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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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The electrical power to a Meggar is provided by
Battery
Permanent magnet D.C. generator
Generator
Any of the above
Permanent magnet D.C. generator
Quick Summary: 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.
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.
V=k⋅Φ⋅N — where V is generated EMF, Φ is flux, and N is rotation speed
Rx=IV — resistance calculated by the deflection ratio of the moving coil system
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.
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.
Portable and self-contained (no external power required).
Direct reading of high resistance values.
Rugged design suitable for field tests.
Requires manual effort for cranking.
Voltage depends on manual cranking speed.
Electronic digital versions are replacing them due to ease of use.
Insulation resistance testing of electrical machinery.
Earth resistance measurement.
Detecting faults in high-voltage cables.
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.
B is correct — The Megger utilizes a hand-driven permanent magnet DC generator to provide the high voltage necessary for insulation resistance measurement.
Always remember that Meggers measure in MΩ (Megaohms); if the needle goes to zero, it indicates a short circuit, while infinity indicates a perfect insulation.