Power Engineering

Question 1

What is the phase difference between a phase voltage and a line voltage in a 3 phase AC system

Answer 1

30 degrees (positive)

Question 2

What is the magnitude difference between a phase voltage and a line voltage

Answer 2

Line voltage = root(3) x phase voltage

Question 3

What is the phase of Vrn

Answer 3

0 degrees

Question 4

What is the phase of Vyn

Answer 4

-120 degrees

Question 5

What is the phase of Vbn

Answer 5

120 degrees

Question 6

What is the phase of Vry

Answer 6

30 degrees

Question 7

What is the phase of Vyb

Answer 7

-90 degrees

Question 8

What is the phase of Vbr

Answer 8

150 degrees

Question 9

For a Star connected 3 phase ac system what type of voltage and current is through/ across the load

Answer 9

Phase voltage

Line current

Question 10

For a delta connected 3 phase ac system what type of voltage and current is through/ across the load

Answer 10

Line voltage

Load current

Question 11

What is the equation for power in a delta using a 3 wattmeter method

Answer 11

P=3xVphxIline x power factor

Question 12

What are the equations to determine the power measurements using the 2 wattmeter method

Answer 12

P1 = Vbr x Ir x power factor
P2 = Vyb x Iy x Power factor

Question 13

For a unbalanced delta 3 phase system what are the equations for the line currents

Answer 13

Ir = Iry - Ibr 
Iy = Iyb - Iry
Ib= Ibr - Iyb

Question 14

What are the equations relating Voltage, turns and currents in an ideal transformer

Answer 14

Vp/Vs = Np/Ns = Is/Ip

Question 15

Does a current flow in the primary current in an ideal transformer if no load is connected to the secondary

Answer 15

No

Question 16

What is flux measured in

Answer 16

Webbers

Question 17

Magnetic flux density measured in and what is the equation

Answer 17

Tesla

B= Flux/(cross sectional area)

Question 18

What is magnetic field intensity measured in and what is the equation

Answer 18

Amps per meter

H = (NxI)/L

Question 19

What is the equation for the max Vrms in a transformer

Answer 19

Vrms_max =4.44 x N x f x Bmax x Ac

Question 20

What is a typical value for Bmax for iron

Answer 20

0.9-1.3T

Question 21

What is the equation for the max current in a Transformer

Answer 21

(Current density limit x coil area x Fill Factor)/N

Question 22

What are the equivalent circuit parameters of the transformer equivalent circuit

Answer 22

R1eq- equivalent copper loss
X1eq- Reactance leakage 
Z1s- Secondary load equivalent
Rc- Iron loss  
Xm- Magnetising reactance

Question 23

For a Delta - Delta or Star - Star 3 phase transformer What is the phase shift between primary and secondary line voltages and how do the bank and phase ratios relate

Answer 23

No phase shift

Bank ratio = Phase ratio

Question 24

For a delta star 3 phase transformer what is the phase shift between primary and secondary line voltages and how do the bank and phase ratios relate

Answer 24

Secondary line voltage leads the primary by 30 degrees

Bank Ratio = phase ratio/(root3)

Question 25

For a Star Delta 3 phase transformer between primary and secondary line voltages and how do the bank and phase ratios relate

Answer 25

Secondary line voltage lags the primary by 30 degrees

Bank ratio = Phase ratio x root 3

Question 26

What are the 2 ways the electromagnets can be switched on and off in a synchronous generator

Answer 26

Power electronic converter or commutator

Question 27

What is synchronous speed in an induction motor

Answer 27

1 period of the mechanical flux rotating 1 full mechanical revolutions

Question 28

What is the equation for the synchronous speed of an induction motor

Answer 28

Ns= (120xFs)/p

Question 29

How is a torque produced by an induction motor

Answer 29

Sinusoidal flux will cause a sinusoidal voltage which will cause a force to be produced which is larger on one side of the rotor which produces torque.