Section D4 " Single Phase AC Circuits "

In parallel line is current is at a minimum while in a series resonant circuit it is at a maximum.

How does parallel circuit resonance differ from series circuit resonance ?

By adding to much capacitance

How is it possible to overcorrect the power factor of a circuit ?

By subtracting VA from Watts vectorially VA squared minus Watts squared, Square root sum and voila!

If you know total values of true power and apparent power in an R-L parallel circuit, how do you calculate reactive power ?

True

In a parallel resonant circuit, true power equals apparent power. True or False ?

Zero

In a purely capacitive circuit when the instantaneous current is zero and the instantaneous voltage is at its positive peak value the instantaneous power value is ?

Leads

In a purely capacitive circuit, current _______ the voltage by 90 degrees.

Leads

In a purely capacitive parallel circuit, current _____ the voltage by 90 degrees.

Negative

In a purely capacitive parallel circuit, when instantaneous current is negative and instantaneous voltage is positive, the instantaneous power value is ?

Positive

In a purely inductive AC parallel circuit, when instantaneous current and voltage are both negative the instantaneous power value is ?

Lags

In a purely inductive circuit, current__________ the voltage by 90 degrees.

Lags

In a purely inductive parallel circuit, current ______ the voltage by 90 degrees ?

They cannot be added directly because they are two currents that are of out phase.

In an AC parallel R-L circuit why don't you add the currents through the resistor and inductor arithmetically ?

False

In an AC series RLC circuit, inductive reactance leads the capacitive reactance by 90 degrees. True or False ?

100%

For a fixed amount of true power, the AC load that draws the lowest value of current would have a power factor of ?

Effective current

For each component of a series circuit the_____ has the same value ?

Impedance

The combined opposition of resistance and reactance to current flow in a circuit.

The cumulative effect of hundreds of uncorrected ballasts becomes important.

The power factors of fluorescent lamp ballasts are often corrected large installations because ?

Capacitance.

The property that of a circuit that opposes any change in voltage ?

Developed almost entirely in the resistance of the circuit.

True power in a series circuit is ?

Avoided at all costs

In industrial circuits a series resonant circuit should be ?

Voltage rating, frequency rating, K VARs of correction and Phase

List four pieces of info on the nameplate of a power factor correction capacitor ?

Induction motors, heavy installations of fluorescent lighting or ballast lighting.

List two types of industrial loads that cause low power factors on electrical systems?

1) Reduced energy costs 2) Increase in system capacity 3) increase in distribution efficiency

Name three reasons for correcting the power factor in AC circuits ?

False, The net VARs is developed in the inductive and capacitive branches combined

Net vars in an RLC parallel circuit is entirely developed in the coil of the inductor. True or false ?

Reactive or wattless power

Quadrature power is the same as ?

180 degrees

What is the phase angle between the current vectors in the reactive branches of an RLC parallel circuit ?

Assuming the resistance is negligible, the currents are out of phase by 90 degrees. ( iL lags Ir )

What is the phase relationship between the currents through the resistor and inductor in a parallel R-L circuit ?

They are 90 degrees out of phase with capacitive current leading.

What is the phase relationship between the currents through the resistor and the capacitor when they are connected in parallel ?

The capacitor supplies all the magnetizing VARs.

What is the relationship between inductive and capacitive reactance when the power factor of a parallel circuit is 100% ?

It represents the proportion of apparent power of a circuit that is usable in the load of the circuit and not used in the reactance of the circuit.

What is the significance of the power factor of an AC circuit ?

100% or unity.

What is the value of the power factor in a resonant circuit ?

Apparent power, total amperage and total voltage.

What type of power is measured in VA and which part of an R-L circuit is it associated with ?

Volt Amps Reactive - Inductive Reactance

What type of power is measured in VARS and which part of an R L circuit is it associated with ?

Increase the value of capacitance

What would reduce the value of line current in an RLC parallel circuit that has a lagging power factor ?

are unaffected

When a load is power factor corrected the watts, vars, and voltage amps of the load device itself ?

Circuit resistance

When a parallel circuit is in resonance circuit impedance equals ?

By switches that facilitate maintenance

When capacitors for power factor correction are used to correct the power factor of an entire service or a main feeder the capacitors will usually be connected how ?

A) Positive

When instantaneous current and voltage are both negative in a purely inductive AC circuit, the instantaneous power value is ? A) Positive B) Negative

In the resistance of the circuit

Where is the true power developed in an R L C parallel circuit ?

Dissipated by the resistance of the circuit.

Where is true pie, measured in watts dissipated in a series R-L circuit ?

False

A circuit that contains both capacitance and inductance tends to cancel out its apparent power ?

False

A series R-C circuit differs from a Series R -L circuit in that voltage across the resistor leads the voltage across the capacitor and can be added to it arithmetically. True or False ?

A circuit is in resonance.

Capacitive Reactance equals inductive reactance when?

Ohms

Capacitive Reactance in measured in ?

A large single induction motor load that is frequently turned on and off.

Capacitors for power factor correction could be connected in parallel with a specific load in which situation ?

The VARs will go down and the current will go down.

If a capacitor bank is rated at 600v, 50 hz and 9.6 KVARs, and if it is hooked up inadvertently to a 50 hz, 550 v system what would happen?

Fall Current will also fall.

If a capacitor bank rated at 240 v 60 hz and 8.4 KVARs is hooked up to a 240 v 45 hz system the VARs will ? What will happen to the current ?

Leading

If the current through the capacitive branch us larger than the current through the inductive branch the phase angle of an RLC parallel circuit is ?

It is the phase angle. It occurs in the voltage, power and impedance triangles.

In the expression PF= Cos0, what is the significance of angle 0 ? Give at least two placed where it occurs.

Where all motors are small, about the same size and fed from a common motor control centre

In what situation is power factor correction of a group of induction motors practical ?

Ohms

Inductive reactance is measured in ?

unstitched

Power factor correction capacitors for fluorescent lamp ballasts are connected ?

Voltage

The KVARs developed by a capacitor are most sensitive to ?

Capacitance.

The ability of a circuit component to store electrical energy in an electrostatic field.

The coil usually has some resistance so the inductive branch has an impedance rather than a pure reactance.

What factor often prevents the currents in the capacitive and inductive branches from being exactly out of phase ?

0 degrees

What is the angle between current and voltage vectors in a purely resistive circuit ?

Low opposition to current flow

What is the danger of a series connection of power factor correction ?

Associated with total circuit impedance.

Which part of an R-C circuit is VA power associated with ?

parallel

Which way are capacitors for power factor correction connected ?

Vector quantities that cannot be added directly. They are out of phase with each other and must take phase angle into account.

Why don't you arithmetically add the voltages across the resistor and inductor in a series AC circuit ?

A risk of over correction may arise.

Why is it not always advisable to try and correct the power factor to unity?

Because voltage is the same across all the branches of a parallel circuit.

Why is the voltage used as a reference vector in an RLC parallel circuit ?