0195136047.pdf

606 ROTATING MACHINES

the evolution of flexible and efficient converters (see Section 16.1). Thus, the inherently good

controllability of a dc machine has been increased significantly.

EXAMPLE 13.4.3

Figure E13.4.3 shows a simplified Ward–Leonard system for controlling the speed of a dc motor.

Discuss the effects of varyingRfgandRfmon the motor speed. Subscriptsgandmcorrespond to

generator and motor, respectively.

+−

Ifg

I

Rfg

+

+

−

−

+

−

Ifm

Rfm

ωm

Eg

Rag + Ram = R

G M

Figure E13.4.3Simplified

Ward–Leonard system.

Solution

IncreasingRfgdecreasesIfgand henceEg. Thus, the motor speed will decrease. The opposite will

be true ifRfgis decreased.

IncreasingRfmwill increase the speed of the motor. DecreasingRfmwill result in a decrease

of the speed.

DC Motor Starting

When voltage is applied to the armature of a dc motor with the rotor stationary, no emf is generated

and the armature current is limited only by the internal armature resistance of the machine. So, to

limit the starting current to the value that the motor can commutate successfully, all except very

small dc motors are started with variable external resistance in series with their armatures. This

starting resistance is cut out manually or automatically as the motor comes up to speed.

EXAMPLE 13.4.4

A 10-hp, 230-V, 500-r/min shunt motor, having a full-load armature current of 37 A, is started

with a four-point starter. The resistance of the armature circuit, including the interpole winding,

is 0.39, and the resistances of the steps in the starting resistor are 1.56, 0.78, and 0.39,in

the order in which they are successively cut out. When the armature current has dropped to its

rated value, the starting box is switched to the next point, thus eliminating a step at a time in the

starting resistance. Neglecting field-current changes, armature reaction, and armature inductance,

find the initial and final values of the armature current and speed corresponding to each step.

Solution

Step 1:At this point, the entire resistance of the starting resistor is in series with the armature

circuit. Thus,