Page 641 - Basic Electrical Engineering
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Figure 7.23 Characteristics of dc compound motors
In cummulative compound motors there is some drop in speed from no
load to full load. For suddenly applied loads the motor speed gets reduced
which may be advantageous in application like punching and shearing
machines, rolling mills, lifts, mine hoists, etc. In differential compound
motors, the resultant flux gets reduced as load increases, and hence the speed
increases. This is seen from the expression for speed, N which is
From the above expression it can be seen that if flux ϕ is reduced, speed N
will increase.
7.13 LOSSES AND EFFICIENCY
The efficiency of a dc machine, like any other machine is the ratio of output
power to the input power. The efficiency can never be 100 per cent because
output is never equal to input. Some energy is lost in the machine during
conversion of energy from mechanical to electrical or vice-versa. To achieve
higher efficiency, the designer of the machine tries to keep the losses as low
as possible.
7.13.1 Losses in a DC Machine
In a dc machine, like any other machines, the whole of input energy does not
get converted into output energy. A portion of the input energy gets lost in
