Electrical Machines Midterm-Theory
what is the cause of residual flux?
hysteresis
what is a domain?
within a ferromagnetic material, there are many small regions called domains in which all the atoms are aligned with their ferromagnetic fields pointing in the same direction
can a 60 Hz transformer operate with 50 Hz?
yes, but only if the max applied voltage is derated so that saturation is avoided
what are the 4 basic principles of magnetic fields?
1. a current carrying wire prodeuces a magnetic field in the area around it. 2. a time changing magnetic field induces a voltage in a coil of wire if it passes through that coil 3. a current carrying wire in the presence of a magnetic field has a force induced on it 4. a moving wire in the presence of a magnetic field has a voltage induced in it
what are the 2 main ways in which the amount of voltage that is generated by a synchronous generator can be increased?
1. increase the speed of rotation of the rotor 2. increase the field current
what is coercive magnetomotive force?
MMF needed to remove residual flux in ferromagnetic material
can a transformer produce a secondary DC voltage if its primary winding conducts DC current?
No because voltage can only be induced in a transformers secondary winding if it is exposed to a time varying magnetic field, not a constant field as would be the case for DC
what is the main problem with the y-y, 3 phase transformer?
a very large 3rd harmonic component of voltage on top of the 50 or 60 Hz fundamental voltage
why does the synchronous generator terminal voltage vs field current curve eventually flatten as field current is increased?
because the rotor becomes saturated so that any additional field current results in little increase of the magnetic field.
How is a permanent magnet created from a block of iron?
by applying MMF to the block of iron so that all the domains in the block of iron are aligned
list all losses that account for the difference between the mechanical power applied to the rotor and electrical power that is produced in a synchronous generator
copper, core, friction and windage
list the 3 types of losses in a transformer
copper, hysteresis, eddy
what is faradays law?
states that if a flux passes through a turn of coil of wire, a voltage will be induced in the turn of wire that is directly proportional to the rate of change in the flux with respect to time
what is lenz's law?
states that the direction of the voltage buildup in the coil is such that if the coil ends were short circuited it would produce a flux opposing the original flux change
what is the armature reaction in a synchronous generator?
the distortion of the air gap magnetic field by current flowing in the stator.
what is Residual flux?
the flux that remains in a ferromagnetic core after MMF has been removed
why are all cores exposed to AC flux variations laminated?
they are laminated to reduce eddy currents
