The electrical power to a Meggar is provided by

Question

Examoogle · Accepted Answer

Permanent magnet D.C. generator

Explanation: A Megger (megohmmeter) is a portable instrument used for measuring high electrical resistance, typically insulation resistance. The required high DC voltage is generated internally by a hand-driven permanent magnet DC generator.

Working Principle: The internal hand-cranked DC generator typically produces a constant voltage (e.g., 500V, 1000V, or 2500V) when turned at a rated speed (usually 160 rpm). The generator is connected to a moving coil instrument containing a pressure coil and a current coil, which operate on the ratiometer principle to indicate resistance values directly on the scale. The use of a permanent magnet ensures constant flux and a stable voltage output independent of the driving torque variability, provided the speed is near the rated value.

Key Formulas: $V = k \cdot \Phi \cdot N$ — where $V$ is generated EMF, $\Phi$ is flux, and $N$ is rotation speed; $R_x = \frac{V}{I}$ — resistance calculated by the deflection ratio of the moving coil system

Key Points: Used for testing insulation resistance of cables, transformers, and motors.; Operates on the Ratiometer principle, making the reading independent of the supply voltage magnitude.; The permanent magnet provides a constant magnetic field, eliminating the need for an external power source.

Advantages: Portable and self-contained (no external power required).; Direct reading of high resistance values.; Rugged design suitable for field tests.

Disadvantages: Requires manual effort for cranking.; Voltage depends on manual cranking speed.; Electronic digital versions are replacing them due to ease of use.

Applications: Insulation resistance testing of electrical machinery.; Earth resistance measurement.; Detecting faults in high-voltage cables.

Additional Info: Modern Meggers may use battery-operated electronic power supplies, but the classic design defined in exams is the hand-cranked PMDC generator.. Option A is incorrect because traditional Meggers are self-excited via mechanical energy.. Option C is incorrect because DC is required for the ratiometer principle.

Answer: B is correct — The Megger utilizes a hand-driven permanent magnet DC generator to provide the high voltage necessary for insulation resistance measurement.

Exam Tip: Always remember that Meggers measure in M$\Omega$ (Megaohms); if the needle goes to zero, it indicates a short circuit, while infinity indicates a perfect insulation.

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