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Examoogle • User • info@examoogle.com • EE-2024-8821
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Electromagnetic attraction relay can be operated on
AC
DC
Both A and B
None of above
Both A and B
Quick Summary: Electromagnetic attraction relays operate on the principle of electromagnetic force pulling a metallic armature towards an electromagnet. Since the force produced is proportional to the square of the current (or flux), the direction of the force is independent of the polarity of the current, allowing the relay to function on both AC and DC supplies.
Electromagnetic attraction relays operate on the principle of electromagnetic force pulling a metallic armature towards an electromagnet. Since the force produced is proportional to the square of the current (or flux), the direction of the force is independent of the polarity of the current, allowing the relay to function on both AC and DC supplies.
F=2μ0Φ2A — where F is attractive force, Φ is flux, A is area, and μ0 is permeability of free space.
The relay consists of a coil wound on a magnetic core. When current flows through the coil, a magnetic flux Φis generated, exerting an attractive force F on a movable armature. This force is defined by F∝Φ2. Because the square of a negative current (in DC) or the square of the instantaneous AC current value remains positive, the net torque is always unidirectional and capable of closing the relay contacts regardless of the supply type.
The operation is based on the force of attraction exerted by an electromagnet on an armature.
The attractive force is proportional to the square of the magnetic flux, making it unidirectional.
For AC applications, shading coils are often added to prevent chatter during current zero crossings.
Simple and robust construction
High operating speed
Versatile for both AC and DC applications
Susceptible to vibration and chatter in AC applications if not properly designed
Lower sensitivity compared to induction relays
Overcurrent protection
Differential protection
Auxiliary relays in control circuits
In AC circuits, the flux pulsates at double the frequency, which may cause armature hum; this is mitigated using a shading ring on the pole face.
Option A and B are both correct because the mechanical force does not change direction with current reversal.
C is correct — The electromagnetic attraction relay operates on the force proportional to the square of the current, which remains unidirectional for both AC and DC.
Always remember that any device operating on the principle of square of current (like hot-wire or electromagnetic attraction) is inherently polarity-insensitive, unlike moving-coil permanent magnet instruments.