ECE 3332a Preliminary Final Exam Theory Questions 2018 PDF

ECE 3332a – Final Exam Theory Questions Synchronous Machines Not including stray losses, list all the losses that account for the difference between the mechanical power applied to the rotor and the electrical power that is produced in a synchronous generator? Copper losses, core losses, friction l...

ECE 3332a – Final Exam Theory Questions Synchronous Machines Not including stray losses, list all the losses that account for the difference between the mechanical power applied to the rotor and the electrical power that is produced in a synchronous generator? Copper losses, core losses, friction losses, windage losses Can a 60 Hz synchronous generator be operated at 50 Hz? If so, how? If not, why? Yes, if the operating voltage is derated to 50/60, or 83.3 percent, of its original value. List the possible steps that can be taken as part of an approximate method for determining the synchronous reactance XS at a given field current. 1. Perform an open circuit test and get the internal generated voltage E A from the open circuit characteristic at that field current. 2. Perform a short-circuit test and get the short-circuit current IASc at that field current from the short-circuit characteristic. 3. Find Xs by dividing the EA from Step 1 by the IAsc in Step 2. How can an approximate winding resistance be determined from an actual synchronous generator? By applying a dc voltage to the windings while the machine is stationary and measuring the resulting current flow. What are the two ways by which the amount of voltage that is generated by a synchronous generator can be increased? The voltage can be increased either by (i) Increasing the speed of rotation of the rotor (ii) Increasing the field current In a synchronous generator, is the terminal voltage V φ greater than, less than, or equal to the internal generated voltage E a? Justify your answer. See phasor diagram below. Why does the synchronous generator terminal voltage vs field current curve below eventually flatten as field current is increased? It does so because the rotor becomes saturated so that any additional field current results in little increase of the magnetic field. Note: Know the power-flow diagram of a synchronous generator. List three ways to start a synchronous motor (note: only answer in italics is required) 1. Reduce the speed of the stator magnetic field to a low enough value that the rotor can accelerate and lock in with it. (This can be done during one half-cycle of the magnetic field 's rotation. by reducing the frequency of the applied electric power.) 2. Use an external prime mover to accelerate the synchronous motor up to synchronous speed, (Bring the machine on the line as a generator. Then, turning off or disconnecting the prime mover will make the synchronous machine a motor.) 3. Use damper windings windings to start the machine as an induction motor. (When the machine reaches synchronous speed, there will be no current flowing in the damper windings.) If the load of a synchronous motor is kept constant and the field current IF is changed, what happens to the operation of the motor. Its power factor changes and its output power is constant. What is a synchronous capacitor (or synchronous condenser) and what is it used for? It is a synchronous motor operated with no load. It is used for power factor correction. Draw a simple phasor diagram of EA, Vφ and IA for the case when a synchronous machine is acting as a generator/motor and supplies/consumes reactive power. Draw the V-curve for a synchronous capacitor (condenser) and label the lagging PF and the leaing PF regions Note: (i) The curve must touch IA = 0 at some point. (ii) The saturation region does not have to be labelled. Induction Machines What are the two main types of inductor motors? 1. Cage rotor 2. Wound rotor How much current flows through the rotor of an inductor motor when it is operating with zero slip (in other words, at synchronous speed) - high, low, or none? Briefly explain why? No current flows through the rotor of an induction motor when it is operating with zero slip / at synchronous speed because no lines of flux are being cut in the rotor as the speed of the field and the rotor is the same. What is the frequency of the rotor current in an induction machine relative to that of the stator frequency when the slip is zero? Zero (because there is no current in the rotor when the slip is zero) Note: Part marks may be given if “lower” is put down as an answer, but only a few marks. What is the frequency of the rotor current in an induction machine relative to that of the stator frequency? fr = sfe where fr = rotor current frequency, fe = stator voltage/current frequency and s = slip. What is the pullout torque in an induction motor? It is the maximum allowable torque of the machine. An induction motor is connected to another induction machine. The motor is made to rotate a bit faster than the synchronous speed by the other machine. What happens? The motor becomes a generator. How can the stator winding resistance of an induction machine be determined from an actual machine? It can be determined by performing a DC test. How can the equivalent rotor resistance R2 in the induction motor model be determined from an actual machine? This can be done by performing a DC test to find the stator winding resistance R1 and a locker rotor test to determine resistance R LR. R2 is then equal to RLR – R1. How can the equivalent magnetizing inductance XM in the induction motor model be determined from an actual machine? This can be done by running the motor at close to synchronous speed to minimize rotor current then determining X = X1 + XM (where X1 is the stator winding leakage inductance). X1 can be determined from a locked rotor test so that XM = X – X1. Not including stray losses, list all the losses that account for the difference between the electrical power applied to the rotor and the mechanical power that is produced in an induction machine? Stator copper losses, core losses, rotor copper losses, friction losses, windage losses Note: Know the power-flow diagram of an induction motor. DC Machines List two ways to reduce armature reaction in DC machines with brushes. 