Page 128 - Pipeline Rules of Thumb Handbook

Electrical Design 115

Electrical design

Most pipeline facilities use electrical equipment of some 3. Speeds—Single speed motors are the most common, but
sort or another, ranging from simple power circuits of a few when a range of speeds is required multispeed motors will
amperes capacity to sophisticated supervisory control and give 2, 3 or 4 fixed speeds.
data acquisition systems. It is beyond the scope of this manual
to provide a complete section on electrical information for 4. Service factors—Open, general purpose motors have a
pipeline facilities. This section is intended to provide some service factor depending upon the particular rating of the
basic data that will prove useful to field personnel responsible motor (usually between 1.15 and 1.25).
for electrical installations.
5. Selection of motor type

Electric motor selection There are three main types:

Based on known facts and calculations, the best selection (a) DC motors are designed for industrial drives requiring a
is made after a close study of the installation, operation and controlled speed range and constant torque output on
servicing of the motor. Basic steps in proper selection are adjustable voltage control systems. Because the speed of rota-
numbered 1 through 8 and are briefly described. tion controls the flow of current in the armature, special
devices must be used for starting DC motors.
1. Power supply
(b) Single phase alternating-current motors also require some
(a) Voltage-NEMA has recommended the following auxiliary arrangement to start rotation.
standards.
(c) Polyphase motors are alternating-current types (squirrel
cage or wound rotor).

Nominal Motor Applications—Some driven machines require a low-
Power System Nameplate starting torque which gradually increases to full-load speed;
Volts Volts
others require higher-starting torque. NEMA code letters A,
B, C, D, etc., on the motor nameplate designate the locked-
240 230
480 460 rotor kVa per horsepower of that particular motor design.
600 575 The diagram shows representative speed-torque curves for
polyphase and single-phase NEMA design motors Types A
through D.

(b) Frequency—Motors rated 200 horsepower or less can
vary not to exceed 5 percent above or below its rated
frequency.

(c) Phases—Three-phase power supplies are found in most
industrial locations; for most residential and rural areas, only
single-phase power is available.

2. HP and duty requirements

(a) Continuous duty—means constant load for an indefinite
period (about 90 percent of all motor applications).

(b) Intermittent duty—means alternate periods of load and 6. Torque definitions
no-load, or load and rest.
(a) Locked rotor torque, or “starting torque” is the minimum-
(c) Varying duty—means both the load and time operation torque which the motor will develop at rest for all angular
vary to a wide degree. positions of the rotor.