AC Machines Flashcards
List the three main types of three-phase AC motors
- Squirrel cage induction motor
- Wound-rotor induction motor
- The synchronous motor
What are the main constructional parts of a three-phase induction motor?
Stator
Rotor
End bells
Bearings
How are the two circuits of an induction motor similar to those of a transformer?
Stator = primary winding Rotor = secondary winding
What is meant by the term “synchronous speed”?
The speed at which the magnetic field rotates produced by the stator winding (poles) moves.
What three basic factors allow the development of a rotating magnetic field in a three phase induction motor?
- The fact that the voltages in a 3 phase system are 120 degrees out of phase w/ each other
- The fact that the three voltages change polarity at regular intervals
- The arrangements of the stator windings around the inside of the motor
What factors determine the synchronous speed of an induction motor?
Speed in RPM
Frequency in Hertz
Number of stator poles per phase
S = 120 x F (frequency)
———–
P (stator poles)
What is meant by the term “slip”?
Difference in RPM between the rotor speed and synchronous speed of the stator
Why cant the rotor of an induction motor turn at sync speed?
There will be no cutting action (relative motion) of stator flux on rotor bars to induce a rotor current necessary for torque
How would you reverse the direction of rotation for a three-phase induction motor?
Interchange any 2 line leads to the stator
What is the purpose of “skewing” rotor bars in induction motors?
To provide for a more equal distance of rotor flux allowing for smoother torque development and quieter operation.
Why is the starting current of an induction motor so much higher than its rated running current?
100% slip = maximum cutting rate of stator flux on rotor bars (no CEMF)
State three fundamental reasons that cause the development of a rotating magnetic field within a three-phase induction motor.
- Physical arrangement of coil groups
- Each coil group energized with AC
- Each phase current time displaced by 120 electrical degrees
List the three main factors that determine the torque developed by an induction motor.
- Stator Flux
- Rotor current (flux)
- Rotor power factor
What effect does slip have on rotor reactance?
Rotor frequency is proportionate to % slip: Fr = Fs x %slip
Rotor reactance is proportionate to frequency: XL = 2(pie)fL
What is meant by the term “speed regulation”?
Difference between no load speed and full load speed of the rotor.
What constitutes the “power losses” for induction motors?
Copper losses (I2R) Iron losses (eddy current and hysteresis) Rotational losses (friction, windage)
Why would increasing the rotor circuit resistance of an induction motor increase its starting torque?
Rotor PF is improved
List one advantage and one disadvantage of starting a large induction motor with “reduced voltage” across the stator windings.
Advantage: Reduced in rush current will cut load shock and system disturbances (lowers O/C setting)
Disadvantage: Reduced starting torque, longer accelerating times, more elaborate control and higher cost.
For a given value of slip, the T output of an induction motor varies as the square of the aplied voltage, explain.
Change in applied voltage to the stator results in the change of stator flux; this also change rotor current.
The combo results in the T being proportionate to the square of the applied voltage.
Explain starting torque
Rotational force developed at the instant of starting
Explain full-load torque
Rotational force required to keep rated load (HP) turning
Developed at about 2% - 5% slip for most SCIMs
Explain breakdown (pull out) torque
Amount of overload torque which will stall the motor
What are the objections to building a squirrel-cage induction motor with high enough rotor R to provide an excellent starting torque?
Poor speed regulation
Higher I2R losses
Poorer efficiency
What advantages do a squirrel-cage induction motor have over other types of induction motors?
Relatively constant speed between no load and full load Initial purchase cost Ruggedness Low maintenance High dependability
Explain the constructional differences between a “double” squirrel cage and a “standard” squirrel cage motor?
Two sets of rotor bars (high and low R) embedded within the rotor iron for a “double”
How is the operating characteristics between the “double” and “standard” squirrel cage motors different?
Double typically has higher starting T but slightly poorer speed regulation
What is an advantage of the “double” squirrel cage motor compared to other types?
Relatively good starting T
Good speed regulation
On what type of driven load would you specify a high R type SCI motor?
Higher torque loads (compressors, conveyors)
If a three phase motor is running at rated load, and one line opens, what happens?
