0195136047.pdf

13.4 DIRECT-CURRENT MACHINES 609

Shunt field

Series field

Field rheostat

(a)

(b)

Input from Electromagnetic power

prime mover

Output power

= EaIa = VtaIa = VtIL

Armature terminal power

No-load rotational loss

(friction, windage, and core)

+ stray-load loss

Armature copper loss

I^2 aRa

+ brush-contact loss

Series-field loss I^2 sRs

+ shunt-field loss I^2 fRf

Is If

Ia (motor)

Ia (generator)

Vta Vt

IL (motor)

IL (generator)

−

+

−

+

Armature

(c)

Input power

from mains Armature terminal power

= VtIL

Output available

= VtaIa = EaIa at shaft

Electromagnetic power

Shunt-field loss I^2 fRf

+ series-field loss I^2 sRs

Brush-contact loss

+ armature loss I^2 aRa

No-load rotational loss

+ stray-load loss

Figure 13.4.13(a)Schematic diagram of a dc machine.(b)Power division in a dc generator.(c)Power
division in a dc motor.

motors (known as traction motors) are utilized for electric locomotives, cranes, and car dumpers.
Universal motors, operating with either dc or ac excitation, are employed in vacuum cleaners,
food processors, hand tools, and several other household applications. They are available in sizes
of fractional horsepower up to, and well beyond, 1 hp, in speeds ranging between 2000 and 12,000
r/min.
The dc shunt generators are often used as exciters to provide dc supply. The series generator
is employed as a voltage booster and also as a constant-current source in welding machines. In
applications for which a constant dc voltage is essential, the cumulative-compound generator
finds its use. The differential-compound generator is used in applications such as arc welding,
where a large voltage drop is desirable when the current increases.