Handouts 1,2,3,5 Flashcards
What happens to the speed of an AC universal motor as the load is decreased ?
Load goes down then the speed goes up
What is the primary difference between a start-type and run-type capacitor?
- start electrolytic
- run oil filled
When creating a magnetic field, do the lines of flux flow from south to north across the air gap?
No, North to south
What is the rotational force produced by a motor called?
Torque
What type of AC machine coverts electrical input to mechanical output?
Motor
In an AC universal motor, are torque and speed directly or inversely proportional to each other?
Inversely
What approximately percentage of rated speed does a centrifugal switch open at?
75%
If the centrifugal switch fails to open as a capacitor-start induction-run motor accelerates to its rated speed, what could happen to the starting windings?
Overloads trip/ windings over heat
In reference to the left hand rule for conductors, what do the fingers and thumb represent?
Thumb- electron flow
Fingers- magnetic lines
In reference to the left hand rule for electromagnets, what do the fingers and thumb represent?
Thumb- north end
Fingers- electron flow
What is a measure of a materials ability to become magnetized called?
Permeability
What component of an AC machine supports the shaft and allows it to rotate freely?
Bearings
Does the stator of an AC machine produce a stationary magnetic field or an alternating magnetic field?
Alternating magnetic field
What are the terminal markings for start windings leads?
T5 and T8
In a capacitance start induction run split phase induction motor start field windings are wounded with ________ turns of __________ gauge wire relative to the run field windings
More, smaller
To reverse a 4 lead permanent slip capacitor motor, which wiring connections are interchanged?
T5 T8
List three types of power losses that occur in a split phase induction motor
Eddy currents
Hysteresis
Heat
Does the counter emf generated increase or decrease as the load is added to an ac series universal motor
CEMF goes up
LOAD goes up
(Increases) is the answer
What type of single phase AC motor has the highest starting torque characteristics?
Universal series motor
A rotating magnetic field is produced in an AC motor by “splitting” the phase through two separate field windings. What type of motor uses this principle?
Split phase induction motor
Why are pole pieces constructed of laminated sheets
To reduced eddy currents or to minimize losses
What are the brushes in an AC universal motor made of?
Carbon graphite
What is the minimum number of terminal leads require for connection of a dual voltage 115/230v resistance start induction run motor?
6 leads
In an AC universal motor when proper commutation is taking place, what colour is the commutator?
Dark chocolate
In a permanent magnet motor, where are magnetic flux lines most densely concentrated?
At the poles
What types of capacitor does a permanent split capacitor motor use?
Oil filled capacitor
What type of capacitor does a capacitor start induction run motor use?
Electrolytic
What are two factors that determine the amount of torque developed by a motor load?
Strength of magnetic field
Current
What is the portion of the stator pole piece is closest to the rotor assembly called?
Pole shoe
How is the capacitor in a capacitor start induction run split phase motor connected?
In series with the start windings
What solid state device can be used to disconnect the start windings in a resistance start induction run split phase motor
Thermistor
Does a current relay use normally open or normally closed contact is connected in series with the start windings?
Normally open
What type of motor has a short circuited time of heavy copper wire wounded around part of the stator pole?
Shaded pole induction motor
When an AC motor is first started, why is the inrush current so high?
Low CEMF
What are two advantages of a resistance start induction run motor
Good for frequent start condition
Constant fixed speed
What are two disadvantages of a resistance start induction run motor?
High starting surge current
Limited to 1/3 HP
What are two advantages of a capacitor start induction run motor?
Low in rush current on start up
High starting torque
What are two disadvantages of a capacitor start induction run motor
Can’t be started more than 8 times per hour
Prolonged current flow will damage capacitor
What are two advantages of a permanent split capacitor motor
Good starting torque
Good running torque
What are two disadvantages of a permanent split capacitor motor
Cap can over heat
Relies on cap
What are two advantages of a capacitor start capacitor run motor
Constant fixed speed
Good running torque
List five methods used in split phase motors to disconnect the start windings
Centrifugal switch Hot wire windings Current relays Potential relays Solid state starting relay
In a resistance start induction run split phase induction motor run field windings are winded with __________ turns of ___________ gauge wire relative to the start field windings
Less, larger
To reverse a 6 lead split phase induction motor which wiring connection are interchanged
T5 and T8
Is the starting torque characteristics of a capacitor start motor greater than or less than the starting torque characteristics of a resistance start motor
Greater
Which type of disconnecting means for a split phase motor functions as both a starting relay and an overload relay
Hot wire relay
Which split phase motor have their start windings disconnected after initial rotation is developed durn start up?
