# ATAT 105 Basic Electricity > # [[T105 Week 7| ◀️ ]]  [[T105 Home| Home ]]  [[T105 Week 9| ▶️ ]]               [[QR T105 Week 8| 🌐 ]] ># [[T105 Week 8|Week 8]] >- [[T105 Week 8#Alternating Current|Alternating Current]] >- [[T105 Week 8#Generators and Alternators|Generators and Alternators]] ># [[T105 Week 8#Lab|Lab]] >- [[T105 Week 8#Oscilloscope Familiarization|Oscilloscope Familiarization]] >[!jbplus|c-blue]- Lesson Intro >### What >In this lesson you will learn about alternating current and the terminology of its parameters, as well as taking a brief look at devices used to create AC. > >### Why > >Alternating current is more complicated than direct current, and so your knowledge must expand as AC is used extensively in aviation. > > >## Approach and Objectives > >By understanding the following topics, you will have achieved the learning outcome for this lesson. Consult your course outline for the learning outcomes and other details of this course. > >### Course Learning Objectives > >CLO 13. Compute the peak, instantaneous and effective (RMS) values of an AC sine wave. >CLO 14. Show using an oscilloscope the measurement of frequency, period and voltage of an AC waveform. > > >### Testing > >You will be tested on this material on the midterm test and the final test. Details [[T105 Intro#Testing and Grades|here]]. >[!jbplus|c-blue]- Prof >### Objectives > >In this lesson we will learn about alternating current and its parameters in order to prepare students for AC circuit analysis > > ### Theory >For the theory course, emphasize the terms, and their relationships to each other. Visuals help a lot: when understanding p versus pp for instance. >### Lab >For the lab, offer guidance on the oscilloscope. They are not so much becoming expert at the oscilloscope, but using it to display aspects of AC that they have just learned about in theory. ## Alternating Current ![[Pasted image 20210808185413.png|350]] So far, every circuit we have looked at, and every component in it, have been discussed in terms of Direct Current. That is, there is a steady current, which flows in only one direction, that is, from the power source, through the circuit, and back to the power source. This you should understand by now. We will now look at alternating current, which operates on the same physics, but which provides a significant increase in complexity. Alternating current is current flow which continually changes its value and periodically reverses direction. It has many advantages over direct current. For example, AC is much easier to generate in the large quantities needed for homes and industries, and for large transport aircraft. ### Generation of AC More important though, is the ease with which AC current and voltage can be changed to get the most effective use of electrical energy. If you recall from our earlier lessons, there is a close relationship between magnetism and electricity. Let's briefly review. Any time electrons flow in a conductor, a magnetic field surrounds the conductor. The amount of electron flow determines the strength of the magnetic field. We also saw that when a magnetic field is moved across a conductor, electrons are forced to flow within the conductor (Faraday's Law), and that the rate at which the lines of magnetic flux are cut by the conductor determines the amount of flow. ![[Pasted image 20210808185514.png|350]] Therefore, electron flow is increased by: - increasing the strength of the magnetic field - increasing the speed of movement of the conductor through the lines of flux. Common household electricity is produced by a rotary generator in which a coiled conductor rotates inside a magnetic field. The changing values of voltage produced as the coil rotates can be observed on an oscilloscope. ![[Pasted image 20210808185551.png|350]] The values start at zero, rise to a peak, and then drop back off to zero. As the coil continues to rotate, the voltage builds up in the opposite direction to a peak and then back to zero. One complete cycle of voltage change is produced with each complete revolution of the coil. The AC wave form produced by a rotary generator is called a sine wave. ![[Pasted image 20210808185619.png|350]] From the wave illustrated you can see that one cycle begins at 0 degrees and ends at 360 degrees. The values of alternating current follow the sine wave. ![[Pasted image 20210808185659.png|350]] This can be seen through the use of a generator consisting of a single loop of wire that is rotated in a magnetic field. When the loop moves parallel with the lines of flux within a magnetic field, it does not pass through any lines of flux, and no voltage is generated. This is the starting point, or the zero-degree angle. As the loop rotates to 45 degrees, it cuts across some of the lines of flux. The voltage generated at this point is 0.707 times the peak amount. As the loop continues to rotate to the 90 degree point, it cuts across the maximum number of flux lines for each degree it rotates. It is here that the peak voltage is produced. Further rotation decreases the number of flux lines cut for each degree of rotation. Once the loop reaches 180 degrees, it cuts no flux lines and the output is again zero. Rotation beyond this point brings the opposite side of the loop down through the flux lines near the south pole of the magnet. ![[Pasted image 20210808185846.png|350]] The voltage builds in the opposite direction and changes in a continuous and smooth manner. Watch this animation and watch for the various points just discussed [[AC Generator Animation|🎞]] ### Cycle and Alternations ![[Pasted image 20210808190120.png|350]] #### Cycle As mentioned earlier, a cycle is one complete sequence of voltage or current change from zero, through a positive peak, back to zero, through a negative peak, and back to zero