The post Transformer Questions and Answers appeared first on Electrical Academia.

]]>**1. What are the following relationships in an ideal transformer?**

**a.** Turns ratio and voltage ratio?

The turn’s ratio is the same as the voltage ratio.

\[\frac{{{N}_{1}}}{{{N}_{2}}}=\frac{{{V}_{1}}}{{{V}_{2}}}\]

**b.** Voltage ratio and the current ratio?

The current ratio is the inverse of the voltage ratio.

\[\frac{{{I}_{2}}}{{{I}_{1}}}=\frac{{{V}_{1}}}{{{V}_{2}}}\]

**c.** Primary and secondary power?

The primary power is equal to the secondary power according to the law of conservation of energy.

**2. A transformer rated at a primary voltage 4,800 volts and a secondary voltage of 240 volts what is the turn’s ratio?**

\[\frac{{{N}_{1}}}{{{N}_{2}}}=\frac{4800}{240}=20:1\]

**3. A transformer that with a 120 volt primary and 12 volts in the secondary, and a primary with 800 turns of wire, how many turns would be required in the secondary?**

\[\frac{800}{{{N}_{2}}}=\frac{120}{12}\Rightarrow {{N}_{2}}=80\]

**4. With a turn’s ratio of 1:2 and a secondary voltage of 960 volts, what would be the primary voltage?**

\[\begin{align} & \frac{{{N}_{1}}}{{{N}_{2}}}=\frac{{{V}_{1}}}{{{V}_{2}}}\Rightarrow \frac{1}{2}=\frac{{{V}_{1}}}{960} \\ & {{V}_{1}}=480V \\\end{align}\]

**5. The power rating of a transformer is in Volt-amps rather watts, why is that?**

Both reactive and resistive power contribute to temperature rise in a transformer. For this reason, the total apparent power or VA is used to rate transformers, rather than watts, as this measurement is independent of power factor so it takes into account both the resistive and reactive factors.

**6. A 60-KVA single phase transformer with a primary voltage of 2,400 volts and a secondary voltage of 240 volts.**

**a.** List the rated current in the primary

\[S={{V}_{1}}{{I}_{1}}\Rightarrow {{I}_{1}}=\frac{60*{{10}^{3}}}{2400}=25A\]

**b.** list the rated current in the secondary.

\[S={{V}_{2}}{{I}_{2}}\Rightarrow {{I}_{2}}=\frac{60*{{10}^{3}}}{240}=250A\]

**c.** list the turns ratio.

\[\frac{{{N}_{1}}}{{{N}_{2}}}=\frac{2400}{240}=10:1\]

**7. Winding taps are used for which applications?**

When the applied primary voltage is slightly higher or lower than its rated value, to compensate for varying line drops where occasional heavy loads are used.

**8. With a secondary transformer output of 1,320 watts and a primary input of 1,800 watts, calculate the efficiency of the transformer.**

\[\eta =\frac{Output}{Input}*100=\frac{1320}{1800}*100=73.33%\]

**9. A transformers secondary no-load voltage is 480 volts and has a full load voltage measuring 465 volts, the transformers regulation percentage would be what?**

\[VR=\frac{{{V}_{NL}}-{{V}_{FL}}}{{{V}_{NL}}}*100=\frac{480-465}{480}*100=3.125%\]

**10. A 37.5 KVA transformer rated at 480 volts in the primary and 208 volts in the secondary, what would be the primary and secondary line current?**

\[S=\sqrt{3}{{V}_{1}}{{I}_{1}}\Rightarrow {{I}_{1}}=\frac{37.5*{{10}^{3}}}{\sqrt{3}*480}=45.105A\]

\[S=\sqrt{3}{{V}_{2}}{{I}_{2}}\Rightarrow {{I}_{2}}=\frac{37.5*{{10}^{3}}}{\sqrt{3}*208}=104.08A\]

**11. A transformer nameplate has the following data:**

**a.** 35 KVA

**b.** 60 Hz

**c.** Single phase

HV 480 V

LV 240 V

Impedance 2.6%

Temperature rise 80 ^{o}C

**What is the determination of the following information?**

**a.** What would be the primary terminal markings?

H1 and H2

**b.** What would be the secondary terminal markings?

X1 and X2

**c.** What is the rated frequency?

60 Hz

**d.** List the values of the turn’s ratio.

\[\frac{{{N}_{1}}}{{{N}_{2}}}=\frac{480}{240}=2:1\]

