5.4.4 Back EMF
By the end of this lesson you can:
- Explain how a motor generates a back emf
- Describe how back emf affects the current in a motor
- Explain why the starting current is high
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).
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.
In a rush?
This video provides a a quick summary - useful for review
Otherwise scroll down.
Check Your Understanding: Back EMF & Motor Performance
2. Current increases: since Inet = (Vsupply − εback) / R, a lower back EMF gives a bigger potential difference across the armature, so it draws more current to deliver the extra torque needed for the heavy load.
1. Back EMF is directly proportional to speed (εback ∝ ω), so slowing down reduces the back EMF.
2. The armature current is Inet = (Vsupply − εback) / R. When the back EMF drops, the numerator increases, so the motor draws more current to produce greater torque.