Ac machines Flashcards
If rotor slip increases what happens to PF of the rotor
PF WILL INCREASE
If the mechanical load on a motor increases what happens to the motors PF
PF improves
What happens to the cemf in the stator if the rotor flux increase
Opposed by 180° so cemf will decrease as rotor flux increases
If rotor slip rises what happens to stator current
Current will increase
If the motors load is lightened what happens to the rotor speed
Increases
If rotor speed is Increased how is induced voltage affected
Output voltage will decrease
How is torque affected by an increase in stator current
Current increases
Torque increases
What will happen to the rotor speed if the frequency iof the stator voltage decreases
Rpm decreases
If the motor stalls what happens to sync speed
Sync speed is resultant of frequency and number of poles so there is not affect on sync
If a motors rated FLT is 50ftlbs but the motors supply voltage is reduced to 75% of rated voltage how much torque is produced
Torque is equal to applied voltage²
.50²x50 =12.5ftlbs
What will happen to the slip% when the load increases on the motor
Increases
When slip% increases what happens to rotor frequency
It will increase
How would a larger %speedreg affect rotor current
Current would increase
If the rotor PF goes down what happens to output torque
Less PF equals less torque
How is line current affected if the induced rotor voltage decreases
Less induced=higher cemf =lowers current
What happens to a motors efficiency when load is reduced
Shouldn’t have much change
If the rotor speed drops from 1795 to 1725 how will this affect the torque
Torque increases
A motor stalls what happens to rotor flux
Rotor flux is only changed by frequency and pole numbers
Does a 3phase scim operate at higher power factor when it is operating no load or full load
Full load
A fully loaded synchronous motor has excitation adjusted so that its power factor is unity. The excitation is decreased slightly without making any other change what would happen?
The pf would become lagging
What are two advantages of a high reactance scim
Low inrush
Low speed reg
What two items found on a nameplate can you use to correctly determine the size of overload device
Service factor
FLA
The rotor of a 3phase double squirell cage induction motor has __________ resistance bars beat the rotor surface
High
What are two advantages of choosing a 3phase scim having high resistance rotor
Low inrush
Strong starting torque
State two disadvantages of a 3phase scim high resistance rotor
High% speed reg
Low efficiency
If you reversed two of the rotor circuit leads for a 3phase wound rotor motor what would happen
Operate as previous
How could you determine if there is a faulty bearing in a scim
Vibration displacement indicator
What would the likely result should the centrifugal switch of a split phase motor fail to open during start
Less full load torque
Slower speed
Poorer speed regulation
Burn out of start winding
What would be the likely result of an open circuit in the rheostat circuit of a 3phase wrim
Motor won’t start
What test can you do to find out the problem with a WRIM if you suspect there is an open in the rheostat circuit
Short out m1 M2 m3
What are two advantages of a scim with a high reactance design over a standard scim
Low inrush
low %speed ref
What is a disadvantage of a high reactance scim
Low starting torque
What is are two advantages of a scim with a low Z
Strongest starting torque
Low%speed reg
What is one disadvantage of a low Z scim
Large inrush current
Should a 3phase wrim be started max or min resistance
Max
What is the two purposes of the amortisseur winding
Start winding
Dampening winding
What is the advantage of a double squirrel cage motor when compared to other types of induction motors
Strong start torque
Low%speed reg
The torque angle between the centre’s of the stator field and rotor field of a synchronous motor are determined by what
The load
A double squirrel cage rotor is what nema design
Nema c
What term is meant by single phasing of a 3phase scim
One of the lines of the motor has developed an open
To reverse rotation of a split phase motor you would
Reverse the star winding leads
When the field rheostat of a synchronous motor is adjusted to produce the lowest stator line current the pf would be
At unity
Is it possible to vary the torque of a wound rotor motor
Yes by adjusting the external resistance
Is it possible to vary the speed of a wound rotor motor
Yes adjusting external resistance
What would happen to the speed of a wound rotor motor if the frequency of the applied voltage was increased
The speed would increase due to speed being directly proportional to frequency in an induction motor
What is the difference between the stator of a scim and the stator of a WRIM
No different from eachother
If we compared the starting winding of a standard split phase motor to the running winding we would see
That the starting winding is placed over the top of the running winding in the stator slots
What would be the main reason one would expense a WRIM over a scim
High start torque
Smoother acceleration of heavy loads
Will a 3phase wrim start if the rotor circuit is opened
No
No current flow in rotor circuit
If a 3phase wrim fails to initially start but rotates when the 3 slip rings are shorted what would the problem be
Open in the secondary resistor circuit
What happens if the centrifugal switch contacts fail to close when the motor stops
The motor will not start again
How would you reverse the rotation of a split phase induction motor
Interchange the start winding terminals
What is the primary difference of a split phase and a split phase capacitor start
Capacitor start has better starting torque
What is one limitation of a capacitor start induction motor
Needs heal time cants be started frequently
A 3phase Ac motor is said to be single phasing when
No current flows in one of the line
Will a universal motor operate faster or slower on a DC or single phase Ac supply
Would be the same
What precaution must be watched when operating a single phase universal motor
Loss of load causes runawayb
If the excitation is adjusted so that the power factor of the fully loaded synch motor is at unity what would happen if excitation is decreased
PF would decrease and start lagging
How could we increase the speed of a synch motor
Increase frequency
If a fully loaded synchronous motor is at unity and only mechanical load is reduced what would happen
PF would lead
When the line current of a synchronous motor is at Minimum what would the pf be
Unity
When should the DC excitation be applied to the rotor of a synch motor
About 95% rated speed
What is meant by the term pull out torque
Torque value that causes synch motor to drop out of synch and drop to a standstill
When operating an alternator at no load a decrease in the speed of the prime mover will cause a
Decrease in frequency and voltage
When a synchronous motor is operated with little or no load and its field excitation is adjusted for a leading pf it can be called a
Synchronous condenser
A fully loaded synchronous motor is operated at unity of if only the mechanical load is reduced you would expect the pf to
Go leading
What rating does a vfd need to be if driving a motor
Inverter rated
Is a synchronous motor an induction motor
No
a synchronous motor is a ______________ motor
Electromagnet
Are the stator winding of a synchronous motor the same as a squirrel cage and a WRIM
Yes
What sizes in hp do synchronous motors come in
200-20000hp
What percentage do you put a synch motor into synch speed
Around 96-98% synch rpm
The ____________ of ______________ produces motor torque
Stretching
Flux linkages
Synchronous motors have an efficiency of around
97-98%
What prevents voltage from travelling down the DC source path of a synchronous motor
A field discharge resistor
How do you reverse a split phase motor
Swap terminals on start winding
What is the slotted ring which holds fixed windings in an AC machine
Stator core
What is the revolving part of an AC machine called
The rotor
Where is the revolving magnetic field produced In an AC machine
Stator iron
What causes the revolving magnetic field in the rotor of an AC machine
Mutual induction
What other Ac equipment can a scim be compared too besides another motor
Transformer
What is the relationship between slip speed and induced rotor voltage
Directly proportional
What is the relationship between reactance and frequency
Directly proportional
What approximate %slip is motor torque the greatest
25%
What is the relationship between resistance and reactance when torque is at its strongest point
Both values should be equal