Typically it will remain running at a decreased speed but will have excessive current and usually a humming noise.
What is it called when a three phase motor has one of its line open?
Single phasing
Equation for sync speed (rpm) ?
N = f x 60 f x 120
———— or —————
pairs of poles poles
Rotor frequency equation?
fr = supply frequency x % slip
Rotor frequency is dependent upon _____ ?
Slip
At standstill for a SCIM, the rotating field cuts the rotor bars at ______ which results in the induced voltage and current being ________.
Full frequency
At a maximum
At standstill for a SCIM, the rotating field cuts the rotor bars at ______ which results in the induced voltage and current being ________.
Full frequency
At a maximum
At standstill for a SCIM, the rotor PF is ______ ?
Low
At standstill for a SCIM, induced voltage is _______, rotor impedance is ______, and rotor reactance will be _______.?
Highest
Highest
Maximum
Torque is produced by the interaction of ______ and _______.?
Stator flux and rotor flux
Rotor flux is proportionate to rotor _______ and therefore torque is a function of rotor _____ and ______.?
Current
Current and Frequency
As the rotor on a SCIM accelerates, the rotating magnetic field will cut _____ of the rotor bars and ____ current will flow in the rotor and the stator.
Less
Less
As the SCIM rotor accelerates the magnetic field will cut _____ of the rotor bars and _____ current will flow in the rotor.
Less
Less
Breakdown torque varies with motor (rotor design) but typically is?
200% - 300% of full load torque.
Torque is maximized when?
Rotor R = Rotor XL
What does rotor design affect?
In rush current
Starting T
Speed regulation
Full load T
Torque characteristics of a low R rotor?
Poor starting torque
Better running torque
Torque characteristics of a high R rotor?
Good starting torque
Low running torque
For a 3 phase motor to be connected to high voltage, how must the phase windings be connected?
Wye or Delta and in series
For a 3 phase motor to be connected for low voltage, how must the phase windings be connected?
Either wye or delta and in parallel
How many leads are brought out if the motor is intended to be used for wye-delta starting and how must it be connected?
12 leads (also windings must be connected in delta)
When a motor is connected for high voltage, why is the current flow be half as much when compared to low voltage operation?
When windings are connected in series for high voltage, the impedance is 4 times greater
The amount of induced voltage for a SCIM is determined by?
- Strength of the magnetic field of stator
- Number of turns of wire cut by the magnetic field
- Speed of cutting action
The amount of T produced by an AC induction motor is determined by?
- Strength of the magnetic field of stator
- Strength of magnetic field of the rotor
- Phase angle difference between rotor and stator fields
When the SCIM speed decreases, the rotating magnetic field cuts the rotor bars at a _____ rate?
Faster
The speed performance of an induction motor is measured in?
Percent slip = sync speed - rotor speed
————————————– x 100%
sync speed
What is a common rotor slip for a SCIM?
2% - 5%
What determines the operating characteristics of a motor?
The type of rotor bars used in the construction of the rotor
The code letter on the nameplate of an AC SCIM indicates what?
The type of bars used in the rotor
A PF of about _____ is common for a SCIM at no load.
10%
The PF of an induction motor at full load is about?
85 - 90% lagging
When does single phasing occur?
When one line of a 3 phase system is open
For a double SCIM, the outer SC winding has ?
High R bars to provide high starting T and low starting current
For a double SCIM, the inner SC bars have?
Low R and a higher inductance (XL)
NEMA rated motors are designed to operate at?
+/- 10% of their rated voltage
What are the 3 main parts of a SCIM?
Stator
Frame
Laminated Core
What is the purpose of the core on a SCIM?
Supports the stator coils
Provides a low reluctance path for the magnetic circuit
Maximum T is produced when the flux and rotor current are?
In phase
What is the most common motor design?
