0195136047.pdf

608 ROTATING MACHINES

Efficiency

As is true for any other machine, the efficiency of a dc machine can be expressed as

Efficiency=

output

input

=

input−losses

input

= 1 −

losses

input

(13.4.17)

The losses are made up of rotational losses (3 to 15%), armature-circuit copper losses (3 to 6%),

and shunt-field copper losses (1 to 5%). Figure 13.4.13 shows the schematic diagram of a dc

machine, along with the power division in a generator and a motor. The resistance voltage drop,

also known asarc drop,between brushes and commutator is generally assumed constant at 2

V, and the brush-contact loss is therefore calculated as 2Ia. In such a case, the resistance of the

armature circuit should not include the resistance between brushes and commutator.

EXAMPLE 13.4.5

The following data apply to a 100-kW, 250-V, six-pole, 1000-r/min long-shunt compound

generator: no-load rotational losses 4000 W, armature resistance at 75°C=0.015, series-

field resistance at 75°C=0.005, interpole field resistance at 75°C=0.005, and shunt-field

current 2.5 A. Assuming a stray-load loss of 1% of the output and a brush-contact resistance drop

of 2 V, compute the rated-load efficiency.

Solution

The total armature-circuit resistance (not including that of brushes) is

Ra= 0. 015 + 0. 005 + 0. 005 = 0. 025 

Ia=IL+If=

100 , 000

250

+ 2. 5 = 402 .5A

The losses are then computed as follows:

No-load rotational loss 4000 W

Armature-circuit copper loss = 402.5^2 ×0.025 4050 W

Brush-contact loss = 2Ia=2× 402.5 805 W

Shunt-field circuit copper loss = 250×2.5 625 W

Stray-load loss = 0.01×100,000 1000W

Total losses 10,480 W

The efficiency at rated load is then given by

η= 1 −

10,480

100,000+10,480

= 1 − 0. 095 = 0. 905 ,or 90.5%

It is usual to determine the efficiency by some method based on the measurement of losses,

according to test codes and standards.

Applications for DC Machines

Dc motors find wide applications in which control of speed, voltage, or current is essential.

Shunt motors with constant speed are used for centrifugal pumps, fans, blowers, and conveyors,

whereas shunt motors with adjustable speed are employed in rolling mills and paper mills.

Compound motors find their use for plunger pumps, crushers, punch presses, and hoists. Series