EE1F24- Three phase induction motors/ synchronous motors Flashcards
(24 cards)
Why do we use three phases?
-Less cable is required compared to 3x2 wire
-Power deluvery more consistent then 1ph
Higher power then 1ph for a given size
What is the relation between Vline and Vphase for a delta star configuration?
Vline is equal to root 3 Vphase
What is the relation between I line and I phase for a delta star configuration?
I line is equal to I phase.
What is the V and I relations for a delta connection?
Vline = Vphase
I line = Root 3 Iphase
What are the relations for a balanced star or delta configuration?
VA=Root 3 Vline (I)line
Power= Root 3 Vline (I)line cosϕ
VAr=Root 3 Vline (I)line sinϕ
Where:
ϕ is the phse angle between the voltage and the current.
What are the two methods for power measurements for a balanced load?
One Watt Meter- method involves taking one watt meter where phase voltage and line current is used to calculate the real power and power factor on a single phase (only works on star connection)
Two Watt Meter- Measure the current of two phases, measure the two voltages from same two, add power for both together(just trust this gives power for 3phases) , since instantaneous measurement Power is equal to VI
How do we determine power factor in two wattmeter method?
ϕ= tan^-1 ( Root3x((P1-P2)/(P1+P2)) )
Power factor =cosϕ
What is a stator?
Stationary part which has coils connected to an ac power supply.
How does an induction motor work in simple terms?
-AC Current flows in stator creating rotating magnetic field
-Magnetic field cuts across rotor
-Creates current in conductor due to faradays law
-Rotor creates own magnetic field due to current
-Rotor rotates in direction of magnetic field set up by stator
What are the advantages of an induction motor?
-cheap and robust
-Explosion proof
-Little to no skilled maintenence
-No self start properties
What are the two laws induction motors are based on?
-Faradays law of electromagnetic induction
-Lenzs law- the direction of an induced current is such that to oppose the change that created it
What is the whiplash rule?
States the flux lines from the rotor will find themselves around the stator conductors in the same direction as the flux lines produced by the current flowing in the conductors.
How does torque angle differ for generators and motors?
Always positive for generators and negative for motors.
What is torque angle?
The angular displacement between the stator magnetic field and the rotor magnetic field expressed in radians.
Why do we need torque angle?
Necessary for transfering mechanical input into electrical power as the rotors magnetic field must lead the stators in order to push magnetic flux across the gap.
Why is synchronous speed significant?
-is equal to steady state rotor speed
-Steady state magnetic field component produced by stator rotates at this speed
-Determines the frequency of induced voltage output
-N of poles in alternator must be matched to meet speed requirements
Why is the relation between stator voltage and rotor magnetic field current non-linear for a practical machine?
Will see a curve due to saturation of the machines magnetic circuit.
What is shown by an MMF vector diagram?
-Total mmf= resultant of rotor and stator mmf’s
-Magnitude of resultant is less then magnitude of rotor mmf
-Torque angle is angle between the rotor mmf axis and resultant air gap mmf axis.
What connection arrangement do the vast majority of altenators use?
Pretty much all altenators are connected in a star layout.
What is the equation for real power per phase?
Real power per phase= (EVsin(δ))/Xs
Answer comes out in Watts
What is the equation for reactive power per phase?
Real power per phase= 1/Xs x (EVcosδ - V^2)
Answer comes out in terms of VAR
What can cause real power output to change for a synchronous generator?
Can only change if the prime mover power changes, real power is proportional to sinδ.
What is reactive power proportional to for a synchronous generator?
Approximately proportional to the arithmetic difference between the internal voltage E and the terminal voltage V.
What are the two unique attributes of synchronous motors?
-Have perfect speed regulation, always rotating at synchronous speed irrespective of loading
-Can be over excited to run with a leading power factor so can be used as a power factor correction device
