0195136047.pdf

760 BASIC CONTROL SYSTEMS

PIV=Vm (16.1.17)

Thedisplacement factorDF is given by cosφ, whereφis the angle between the fundamental

components of the input current and voltage.

Theharmonic factorHF of the input current is given by

HF=

(

Is^2 −I 12

I 12

) 1 / 2

=

[(

Is

I 1

) 2

− 1

] 1 / 2

(16.1.18)

whereI 1 is the fundamental rms component of the input current.

Theinput power factoris given by

PF=

I 1

Is

cosφ (16.1.19)

For an ideal rectifier,

η= 1. 00 ; Vac= 0 ; FF= 1. 0 ; RF= 0 ;

TUF= 1. 0 ; HF= 0 ; PF= 1. 0 (16.1.20)

Solid-State Control of DC Motors

Dc motors, which are easily controllable, have historically dominated the adjustable-speed

drive field. The torque–speed characteristics of a dc motor can be controlled by adjusting

the armature voltage or the field current, or by inserting resistance into the armature circuit

(see Section 13.4). Solid-state motor controls are designed to use each of these modes. The

control resistors, in which much energy is wasted, are being eliminated through the develop-

ment of power semiconductor devices and the evolution of flexible and efficient converters.

Thus, the inherently good controllability of a dc machine has been significantly increased

in recent years byrectifier control, chopper control,andclosed-loop controlof dc motors.

When a dc source of suitable and constant voltage is already available, designers can employ

dc-to-dc converters or choppers. When only an ac source is available, phase-controlled rec-

tifiers are used. When the steady-state accuracy requirement cannot be satisfied in an open-

loop configuration, the drive is operated as a closed-loop system. Closed-loop rectifier drives

are more widely used than chopper drives. Only rectifier control of dc motors is consid-

ered here.

Controlled rectifier circuits are classified as fully controlled and half-controlled rectifiers,

which are fed from either one-phase or three-phase supply. Figure 16.1.11 shows a fully controlled,

rectifier-fed, separately excited dc motor drive and its characteristics. A transformer might be

required if the motor voltage rating is not compatible with the ac source voltage. To reduce ripple

in the motor current, a filter inductor can be connected in series between the rectifier and the motor

armature. The field can be supplied from the same ac source supplying the armature, through a

transformer and a diode bridge or a controlled rectifier. While single-phase controlled rectifiers

are used up to a rating of 10 kW, and in special cases even up to 50 kW, three-phase controlled

rectifiers are used for higher ratings.

VaandIain Figure 16.1.11 denote the average values of the converter output voltage

and current, respectively. Assuming continuous conduction when the armature currents flow

continuously without becoming zero for a finite time interval, the variation ofVawith the firing

angle is shown in Figure 16.1.11(b). Providing operation in the first and fourth quadrants of

theVa–Iaplane, as shown in Figure 16.1.11(c), the fully controlled rectifiers are two-quadrant