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Generated EMF and Back EMF
Name: Ankur
Status: student
Age: N/A
Location: N/A
Country: N/A
Date: N/A
Question:
In a generator, emf induced is called generated emf; but
in motor, it is called back emf. Why?
Replies:
The term "emf" refers to "electromotive force". In a generator the
"electromotive force" is "generated" by the "generator" and is assigned a
positive, or forward, value. In contrast, a motor converts electrical power
to do some sort of work (that is, to generate some sort of mechanical force
or mechanical energy ). Since this "uses" electromotive energy (or provides
some sort of electromotive force) the change is assigned a negative value
(or "backward" force).
This is all a convention that could be assigned the opposite way, but that is
the way the "forward" and "backward" convention have been assigned. It is
purely a convention.
Vince Calder
There is no intellectual reason, Ankur.
Both are names for exactly the same physical phenomenon.
Even their polarities (voltage vs. direction of rotation) are the same.
I guess the distinction is a "common sense" thing, an idiom:
in a motor, the main electrification that dominates the user's awareness
is the applied power which is driving the motor.
Its voltage and current tend to be taken as positive in polarity.
Once the drive current accelerates the motor to substantial rotation rates,
the induced EMF resulting from that rotation
always has a polarity which opposes the drive current,
"pushing back" in a common way of thinking.
Shortening and combining phrases gives the phrase "back-EMF".
Induced EMF would be the more general and scientific term,
and seems clearer to me when I'm trying to model a motor's behavior mathematically.
Back--EMF would be an intuitive name applied to induced EMF
on occasions when there is a larger pre-existing drive current.
Jim Swenson
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Update: June 2012
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