AC machines midterm Flashcards
What is flux density
The amount of field lines in a given area
What is flux density unit measured in
Tesla’s (T)
What is flux unit measured in
Weber’s (B)
What is permeability
Ability of an object to be magnetized
What is Reluctance
Opposite of permeability,
Inability to be magnetized
What is residual magnetism
After coil returns to 0 current, the magnetic domains become disorganized but some remain aligned creating residual magnetism
What is retentivity
Ability for a core material to retain magnetism
What is an ideal core material and what attributes would it have
High permeability, low retentivity. Often made it silicon steel
Problems with core material
Saturation
Only use rated voltage for machines as additional voltage only creates heating
Problems with core material
Hysteresis losses
Causes heating in core. Use silicon steel core to reduce losses
Problems with core material
Eddy currents, how to reduce
Use laminated sheets to reduce losses in core
How to limit hysteresis loops
Use core material with low retentivity to reduce losses
Best material for a core
Silicon steel
What are Eddy currents
Circulating currents in a core that create heat and i²r losses
What is Faraday’s law
A moving or changing field will induce a voltage and current into an inductor
What is Lenz’s law
The polarity of an induced current will always oppose the originating field
What material is a stator made of
Made of stacked silicon alloy steel laminations
Right hand rule
Generators,
Thumb - thrust
Pointer finger - direction of field North to South
Center finger- direction of current
Left hand rule
Motors
Thumb- force on conductors
Pointer finger- field direction north to south
Center finger - direction of current
Formula for percent slip
(NS - NR) divided by NS
Formula for speed regulation
(Speed no load - speed full load) divided by speed full load
What kind of cap does a cap start induction run motor use
Electrolytic to shift start winding current
What is a cap start motor used for and how does it’s current draw compare to an inductive start
High starting torque loads, and has less starting current
What motor is not reversible
Shaded pole
What is Lorentz force
A current carrying conductor in a field will experience a force
What is Faraday’s law
A moving conductor through a field will have current induced or a moving or changing field will induce voltage and current in a conductor
Four methods to disconnect start winding
Thermal sensor
Current sensor
Centrifugal switch
Voltage sensor
What type of AC motor does not use a squirrel cage rotor assembly
AC universal
Why are pole pieces constructed from laminated steel sheets?
To reduce eddy currents
What components does a cap start induction run split phase motor have in the start winding circuit
Centrifugal switch, start winding, electrolytic capacitor
What’s single phase AC motor has a high starting torque
Cap start
At what percentage of speed does a centrifugal switch open
75 to 85 percent
Which split phase motors have their start winding disconnected at 75% rated speed
Cap start and resistance start
What happens if a centrifugal switch fails to reclose
It won’t restart
Which split phase motor has their run winding on at all times
Cap run
What is the function of the centrifugal switch
Remove start winding
What motor is best suited for refrigerator
Cap start hermetically sealed
In a universal AC motor are the armature and field coils connected in series or parallel?
Series
How can you reverse the direction of an AC universal motor
Change connections a1 and a2 or S1 and S2
What is the typical application of a shaded pole motor
Small fans
List three types of capacitors used with AC motors
Electrolytic, metal film, paper and oil
Can a 2 lead motor be reversed
No
