Types of Ac Power

TM 11-5895-1012-10

However, equipment that requires large amounts of

c. Auxiliary Power. Auxiliary power is provided at

power, such as a high-powered radio transmitter,

each DCS Station to replace all or part of the primary

generally is designed to operate on 240 or 480 volt,

power in case of failure. This auxiliary power is usually

three phase power. These are nominal voltages, and a

supplied by engine-generators and storage batteries.

variation of 5 percent usually can be tolerated.

These engine-generators are installed and operated as

(1) Some equipment is built with power input

part of the DCS Station. Auxiliary power is the

transformers, that have tapsbrought out to terminals.

currently-approved designation for back-up or altern-

These taps are on the primary (input) side of the trans-

ate power sources frequently referred to as emergency

former, and are marked with a common voltage value.

power sources.

For example, a transformer in a piece of equipment

d. Emergency Lighting Power. Emergency power

may have-either three or five taps. If there are three

requirements at DCS Stations are normally limited to

tap, they usually are marked 105, 115, and 125 volts;

battery-powered lighting systems for use while

if there are five taps, they are usually marked 105,

auxiliary power equipment is being activated after

110, 115, 120, and 125 volts. Transformers for higher

failure of the primary power supply. These emergency

systems are normally automatically activated im-

mediately upon failure of the primary or auxiliary

(2) Three-phase power requirements at DCS

power supply, and are removed from service as soon as

Stations are usually at a nominal 120/208 volt po-

primary or auxiliary ac power is restored.

tential. However, some RF power amplifiers etc., may

2-24. Classes of Power

be designed to operate on different voltages, such as

240 or 416 volts. These voltages are supplied by

Station power is divided into four classes as follows:

special transformer configurations.

a. Class A Primary Power. A primary power plant

c. Phase. Equipment that requires 120-volt input

which provides an essentially continuous supply of

power is generally designed for single-phase power

electrical power.

most equipment that requires 240-volt input power is

b. Class B Auxiliary Power. A standby power plant

designed for three-phase power. The generation of

to cover extended outages (usually days in length) of a

three-phase power is accomplished by three separate

primary power plant.

induction coils in the generator. The method of

c. Class C Auxiliary Power. A quick start (10 to 60

connecting these three coils results in various

seconds) unit(s) to cover short-term outages (usually

configurations, and may be arranged to give different

hours in length) of a primary power plant.

voltages in the range of 208 to 480 volts. The three-

d. Class D Auxiliary Power. An uninterruptible (no

coils, and the individual phases they produce, are

break) power unit(s) using stored energy to provide

separated by 120 degrees so that there is equality be-

continuous power within specified voltage and fre-

tween the three phases. Higher powered generating

quency tolerances.

plants normally produce three-phase power. The

2-25. Types of Ac Power

majority of the DCS Station equipment uses only

single-phase power. Distribution of primary power

AC power is available in many different forms (voltage,

within the station is important; therefore, disparity

phase, and frequency). The type required by a particu-

between loads on individual phases creates problems

lar DCS Station is determined by the equipment in-

and should be balanced within reasonable limitations.

stalled at that DCS Station.

a. Frequency. All ac powered communications

2-26. Generating Equipment

equipment manufactured in the United States is avail-

a. General. The engine-generator sets provided at

able in models that require an input frequency of 60-

the DCS Stations are used for primary or auxiliary

Hertz (Hz). Some equipment is also available in models

that can operate on ac power supplied at different fre-

power requirements. For use as either a primary power

source or an auxiliary power source, the electrical

quencies, such as 25 or 50 Hz. Most foreign-made

characteristics are the same. Normally, the only dif-

equipment is designed to operate only from 50 Hz

ference is that more sets of equipment are required if

power sources. Since most of the equipment at any

DCS Station will be manufactured in the United

they are used to produce power at all times. For

example, the general practice for auxiliary power is to

States, the primary ac power source should be 60 Hz.

So that equipment designed for other frequencies can

provide two complete sets of power generating equip-

be operated, a rotary or solid-state static type con-

ment. When they are used to provide all power for the

verter is used to change the frequency of the primary

station, at least three complete sets of equipment are

power.

required.

b. Voltage. Most communications and test equip

b. Engines. The engine, or prime-mover is designed

nave more horsepower than the generator rating;

ment is designed to operate on 120 volts single phase.

2-28