# Electrical Systems coursework help

Discussion in 'Homework Help' started by curzon125, Jul 27, 2011.

1. ### curzon125 Thread Starter New Member

Jul 27, 2011
1
0
I have some uni work that needs to be completed, but i am struggling to answer it. I am aware you can't tell me the answers, but if anyone can give me equations or web pages with similar problems on I'd be grateful.

Q1
A balanced delta load consisting of 3 resistors is connected to a 400V 3-phase supply. The total power dissipated in the load is 25kW.

b) Determine the value of the 3 resistors.
c) sketch the waveform in each resistor
d) If the resistive load is replaced by a resistive load that dissipates the same total power, determine the value of each of the 3 resistors in the star load

Q3
An unbalanced star 3-phase load connected to a 400V, 50Hz, 3-phase supply is shown below.
line 1: 300ohms
line 2: 200ohms and 500mH
line 3: 200ohms and 10uF

a) determine the magnitudes and phases of each line current
b)Determine the power dissipated in each branch

Q5 (related to Q3)
If the length of the line between the generator and the load is 5km and the impedance of each core of the line at the frequency of 50Hz is 1.8+j2.2 ohms per km, determine the value of the power factor correction components that would be required so that the power factor of the system is equal to 1.

Q7
A single phase 10kVA transformer has 400turns on the primary winding and 80 turns on the secondary winding. the primary winding is connected to a 400V, 50Hz, supply. The primary and secondary resistances are 0.4 and 0.015 respectively, with corresponding leakage reactance' of 1mH and 50uH respectively and the magnetisaion current, hysteresis and eddy current loses referred to the primary winding are equivalent to a parallel combination of a 40mH inductance and a 60ohm resistance.

c) The output of the transformer is connected to a resistive load that is nominally rated at 10kW when connected to a 80V supply. Neglecting the magnetisation current, hysteresis and eddy current loses, calculate the power that would be delivered to the load.

Q11
c)A 4 pole, 50Hz, 400V 3-phase induction machine is rated at 5kW when operating at a slip of 5%, has a starting torque of 25Nm and exhibits a peak torque of 50Nm which corresponds to a slip of 15%.
i) Determine the shaft speed and torque when the machine is operating as a motor at rated power
ii) Determine the shaft speed and torque when the machine is operating as a generator at rated power
iii) Sketch the torque/speed characteristics of the machine to include the locked rotor and both motoring and generating modes
iv) draw the per phase equivalent circuit referred to the stator
v) describe the power flow within the machine, accounting for all power losses

Q12
A constant V/f control topology is used to rive a practical 4 pole asynchronous a.c. (induction) machine where the stator referred airgap voltage, E1, is kept proportional to frequency. The per phase equivalent circuit parameters of the machine referred to the stator are:
R1 = 0.75, L1 = 4mH, R2 = 0.5, L2 = 4mH, Rm = 150, Lm = 70mH
At a supply frequency of 50Hz, the stator referred airgap voltage, E1, is 200V rms.

Draw and label the per phase equivalent circuit of a 3-phase induction machine.

For an operating supply frequency of 50Hz, determine the shaft speed, the torque and mechanical power of the machine operating as a motor under the following conditions:
i) a stationary rotor
ii) Synchronous speed
iii) a slip of 2.5%
iv) a slip of 5%

For an operating supply frequency of 30Hz, determine the shaft speed, the torque and mechanical power of the machine operating as a motor under the following conditions:
i) a stationary rotor
ii) Synchronous speed
iii) a slip of 2.5%
iv) a slip of 5%

Thanks for any help guys!

2. ### narasimhan Member

Dec 3, 2009
72
6
Here refer to this AAC page polyphase circuits
Or get hold of Engineering circuit analysis by William hayt. There is a separate chapter dealing with these exact type of problems