D5-3 Phase AC And Capacitors Flashcards
D5-LT1
How do you reverse the phase sequence?
By interchanging any of the two lines or by reversing the prime motor
D5-LT1
Define phase sequence
The order in which the three phases reach their positive peaks respectively
Ex: CBA or ABC
D5-LT1
Advantages of three phase systems
- Smaller machines with the same KVA output as a single phase machine
- Needs less copper to manufacture
- Three sine waves give a three phase system a smoother, less pulsating wave
- Three phase motors require no starter coil
D5-LT1
Define polyphase
A system with more than one phase. The most common kind of polyphase system is a three phase system.
D5-LT2
List six reasons for grounding the Starpoint in a Y system
- Reduces magnitude of transient overvoltages
- Lightning protection
- easy to find ground fault
- better system and equipment fault protection can be used
- less stress on electrical insulation
- maximum volts to ground is limited to phase voltage
D5-LT3
Ungrounded systems and ground fault protection
Three lights are connected in WYE to a Delta system.
Under normal conditions all three lights will glow dimly at about 58% of its maximum rating.
Under ground fault conditions one light goes dark and the two remaining lights glow brightly
D5-LT3
Define high leg and the CEC code for what is required to be done for the high leg
A line is the high leg because it has higher voltage to ground with respect to the other lines volts to ground.
CEC requires to keep the high leg separate from the grounding compartment
D5-LT4
define balanced load
A load in which each phase has the same values of impedance and power factor, as well as the same three phase voltage impressed upon them
D5-LT4
What are some things that can cause a balanced load to become unbalanced?
If one load has a single phase system added to its line
Or
if there is a ground fault
D5-LT5
advantages of using a neutral wire in WYE circuits
A three phase 4 wire circuit can supply both three phase motor loads and single phase load with the line and the neutral
As well as the convenience of using the neutral as a grounding point which improves safety.
D5-LT5 Different designations for a neutral depending on: three phase 4 wire single phase 3 wire single phase 2 wire
three phase 4 wire: Neutral conductor
single phase 3 wire:Common conductor
single phase 2 wire:Identified conductor
D5-LT6
Describe effects of an open in line or load in a three phase 4 wire WYE circuit
Open phase with neutral situation
The neutral acts as a line if one of the lines or phases break
Power will be about 66% of that which would be under normal conditions
D5-LT6
Describe the effects of an open line or load in a three phase 3 wire circuit in delta
The circuit will be single phasing, with three loads and one being in parallel with the other two.
If the load is broken then it would be one load in parallel with the other load
D5-LT6
Describe single phasing in a three phase 3 wire circuit inWYE
And open in line or load in a three phase 3 wire circuit causes a low to experience single phasing where the two remaining lines are the only two working.
Power will be less than 50% of power under normal conditions.
D5-LT7
Measuring power using 3 wattmeter method
- Calculate power factor accurately -wattmeters are connected in WYE
- can measure power in both WYE and delta circuits
- reads phase voltage
