Which of the following law/rule can be used to determine the direction of rotation of DC motor?

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Examoogle · Accepted Answer

Fleming's left-hand rule

Explanation: Fleming's left-hand rule is specifically used to determine the direction of force (and thus rotation) on a current-carrying conductor placed within a magnetic field, which is the foundational principle of a DC motor. When current flows through the armature conductors in a magnetic field, the motor action is described by the interaction between the field flux and the conductor current.

Working Principle: According to the rule, if you hold the thumb, index finger, and middle finger of your left hand mutually perpendicular to each other, the index finger points in the direction of the magnetic field, the middle finger points in the direction of the current, and the thumb indicates the direction of the resultant mechanical force. In a DC motor, this force creates the torque that causes the armature to rotate.

Key Formulas: $F = BIl \sin	heta$ — Force on a current-carrying conductor; $	au = K_t \Phi I_a$ — Electromagnetic torque of a DC motor

Key Points: Used for DC motors (motor action).; Thumb represents the direction of motion or force.; Index finger represents the direction of the magnetic field.; Middle finger represents the direction of the current flow.

Advantages: Simple mnemonic for field-current-force relationship; Applicable to all DC machine calculations

Disadvantages: Only provides direction, not magnitude

Applications: DC motor design and analysis; Electromagnetic actuators; Relays and solenoid mechanisms

Additional Info: Lenz's Law is used to determine the direction of induced EMF in a circuit (Generator action).. Faraday's Law describes the magnitude of induced EMF.. Coulomb's Law relates to electrostatic force between charges.

Answer: D is correct — Fleming's left-hand rule is the standard method for determining the direction of force on a current-carrying conductor in a magnetic field.

Exam Tip: Always remember: 'Left for Load' (Motors) and 'Right for Result' (Generators/Induced Current) to distinguish between Fleming's rules.

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