EET272 Worksheet Week 9

Transcription

1 EET272 Worksheet Week 9 answer questions 1-5 in preparation for discussion for the quiz on Monday. Finish the rest of the questions for discussion in class on Wednesday. Question 1 Questions AC s are becoming much more popular as an industrial. One thing that has led to this increase in popularity is the availability of AC drives. Recall that speed control of an AC requires changing the frequency that is input to the. This can be done with normal control devices but it is complicated, expensive, and not very accurate. Digital AC drives are designed to provide speed control for AC s. They are also able to provide complex acceleration, deceleration, and stopping functions. AC drives can be used to drive a 3-phase with only single phase power. To become more familiar with AC drives, what they do and how they do it, read and outline chapter 9 AC Drives on pages of the textbook. AC drives are often referred to by the acronym: VFD. This stands for Variable Frequency Drive. This terms comes from a basic description of the function the AC drive performs. file q0063 Question 2 The AC drive is composed of 4 main blocks. This figure shows how these blocks are connected. Converter Intermediate Circuit Inverter Vin V1 V2 T1 T2 T3 Control Circuit In your own words describe how each block works. Suggestions for Socratic discussion Which block contains the user interface The AC drive converts AC to DC and back to AC. Is this efficient file q0064 1

2 Question 3 A single-phase bridge rectifier circuit is made of four diodes, arranged like this: Single-phase AC power source DC Trace the directions of current through all four diodes, and determine the polarity of DC voltage across the terminals. A three-phase bridge rectifier circuit is made of six diodes, arranged like this: Three-phase AC power source DC Once again, trace the directions of current through all six diodes, and determine the polarity of DC voltage across the terminals. Kuphaldt file i

3 Question 4 Determine the directions of electric current where you see question marks in the following schematic diagram for a variable-speed AC drive, at the moment in time (t 1 ) specific on the oscillograph: Three-phase AC power source (60 Hz) L1 L2 L3 T1 T2 T3 AC Hz L1 L2 L3 Input current 0 T1 T2 T3 Output current 0 t 1 Use conventional flow notation to show current direction (a positive current flowing from power source to, and a negative current flowing from to power source). If there is no current going through a labeled wire or component, just write NO instead of drawing an arrow on the diagram. Also, estimate the speed of the electric based on the waveforms shown in the oscillographs. Kuphaldt file i

4 Question 5 PWM modulation is used typically used to convert the DC signal back to an AC signal. In microcontrollers you have used PWM to control the speed of. In microcontrollers you created a PWM signal by varying the duty cycle of enable to the. For an inverter another form of PWM is used. In this case, a very high speed clock is used. The controller then decides whether to enable the switch for that cycle or not. To create a 60Hz output signal, the inverter is often clocked at 1-10KHz. A simple circuit that could be used as a single phase inverter is shown: + A Vdc T1 (AC out) B Motor Coil - Explain how A and B are controlled to create an AC output wave. Suggestions for Socratic discussion Why does this create a sine wave and not just a string of pulses on T1 What is the purpose of the reverse bias diodes file q0064 4

5 Question 6 This variable-frequency drive (VFD) circuit converts three-phase AC power at 60 Hz into rectified and filtered DC, then switches that DC into three-phase AC of whatever frequency desired. The control circuitry for triggering the MOSFETs is not shown in this diagram, for the sake of simplicity: Three-phase AC power source (60 Hz) L1 L2 L3 Q 1 Q 3 Q 5 T1 T2 T3 AC Q 2 Q 4 Q 6 Your task is to determine the states (ON or OFF) of those six transistors during each of the time periods shown in the oscillograph: T1 T2 T3 Output current 0 t 1 t 2 t 3 t 4 t 5 t 6 t 7 Time period Q 1 Q 2 Q 3 Q 4 Q 5 Q 6 t 1 t 2 t 2 t 3 t 3 t 4 t 4 t 5 t 5 t 6 t 6 t 7 Assume a positive current on the graph is one where the drive sources current to the, and a negative current on the graph is one where the drive sinks current from the. Suggestions for Socratic discussion What would be different, if anything, about the switching of these six power transistors to make the spin faster What would be different, if anything, about the switching of these six power transistors to make the spin in reverse rather than forward Kuphaldt file i