**e.** List the maximum permit-able current the can be delivered to the load.

\[S={{V}_{2}}{{I}_{2}}\Rightarrow {{I}_{2}}=\frac{35*{{10}^{3}}}{240}=145.8A\]

**f.** The current at the output terminal under a short condition would be how much?

\[{{\text{I}}_{\text{sc}}}\text{=}\frac{\text{Seconday Current}}{\text{Percentage Impedance}}\text{=}\frac{{{\text{I}}_{\text{secondary}}}}{\text{Z }\!\!%\!\!\text{ }}\text{=}\frac{\text{145}\text{.8}}{\text{2}\text{.6 }\!\!%\!\!\text{ }}\text{=5607}\text{.69A}\]

**g.** When operating at full load in a 40 ^{o}C ambient temperature, the winding temperature should be what amount?

$\text{Winding Temperature=Ambient Temperatre+Temperature Rise=4}{{\text{0}}^{\text{o}}}\text{+8}{{\text{0}}^{\text{o}}}={{120}^{o}}$

**12. Which winding of the transformer has the larger conductor size if a 10-KVA transformer with a primary rating of 480 V and a secondary rating of 24 V? Why?**

The 24 V secondary windings because it would be rated for a higher current than the primary winding.

**13. What are buck-boost transformers used for?**

A buck-boost transformer can be used when the available supply voltage does not match the voltage required by the load.

**14. What are three advantages of the three-phase transformer over three single-phase transformers when transferring three-phase voltage?**

A single three-phase transformer is cheaper, easier to install, and will operate more efficiently than three single phase units.

**15. What would safety reasons explain why the secondary circuit of an instrument transformer should be closed whenever there is primary current? Why?**

If the secondary is not loaded, this transformer acts to step up the primary voltage to a dangerously high level.

**16. Why is a megger preferred to an ohmmeter when measuring the insulation breakdown of a distribution transformer?**

To make this resistance test for insulation breakdown, a very high voltage is necessary beyond the range of a standard ohmmeter.

**17. A transformer vault serves what main functions?**

Transformer vaults serve the following main purposes: Provide a means to isolate a potentially hazardous electrical component from unqualified personnel and contain any fire or combustion that may occur as a result of a transformer malfunction.

**18. Give a compared analysis of the extent of overcurrent for a transformer overload condition as opposed to a short circuit condition.**

A typical transformer overload condition may have a current flow that is from one to six times that of normal full load current. Short circuits currents can reach levels that are hundreds of times greater than the full load current.

**19. Harmonics are created by what characteristics of non-linear loads?**

The characteristics of non-linear loads that they demand current only during part of the cycle creates harmonic effects.

**20. List some ways that harmonics shortens the service life of a transformer.**

Harmonics shorten the service life of a transformer by causing additional heat in the transformer windings.

The post Transformer Questions and Answers appeared first on Electrical Academia.

]]>The post RLC Parallel Circuit Problems with Solutions appeared first on Electrical Academia.

]]>These questions are related to Parallel RLC Circuit which is covered in detail here:

**1. What are the three characteristics of the voltage across each branch of a parallel RL circuit?**

The voltage across each of the branches is the same value, equal in value to the total applied voltage, and all in phase of each other.

**2. The total current in a parallel RL circuit is Equal to the vector sum rather than the arithmetic sum. Why?**

Because the branch currents are out of phase with each other.

**3. The terms apparent power, reactive power, and true power as they apply to the parallel RL circuit are defined as:**

a. Apparent power (VA) includes both the true power (Watts) and the reactive power (VARs), the true power (WATTs) is that power dissipated by the resistive branch, and the reactive power (VARs) is the power that is returned to the source by the inductive branch.

**4. The current measurements of a parallel RL circuit show a current flow of 2 amps through the resistive branch and 4 amps through the inductive branch, determine the value of the total current flow.**

${{I}_{T}}=\sqrt{I_{R}^{2}+I_{L}^{2}}=4.47A$

**5. For an RL circuit with a 240-V supply and 20 Ω resistor and a 48 Ω inductor calculate:**

**a.** Apparent power.

$\begin{align} & {{I}_{R}}=\frac{{{V}_{R}}}{R}=\frac{240}{20}=12A \\ & {{I}_{L}}=\frac{{{V}_{L}}}{{{X}_{L}}}=\frac{240}{48}=5\angle -{{90}^{o}}A \\ & \left| {{I}_{T}} \right|=\sqrt{{{\left| {{I}_{R}} \right|}^{2}}+{{\left| {{I}_{L}} \right|}^{2}}}=\sqrt{{{12}^{2}}+{{5}^{2}}}=13A \\ & S=V\left| {{I}_{T}} \right|=240*13=3120A \\\end{align}$

**b.** True power.

$\begin{align} & {{I}_{R}}=\frac{{{V}_{R}}}{R}=\frac{240}{20}=12A \\ & {{P}_{R}}=V{{I}_{R}}=240*12=2880W \\\end{align}$

**c.** Reactive power.

$\begin{align} & {{I}_{L}}=\frac{{{V}_{R}}}{R}=\frac{240}{48}=5\angle -{{90}^{o}}A \\ & \left| {{I}_{L}} \right|=5A \\ & {{P}_{L}}=V{{I}_{L}}=240*5=1200\text{ }VAR \\\end{align}$

**d.** Circuit power factor.

$\begin{align} & \theta ={{\tan }^{-1}}\left( \frac{Q}{P} \right)={{\tan }^{-1}}\left( \frac{1200}{2880} \right)={{22.6}^{o}} \\ & Cos\theta =Cos({{22.6}^{o}})=0.923\text{ Lagging} \\\end{align}$

**6. Explain the difference between RL circuit and an RC circuit.**

The principal difference between two of them is the phase relationship. In a purely capacitive circuit, the current leads the voltage by 90^{o}, while in a pure inductive the current lags the voltage by 90^{o}.

**7. Using a parallel RC circuit which has a power supply of 100 –V, 60 Hz, and a current flow through the resister of is 10 amps and the current flow through the capacitor is 10 amps. What are the following values?**

**a.** Line current (I_{T}).

\[{{I}_{T}}=\sqrt{I_{R}^{2}+I_{C}^{2}}=\sqrt{{{10}^{2}}+{{10}^{2}}}=14.14A\]

**b.** Impedance (Z).

\[{{Z}_{T}}=\frac{{{V}_{s}}}{{{I}_{T}}}=\frac{100}{14.14}=7.07\Omega \]

**c.** True power (W).

\[{{P}_{R}}=V{{I}_{R}}=100*10=1000W\]

**d.** Reactive power (VARs).

\[{{Q}_{L}}=V{{I}_{L}}=100*10=1000W\]

**e.** Apparent power (VA).

\[{{S}_{T}}=\sqrt{P_{R}^{2}+P_{L}^{2}}=\sqrt{{{1000}^{2}}+{{1000}^{2}}}=1414.21\text{ }VA\]

**f.** Power Factor

\[\begin{align} & \theta ={{\tan }^{-1}}\left( \frac{Q}{P} \right)={{\tan }^{-1}}\left( \frac{1000}{1000} \right)={{22.6}^{o}} \\ & Cos\theta =Cos({{45}^{o}})=0.707\text{ Lagging} \\\end{align}\]

**8. For a RC parallel circuit with a supply voltage of 120-V and total watt of 9604 no value for the resistor and a capacitor valued at 1500µF, determine the following:**

**a.** The amount of current through the resistor.

\[\begin{align} & {{P}_{R}}=\frac{V_{S}^{2}}{R}\Rightarrow R=\frac{V_{S}^{2}}{{{P}_{R}}}=\frac{{{120}^{2}}}{9604}=1.5\Omega \\ & I=\frac{V}{R}=\frac{120}{1.5}=80A \\\end{align}\]

**b.** The capacitive reactance of the capacitor.

\[{{X}_{C}}=\frac{1}{2\pi fC}=\frac{1}{2\pi *60*1500*{{10}^{-6}}}=1.77\Omega \]

**c.** The amount of current flow through the capacitor.

\[{{I}_{C}}=\frac{{{V}_{s}}}{{{X}_{C}}}=\frac{120}{1.77}=67.79A\]

**d.** The line current.

\[{{I}_{L}}=\sqrt{I_{R}^{2}+I_{C}^{2}}=\sqrt{{{80}^{2}}+{{67.79}^{2}}}=104.85A\]

**e.** Apparent power.

\[S=V{{I}_{L}}=120*104.85=12582\text{ }VA\]

**f.** PF

\[\begin{align} & \theta ={{\tan }^{-1}}\left( \frac{R}{{{X}_{C}}} \right)={{\tan }^{-1}}\left( \frac{1.5}{1.77} \right)={{40.28}^{o}} \\ & Cos\theta =Cos({{40.28}^{o}})=0.76\text{ Leading} \\\end{align}\]

**9. 4-A inductor and a 12-A capacitor in parallel, what is the total current?**

\[\begin{align} & {{I}_{L}}=\text{ }4A\text{ } \\ & {{I}_{C}}=\text{ }12A \\ & {{I}_{C}}-\text{ }{{I}_{L}}=\text{ }{{I}_{T}}\text{= }8A \\\end{align}\]

**10. With a source voltage of 208-V, frequency of 60 Hz and a LC parallel circuit that has XC = 16 Ω and XL = 8 Ω, what would be:**

**a.** Impedance value be.

\[{{Z}_{T}}=\frac{{{Z}_{C}}*{{Z}_{L}}}{{{Z}_{C}}-{{Z}_{L}}}=\frac{16*8}{16-8}=16\Omega \]

**b.** Current through the capacitor.

\[{{I}_{C}}=\frac{Vs}{{{X}_{C}}}=\frac{208}{16}=13A\]

**c.** Current through the inductor.

\[{{I}_{L}}=\frac{Vs}{{{X}_{L}}}=\frac{208}{8}=26A\]

**d.** Line current.

\[{{I}_{T}}=\text{ }{{I}_{L}}-\text{ }{{I}_{C}}\text{= }13A\]

**e.** The impedance (Z).

\[{{Z}_{T}}\text{ }=\frac{{{V}_{s}}}{{{I}_{T}}}=16A\]

**11. You have a parallel RLC circuit with 6-A trough the resistor, 8-A through the inductor, 5-A through the capacitor, calculate total line current. Is the circuit capacitive or inductive?**

${{I}_{L}}=\sqrt{I_{R}^{2}+{{\left( {{I}_{L}}-{{I}_{C}} \right)}^{2}}}=\sqrt{{{6}^{2}}+{{3}^{2}}}=6.7A$

Since more current is through an inductor so circuit is inductive in nature.

**12. You have a parallel RLC circuit with a 16 Ω resistor, 8 Ω inductor, 20 Ω capacitor, and a 120-V power supply what are the following values?**

**a.** Current through the resistor (I_{R}).

\[{{I}_{R}}=\frac{{{V}_{s}}}{R}=\frac{120}{16}=7.5A\]

**b.** Current through the inductor (I_{L}).

\[{{I}_{L}}=\frac{{{V}_{s}}}{{{X}_{L}}}=\frac{120}{8}=15A\]

**c.** Current through the capacitor (I_{C}).

\[{{I}_{C}}=\frac{{{V}_{s}}}{{{X}_{C}}}=\frac{120}{20}=6A\]

**d.** Net reactive current (I_{X}).

\[{{I}_{X}}={{I}_{L}}-{{I}_{C}}=9\text{ }A\]

**e.** Total line current (I_{T}).

\[{{I}_{T}}=\sqrt{I_{R}^{2}+{{\left( {{I}_{L}}-{{I}_{C}} \right)}^{2}}}=\sqrt{{{7.5}^{2}}+{{9}^{2}}}=11.71A\]

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]]>The post Relay Questions and Answers appeared first on Electrical Academia.

]]>These questions are related to Solid State Relay Timers, ON and OFF Delay Timers, Magnetic Motor Contactors and Starters which are covered in detail here:

ON Delay Timer and OFF Delay Timer

**Electromagnetic relays operate in which way?**

An electromechanical relay consists of a coil and contacts. It turns a load circuit ON or OFF by energizing an electromagnet that opens or closes contacts in the circuit.

**Relays are associated with two types of circuits, how do they interact with each other?**

A relay is made up of two circuits: the coil input or control circuit and the contact output or load circuit. Closing the switch in the control circuit energizes the electromagnet, which in turn closes the relay contacts in the load circuit to switch the load on.

**What is the difference between a normally closed (NC) and a normally open (NO) relay contact?**

Normally open contacts are those contacts that are open when the coil is DE energized and closed when the coil is energized. Normally closed contacts are those contacts that are closed when the coil is de energized and open when the coil is energized.

**There are three common relay control applications, describe them.**

(1) To control a high voltage load with a low voltage control circuit. (2) To control a high current load with a low current control circuit. (3) To control multiple switching operations by a single, separate current.

**For trouble shooting purposes, what are two commonly used relay options?**

Two common relay options used for troubleshooting are an on/off indicator to indicate the state of the relay coil and a manual override button to move the contacts into their energized position for testing.

**Relays are specified in six different ways, list them.**

(1) Type of operating current (AC or DC). (2) Normal operating voltage or current. (3) Permissible coil voltage variation (4) Coil resistance (5) Power consumption. (6) Contact rating (AC or DC) maximum current rating at specific voltage

**In a relay contact switching arrangements, the terms poles, throw, and break are defined as:**

Pole is the number of switch contact sets. Throw is the number conducting positions, single or double. Break designates the number of points in a set of contacts where the current will be interrupted during opening the contacts.

**Solid state relays have what main advantage over electromechanical relays?**

More reliable

**How is the electrical isolation of the input and output of a solid state relay is accomplished?**

Electrical isolation of the input and output sections of a solid state relay is accomplished by using an LED in the control circuit and a photodetector in the load circuit.

**Explain how output contacts in conventional relays are switched differently than that of time delay relays.**

In a conventional control relay, the contacts immediately change when the control circuit is energized. With a time-delay relay, the contacts do not change state until a predetermined time after the input is either energize or de energized.

**What is the difference between an on-delay timer and an off-delay timer?**

The contacts of an on-delay timer change state a fixed time after the control circuit is energized. The contacts of an off-delay timer change state after a fixed time after the control circuit is de energized.

**A two coil latching relay state is changed by what means?**

In a two coil latching relay, energizing one coil will latch the contacts closed and they will stay in that position. When the second coil is energized the contacts will change state and stay in that position even if power is removed.

**A single coil latching relay state is changed by what means?**

In a single-coil latching relay, the direction of current through the coil determines if the contacts will be latched or unlatched state.

**A relay and a contactor differ in which way?**

They both operate on the same principle, but the contactor is capable of handling heavier loads currents.

**You must combine two components to form a magnetic motor starter, what are they?**

A magnetic motor starter is a contactor with an overload protective device attached.

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]]>The post Inductor Questions and Answers appeared first on Electrical Academia.

]]>These questions are related to Capacitor Circuit, Capacitor Connections, Capacitive Reactance, and RC Circuit Time Constant which are covered in detail here:

Inductors in Series | Inductors in Parallel

**1. Inductance is defined as what?**

Inductance is the ability of a component to oppose any change in (increase or decrease) in the current.

**2. Name the base unit used when measuring inductance.**

Henry (H)

**3. State the relationship between the inductance value of a coil and the amount of emf it produces.**

The greater inductance value or the faster the rate of change of current, the greater the emf induced in the circuit.

\[{{V}_{L}}=L\frac{di}{dt}\]

**4. What effect (increase or decrease) would the following changes have on the inductance of a coil?**

a. Increase in the number of turns of wire. **Increase**

b. Removal of its iron core. **Decrease**

c. Spacing the turns of wire farther apart. **Decrease**

**5. For a coil that has an inductance of 5H and a DC resistance of 10 Ω:**

**a.** Calculate the RL time constant.

\[\tau =\frac{L}{R}=\frac{5}{10}=0.5\operatorname{seconds}\]

**b.** When the DC voltage is applied to this coil, approximately how long will it take for the current to reach its maximum value?

Five-time constants.

**6. Define the term inductive reactance.**

The opposition to AC current flow is called inductive reactance.

${{X}_{L}}=2\pi fL$

**7. What is the base unit used to measure inductive reactance?**

Inductive reactance is measured in ohms Ω.

**8. State whether the inductive reactance (increases or decrease) with each of the following changes:**

**a.** Increase in the frequency of the AC supply source. **Increase.**

**b. **Decrease in the inductance of the coil. **Decrease.**

The above results are based on the following formula:

${{X}_{L}}=2\pi fL$

**9. Calculate the inductive reactance of a 2.5 H inductor when operated at a frequency of 50 Hz.**

\[{{X}_{L}}=\text{ }2\pi fL;\text{ }2\pi *50*2.5\text{ }=\text{ }785.39\Omega \]

**10. A 6 H inductor is connected to a 12 VDC source. What is the value of its inductive reactance? Explain.**

An inductor in a DC circuit has no inductive reactance according to the following formula:

${{X}_{L}}=2\pi fL=2\pi *0*6=0\Omega $

**11. An AC voltage of 240 volts with a frequency 60 Hz is applied to a 0.5 H inductor. Neglecting its small amount or wire resistance, how much current would flow through it?**

\[\begin{align} & {{X}_{L}}=2\pi fL=2\pi *60*0.5=\text{ }188.5\Omega \Omega \\ & I\text{ }=\frac{{{V}_{s}}}{{{X}_{L}}}=\frac{240}{188.5}=1.27A \\\end{align}\]

**12. Determine the total inductance of a 6 H and a 4 H inductor connected in:**

**a.** Series.

\[{{L}_{T}}={{L}_{1}}+{{L}_{2}}=\text{ }6+4=10H\]

**b.** Parallel.

\[{{L}_{T}}=\frac{{{L}_{1}}*{{L}_{2}}}{{{L}_{1}}+{{L}_{2}}}=\frac{4*6}{4+6}=2.4H\]

**13. Inductors of 1H and 2H are connected in series to a 440V, 60Hz power supply.**

**a.** Determine the total current flow for the circuit.

$\begin{align} & {{X}_{T}}=2\pi f{{L}_{T}}=2\pi *60*3=1131\Omega \\ & I=\frac{{{V}_{s}}}{{{X}_{T}}}=\frac{440}{1131}=0.389A \\\end{align}$

**b.** Repeat for the two inductors connected in parallel to the power supply.

$\begin{align} & {{X}_{T}}=2\pi f{{L}_{T}}=2\pi *60*\left( \frac{1*2}{1+2} \right)=251.33\Omega \\ & I=\frac{{{V}_{s}}}{{{X}_{T}}}=\frac{440}{251.33}=1.75A \\\end{align}$

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]]>The post Basic Electronics Questions and Answers appeared first on Electrical Academia.

]]>**1. A diode has a certain characteristic when operating. Explain this characteristic.**

The main operating characteristic of a diode is that it allows current in one direction and blocks current in the opposite direction.

**2. What must the conditions be for a LED to emit light?**

An LED emits light when the diode is forward biased allowing current to flow.

**3. Transistors have two main functions, what are they?**

Amplification and switching.

**4. A bipolar junction transistor has three semiconductor sections, what are they?**

Emitter (E), base (B), and collector (C).

**5. Give a detailed explanation on how a bipolar junction transistor amplifies current.**

The BJT is a current amplifier in that a small current flow from the base to emitter results in a larger flow from collector to emitter.

**6. What are the names of the three leads attached to a junction field-effect transistor?**

Gate, source, and drain.

**7. What are the similarities between a thyristor and a mechanical switches operation?**

Similar to a mechanical switch, thyristors have only two states: on (conducting) and off (non-conducting).

**8. List some of the similarities and differences between an SCR and a diode?**

Silicon controlled rectifiers (SCRs) are similar to diodes except for a third terminal, or gate, which controls, or turns on the SCR.

**9. The control of an SCR is different when operated from an AC source than a DC source, explain the difference.**

When operating from a DC source, once the SCR is turned on it stays on.

With an AC source, the SCR will automatically switch to off when the sine wave goes through the zero volts.

**10. Explain how the operation concerning an SCR which is unidirectional and a Triac which is bi-directional is different.**

An SCR can only control the power delivered to a load from 0 to 50%. The triac can deliver power to the load from 0 to 100%.

**11. A single wave and half wave rectifier change AC to DC, what is the difference between the two?**

During the positive half cycle of the AC input wave, the anode side of the diode is positive.

**12. What is the difference in the output when a single phase half-rectifier is replaced by a full-wave rectifier?**

The diode is forward biased, allowing it to conduct a current to the load. Because the diode acts as a closed switch during this time, the positive half cycle of the AC wave form is developed across the load. During the negative half cycle of the AC input, the anode side of the diode is negative. The diode is now reversed biased; as a result, no current can flow through it. The diode acts as an open switch during this time, so no voltage is produced across the load.

**13. Transistors can be a switching device or an amplifying device, how do the operations compare?**

When a transistor is used as a switch, it has only two operating states, on and off.

**14. A MOSFET has certain operating characteristics that are utilized for providing long time-delay periods for electronic timers, explain them.**

The high input impedance, the low current into the gate, are the characteristics of the MOSFET that are utilized to provide long time delay periods for electronic timers.

**15. To provide a varying amount of power to a three phase, reduced voltage starter there is a certain SCR control utilized to accomplish this, explain. **

Phase angle control.

**16. For switching AC power loads, there are certain characteristics of a triac that make it a perfect electronic switch, define this characteristic.**

The operating characteristic of being bi-directional makes the triac an ideal component for switching AC power loads.

**17. Diacs can be utilized to control power in a triac lamp dimmer circuit, explain how this is accomplished.**

The diac is bi-directional and when the control voltage charges to the break over voltage the diac triggers the triac into conduction for the remainder of the half cycle.

**18. An electronic motor drive has what primary function?**

The primary function of an electronic motor drive is to control speed, torque, direction, and resulting horsepower of a motor.

**19. An electronic frequency drive has three major sections, list them and state the main function of each.**

**Rectifier section:** The full-wave three phase diode rectifier converts the 60 Hz power from a standard utility supply to either fixed or adjustable DC voltage.

**Inverter section:** Electronic switches, switch the rectified DC on and off, and produce a current or voltage waveform at the desired new frequency.

**Control section:** An electronic circuit receives feedback information from the driven motor and adjusts the output voltage or frequency to the selected values.

The post Basic Electronics Questions and Answers appeared first on Electrical Academia.

]]>The post Capacitor Questions and Answers appeared first on Electrical Academia.

]]>These questions are related to Capacitor Circuit, Capacitor Connections, Capacitive Reactance, and RC Circuit Time Constant which are are covered in detail here:

Capacitor in Series | Capacitors in Parallel

**1. Define capacitance.**

The ability of an electric circuit or component to store electric energy by means by means of an electrostatic field.

**2. Compare between an inductor and a capacitor the manner in which energy is stored.**

The capacitor stores energy in an electrostatic field, the inductor stores energy in a magnetic field.

**3. Common practical applications for capacitors list four.**

**1.** Power factor correction of an electrical system.

**2**. Improving torque in motors.

**3**. Filters in AC circuits.

**4.** Timing of control circuits

**4. The base unit used to measure capacitance is what?**

Farad (F)

**5. What three factors determine the amount of capacitance in a capacitor?**

**1.** Area of the plates

**2.** Type of dielectric

**3.** Spacing between plates

**6. What factors determine the voltage rating of a capacitor?**

The voltage rating of a capacitor indicates the maximum voltage that can be safely applied to its plates and depends on the insulation strength of its dielectric.

**7. To provide a total capacitance of 100µF, how would you connect two 50µF capacitors?**

In parallel.

**8. Calculate to total capacitance at a maximum voltage for two 220µF, 300-V capacitors connected in series.**

110µF.

**9. With a 25KΩ resistor connected in series with a 1,000µF capacitor and operated from a 12-VDC source:**

**a. **Calculate the RC time constant.

Let’s calculate it using the following formula:

$\tau =RC=25*{{10}^{3}}*1000*{{10}^{-6}}=\text{25seconds}$

**b. **When a voltage is applied to the circuit approximately how long will it take for the voltage across the capacitor to reach 12V?

125 seconds. Approximately, it will take 5-time constants which are equal to 125 seconds in this case.

**c. **If the fully charged capacitor is discharged through a 25Ω resistor, what would the value of the voltage across the capacitor be after the first 25 seconds of discharge?

4.416 V. After first time constant, the capacitor voltage decreases to 36.7 % of the fully charged voltage value.

**10. Define capacitance reactance.**

The opposition to AC current flow by a capacitor.

**11. When measuring capacitance reactance what is the base unit used?**

Ohm (Ω)

**12. Capacitance reactance is affected in what way by capacitance?**

As capacitance goes up capacitance reactance goes down

\[{{X}_{C}}=\frac{1}{2\pi fC}\]

**13. Capacitance reactance is affected in what way by frequency?**

As frequency goes up capacitance reactance goes down.

\[{{X}_{C}}=\frac{1}{2\pi fC}\]

**14. With a frequency of 60Hz and an AC voltage of 240 Volts applied to a 50µF capacitor. Calculate the amount of AC current flow in the circuit.**

First, let’s calculate reactance by the following formula:

\[{{X}_{C}}=\frac{1}{2\pi fC}=\frac{1}{2\pi *60*50*{{10}^{-6}}}=53.1\Omega \]

Now, calculate current from the following formula:

\[I=\frac{V}{{{X}_{C}}}=\frac{240}{53.1}=4.52A\]

**15. A 10µF(C1) and a 40µF(C2) capacitor are connected in series to a 230V, 60Hz source. Calculate the value of the voltage drop across each capacitor.**

