9kW.4(b).Basic electrotechnical units and theory
91
The ratio the output power the input power known the efficiency
of the machine. 4. Note that efficiency has units, but simply expressed
as percentage. 4.c. The symbol for effi-
ciency the Greek letter ‘eta’ (η).
The input power the transformer was found 10.
Electrical machines
Electrical machines are energy converters. In
general,
Power output
Power input
Since efficiency usually expressed percentage modify the general
formula follows.
Definition
The ratio the output power the
input power known the efficiency
of the machine.4(a). In
general,
Power output
Power input
.
Power output
Power input
100
Example
A transformer feeds the 9.
Simple a.
Power output
Power input
kW
kW
100
9 81
10 9
100 90
. 4. Find the efficiency the
transformer.81kW motor driving the mechanical hoist the previous example.
.
%
Thus the transformer 90% efficient. Alternatively, the machine input electrical
energy and the output mechanical energy then the machine motor, as
shown Fig.5, the loop wire will cut the lines magnetic flux
Effort Machine Load
FIGURE 4. the machine input mechan-
ical energy and the output electrical energy then that machine genera-
tor, shown Fig. generator alternator
If simple loop wire rotated between the poles permanent mag-
net, shown Fig.3
Simple machine concept. The symbol for efficiency the Greek letter ‘eta’ (η).
An electrical machine may used motor generator, although in
practice the machine will operate more efficiently when operated the
mode for which was designed