6 Question 7 Most AC drives modulate voltage to the in proportion to frequency. This is called the volts-per-hertz ratio (V/Hz), and it is often configured as a constant value for the full operating range of the. In order to understand why voltage must be modulated in proportion to frequency as we vary its speed, it is helpful to model the electric as a simple inductor (powered by a variable-voltage, variable-frequency AC power source): Variable V Variable f L What would happen to the inductor if we decreased the AC frequency while holding AC voltage constant Kuphaldt file i01441 Question 8 Suppose a large grinding machine used in a production machine shop is powered by an induction, which in turn receives its electrical power from a VFD. The time for this machine to coast to a stop after running at full speed is quite long, owing to the mass of the spinning griding wheel. This coast time has a negative effect on production, because the operators must wait until the wheel finally stops before they can take the freshly-ground parts off the machine and replace them with new parts to be ground. Your supervisor would like to shorten this stopping time by using the dynamic braking feature of the VFD, which up to this point in time had never been configured for use. Explain where the stored (kinetic) energy of the spinning grinding wheel goes when the VFD dynamically brakes it to a quick stop. Suggestions for Socratic discussion What are some alternative braking techniques to dynamic braking In each of these techniques, where does the grinding wheel s kinetic energy go during the braking process Kuphaldt file i

7 Question 9 The following variable-speed drive receives a variable DC voltage from a potentiometer as a speed-command signal from a human operator. In this case, the potentiometer s full range commands the to spin from 0 RPM to 1800 RPM (the wiper here is drawn in a position nearer 100% speed: 3 phase line power Motor L1 L2 L3 VFD T1 T2 T3 10 kω One day the operations manager approaches you to request you modify this speed-command system so that the operators cannot call for a speed less than 100 RPM or greater than 1670 RPM. You consult the manual for the drive, and are surprised to find it lacks this sort of capability: a resistance input of 0 to 10 kω will only translate to a speed range of 0 to 1800 RPM. This means you must figure out a way to set the adjustable speed range limits externally to the drive (i.e. by limiting the range of the potentiometer s resistance adjustment). You know you cannot mechanically limit the turning of the potentiometer knob, but you can connect fixed-value resistors to the potentiometer to electrically limit its range, so that full clockwise will only command the drive to go as high as 1670 RPM, and full-counterclockwise will only command the drive to go as low as 100 RPM. Modify this diagram to include any necessary fixed-value resistors, and also calculate their necessary values. Kuphaldt file i

8 Question 10 A common accessory device for a variable-frequency drive (VFD) is a line reactor, which is nothing more than a large inductor connected in series with each of the drive s power line conductors. The purpose of a line reactor is to act as a low-pass filter, allowing 60 Hz power to the VFD but blocking harmonic frequencies generated by the VFD from corrupting the AC power supply system. 3-phase 480 VAC Line reactor L1 L2 L3 VFD T1 T2 T3 AC T1 T2 T3 Suppose each winding of a line reactor for a 10 horsepower VFD has Ω of resistance and 1.5 mh of inductance. Calculate the amount of impedance offered by each winding to the following harmonics: Frequency (f) 60 Hz (1st harmonic) 180 Hz (3rd harmonic) 300 Hz (5th harmonic) 420 Hz (7th harmonic) 540 Hz (9th harmonic) Impedance (Z) Hint: you may consider each reactor coil to be a series-connected inductor and resistor, together producing a certain amount of impedance for each frequency. Kuphaldt file i

9 Question 11 An instrument technician finds a great deal on a lathe for his home machine shop. The only problem is, this lathe has a three-phase and his shop only has single-phase electric power. This technician knows, however, that you can wire most VFDs to input single-phase AC power and output three-phase AC power, so he buys a used VFD and wires it up in this manner to power his lathe. Not only does this fix allow him to run his lathe on single-phase power, but it also gives him the ability to vary the lathe s speed, and also to suddenly stop it if needed. Now, this particular VFD is an inexpensive model, and it has no braking resistor connected to it. Based on this information, identify the likely technique this VFD uses to brake the lathe, and identify where the lathe s kinetic energy will be dissipated. Suggestions for Socratic discussion What are some alternative braking techniques to dynamic braking In each of these techniques, where does the grinding wheel s kinetic energy go during the braking process Kuphaldt file i

