Ac Machines Final Flashcards
2 basic categories of alternators
Rotating armature type
Rotating field type
How is voltage controlled while maintaining the frequency of an alternator
Rheostat controls the amount of dc excitation current
Is field excitation current of an alternator AC or DC
Always DC
Is excitation current and output voltage directly or inversely proportional in an alternator
Directly
If resistance is removed from the excitation circuit of an alternator will the magnetic field strength increase or decrease?
Increase
If resistance is added to the excitation circuit of an alternator will the output voltage increase or decrease?
Decrease
Two types of rotor assemblies found in rotating field type alternators
Salient pole
Cylindrical pole
Does a rotating field type permanent magnet alternator require slip rings or brushes
No
Mechanical force used to turn an alternator
Prime mover
Resistance to magnetism
Reluctance
Two types of magnets used in the rotor construction of a rotating field type alternator
Electro magnet
Permanent magnet
Squirrel cage induction motor is drawing more current than the nameplate full load current list two conditions that can cause increased current
Single phasing
Overloaded motor
When is a synchronous motor considered a synchronous condenser
When it’s DC field is over excited, is operated with no load and used for power factor correction only
Can a synchronous motor be started with DC connected to the rotor
Never, can only be connected when the motor reaches close to synchronous speed
How can a synchronous motor be made to have a leading power factor
When the rotor circuit is overexcited
What detrimental condition causes reduced voltage and increased current flow to the stator windings
Single phasing
If the number of poles is reduced in a synchronous motor does the speed increase or decrease
Increase
Formula for speed regulation
(NL speed - FL speed) / FL speed x100
Formula for efficiency
Output watts / input watts
Is a field winding a load or a source
Load
What type of alternator is used for main electrical distribution generation
Rotating field alternator
What are the brush leads labeled in a rotating field type alternator
F1 and F2
What instrument is used to indicate phase position and frequency
Synchroscope
What rotor bar design has the highest starting torque and lowest starting current
Type A
Formula for number of poles
Freq x 120 / RPM
Formula for slip %
Sync speed - rotor speed / sync speed x100
Formula for rotor speed
120xhz/ poles-slip
Formula for synch speed
120 x hz/ poles
Regulation percentage formula
NLS-FLS/FLS
Type of material used for squirrel cage rotor bars
Copper or aluminum
Is the external resistance applied to the rotor circuit or the stator circuit of a wound rotor motor
Rotor circuit
How can the speed of a wound rotor motor be controlled
External speed controller
What components ride against the slip ring
Brushes
How many stator leads to add for dual voltage and how many for dual configuration
Dual voltage- 6
Dual configuration- 3
Ex. A 3 phase dual voltage dual configuration = 12 leads
What is the purpose of an amortisseur
Provided for starting
