AC theory part 1

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 Author: johnbowens ID: 116950 Filename: AC theory part 1 Updated: 2015-06-25 15:14:38 Tags: AC theory Folders: Description: AC theory 2nd year IBEW JATC Show Answers:

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1. which is easier to transform? AC or DC? NJATC, AC Theory 2nd edition, 2009, WB 9
AC
2. Each time an alternator completes one complete 360 degree rotation it is called what? NJATC, AC Theory 2nd edition, 2009, WB 10
A cycle
3. In an AC generator what changes to cause polarity changes? NJATC, AC Theory 2nd edition, 2009, WB 10
The flux field (left hand rule)
4. 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
5. The time required for one cycle or two alternations is what? NJATC, AC Theory 2nd edition, 2009, WB 12
A period
6. 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.
7. Current flow from a battery is generally considered from what polarity to what? NJATC, AC Theory 2nd edition, 2009, WB 34
- to +
8. 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
9. 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.
10. 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
• 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)
11. Current and voltage in an AC purely resistive circuit are in or out of phase? NJATC, AC Theory 2nd edition, 2009, WB 37
in
12. 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.
13. 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.
14. 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
15. Capacitors oppose a change in?
Voltage.
16. 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.
17. 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%
18. 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
19. 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.
20. 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 changingAC 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
21. In a purely inductive circuit the current lags behind the applied voltage by? NJATC, AC Theory 2nd edition, 2009, PP 57
90 degrees
22. 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
23. 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.
24. 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
25. 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.
26. 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
27. 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.
28. 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.
29. 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
30. 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.

31. 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
32. In a voltage triangle, what are the following parts? AC Theory, NJATC 2009 PP 77
Hypotenuse ?
Opposite ?
33. 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
34. 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
35. power factor is the cosine of theta in which triangles? AC Theory, NJATC 2009 PP 80
• impedance, voltage, and power
36. 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
37. 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
38. 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.
39. 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.

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