Design B
Characteristics of a Design D rotor
High R rotor design Small rotor bars High starting T Low starting current Poor speed regulation
Characteristics of a Design B rotor
High reactance rotor design Deep bars Low R Relatively low starting current Fair starting torque Better speed regulation
Characteristics of a Design A rotor
Standard rotor design Bars are a compromise btwn small/deep Relatively low R Low inductive reactance High starting current Fair starting T Best speed regulation
Characteristics of a Design C rotor
Double squirrel cage rotor
Combo of small bars for high R and deep bars for low R
High starting T
Good speed regulation
Has the deep low R bars for high XL for running
A linear induction motor moves in a _____?
Line
A linear induction motor consists of a ?
Flat stator and a flat rotor
The rotor for a linear motor is normally made out of ?
Conductive material, normally aluminum (inexpensive and light weight)
Describe the difference between the conventional motor and the linear-induction motor.
Conventional motor, the stator is stationary and the rotor moves
Linear, the stator moves and the rotor is stationary
How is the sync speed of a linear induction motor measured?
Meters per second
What is the sync speed equation for a linear-induction motor?
Sync speed = 2 x pole pitch x frequency
What are the main constructional differences btwn a “SCIM” and a “WRIM”?
Rotor construction: 3 separate insulated windings 120 electrical degrees displaced
Usually Y connected internally with ends brought out to slip rings for external brush connection to resistor grids.
What 2 major advantages does a “WRIM” have that are not available with a “SCIM”
High starting torque
Low starting current with R in rotor circuit
May be operated at variable speed (torque)
Smooth acceleration under heavy loads
How should a WRIM be started?
With full resistance in rotor circuit and rated voltage across the stator
Would a WRIM have a higher operating eff. at 80% slip or 5% slip?
5% slip, lower rotor currents therefore smaller I2R losses
List 2 disadvantages of a WRIM
High initial cost
More maintenance
More elaborate control
List three examples of where a WRIM may be used
Elevators
Cranes
Compressors
How many slip rings are on a WRIM?
3
How is the starting current in a large WRIM limited to a safe and reasonable value?
External resistors during starting
What constructional features make a 3 phase Sync motor different from a SCIM?
Rotor construction: Salient DC field poles, 2 slip rings, brushes, ammortisseur winding
What factors determine the rotor speed of a 3 phase sync motor?
Number of stator poles
Frequency of the applied voltage
How is a 3 phase sync motor most commonly started?
By means of ammortisseur windings (cage rotor)
What TWO important charac. are possessed only by the Sync motor and not by an other 3 phase motor?
0% speed regulation (constant speed) Variable PF (lead, lag or unity)
For a constant mech output (load), how can the PF at which a sync motor operates change?
By varying the DC field excitation (increased: leadings vars, decreased: lagging vars)
What is a sync condenser (sync capacitor/rotating caps) ?
Sync machine used solely for PF correction
What is meant by the torque angle of a 3 phase sync motor?
The angular displacement between the centers of the rotor poles and the rotating stator poles.
Why will a sync motor not operate properly if the field circuit is energized before the rotor speed approaches near sync speed?
Torque angle is too large to allow the rotor to pull in to synchronism
Define pull-in torque
Rotational force required to pull the rotor into synchronism
Define pull-out torque
Force required to pull rotor out of synchronism
What is the purpose of the SCIM type winding on the face of the salient poles?
to allow the motor to be self starting (amortisseur winding)
At what PF does a sync motor draw the least stator current for a given load?
Unity PF
A “bump” when a sync motor pulls into synchronism would be caused by?
Rotor speed (torque angle) is too low at point of sync
What would be considered as an acceptable percent slip on a sync motor at the moment of field application?
3 - 5 % slip
For a sync motor at constant mech load, what is an indicator for unity PF?
Minimum line current
Define under excited
Decreased field current (lagging PF)
Define over excited
Increased field current (leading PF)
What is the purpose of a “field discharge resistor” used with a 3 phase sync motor?
To reduce the level of induced voltage during starting and dissipate heat energy
What type of 3 phase induction motor has the best speed regulation?
Sync motor
What type of 3 phase induction motor has the best PF?
Sync motor
What type of 3 phase induction motor has the best starting T?
WRIM
What type of 3 phase induction motor has the lowest initial cost and maintenance cost?
SCIM
What creates the CEMF in a 3 phase sync motor?