Resistance start induction run
Capacitor start induction run
Which split phase motors have their start windings energized at all times
Permanent split capacitor
Capacitor start capacitor run
What is the maximum phase displacement angle between the start and run windings of a capacitor start induction run split phase motor
90°
What is the function of a centrifugal switch
Mounted inside a motor that serves as a disconnect the start windings
What type of single phase motor would be best suited to be used for a residential refrigerator
Capacitor start induction run
What is a thermistor
Device that exhibits a change of resistance with change of temperature
Is the resistance of a start field windings higher or lower than the resistance of the run field windings in a capacitor start induction run split phase induction motor
Higher
What are the minimum number of stator leads required for a single voltage, reversible split phase induction motor
4
What are the minimum number of stator leads required for a dual voltage reversible split phase induction motor
6
What are the minimum number of stator leads required for a single voltage non reversible split phase induction motor
2
List three application of permanent split capacitor motors
Central air conditioners
Air handlers
Blowers
List three applications of resistance start induction run motors
Washer machine
Dishwasher
Clothes dryer
List three applications of capacitor start induction run motors
Fridges
Freezer
Wine cooler
List two types of capacitors used in split phase induction motors
Oiled filled
Electrolytic capacitors
When an AC motor is first started why is the in rush current so high?
The rotor current is only limited by the very low rotor circuit resistance
What type of single phase motor does not use a squirrel cage rotor assembly
Universal series motor
List two factors that determine the amount of torque developed by a motor
Strength of the main magnetic fields
Amount of current flow
Why are pole pieces laminated
To minimizes the circulation of induced eddy currents within the core and hysteresis
What type of single phase motor is typically not reversible
Shaded pole induction motor
In an AC universal motor are torque and speed directly or inversely proportional to each other
RPM goes down
Torque goes up
So torque and speed are inversely
If the number of poles in a shaded motor is double does the speed of the motor increase or decrease
Decreases
What is the typical phase displacement angle in a shaded pole motor
10°
What happens to the torque of an AC universal motor if the load is increased
If the load increases the torque will increase as well
The amount of CEMF produced in a rotor assembly is proportional to what three factors
Amount of conductors
The magnetic field strength
The speed of rotation
Is the shaded pole an induction motor
Yes
Why are pole pieces constructed with silicon steel
To reduced hysteresis
In universal motors, are the armature and field coils connected in series or parallel?
Series
Which single phase motor is considered to be the most simple and least expensive?
Shaded pole induction motor
What is the formula for synchronous speed
RPM=120xhz/# of poles
What is the formula for rotor speed
RPM=120xhz/# of poles -%slip
How should the direction of rotation of an AC universal motor be revered
Interchange armature leads A1 and A2
Does a shaded pole induction motor rotate in the direction towards the shading coil potion of each pole piece or away from the shading coil portion of each pole
Piece
Towards
Does the CEMF generated increase or decrease as load is added to an AC series universal motor
Load goes up
CEMF goes up
It increases
List three applications of shaded pole motors
Small fans
Timers
Clocks
List three applications of AC universal motor
Power tools
Sump pumps
Razors
List three types of power losses that occur in AC machines
Mechanical
Electrical
Magnetic
When creating a magnetic field, do the lines of flux flow from the north to the south across the air gap?
Yes
What are the three factors that determine the strength of an electromagnet
Conductors
Current
Core
Where are magnetic flux lines most densely concentrated
At the poles
What is measure of a material ability to become magnetized called
Permeability
What is resistance to magnetism called
Reluctance
When an electromagnet reaches a point where strength increases only slightly with an increase in current, it is known as a point of ___________
Saturation
What is the amount of magnetism left in a material after magnétisant force has stopped called
Residual magnetism
In reference to the left hand rule for conductors, what do the fingers and thumbs represent
Thumb- current
Fingers- field
In reference to the left hand rule for electromagnets what do the fingers and thumb represent
Thumb- north field
Finger- current
Does an increase in current flow in a conductor result in an increase of magnetic field around the conductor
Yes
What are two types of magnets
Permanent magnets
Electromagnets
What is the rotational force produced by a motor called
Lorentz force
When a conductor cuts through magnetic lines of flux a voltage is induced into the conductor. What is this principle called?
Electro magnetic induction
In reference to the right hand rule motors what do the thumb, forefinger and middle finger represent
Thumb- thrust
Forefinger- field
Middle finger- current
Does the stator of an AC motor produce a stationary magnetic field or a rotating magnetic field
Rotating magnetic field
What is the purpose of a field winding in an AC machine
Create magnetic lines of flux
Is a field winding in an AC motor considered an electrical load or an electrical source
Electrical load
What are two types of bearings found in an AC machine
Sleeve and rolling
Provide two reasons why the poles shoe of every pole piece is curved to conform to the surface of the rotor assembly
Wider and more even flux distribution
What is the portion of the pole piece closest to the rotor called
Pole shoe
What are the three components that comprise a rotor assembly
Shaft
Iron core
Rotor windings
What components of an ac machine supports the shaft and allows it to rotate freely
Bearings
List with electrical input requirements that are found on an AC machine name plate
Manufacture name Model number Serial number Rated voltage Current (FLA) kilowatts Horsepower Speed
How are eddy currents minimized in an AC machine
Thin coat of enamel within the core
How are hysteresis minimized in an AC machine
Silicone steel alloys
What components forms the axis for rotation of the rotor assembly in an AC machine
Shaft
List 4 components of the magnetic circuit in an AC machine
Air gap
Pole shoes
Shaft
Frame
List 4 components of the electric circuit in an AC machine
Brushes
Field windings
Lead
Series field windings
List 4 mechanical components used in an AC machine
Frame
End plates
Bearings
Base
What are the four types of enclosures for AC machines
Open type
Weather proof
Drip proof
Totally enclosed type
