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The back EMF in a DC motor opposes the supply voltage. This is explained by _________________.
Fleming’s right hand rule
Fleming’s left hand rule
Faraday’s laws of electromagnetic induction
Lenz’s law
Lenz’s law
Lenz's Law states that the direction of an induced electromotive force (EMF) is such that it opposes the change in magnetic flux that produced it. In a DC motor, the armature conductors rotate in a magnetic field, inducing a back EMF that acts in opposition to the applied supply voltage, thereby adhering to the principle of conservation of energy.
IS 4722:2001 (Rotating Electrical Machines)
Lenz's Law states that the direction of an induced electromotive force (EMF) is such that it opposes the change in magnetic flux that produced it. In a DC motor, the armature conductors rotate in a magnetic field, inducing a back EMF that acts in opposition to the applied supply voltage, thereby adhering to the principle of conservation of energy.
Eb=60APΦZN — Formula for back EMF (Eb) where P=poles, Φ=flux/pole, Z=total conductors, N=speed in rpm, A=parallel paths
V=Eb+IaRa — Voltage balance equation for a DC motor where V is supply voltage and IaRa is armature drop
As the motor armature rotates, the conductors cut the magnetic flux lines. According to Faraday's Law, an EMF is induced. Lenz's Law dictates that this induced EMF must oppose the cause (supply voltage), creating a current that opposes the rotation if it were a generator, or limiting the input current to maintain energy balance in a motor.
The back EMF is proportional to the speed of the armature (Eb∝N).
Back EMF regulates the armature current (Ia=RaV−Eb), acting as a self-regulating mechanism for the motor.
Without back EMF, the armature current would be excessively high, potentially damaging the motor windings.
Lenz's Law is essentially the application of the Law of Conservation of Energy to electromagnetic induction.
Self-regulation of motor speed
Prevents excessive surge current during operation
Enables efficient conversion of electrical to mechanical power
Reduces the effective torque capacity relative to total supply voltage
Zero back EMF at startup leads to high starting current
DC Shunt Motors
DC Series Motors
DC Compound Motors
Option A (Fleming's Right Hand Rule) is used for generators to determine the direction of induced EMF.
Option B (Fleming's Left Hand Rule) is used for motors to determine the direction of force on a current-carrying conductor.
Option C (Faraday's Law) determines the magnitude of the induced EMF, but not its direction/opposition.
D is correct — Lenz's law dictates that the induced back EMF must oppose the supply voltage that creates the rotation, ensuring energy conservation.
Remember: Fleming's Left Hand Rule is for motor action (Force), while Lenz's Law explains the inherent opposition (Back EMF) generated by the motor's motion.
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