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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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Which of the following equation is used to find magnetizing current in transformers and in salient pole machines?
I m = 0 . 707 A T T
I m = 0 . 37 A T p h P k w T p h
I m = 0 . 427 A T 60 P k w T p h
All of these
Im=0.707 ATT
Quick Summary: The magnetizing current ($I_m$) in a transformer or electrical machine is the portion of the no-load current responsible for producing the alternating magnetic flux in the core. The formula $I_m = \frac{0.707 \cdot AT}{T}$ relates the magnetizing current to the required ampere-turns (AT) and the number of turns per phase (T) by accounting for the RMS conversion of sinusoidal excitation.
The magnetizing current (Im) in a transformer or electrical machine is the portion of the no-load current responsible for producing the alternating magnetic flux in the core. The formula Im=T0.707⋅AT relates the magnetizing current to the required ampere-turns (AT) and the number of turns per phase (T) by accounting for the RMS conversion of sinusoidal excitation.
Im=T0.707⋅AT — RMS magnetizing current calculation
AT=Φ⋅R — Basic MMF requirement for a core with flux Φ and reluctance R
In a magnetic circuit, the required Magnetomotive Force (MMF) or Ampere-Turns is dictated by the flux density and reluctance of the magnetic path. Since electrical machines operate with sinusoidal excitation, the peak value of current is related to the RMS value by a factor derived from the sinusoidal waveform. The factor 0.707 represents the reciprocal of 2, effectively converting the peak magnetizing force requirement into the RMS current required to drive the flux through the core reluctance.
The constant 0.707 is the RMS multiplier (1/2) for a sinusoidal waveform.
Magnetizing current is reactive in nature and lags the supply voltage by 90 degrees.
Higher core permeability reduces the required AT for a given flux density.
In salient pole machines, the magnetic circuit path is non-uniform, necessitating higher AT compared to uniform air-gap machines.
Allows accurate prediction of no-load power factor.
Essential for calculating transformer excitation impedance.
Assumes a constant reluctance, which is physically limited by core saturation.
Requires knowledge of magnetic path geometry and B-H curve properties.
Transformer design and efficiency analysis.
Steady-state analysis of synchronous machines.
The factor 0.707 strictly applies to sinusoidal excitation. If the excitation contains harmonics (due to saturation), the effective multiplier changes.
Options B and C contain empirical constants often used for induction machines (involving winding factors kw), but are not the fundamental definition for general magnetizing current as asked.
A is correct — The equation Im=T0.707⋅AT correctly expresses the RMS magnetizing current in terms of Ampere-Turns and turns per phase.
Always verify if an engineering formula is derived for RMS values or peak values; in electrical power engineering, RMS is the standard unless specified otherwise.