0195136047.pdf

13.4 DIRECT-CURRENT MACHINES 595

Commutating pole (interpole)

Shunt field

winding

Series field

winding

Compensating field winding

located in main-pole shoes

+

+

+

++

+

+ +

+

+

Main

pole

Main

pole

Armature with

armature winding

Commutating (interpole)

field winding

Figure 13.4.1Section of a dc machine illustrating the arrangement of various field windings.

Armature

Commutating

(compole or interpole)

winding

Shunt field

Series field

Field rheostat

Compensating winding

Figure 13.4.2Schematic con-

nection diagram of a dc machine.

Ea=Kaφωm (13.4.3)

which is the speed (motional) voltage induced in the armature circuit due to the flux of the
stator-field current. The electromagnetic torqueTeis given by Equation (12.2.18) as

Te=KaφIa (13.4.4)

whereKais the design constant. The productEaIa, known as the electromagnetic power being
converted, is related to the electromagnetic torque by the relation

Pem=EaIa=Teωm (13.4.5)

For a motor, the terminal voltage is always greater than the generated emf, and the electromagnetic
torque produces rotation against a load. For a generator, the terminal voltage is less than the
generated emf, and the electromagnetic torque opposes that applied to the shaft by the prime
mover. If the magnetic circuit of the machine is not saturated, note that the fluxφin Equations
(13.4.3) and (13.4.4) is proportional to the field currentIfproducing the flux.

Commutator Action

As a consequence of the arrangement of the commutator and brushes, the currents in all conductors
under the north pole are in one direction and the currents in all conductors under the south pole