Rotating-Coil Generators, Back EMF, and Motor Power

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Back emf

The emf induced in a rotating motor coil that opposes the applied supply voltage.

How does motor speed affect back emf and current?

As motor speed rises, back emf rises, reducing the voltage across the coil resistance and lowering current.

What happens when a motor’s mechanical load increases?

The motor slows, back emf falls, and current and torque increase; resistive heating also rises.

How is motor input power divided in the simplified model?

Input power is divided between mechanical output and resistive heating: $P_s=P_m+P_R$.

Why are motor iron cores laminated?

Thin insulated sheets limit eddy-current paths and reduce unwanted heating and energy loss.

Quiz questions

1. What type of voltage does a simple rotating-coil generator produce?Answer: Sinusoidal alternating voltage
2. A rotating coil has angular velocity $\omega$. Which expression gives its rotation frequency $f$?Answer: $f=\omega/(2\pi)$
3. A 200-turn coil of area $7.85\times10^{-3}\,\text{m}^2$ is in a $0.80\,\text{T}$ field. If the flux change per turn has magnitude $BA$ and takes $15.0\,\text{ms}$, what is the approximate average-emf magnitude?Answer: About $84\,\text{V}$
4. How often does the emf reverse sign as a simple rotating coil turns at constant speed?Answer: Every half-turn
5. For a rotating-coil generator, what change increases peak emf if the turn count, area, and field stay fixed?Answer: Increasing angular velocity