The magnetic field produced by magnetizing current called excitation current
The rotor circuit in a WRIM is often referred to as what?
Secondary circuit of the motor
What would happen to the rotor speed of a SCIM if the frequency of the applied voltage was increased?
Speed increases
What happens to the rotor of a WRIM when the load is increased?
Increases
A 3 phase sync motor may be used wherever a ______ motor is required?
NEMA Class B
A Sync motor is particularly well-suited to loads that operate ?
Below speeds of 500 rpm
Sync motors require a ?
Starting aid
Two ways to start a sync motor?
Amortisseur windings embedded in the rotors pole face
WRIM called a “pony motor” for high inertia loads
What are the 3 common methods of supplying the sync motor with DC field current?
External converters
Built-in exciters
Brushless exciters
Typical angles at which pull-out T occurs?
May be as high as 60-70 degrees
How do you locate a shorted stator coil?
Growler
How do you locate a grounded stator coil?
Smoking at the point of grounding
How do you locate an open rotor bar?
Variations in the ammeter
Another name for a sync shaded-pole motor?
Hysteresis Motor
List the essential parts of a split phase induction motor?
Stator Rotor Centrifugal switch End bells/bearings Nameplate
What are the differences in the start and run windings on a single phase motor?
Start = High R, smaller wire, more turns, top of stator slots Run = Low R, larger wire, less turns, bottom of stator slots
What would happen if the centrifugal switch on the single phase motor fails to close when the motor stops?
Will not start
At what approx. percentage of full-load speed does the centrifugal switch for a phase motor open?
`75%
How is the direction of rotation reversed on a split phase induction motor?
Interchange leads of either start or stop windings, typically start windings T5 and T8
What is the main function of the capacitor in the capacitor start induction motor?
To improve the starting T
List two methods of controlling the speed on a single phase universal motor?
Series resistor
Electronic control
How is the direction of rotation changed on a universal motor?
Interchange armature leads
The direction of flux on the stator for a shaded pole motor is from?
Main pole face to the shaded pole face
Name 2 applications of shaded pole motors?
Bathroom/Hood fans
Small appliances
Why is the starting T of a capacitor start single phase motor better than that of a resistance start motor?
Greater phase displacement between start and run winding currents, resulting in smoother rotating field
What is another name commonly applied to the AC series motor?
Universal motor
What are some common applications of a universal motor?
drills, vacuum cleaners, reciprocating saws and sanders
What are the 2 general classes of alternators?
Rotating field type
Rotating armature type
Which alternator is used more commonly?
Rotating field type
What type of alternator rotor is commonly used for high speed machines?
Cylindrical (non-salient) type
What type of alt. rotor is commonly used for low speed machines?
Salient pole rotors
When paralleling alternators, what indicates wrong phase displacement and how is it corrected?
Lights flashing at different times
Interchange 2 leads of the incoming alternator or reverse prime mover
When paralleling alternators, what indicates wrong frequency and how is it corrected?
Lights flashing
Changing speed of prime mover (varying DC field of DC motor)
When paralleling alternators, what indicates wrong voltage and how is it corrected?
Lights on (dim to bright), voltage across switch Adjust field excitation of 3 phase alternator
When paralleling alternators, what indicates wrong phase displacement and how is it corrected?
Lights on or some voltage across the switch
“tweak” the excitation of prime mover to change the phase displacement
What is “hunting” when paralleling alternators and how is it reduced?
Surging/Oscillating of power between 2 alternators, not desired.
“damping” the prime mover governor controls, heavy fly wheels or amortisseur windings.
Lamps have to be rated for _____ phase voltage.
Double
When paralleling alts., a change in field excitation causes a change in?
Ampere load, KVARS of alt.
When paralleling alts., a change in the prime mover governor controls causes a change in?
Kilo watts btwn sync gens.
Operation of paralleling alts., a lagging F causes terminal voltage to ?
Drop on alts.
Operation of paralleling alts., leading PF causes Vt to?
Go up on alts.
When paralleling alts., the prime mover must have relatively ___________?
Similar speed-load characteristics
