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why the efficiency of the dc machine is high ,when it is
operated as a generator?
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Answer Posted / guest
Effiiency of any machine is defined as the ratio of output
to the output.
Efficiency=(O/P)/(I/P)
MATHEMATICAL PROOF
Since electrical quantities are easier to measure in
comparison to mechanical quantities.
So in the determination of efficiency of DC machines,
we first find electrical quantities(input or output)and
subsequently find mechanical quantities(output or input)by
subtracting or adding the losses.
Efficiency of Generator=Output/input=(V*I)/(V*I+loss)
Efficiency of Motor = Output/input=(V*I-loss)/(V*I)
As long as machine is same, the rating and losses are not
going to change.
Clearly the ratio of above two equation is greater than 1
meaning there by efficiency of generator is greater than
motor.
PHYSICAL CIRCUIT PROOF
If we look into the circuit model,In case of generator back
emf causes less amount of current to flow through the
armature.So electrical losses(Cu loss) are less.Electrical
losses are function of current
In case of motor counter torque is produced so speed of
motor is less.So rotatinal loss(Core and friction) is
less.Rotational losses are function of speed.
Since loss:electrical loss > Mechanical loss.
So efficiency:Generator > Motor
Author:arbelectbit@yahoo.com
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