AE ADVANCEMENT EXAM/ MODULE 2/ pt. 4
A transformer secondary has 20 amperes of current flowing at 60 volts potential. The applied voltage is 10 volts. What is (a) the turns ratio of the transformer and (b) what total current is flowing in the primary?
(a) 1:6, (b) 120 amperes
A capacitor will (a) conduct what type of current, and (b) block what type of current?
(a) All ac (b) Dc
Inductance has what effect, if any, on a change in (a) current, and (b) voltage?
(a) Opposes it (b) no effect
What is the opposition offered by a coil to (a) the flow of alternating current and (b) a change in current?
(a) Reactance (b) Inductance
A series ac circuit has a XC of 110 ohms, an X L of 30 ohms, and a circuit resistance of 22 ohms. What is the power factor of this circuit?
.27
An inductive circuit contains a 200-μH coil and the ac voltage applied is at a frequency of 120 Hz. What is the value of reactance for the circuit?
0.15 Ω
A transformer has a unity coefficient of coupling with a 5:1 turns ratio; 20 volts are induced in the secondary. What is the primary voltage?
100 volts
An ac circuit dissipates 800 watts across its resistance and returns 600 var to the source. What is the value of the apparent power of the circuit?
1000 VA
An electrical circuit contains a 25-μF capacitor and operates from a 60-Hz ac source. What is the value of capacitive reactance of the circuit?
106.2 Ω
An ac series circuit has the following values: I = 7.5 amps, X L = 80Ω, and XC = 35Ω. What is the value of reactive power for the circuit?
2531 var
If the frequency applied to a circuit with a 200-μH coil is increased from 120 Hz to 50 kHz, what will be the value of reactance for the circuit?
3.62.8 Ω
A transformer has a source voltage of 50 volts ac, with a turns ratio of 1:6. The coefficient of coupling is 1.0. What is the voltage of the secondary winding?
300
A transformer has a unity coefficient of coupling. Thirty-five volts applied to its primary induces 105 volts in its secondary. The secondary is composed of 99 turns. What is the number of turns in the primary?
33 turns
A 2:1 transformer delivers 30 watts to the load and 3 watts of power are lost to internal losses. What total power is drawn from the source?
33 watts
An ac series circuit has the following characteristics: R = 8 ohms, XC = 100 ohms, XL = 70 ohms, and E = 220 V. What is the value of true power for the circuit?
402 W
A series circuit contains an inductor having 12 ohms of resistance and 30 ohms of inductive reactance in series with a capacitor having 21 ohms of capacitive reactance. The applied voltage is 100 volts. What is the value of current for the circuit?
6.6 amps
A series ac circuit has the following values: XL = 5Ω, XC = 6Ω, and R = 7Ω. What is the value of Z?
7.07 Ω
A series circuit contains an inductor having 12 ohms of resistance and 64 ohms of inductive reactance in series with a capacitor having 69 ohms of capacitive reactance. If the current through the circuit is 6.5 amperes, what is the value of the voltage applied to the circuit?
84.5 volts
What is the principal difference between a high-voltage transformer and a low-voltage transformer?
A high-voltage transformer has more insulation between the layers of windings than does a low-voltage transformer
Which of the following is NOT a necessary element in a basic transformer?
A magnetic shield
What three materials are most commonly used for transformer cores?
Air, soft iron, and steel
Before starting to work on any electrical equipment, you should first determine that the equipment is in which of the following conditions?
Deenergized
What term applies to the current in the primary of a transformer that creates the magnetic field?
Exciting current
What term is used to express the total opposition to ac in an electrical circuit?
Impedance
A transformer designed for a low frequency will NOT be damaged when used at higher frequencies. What change within the transformer, limits transformer current to a safe value at higher frequencies?
Increased inductive reactance
Voltage leads current in which of the following types of circuits?
Inductive
The formula 2πfL is used to determine what electrical quantity?
Inductive reactance
In a purely reactive circuit, what happens to power?
It is returned to the source
What is the main cause for the coefficient of coupling of a transformer being less than unity?
Leakage flux
Which of the following terms applies to the flux from the primary that does NOT cut the secondary
Leakage flux
When the secondary of a transformer is NOT connected to a circuit, the transformer is said to be operating under which of the following conditions?
No-load
What is the source of the magnetic flux that develops secondary voltage in a transformer?
Primary exciting current
Apparent power in an ac circuit is a combination of which of the following factors?
Reactive power and true power
What wire colors conventionally identify the secondary center tap of a power transformer?
Red and yellow
True power in a circuit is dissipated in what circuit element?
Resistance
When working on electrical equipment, why should you use only one hand?
The free hand will minimize the possibility of creating a low resistance path to ground through your body
What is the major difference between the primary and secondary windings of a transformer?
The primary is connected to the source; the secondary is connected to the load
In the primary of a transformer, what opposes the current from the source?
The self-induced emf
A person is working on electrical equipment. The power is secured and tagged. The technician receives a shock on the hand. What safety precaution was overlooked?
The technician had not discharged the equipment's capacitors
Which of the following defines transformer action?
The transfer of energy from one circuit to another through electromagnetic action
When impedance is calculated for a parallel ac circuit, an intermediate value must first be calculated. The intermediate value must then be divided into the source voltage to derive impedance. What is this intermediate value?
Total current
What is the unit of measurement for reactive power?
Var
What is the unit of measurement for apparent power?
Volt-ampere
True power is measured in what unit?
Watt
Opposition to the flow of current by a coil in an ac circuit is represented by what symbol?
XL
The two types of transformer cores most commonly used are the shell-core and the
hollow-core
