Characteristics of Three Phase AC Flashcards
What are three advantages of three-phase systems?
1) Possible to produce smaller machines with the same kVA capacity
2) Requires about 3/4 as much copper to distribute same energy with the same efficiency as single phase system
3) Power is constant at all times, where as single phase is pulsating
4) Designs of three phase motors are much simpler than single phase motors
If a three-phase generator or motor is built in the same frame size as a single-phase motor, how much greater is the kVA rating than that of a single-phase machine?
About 1.5x
What is a polyphase system?
A system that has two or more phases
What is the voltage induced in each winding dependant on?
The rate of change of flux surrounding armature conductor
What are the configurations of a three-phase alternator?
Wye or Delta
The order in which the three phases reach their positive peaks is referred to as the __________ _________
Phase sequence
How many phase sequences are there in a three phase system?
- ABC or CBA
How can the phase sequence be reversed?
1) Direction of the prime mover can be reversed so the rotor turns in the other direction
2) Any two of the line leads may be interchanged
The positive direction of phasor rotation is __________
Counter-clockwise
Voltage phasors are generally drawn with _____ arrowheads
Open
When subscript notation is used to represent the voltage across a source, the first subscript has ______ polarity with respect to the second.
Positive
When subscript notation is used to represent the voltage across a load, the first subscript has ________ polarity with respect to the second.
Negative
The first subscript always represents the end of the circuit component at which the current __________ the circuit component
tracing-loop enters
Reversing the order of the subscripts causes the direction of the phasor to change by _________ degrees.
180
