AC Theory
A circuit can be purely inductive. A. True B. False
False
There are two common types of AC power systems: single phase and double phase. A. True B. False
False
When CEMF opposes the applied voltage in an AC circuit, it has the effect of increasing current. A. True B. False
False
Before a capacitor can store energy, it must first be ____________________.
charged
The physical property of all AC circuits that opposes any change in _______________ is called inductance.
current flow
What are the three main components of a capacitor?
1. Plate 2. Insulator (dielectric) 3. Plate
To calculate the line voltage of a wye-connected, 3φ system, multiply the phase voltage by ___________
1.73
Given a wye-connected transformer with a phase voltage of 277 V and phase current of 5 amps, calculate the line voltage and current. Answer:
480 volts, 5 amps
Given a delta-connected transformer with phase voltage of 480 V and phase current of 5 amps, calculate the line voltage and current. Answer:
480 volts, 8.65 amps
List two ways to increase inductive reactance in an AC current.
A. Coil the conductor B. Place a metal core inside the coiled conductor
List the three requirements for inducing a voltage. A. B. C.
A. Conductor B. Magnetic field C. Relative motion
Capacitance causes (A) ________________ and (B) __________________ to be out of phase.
A. Current B. Voltage
The number of cycles completed each second by a given AC voltage is called (A) _________________ and is measured in (B) _______________________.
A. Frequency B. Hertz
Two types of connections commonly used for 3φ systems are _______________ and ____________connections
Delta and Wye
When a capacitor is fully charged, the source voltage and the voltage across the capacitor are equal in amount but opposite in polarity. A. True B. False
False
The combined effect of resistance, inductance, and capacitance on current flow in an AC circuit is called _____________.
Impedance
In a purely inductive circuit, _______________________________ is the only current-limiting factor.
Inductance reactance
__________________________ is the measure of the opposition to current flow that is created by inductance.
Inductance reactance
Capacitive reactance is measured in ___________________.
Ohms
The _________________ of an AC power source changes periodically.
Polarity
In most cases, true power in AC circuits is calculated with the aid of the _______________________ associated with the specific circuit.
Power Factor
Operating voltages on almost all AC equipment are values. A. RMS B. peak C. peak-to-peak
RMS
The letters used to express effective AC values are ____________.
RMS
AC power sources do not have a fixed polarity. A. True B. False
True
Apparent power is the result of multiplying voltage times current in any circuit. A. True B. False
True
Capacitance is a physical property of all AC circuits that opposes a change in voltage. A. True B. False
True
Capacitance is often added to AC circuits to counter the effects of inductance. A. True B. False
True
In a delta-connected system, phase voltage equals line voltage, but phase current does not equal line current. A. True B. False
True
Insulation rating on equipment is based on peak voltage. A. True B. False
True
Most meters read RMS values; unless the data plate on a meter or piece of equipment indicates otherwise, all values are RMS. A. True B. False
True
There are different kinds of power in AC circuits: true power, reactive power, and apparent power. A. True B. False
True
True power is the amount of power actually used to do work. A. True B. False
True
When a sine wave is used to represent voltage, voltage below the horizontal line is negative. A. True B. False
True
The common symbol for inductive reactance is: A. XL B. LX
XL
The common symbol for capacitive reactance is ___________________.
Xc
Reactive power is power that (does, does not) do useful work.
does not
In a wye-connected, 3φ system, phase current and line current are (equal, unequal
equal
Positive power is power that is: A. going to a load from a power source. B. returning to a power source from a load. C. the product of a negative current and a positive voltage. D. equal to voltage times resistance.
going to a load from a power source
A dielectric in a capacitor: A. increases the flow of protons in the positive terminal. B. keeps electrons from flowing from one place to the other. C. aids electrons in their flow from positive to negative. D. decreases the number of electrons on any plate.
keeps electrons from flowing from one place to the other.
In a purely inductive circuit, current ___________________ voltage. A. leads B. lags
lags
When a conductor rotating in a magnetic field is cutting through the maximum number of flux lines: A. the conductor stops moving. B. no voltage is induced. C. maximum voltage is induced. D. None of the Above
maximum voltage is induced
The voltage and current sine waves for reactive power are always (in, out of) phase
out of
Whenever voltage and current increases or decreases at different times, they are said to be _____________________________.
out of phase
In a 3φ system, the current that flows through the coils or resistors is called (phase, line) current.
phase
When current flowing through a conductor is increasing: A. the magnetic field around the conductor is building up. B. the magnetic field around the conductor is collapsing. C. no voltage is induced. D. there is no magnetic field around the conductor.
the magnetic field around the conductor is building up.
What is capacitive reactance?
the measurement of the opposition flow that is created by capacitance
The middle line on the secondary side of a three-wire, 1φ system is: A. the neutral line. B. connected in a delta pattern. C. never used. D. 110 volts.
the neutral line.
The component that changes a two-wire, 1φ system into a three-wire, 1φ system is a _________________.
transformer