First, we will calculate total capacitance:

\[{{C}_{T}}=\frac{{{C}_{1}}*{{C}_{2}}}{{{C}_{1}}+{{C}_{2}}}=\frac{10*40}{10+40}=8\mu F\]

Let’s use the following formulas to compute voltage drop across each capacitor:

\[\begin{align} & {{V}_{{{C}_{1}}}}=\frac{{{C}_{T}}}{{{C}_{1}}}*{{V}_{s}}=\frac{8}{10}*230=184V \\ & {{V}_{{{C}_{2}}}}=\frac{{{C}_{T}}}{{{C}_{2}}}*{{V}_{s}}=\frac{8}{40}*230=146V \\ & {{V}_{s}}={{V}_{{{C}_{1}}}}+{{V}_{{{C}_{2}}}}=184+46=230V \\\end{align}\]

**16. A 10µF(C1) and a 40µF(C2) capacitor are connected in parallel to a 280V, 60Hz source. **

**a. What is the capacitive reactance and current flow through C1?**

We will use the following formulas to compute reactance and current flow:

$\begin{align} & {{X}_{{{C}_{1}}}}=\frac{1}{2\pi f{{C}_{1}}}=\frac{1}{2\pi *60*10*{{10}^{-6}}}=265.258\Omega \\ & I=\frac{{{V}_{s}}}{{{X}_{{{C}_{1}}}}}=\frac{280}{265.258}=1.055A \\\end{align}$

**b. What is the capacitive reactance and current flow through C2?**

We will use the following formulas to compute reactance and current flow:

$\begin{align} & {{X}_{{{C}_{2}}}}=\frac{1}{2\pi f{{C}_{2}}}=\frac{1}{2\pi *60*40*{{10}^{-6}}}=66.314\Omega \\ & I=\frac{{{V}_{s}}}{{{X}_{{{C}_{2}}}}}=\frac{280}{66.314}=4.22A \\\end{align}$

**17. What type of power is associated with a capacitor called and in what units is it measured?**

Capacitive reactive power, which is measured in VARs

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]]>**What is the definition of a generator?**

A generator is a machine which converts mechanical energy to electrical energy.**What is the definition of an alternator?**

An AC generator is also called an alternator.**With a six pole alternator spinning at 1200 rpm, what would the frequency of the voltage be?**

\[f=\frac{n*p}{120}=\frac{1200*6}{120}=60Hz\]

**To generate an output frequency of 6oHz, what would be the rpm speed need to be with an eight pole alternator?**

\[n=\frac{120*f}{p}=\frac{120*60}{8}=900RPM\]

**In North America what is the standard frequency of AC voltage generated?**

The standard frequency in North America is 60 Hz**With a voltage of 240-V RMS, what would be the peak voltage?**

${{V}_{p}}=\sqrt{2}{{V}_{rms}}=1.414*240=339V$

**Using an AC volt meter we found the AC sine wave voltage to be 10-Volts.****Determine the peak value of this voltage.**

${{V}_{p}}=\sqrt{2}{{V}_{rms}}=1.414*10=14.14V$**Determine the RMS value of this voltage.**

Measured voltage is RMS voltage; 10-V rms.**Determine the peak to peak value of this voltage.**

Peak x 2 = peak to peak; 14.14 x 2 = 28.28 volts peak to peak.

**List the two single phase voltages in a residential system.**

120-V and 240-V AC**The direction of rotation of an induction motor is affected in what way by interchanging the supply phases?**

By interchanging the supply phases the motor will run in reverse.**When connecting a three phase alternator to the power system, what conditions must be met?**

**1.**The phase sequence or rotation of the machine must be the same as that of the system.

**2.**The alternator voltage must be in phase with the grid system.

**3.**The alternator frequency must be the same as the grid system frequency.

**The three stator coils of a three phase alternator are positioned apart by how many electrical degrees?**

The stator coils must be set to 120 degrees apart.**There are two basic types of three phase alternator coil connections, what are they?**

The Wye and Delta connections.**In an AC resistive circuit, what is the phase relationship between the voltage and current?**

The current and voltage are in phase.**A 10 ohm heater is connected to an AC 340-V peak-to-peak voltage determine:****The peak value of the voltage.**

340-V/2 = 170-V peak**The effective value of the voltage.**

$\frac{170}{\sqrt{2}}=120V$**The wattage.**

120V/10Ω = 12Amps;

12A x 120V = 1440 Watts

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