again. It then repeats. #### Alternation An alternation is one-half of an AC cycle in which the voltage or current rises or falls from zero to a peak and back to zero. Therefore, there is a positive and a negative alternation. ### Period and Frequency ![[Pasted image 20210808190141.png|350]] #### Period (T) The time required for one cycle of events to occur is called the period of the alternating current or voltage. #### Frequency (f) The frequency of AC is the number of cycles completed in one second. Frequency is expressed in Hertz (Hz) with one hertz equal to one cycle per second. Therefore the formula is: $ f = \frac {1}{T}$ Where: $f = Frequency$ $T = Period$ ### Factors of Frequency The frequency of alternating current produced by a generator is determined by the number of pairs of magnetic poles in the generator and the number of revolutions completed per minute by the rotating coils. ### Peak Value ![[Pasted image 20210808190356.png|350]] As discussed, the peak value of a sine wave is the maximum value of voltage or current in either the positive or the negative direction. The difference between the positive and the negative peak values is called the peak-to-peak value and is equivalent to twice the peak value. ### Average Value If all of the instantaneous values of current or voltage in one alternation of a sine wave are averaged together, they have a value of 0.637 times the peak value. This is referred to as an average value. It actually has very little practical use for making computations, but we want to be thorough in our coverage of the parameters of alternating current. ### Effective Value (RMS) ![[Pasted image 20210808194120.png|350]] The effective value of AC is the value that produces the same amount of heat as a corresponding amount of DC. The effective value is sometimes referred to as the root mean square (successive mathematical operations) or RMS value and is 0.707 of the peak value. Therefore, an effective value is always less than the peak value. To determine the effective value, square all of the instantaneous values in one alternation, find the average of these squared values, and calculate the square root of this average. ![[Pasted image 20210808194143.png|350]] A peak value of 100 volts is the same as 70.7 volts RMS on a voltmeter. ### Phase ![[Pasted image 20210808194636.png|350]] An oscilloscope traces sine wave patterns of AC voltage and current on its screen. When the sine waves cross the zero line at the same time, the voltage and current are said to be in phase. In other words, both voltage and current follow the identical sine wave. #### Phase Angle ![[Pasted image 20210808194705.png|350]] In alternating current where the values are constantly changing, certain circuit components cause a difference in zero line crossing (phase shift) between the voltage and the current. The amount of shift is referred to as the phase angle. For example, some electrical components cause the current to reach its maximum value 90 degrees before the voltage. ![[Pasted image 20210808194737.png|350]] In this situation, there is a 90 degree phase angle between the current and voltage and the current leads the voltage. ![[Pasted image 20210808194752.png|350]] Other components cause the voltage to change before the current, and the current is said to lag the voltage. We covered inductive and capacitive circuits, where we learned about ELI the ICE man, a mnemonic that helps us to remember whether voltage or current leads, depending on inductance or capacitance. ### Apparent Power In the study of direct current, electrical power is the product of voltage and current and is measured in watts. In alternating current, the values for both voltage and current are given in effective (RMS) values. The product of these effective values is called the apparent power and is expressed in volt-amps rather than in watts. ![[Pasted image 20210808195024.png|350]] In a circuit that contains only resistance, the current is in phase with voltage and the power developed at any instant is the product of the voltage and the current. As long as the voltage and the current are in phase, the power is positive. ![[Pasted image 20210808195111.png|350]] [[True power]] is the actual AC power when phase is taken into account. If the current either leads or lags the voltage, there is at least part of a cycle in which the voltage or current is positive and the other is negative. [[True power]] is expressed in Watts and is the product of voltage and that portion of the current that is in phase with the voltage. ### Power Factor The ratio of [[true power]] to apparent power is called the power factor (PF). A [[true power]] of 50W with an apparent power of 100VA has a power factor of $ \frac {50}{100} = 0.5 $ When the PF is multiplied by the current, it indicates the amount of current that is in phase with the voltage. In the above example, the power factor is 0.5, meaning only 50 percent of the current is in phase with the voltage. ![[Pasted image 20210808195157.png|350]] Power factor can also be calculated when the amount of phase shift (Ө) between the voltage and the current is known. $ Power factor = cos(Ө)$ For example, if the phase angle between voltage and current is 30 degrees, then the power factor is equivalent to the cosine of 30 degrees. $ Power factor = cos (30)$ Therefore $ PF = 0.866 $ If 100% of the current is in phase with the voltage (zero phase angle), as it is in a circuit having only resistance, the power factor is 1. $PF = cos (0) = 1$ To bring this back to something we can compare with DC power, we can represent the equation for [[true power]] as the product of the voltage and current and the power factor: $ True power = E \times I \times Power Factor$ or $ True power = E \times I \times cos(Ө)$ * Note that in a capacative or or inductive circuit, [[true power]] is always less than apparent power. ![[Pasted image 20210808195304.png|350]] As mentioned earlier, when the current and voltage are in phase, the phase angle is zero and the power factor is 1. Under these circumstances, the [[true power]] is equal to the apparent power. ![[Pasted image 20210808195404.png|350]] However, with a 45 degree phase angle, the [[true power]] is only 0.707 of the apparent power. ![[Pasted image 20210808195433.png|350]] When the current and voltage are 90 degrees out of phase, the cosine of 90 degrees is zero. Therefore the power factor is zero. In this situation, there is no [[true power]] produced in the circuit, even though voltage is present and current is flowing ### Resistive Circuits ![[Pasted image 20210808200241.png|350]] Circuit components such as light bulbs, heaters, and composition resistors provide resistance to an AC circuit. Circuits containing only these types of devices are called resistive circuits. In a resistive circuit, the current and voltage are in phase. In other words, the current and voltage both pass through zero in the sine wave at the same time and go in the same direction. The power factor in a purely resistive circuit is one, so the apparent power and [[true power]] are the same. To calculate the power in watts in a resistive circuit, multiply the effective value of voltage by the effective value of current. $ Power = Eeff \times Ieff$ ## Electrical Generation Components ![[Pasted image 20210808200339.png|350]] The main sources of electrical power on aircraft are: - Alternators - Generators - Starter Generators - External Power ### Power Generation Principles ![[Pasted image 20210808200406.png|350]] Alternators and generators produce electrical power by means of electromagnetic induction. Alternators and generators convert mechanical energy (from the engines) into electrical energy. You can review what we learned about induction [[T105 Week 9#Inductance|here]]. ![[Pasted image 20210808200446.png|350]] ## Generators and Alternators ![[Pasted image 20210808200512.png|350]] Generators and alternators employ rule #1 from the above graphic. During the discussion that follows, note that the rotating part (rotor) and the stationary part (stator) can change their roles depending on whether we are talking about a generator or an alternator. #### Generators A generator consists of a number of conductors (called an armature) which move (rotate) within a magnetic field. Because these rotate, they are called the rotor. This causes current to flow in the windings of the armature. Aircraft use the mechanical power of the engine to move (rotate) the generator's armature. ![[Pasted image 20210808200636.png|350]] #### Alternators An alternator consists of a rotating magnetic field which causes current to be induced into a stationary conductor called a [[stator]]. Aircraft use the mechanical power of the engine to move (rotate) the alternator's field. ![[Pasted image 20210808200709.png|350]] You will see that the magnetic field in a practical generator is not created by natural magnets, but by electromagnets, and thus can be referred to as electromagnetic field. The generator uses its own output to supply current through the "Field Windings" - this allows changing of the output voltage. #### M1 Alternators ![[Pasted image 20210808200721.png|350]] Smaller, M1 aircraft use alternators similar to automotive belt driven units. #### M2 Alternators: ![[Pasted image 20210808200736.png|350]] Large, M2 aircraft use high output alternators, some capable of power outputs in the area of 100 KVA. You will learn much more about motors, generators, alternators and more as you progress in the program. ## In the Lab This week you will become familiar with AC waveforms, using the oscilloscope to measure. Download the user's manual for our oscilloscope [[9018-19215.pdf|here]] Familiarize yourself with the controls and connectors. This information comes with more detail in the user's manual on page 22: ![[Pasted image 20211108111553.png|350]] Familiarize yourself with the display, details available on page 29 of the manual: ![[Pasted image 20211108111916.png|350]] Pay attention to the settings on the waveform generator menu, available on page 77: ![[Pasted image 20211108111736.png|350]] We will be injecting waveforms into the oscilloscope, and then we will observe what the oscilloscope can show us about an ac waveform. Watch the following video in its entirety: [[Introduction to the Oscilloscope|🎞Introduction to the Oscilloscope]] This one goes into more detail, and our friend Mehdi is entertaining: [[What is an oscilloscope|🎞What is an oscilloscope?]] Let's remind ourselves of the basics of what we want to see in the lab: We will look at voltage measurements over time. So, we will examine: - Voltage - Frequency - Period # Lab ## Oscilloscope Familiarization Using the internal signal generator of the oscilloscope, generate a sine wave of 10V_pp with a frequency of 1KHz. ![[Pasted image 20211108120952.png|350]] Connect the output of the signal generator to input 1 with a double ended BNC connector: ![[Pasted image 20211108114303.png|350]] ![[Pasted image 20211108114200.png|350]] Press Autoscale key: ![[Pasted image 20211108114418.png|350]] Answer the questions on your worksheet. ## Adjusting the waveform ### Voltage (amplitude) Decrease the voltage of the waveform generator to 5V_pp. Anwer the questions on your worksheet ### Frequency Increase the frequency of the waveform generator to 2KHz. Answer the questions on your worksheet. Use your remaining time to explore the waveforms that the internal waveform generator can produce. Practice manipulating the waveform on the screen to get an accurate reading, and confirm at every step that the oscilloscope is showing correct measurements.