Ac Machines Midterm Flashcards
(36 cards)
Which direction do magnetic field lines point?
North to South
What is the SI unit for magnetic flux?
Units of Webers
What is the SI unit for magnetic flux density?
Tesla(T)
What 3 factors determine the strength of an electromagnetic?
Number of turns, Core material, the magnitude of current
A material that is easily magnetized is said to have a higher__________. An example of this material is___________.
Permeability
Iron/steel
A material that is not easily magnetized is said to have a high_________. An example of this material is__________.
Reluctance
Plastic/Glass
When an iron core electromagnetic reaches a point where strength increases only slightly with an increase of current, it is known as___________.
Saturation
Core materials that tend to retain residual magnetism are said to have a high ___________.
Retentivity
The extra power used in removing/reversing the magnetic field in a core material is known as___________ loses.
Hysteresis
What are 2 type of losses associated with magnetic circuits and how are they caused?
A) eddy currents, caused by magnetic flux inducing currents in core materials that are conducive
B) hysteresis loss, caused by extra power required to re-orient the magnetic domains of the core
List 4 methods used in a single phase motor to disconnect the start winding
1) Centrifugal switch
2) current relay
3) potential relay
4) thermal relay
Which type of single phase motor does not use a squirrel cage rotor assembly?
The AC universal motor
Why are pole pieces constructed of laminated steel sheets?
They have a smaller cross-sectional area, which increases core resistance and reduces eddy loses
What components does a capacitor start induction run split phase motor have in the start winding circuit?
1) A winding
2) electrolytic capacitor
3) a switch to disconnect the start winding
What type of single phase motor has the highest starting torque characteristics?
Capacitor start motor- because the angle is close to 90 degrees which is the maximum torque angle for single phase motors
What approximate percentage of rated speed does a centrifugal switch open at?
75%-85%
Motor 2 has double the poles motor 1 has. Which motor will be faster?
Motor 1 will be twice as fast since it has half the poles
In a resistance start induction run split phase motor, run field windings are wound with_______ turns of_________ gauge wire relative to the start field windings.
A) less
B) thicker
In a capacitor start induction run split phase motor, start field windings are wound with________ turns of__________ gauge wire relative to the run windings
A) less
B) thinner
To reverse a split phase induction motor, which wiring terminals have to be interchanged?
The start windings T8 and T5 to T5 to T8
Which type of single phase motor is typically not reversible?
Shaded pole motor- because the shading rings is fixed to one side of the pole face
Which split phase motor have their start winding disconnected at 75% speed?
Resistance/ Capacitor start motors
What happens is the centrifugal switch fails to reclose when a resistance start induction run split phase motor stops?
The motor will not be able to start again since the start windings are out of the circuit
Which split phase motor have their start windings energized at all times?
Capacitor run motors
