Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne 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



Click here to return to the Physics Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory