AC theory Part 2

  1. Capacitance is measured in what? AC Theory, NJATC 2009 PP 91
    Farads
  2. Capacitors have the characteristic to oppose what? AC Theory, NJATC 2009 PP 91
    To oppose a change in voltage. They are used in motor variable frequency drive applications for this reason.  They are said to block DC and pass AC.  They are also used in AC to DC full wave bridge rectifiers to filter the DC and also on generators for this same reason on the voltage regulator.
  3. Capacitors are used to block and pass what? voltage, current, static, etc. Which one? AC Theory, NJATC 2009 PP 91

    Regarding the above statement, do the electrons pass through the insulating dielectric and/ or exert electrical pressure? AC Theory, NJATC 2009 PP 91
    Block DC and pass AC. Direct current doesn’t flow through a capacitor, in an alternating-current circuit, the electrons in the circuit move back and forth to alternately charge the capacitor, first in one direction, and then in the other. A capacitor permits current to flow because of its ability to store ‘ and then discharge the energy as the alternating current flows the opposite direction. Source: Mike Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 NEC. PP 69  

    The electrons do not pass through the dielectric plate but do exert electric pressure.
  4. Using scientific notation, what is Micro, Nano, and Pico? AC Theory, NJATC 2009 PP 91
    • Micro 10>(-6)
    • Nano 10>(-9)
    • Pico 10>(-12)
  5. The symbol for a fixed and variable capacitor is what? AC Theory, NJATC 2009 PP 91
    Image Upload 1
  6. The general formula for capacitance is what? AC Theory, NJATC 2009 PP 92

    This formula is used to do what? AC Theory, NJATC 2009 PP 92
    • C = Q/V
    • where:
    • C= capacitor value in farads
    • Q= charge in coulombs
    • V= voltage applied

    This formula is used to compare the charge a capacitor can hold per volt that is applied to the capacitor. The larger the plates the more the voltage.
  7. Capacitors appear to oppose a change in ______. AC Theory, NJATC 2009 PP 93
    voltage: block dc but pass ac as well.
  8. The formula for calculating the capacitance of a capacitor measured in farads is? AC Theory, NJATC 2009 PP 92
    • C = .225AK/10^6D
    • where:
    • C = capacitance in microfarads
    • A = cross-sectional area of one plate in square inches
    • K = dielectric constant of the material between the plates (for air K is 1)
    • d = the thickness of the dielectric (or the distance between the plates) measured in inches
  9. What is the relationship between the current and voltage in a capacitor? AC Theory, NJATC 2009 PP 97
    The current in the capacitor circuit leads the voltage across the capacitor by 90 electrical degrees. (ELI the ICE man)
  10. Capacitors designed for use in AC circuits are considered what? AC Theory, NJATC 2009 PP 98
    Non-polarized, meaning no specific voltage polarity. Polarized would be indicated WVDC (working volts DC) or a +/- stamp.
  11. What abbreviation means that a capacitor is rated for a maximum voltage in DC terms? AC Theory, NJATC 2009 PP 99
    WVDC: working volts DC
  12. Polarized capacitors are indicated how and used in what type of voltage? AC Theory, NJATC 2009 PP 99
    By a positive or negative marked leads and used only in DC voltage.
  13. The plate with a lack of electrons in a capacitor has a ______charge, where the plate with an excess of electrons has a ______ charge. AC Theory, NJATC 2009 PP 100
    • positive
    • negative
  14. What is the relationship between the voltage rating and the thickness of the dielectric in a capacitor? AC Theory, NJATC 2009 PP 104
    The voltage rating is decreased proportionally to the thickness of the dielectric.
  15. What is a shorted capacitor and how does this happen?
    Shorted: current flows = really low ohmic value….the more the capacitor is charged, the stronger the electrical field. If the capacitor is overcharged, the electrons from the negative plate could be pulled through the insulation to the positive plate- If this happens, the capacitor is said to have broken down (shorted). Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 NEC PP 70.
  16. How do you test a capacitor? AC Theory, NJATC 2009 PP 105

    What are the symptoms of a leaky / shorted capacitor?
    By using an ohm-meter and watching for an ohmic spike as he capacitor charges and current flow decreases, and then as current ceases an infinite ohmic value is given.

    • Leaky: current flows = never reaches infinite ohms
    • Shorted: current flows = really low ohmic value….the more the capacitor is charged, the stronger the electrical field. If the capacitor is overcharged, the electrons from the negative plate could be pulled through the insulation to the positive plate- If this happens, the capacitor is said to have broken down (shorted). Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 NEC PP 70.
  17. Capacitors charge and discharge at an exponential rate when connected with what? AC Theory, NJATC 2009 PP 106

    Proven by the Time Constant formula for capacitors which is what?
    When connected with resistors or in series with resistors

    • T=RxC
    • Versus T=L/R
  18. Is there Capacitive reactance in a D.C. circuit and what does it oppose? AC Theory, NJATC 2009 PP 110

    What is the formula for capacitive reactance?

    What does this formula show?
    NO, capacitive reactance only exists in an AC circuit and it opposes a change in voltage.

    XC= 1/ (2pieFC)

    That as the frequency increases XC decreases, or that they are inversely proportional to each other.
  19. What is the formula for Capacitance? AC Theory, NJATC 2009 PP 113
    C=1/(2pieFXc)
  20. Inductors store energy in a _________, and capacitors store energy in a__________. AC Theory, NJATC 2009 PP 91
    • magnetic field
    • electrostatic field
  21. It has two leads, the anode and the cathode, which is where it gets its name from. http://www.funtrivia.com
    Diode.  The greek words ana and hodos means up and way, so this is the negative lead.  The greek words katta and hodos means down and way, and therefore is the positive lead, using conventional electron theory.
  22. ______ were used for logic circuits in some of the earliest computers. They were later replaced by modern semiconductors and are used to switch high voltage circuits. http://www.funtrivia.com
    • Relays.  Modern semiconductors such as the PNP transistor or the NPN transistor use a base collector and emitter to switch a circuit.
    • Image Upload 2
  23. This is a type of semiconductor in which the flow of current can be controlled though a base or gate terminal it is usually used for electronic switching or amplifiers. http://www.funtrivia.com
    Transistor
  24. This consists of usually two coil windings which usually share a magnetic core. It is used to convert an ac source or signal into another ac source or signal with same power (ideally) but different voltage and current. http://www.funtrivia.com
    transformers
  25. This is a special type of diode that breaks down when a critical reverse voltage is applied. it is most commonly used to regulate voltage. http://www.funtrivia.com
    Zener diode
  26. This is a special type of diode that lights up when a small voltage is applied. They are usually used as indicators or in numerical or alphanumerical displays. http://www.funtrivia.com
    LED. LED's use mili-amps and therefore large amounts of power is displays and therefore LCD's using milliamps are preferred.
  27. A multi-segment rotating connection that is connected to the armature windings defines what term? AC Theory NJATC 2nd Ed. 2009, PP 341
    A Commutator is connected to this.
  28. Impedance is a combination of what 3 components in AC theory?
    • resistance, inductive reactance, and capacitive reactance compose the 3 parts of this.
    • Mike Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 NEC. PP63
  29. Alternating current is primarily used because it can be transmitted ________ by transforming it to high-transmission level voltages then converting this voltage back to lower distribution voltage levels. High-voltage transmission lines allow for the transmission of alternating-current at high voltage, and relatively low current. High-voltage transmission, with the outcome of lower current, results reduced voltage drop in the transmission lines. Conductors and electrical equipment are sized based on the current in the lines, so the lower current levels also allow the use of smaller wiring smaller electrical equipment.
    • inexpensively.
    • Mike Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 NEC. PP64

    Although it is important to note that the voltage drop calculation for AC requires the use of VD=1.73KLI/CM versus 2KLI/CM for DC systems.  This means that a 120volt DC system versus a 120volt AC system with the same distance and wiring will have a smaller number as the resulting voltage drop in an AC system.  As illustrated by the calculations below for a 100ft long and 15 amp circuit using number 12AWG respectively.

    • VD=2KLI/CM        or.       VD=1.73KLI/CM
    • VD=2x12.9x100x15/6530=5.93 vd
    • VD=1.73x12.9x100x15/6530=5.17 vd
  30. magnitude of the voltage produced depends upon? In AC voltage.
    • The number of turns of wire, the strength of the Magnetic field, and the speed at which the coil rotates determines this. 
    • Mike Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 NEC. PP 64
  31. The rotating conductor loop is called a “_____” or “_________.” “_____” or " _______” rings and carbon brushes are used to connect the output voltage from the generator to an external circuit.
    rotor or armature and slip or collector rings accomplish this loop to output voltage. Mike Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 NEC. PP 64
  32. Small single-phase ac generators have a conductor loop rotating inside a magnetic field. 
    Note: Larger ac generators have a rotating ____________
    magnetic field inside stationary conductors. Mike Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 NEC. PP 65
  33. Using electromagnets permits the strength of magnetic field (and thus the lines of force) to be modified, thereby controlling __________.
    • the output voltage can be controlled using this for magnetism in a generator.
    • Mike Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 NEC. PP 65
  34. _____ generators are separately excited. Batteries or small dc generator are typically used with the ac generator.
    • All of these are separately excited. 
    • Mike Holt’s Illustrated Guide to Electrical Exam Preparation PP 65
    • Image Upload 3
  35. Devices that intentionally introduce capacitance into circuits are called “capacitors.” An older term for these components is “_________.”
    • condensers is another term for this.
    • Mike Holt’s Illustrated Guide to Electrical Exam Preparation, Based on the 2014 NEC PP 69
  36. Be careful when working on a circuit that capacitors; such as, those found in ________. Capacitors can store large amounts to energy a long period of time. They can present a shock hazard after the power has been removed.
    • Such as those found in variable speed drives.  You must be careful when working around these as they can carry a charge after the circuit is off.
    • Mike Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 PP70
  37. Capacitance is inversely proportional to _________. The closer the plates of the capacitor are to each ‘ the greater the capacitance and, conversely, the greater the distance is between the plates, the lower the capacitance.
    • The distance between the capacitor plates is inversely proportional to this.
    • Mike Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 PP 71
  38. the voltage induced within conductor caused by its own expanding and collapsing magnetic field known as “_________”. 
    Mike Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 PP73
    self-induced voltage is this in regards to the expanding and collapsing field in AC.  This is also known as VAR or Volt Amps Reactive which is the energy stored in the collapsing field.
  39. CEMF aids or opposes current flow?
    • This either opposes or aids conductor current flow.  As alternating current increases, the polarity of the induced volt(CEMF) within the conductor opposes the conductor’s current. As it tries to prevent the current from increasing. However, as alternating current decreases, the polarity of the induced voltage (CEMF) within the conductor aids the conductor’s current and tries to prevent the current from decreasing.
    • Mike Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 PP73
  40. Connecting capacitors in parallel and series does what regarding total capacitance?
    Connecting these in parallel has the effect of increasing the plate surface area, and increasing the capacitance by the sum of the capacitors. Connecting capacitors in series has the effect of increasing the dielectric and decreasing the capacitance. Mike Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 NEC PP 71
  41. To ground a capacitor you do what?
    To ground a capacitor you need a conducting path across BOTH of the terminals of the capacitor.  Not just on one terminal. Mike Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 NEC PP 70
  42. Does current flow through a charged capacitor?
    Except for a small amount of current leakage through the dielectric material, current does not flow through a charged capacitor. Mike Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 NEC PP 69
  43. What is a parallel inductor / capacitor circuit called?Image Upload 4
    Parallel inductor/capacitor circuits are commonly known as tank circuits.
  44. What is a tank circuit’s natural frequency called?  
    Image Upload 5
    A tank circuit’s natural frequency, called the resonant frequency, is determined by the size of the inductor and the size of the capacitor, according to the following equation:Image Upload 6
  45. Where the magnetic field is at the zero reference line (0°. 180°, and 360°), is voltage is being induced back onto the conductor?
    Where the magnetic field is at the zero reference line (0°. 180°, and 360°), no voltage is being induced back onto the conductor. Mike Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 NEC PP 73
  46. Where the magnetic field is at the zero reference line (?°. ?°, and ?°), no voltage is being induced back onto the conductor.  What are the zero reference lines in degree form?
    Where the magnetic field is at the zero reference line (0°. 180°, and 360°), no voltage is being induced back onto the conductor. Mike Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 NEC PP 73
  47. just remember that alternating-current flow contains additional element (reactance) that opposes the flow of electrons, besides conductor resistance. What is the difference between Voltage and current fundamentally in a circuit regarding flux and electrons?  You do not need to memorize this, but just understand it.
    Voltage is electrical pressure usually expressed as CEMF or as an induced voltage by the relative motion between a conductor and a magnet or the collapsing field creating the same relationship; whereas, flux is current and is taking into account the “R” relationship, which may be the 180 degree out of phase relationship with the applied voltage and the collapsing field opposing a change in current, becoming a VA number and not P.
  48. which is easier to transform? AC or DC? NJATC, AC Theory 2nd edition, 2009, WB 9
    AC
  49. Each time an alternator completes one complete 360 degree rotation it is called what? NJATC, AC Theory 2nd edition, 2009, WB 10
    A cycle
  50. In an AC generator what changes to cause polarity changes? NJATC, AC Theory 2nd edition, 2009, WB 10
    The flux field (left hand rule)
  51. When a conductor cuts the magnetic flux lines at ___ degrees, the maximum voltage will be induced into the conductor. NJATC, AC Theory 2nd edition, 2009, WB 11
    90 degrees
  52. The time required for one cycle or two alternations is what? NJATC, AC Theory 2nd edition, 2009, WB 12
    A period
  53. What is the average AC voltage for a full cycle of a perfect sine wave? NJATC, AC Theory 2nd edition, 2009, WB 12
    zero, because the average is the actual average of all peak values across a full cycle.
  54. Current flow from a battery is generally considered from what polarity to what? NJATC, AC Theory 2nd edition, 2009, WB 34
    - to +
  55. The left hand generator rule uses the thumb, forefinger, and the next finger, representing what? NJATC, AC Theory 2nd edition, 2009, WB 34
    • thumb: motion
    • Forefinger: field
    • Next finger: current
  56. A general rule of thumb for ignoring inductance or capacitance in field resistive calculations is what? NJATC, AC Theory 2nd edition, 2009, WB 37
    Generally at 50-60 HZ electricians in the field will ignore inductive or capacitive elements in resistive calculations, but at higher frequencies you cannot. Also, if you answered the resistance is 1/10th or less the inductance then you are also correct.
  57. An AC circuit is solved at 50 to 60 HZ like a DC circuit but uses what type of AC value? NJATC, AC Theory 2nd edition, 2009, WB 37

    However: Effective alternating-current voltage or effective ac current is the equivalent value of direct-current voltage or dc current that will produce the same amount of heat in a resistor.
    Mike Holt’s Illustrated Guide to Electrical Exam Preparation, Based on the 2014 NEC PP 68
    • RMS or effective
    • Peak = 1.414(RMS/eff) a.k.a. max instantaneous value
    • Effective (RMS) = .707(peak)
    • Average = .637(peak)
    • Peak to peak = 2(peak)
  58. Current and voltage in an AC purely resistive circuit are in or out of phase? NJATC, AC Theory 2nd edition, 2009, WB 37
    in
  59. The opposition to current change is called what?
    This opposition to current change is called reactance, rather than resistance; which would be the opposition to current.
  60. What two factors not present in a DC circuit affect the opposition to current flow in an AC circuit? NJATC, AC Theory 2nd edition, 2009, WB 38
    Eddy currents and skin effect: remember that skin effect increases as the frequency increases. I.E. harmonics.
  61. Concerning induced voltage and applied voltage, these two elements are how far out of phase with one another? NJATC, AC Theory 2nd edition, 2009, PP 49
    Applied voltage and induced voltage (CEMF) are 180 degrees out of phase with each other
  62. Capacitors oppose a change in?
    Voltage.
  63. Inductors oppose a change in what? NJATC, AC Theory 2nd edition, 2009, PP 50
    basic rule: Inductors always oppose a change in an AC current.
  64. How many time constants does it take for circuit current to reach its maximum value in an inductive circuit? What is this Maximum value? NJATC, AC Theory 2nd edition, 2009, WB 42
    • 5 time constants
    • Maximum value is considered 99.5%
  65. Does a coil change its henry value with a change of frequency or a change in the current value? NJATC, AC Theory 2nd edition, 2009, PP 55
    NO
  66. The Henry is the unit of measure for inductance. A coil has an inductance of 1 henry when...? NJATC, AC Theory 2nd edition, 2009, PP 52
    ...a current change of one ampere per second causes an induced voltage of 1 volt.
  67. The formula for determining how much CEMF is produced is what? Remember: The voltage created by an inductive load is not proportional to the amount of current flow, but rather to the rate at which current is changing. AC Theory, NJATC 2009 PP 54
    • CEMF = -L x Δ I
    • Δ T
    • where L is inductance, I is current, and T is time
    • The greater the inductance, or the faster the rate of change of current, the greater the CEMF
  68. In a purely inductive circuit the current lags behind the applied voltage by? NJATC, AC Theory 2nd edition, 2009, PP 57
    90 degrees
  69. What is the phase relationship between CEMF to applied voltage and current to applied voltage? AC Theory, NJATC 2009 PP 58
    • CEMF is 180 degrees out of phase with applied voltage and the current lags the applied line voltage by 90 degrees. However:CEMF either opposes or aids conductor current flow. As alternating current increases, the polarity of the induced volt(CEMF) within the conductor opposes the conductor’s current and tries to prevent the current from increasing. But, as alternating current decreases, the polarity of the induced voltage (CEMF) within the conductor aids the conductor’s current and tries to prevent the current from decreasing. This is also known as VAR or volt amps reactive.
    • Mike Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 PP74
  70. The angle that the current lags the voltage through the coil is what? AC Theory, NJATC 2009 PP 61
    • The same angle the impedance vector makes in relation to the resistance vector.
    • Image Upload 7
  71. When doing quick approximations for the impedance of a coil the ratio of the inductive reactance to the resistance is considered. If the resistance is less than ______ of the Xl, then we disregard the R and claim the total opposition of the coil is made up of reactance. AC Theory, NJATC 2009 PP 67
    1/10th
  72. Concerning coefficient of coupling, when voltage across the primary of a transformer does not reach the secondary in its entirety, what is this defined as? Source: http://www.tpub.com/neets/book2/5f.htm 2015
    LEAKAGE INDUCTANCE is assumed to drop part of the applied voltage, leaving less voltage across the secondary.
  73. Concerning a transformer and coefficient of coupling; since the counter emf in the primary is equal (or almost) to the applied voltage, a proportion may be set up to express the value of the voltage induced in terms of the voltage applied to the primary and the number of turns in each winding. This proportion also shows the relationship between the number of turns in each winding and the voltage across each winding. This proportion is expressed by the equation: Source: http://www.tpub.com/neets/book2/5f.htm 2015
    Image Upload 8
  74. The total amount of mutual induction M is calculated by the formula? AC Theory, NJATC 2009 PP 68

    • M=K x √(L1xL2)
    • Where M=mutual induction in henrys
    • K=decimal equivalent of coefficient of coupling
    • The COEFFICIENT OF COUPLING of a transformer is dependent on the portion of the total flux lines that cuts both primary and secondary windings.
  75. Two coils need to be oriented to one another in what way to provide coefficient of coupling and in what way to avoid this. AC Theory, NJATC 2009 PP 68 Tech Tip
    Two coils placed parallel to one another to create a mutual induction or perpendicular to avoid it.
  76. What is referred to as the Q of a coil? AC Theory, NJATC 2009 PP 70

    Represented by what formula?
    The quality factor

    Q=XL/R
  77. In an impedence triangle the angle theta represents what? AC Theory, NJATC 2009 PP 71
    It represents the actual angle of lag that the current lags behind the voltage in the circuit. Also known as power factor.

    Image Upload 9
  78. Normally the lagging angle of current is not measured in the residential application, but that same angle is measured and may be charged for if it is too large in what type of accounts? AC Theory, NJATC 2009 PP 72 Tech Tip
    commercial accounts
  79. In a voltage triangle, what are the following parts? AC Theory, NJATC 2009 PP 77
    Hypotenuse ?
    Adjacent ?
    Opposite ?
    Image Upload 10
  80. When a circuit containing both resistance and inductance is connected to alternating current, the total current will lag the applied voltage at an angle between 0 and 90. The exact amount of the phase angle difference is determined by what ratio? AC Theory, NJATC 2009 PP 78
    ratio of XL/R
  81. What is defined as energy stored in a magnetic field or an electrostatic field and is returned to the circuit as the fields diminish. AC Theory, NJATC 2009 PP 80
    Reactive power
  82. power factor is the cosine of theta in which triangles? AC Theory, NJATC 2009 PP 80
    • impedance, voltage, and power
    • Image Upload 11
  83. The movement of the magnetic field is energy and that energy is stored in the magnetic field is a statement that best describes what term? AC Theory, NJATC 2009 PP 80
    Reactive power measured in volt amps
  84. The power factor is an angular measure. True or False. AC Theory, NJATC 2009 PP 82
    False, the power factor is a ratio.

    • T/A
    • R/Z
    • W/VA
  85. The amount of time required for the current flow to reach the maximum value is expressed in time constants. In a circuit with inductance and reactance, the time constant is defined from the following formula. AC Theory, NJATC 2009 PP
    • T=L/R
    • Where T= time constant in seconds
    • L= Circuit inductance in henrys
    • R= Circuit resistance in ohms

    Note: The first time constant is 63.2 percent of max current and takes 5 time constants for 99.2 percent of current to rise or decay.
  86. The time constants are used for what purpose? AC Theory, NJATC 2009 PP 85
    The time constants are used to protect a circuit from the voltage spike caused by a collapsing inductor magnetic field, which may ruin other circuit components.
  87. The general formula for Voltage Drop in a single phase system is? And in a 3 phase system? And lastly in a DC system?
    • single phase VD=2KLI/CM
    • Three phase VD=1.73KLI/CM
    • DC system VD=I(R)
Author
johnbowens
ID
124557
Card Set
AC theory Part 2
Description
Ac theory in partnership with the NJATC 2009 copyright
Updated