10 Question 12 An electrician wires a reversing control circuit as follows: 480 VAC power Fwd Off Rvs 120 VAC control power Motor F R Reset Reset Awiserelectricianwarnstheonewhowireditthatitiswrongtohavemultipleoverloadheaterassemblies in a reversing control circuit. For one, he says, there should be only one overload heater block. The first electrician ignores the second one s advice, and puts this reversing control system in service. Several months later, the fails from overheating, despite the overload heater elements being properly sized for this. Explain how it is possible for the to overheat in this system. Suggestions for Socratic discussion How would you have chosen to communicate the flaw to the first electrician so that your advice would be better heeded What is the proper way to wire a single thermal overload heater in a reversing control circuit so that the gets the protection it needs What sort of operating scenario might stress this particular (mis-wired) more than others, given the improper overload heater installation Kuphaldt file i

11 Question 13 An AC electric power system has a bank of capacitors connected to correct for low power factor. One day a new VFD is installed to provide variable-speed control for an existing AC. The VFD has its own line reactors connected on the input side to help filter harmonics from the rest of the AC power system. The problem is, the line reactors and the power factor correction capacitors now form a resonant circuit that may produce high currents and/or voltages at a certain frequency: 3-phase 480 VAC 60 Hz Fuses Fuses Line reactor (0.46 mh each) Power factor correction capacitors (1700 uf each) L1 L2 L3 VFD T1 T2 T3 AC T1 T2 T3 Calculate the resonant frequency of the circuit formed by the reactor coils and power factor correction capacitors, then determine whether or not resonance will be a problem in this system. Explain why or why not, showing all your mathematical work. Note: for the sake of simplicity, you may model each resonant circuit as simple pairs of one reactor coil and one capacitor in series with each other. Kuphaldt file i

12 Answer 1 Answers Answer 2 The converter and intermediate circuit are advanced versions of the rectifier circuits you built in 221. The inverter uses transistors as switches to convert the DC back to AC. The controller controls the frequency of the switching and is programmable by the user. Answer 3 Single-phase AC power source DC Arrows drawn in the direction of conventional flow notation Three-phase AC power source DC Arrows drawn in the direction of conventional flow notation 12

13 Answer 4 Three-phase AC power source (60 Hz) L1 L2 L3 no no no no no T1 T2 T3 no AC Hz Speed 860 RPM Answer 5 Every time A turns on, current is driven into the inductor. The more often A turns on the more current that goes into the inductor. This results in a larger voltage. If A is only turned on every so often then the voltage at T1 is smaller. Switch A controls the positive half of the cycle and switch B controls the negative half. If A switches at the output rate, the output waveform would look more like a square wave. Either full voltage or no voltage. By pulsing A on and off at a very fast rate you get an averaging affect. This means that the average voltage across the coil is a function of how long or often A is turned on. For the diodes, remember that inductors don t like to have their current changed. When A turns off, it removes the current from the inductor. But the inductor is storing energy and tries to keep the current flowing. It does this by raising the voltage of T1. The diode provides a way of reducing this voltage spike. It protects the coil and transistor from over voltage. Answer 6 Time period Q 1 Q 2 Q 3 Q 4 Q 5 Q 6 t 1 t 2 pulse off off pulse pulse off t 2 t 3 off pulse off pulse pulse off t 3 t 4 off pulse pulse off pulse off t 4 t 5 off pulse pulse off off pulse t 5 t 6 pulse off pulse off off pulse t 6 t 7 pulse off off pulse off pulse Answer 7 The inductor would most likely overheat and burn up. As frequency decreases, so does inductive reactance (X L ). As reactance decreases, there is less opposition in the inductor to AC current, so current increases proportionately. This increased current will overheat the inductor (as well as saturate its magnetic core!). Thus, we limit inductor current to its normal value through the inductor by reducing voltage as we reduce frequency. Answer 8 13

14 Answer 9 The VFD is designed for a 0-10V input to determine the speed. You must adjust the pot circuit such that minimum pot is not 0V and maximum pot is not 10V. Answer 10 Frequency (f) Impedance (Z) 60 Hz (1st harmonic) Ω 180 Hz (3rd harmonic) Ω 300 Hz (5th harmonic) Ω 420 Hz (7th harmonic) Ω 540 Hz (9th harmonic) Ω Answer 11 This VFD most likely uses DC injection braking to slow down the lathe. Another possibility is plugging. Answer 12 Answer 13 These components will indeed resonate, at 180 Hz. This is a problem because 180 Hz is the 3rd harmonic of a 60 Hz AC power system, and we expect significant odd-harmonic frequencies to come from an operating VFD! 14