phase units may be found for housing or
stalled on a transformer provides automatic tap
other small loads.
changing under load, and normally varies the volt-
age to plus or minus 10 percent of the system's rated
7-2. Defining transformer and regulator char-
voltage by changing tap connections using a motor-
driven, tap-changing switch.
(2) Voltage regulators. Sometimes voltage
tween circuits of the same frequency, usually with
regulation is needed and the system transformers
changed values of voltage and current. All trans-
do not include the LTC feature. Voltage regulators
formers covered in this chapter are constant-voltage
are used to supply the control for the variations in
type. That is, they maintain an approximately con-
load. A voltage regulator needs similar servicing to
stant voltage ratio over loads from zero to the rated
that required for a power transformer. A step-
output. Constant-current transformers are de-
voltage regulator operates on the same principal as
scribed in chapter 6, section IV Transformers can be
the LTC mechanism. An induction voltage regulator
classified in various ways, but their basic construc-
has a series winding and a shunt winding, and uses
tion consists of windings, magnetic cores on which
a motor to rotate the shunt winding to either add to
windings are coiled, insulation, and any special con-
(boost) or subtract from (buck) the series winding
nections applying to the type of load.
voltage. The action provided is dependent upon the
a. Winding terminology. Winding terminology
voltage induced in the series winding and the re-
given below is based on the voltage flow, rating, or
spective polarities of each winding (that is, the re-
spective instantaneous directions of currents enter-
(1) A primary winding has input from the
ing the primary and leaving the secondary
power source and a secondary winding supplies in-
terminals during most of each half cycle). The
put to the loads.
switching mechanism in most new voltage regula-
(2) A high-tension winding has a higher volt-
tors is practically maintenance free, but many of the
age than a low-tension winding. Most transformers
older units require considerable servicing. The
have high-tension primary windings and are there-
manufacturer's recommendations should be fol-
fore step-down transformers. If the same trans-
lowed for all maintenance and servicing require-
former utilized the low-tension winding as the pri-
mary winding it would be a step-up transformer.
7-3. Transformer classification.
(3) Most transformers have two windings,
which are electrically insulated from each other.
Transformers are generally classified by size, insu-
H i g h - v o l t a g e power transformers found in
lation, and location.
transmission-to-distribution substations may have
a. Size. Transformers rated above 500 kVA are
a single winding (autotransformers) or a tertiary
classed as power transformers. Transformers rated
winding to eliminate voltage problems and/or to
at 500 kVA or less are classed as distribution trans-
supply a second load voltage economically.
formers, as they usually have low-tension windings
b. Regulation. Transformers can maintain an ac-
of less than 600 volts. Instrument transformers,
covered in chapter 3, section VI, are not considered
ceptable voltage ratio of about a 2 percent voltage
distribution transformers since they do not serve
drop from zero to rated output in most cases. Most
distribution transformers and smaller power trans-
b. Insulation. There are two types of insulating
formers have tapped windings, which permit ad-
classifications recognized by ANSI/IEEE C57.12.80.
justing the output voltage to broaden the range of