Parallel AC Circuits
What is the phase angle in an AC resistive parallel circuit?
0 degrees
Name 2 applications of Parallel RLC circuits
Electronic filters, signal processing, voltage and current regulation in power transmission systems
Name 2 common types of single phase motors
Resistance-start split-phase, two value capacitor motor
Name 2 devices that are commonly used for power factor correction.
Capacitors, synchronous motors
At what threshold of power factor will electrical utility companies typically add a surcharge or penalty for commercial and industrial customers?
Power factor less than 0.9
What is the phase angle in a purely inductive AC parallel circuit?
-90 degrees OR 90 degrees lagging
A resistive load of 10A is connected in parallel with a capacitor that draws a current of 10A. What is the total circuit current?
14.1A
Pythagoras: 10^2 + 10^2
What type of single-phase motor has the best overall performance?
Two-value capacitor motor OR cap-start/cap-run motor
How many net VARS are in a circuit that has a unity power factor?
0 - they must cancel out
Name a disadvantage of low operating power factor
Larger wire size is required for the same power
Higher line current is drawn from the utility
Higher line lossesHigher line drop
What is the true power in an AC capacitive parallel circuit?
0 W
The power factor of a parallel RLC circuit is 1. If the supply frequency is decreased, what happens to the circuit current?
Increases
Name the 4 basic components of a resistance-start split-phase motor
Frame, stator, rotor and starting switch (cut-off switch)
When a capacitor is connected in parallel with an induction motor for power factor correction, the power factor angle of the circuit will _____. (increase, decrease, stay the same)
Decrease
If the overall power factor of a parallel network is 1, what happens to the incoming line current if the amount of capacitive VARs is increased?
Line current increases
24.5 + 22 + 44 = 90.5uF
A single phase motor is connected in parallel with a heater. If a certain amount of load is taken off the motor, then the total line current will? (increase, decrease, stay the same)
Decrease
What is the term given to the difference between synchronous speed and rotor speed?
Rotor-slip
An induction motor is operating at a power factor of 0.5 lag and draws 50A. A capacitor is connected in parallel and corrects the circuit power factor to 0.85. What current does the induction motor draw now?
50A. Actual motor current does NOT change.
When a power factor correction capacitor is connected in parallel with an induction motor, what effect does it have on the efficiency of the motor?
None. It stays the same.
Two coils are connected in parallel. They have inductive reactances of 14 ohms and 84 ohms. What is the total inductive reactance?
1 / [1/14 +1/84] = 12 ohms
A motor and heater are connected in parallel. If the resistance of the heater increases, what happens to the true power?
Decreases.
Rh increases and P = V^2/R then P will decrease
Which of the 2 capacitors in a cap-start/cap-run motor is oil filled?
The run capacitor since oil-filled is rated for continuous duty. Electrolytic is rated for short-time duty (~3 seconds) only.
The power factor of a parallel RLC network is unity. If the frequency of the supply voltage is increased, what happens to the incoming line current? (Increase, decrease, stay the same)
f up means XL up and Xc down. Q is now unbalanced and circuit current increases.
A capacitor is connected in parallel with an induction motor to correct the power factor to 0.89. What effect does this have on the incoming line current?
It decreases