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In previous lessons, we explored how an electric motor operates: supplying an electric current to a coil within a magnetic field produces a mechanical torque (the motor effect), causing the armature to rotate.
However, as the armature coil rotates inside the magnetic field, it continuously cuts through magnetic field lines, experiencing a changing magnetic flux over time (ΔΦ⁄Δt). According to Faraday's Law of Induction, this changing flux induces an electromotive force (EMF) within the very same rotating coil.
The Concept of Back EMF
By Lenz's Law, the induced EMF acts in direct opposition to the applied supply voltage that powers the motor. This opposing potential difference is known as Back EMF (εback).
Inet = Vsupply - εback⁄R
Where:
Inet is the net armature current in Amperes (A)
Vsupply is the external voltage applied to the motor in Volts (V)
εback is the induced back EMF in Volts (V)
R is the internal electrical resistance of the armature coil in Ohms (Ω)
Speed Dependence & Startup Currents
Because back EMF is proportional to rotational speed (εback ∝ ω):
At Startup (ω = 0): Back EMF is zero (εback = 0), resulting in a large surge of current (Istart = Vsupply⁄R) that can overheat motor windings if not controlled.
At Operating Speed: As the motor accelerates, back EMF increases toward the supply voltage, reducing the net current (Inet) and stabilizing power consumption.
The video below breaks down these simultaneous motor and generator effects and explains how back EMF regulates motor speed under varying mechanical loads.
Check your understanding
Check Your Understanding: Back EMF & Motor Performance
Question: A DC electric motor is running at full operating speed when a heavy mechanical load is suddenly applied to its shaft, causing the motor to slow down. What happens to the Back EMF (εback) and the armature current (Inet) as a result?