1. Using interpoles (between the main poles) 2. Using compensating windings (in slots carved in the faces of the poles) The induced voltage in any given machine depends on what three factors? 1. The flux φ in the machine 2. The speed ω of the machine's rotor 3. A constant depending on the construction of the machine When should DC machines with lap armature windings be used? They should be used in applications where low voltage and high current machines are needed. When should DC machines with wave armature windings be used? They should be used in applications where high voltage and low current machines are needed. What is armature reaction? Armature reaction is the magnetic field caused by current flowing in the armature (or rotor) Does armature reaction result in a stronger field or a weaker field in a DC machine? Armature reactions results in a weaker field. There are five major types of dc generators. List three of them. 1. The separately excited dc generator 2. The shunt dc generator 3. The series dc generator 4. The cumulatively compounded dc generator 5. The differentially compounded dc generator List the two possible ways to control the voltage of a separately excited DC generator. 1. Change the speed of rotation. 2. Change the field current. What is the effect of armature reaction on the output terminal voltage V T of a separately excited DC generator (increases voltage, decreases voltage, no effect on voltage)? Briefly explain why. Armature reaction causes the output voltage to decrease because it results in less field and thus less induced voltage EA. How does a shunt DC generator get the initial field flux to start when it is first turned on? It gets its initial field flux from the residual flux in the poles of the generator. If a shunt DC generator is started and no voltage builds up then what could be wrong? List three possible causes for the voltage to fail to build up during starting. 1. There may be no residual magnetic flux in the generator to start the process going 2. The direction of rotation of the generator may have been reversed. (Another answer: The connections of the field may have been reversed). 3. The field resistance may be adjusted to a value greater than the critical resistance. When does “flashing the field” need to be done? What would happen if it needed to be done but wasn’t done? “Flashing the field” needs to be done when there is no residual flux in the poles of a shunt DC generator. If there is no residual flux in the poles, then the shunt DC motor will not be able to generate any voltage. When does “flashing the field” need to be done? How is it done and why does it work? “Flashing the field” needs to be done when there is no residual flux in the poles of a shunt DC generator. To flash the field, disconnect the field from the armature circuit and connect it directly to an external dc source. The current flow from this external dc source will leave a residual flux in the poles, which will then allow normal starting. List the two possible ways to control the voltage of a shunt DC generator. 1. Change the speed of rotation. 2. Change the field resistance (which changes the field current). Is the terminal characteristic of a series DC generator better or worse than that of a shunt DC generator? Briefly explain why. It is worse because the machine saturates more easily than a shunt generator so that E A is less and thus VT is less at heavier loads. Sketch the terminal characteristic of a shunt DC generator and a cumulatively compounded generator that is undercompounded/flat compounded/ overcompounded. See the graph below. Note that for full marks, you must show that:  - An undercompounded cumulatively compounded generator has VTfull-load < VTno-load. - A flat compounded cumulatively compounded generator has VTfull-load = VTno-load. - An overcompounded cumulatively compounded generator has VTfull-load > VTno-load. depending on what you are asked. Regardless of how the machine is compounded, VTfull-load is greater than the VTfull-load of the shunt DC generator. Draw the equivalent circuit of a cumulatively compounded DC generator with a long shunt connection. Does a differentially compounded DC generator have a better, worse, or the same terminal characteristic as a cumulatively compounded DC generator? It has a worse characteristic because the field is severely weakened as the load is increased so that less EA and thus less VT is generated. What is the main drawback of a differentially compounded DC generator? It has a poor terminal characteristic. What would happen if the field is removed in a shunt DC motor after it has been started up? The machine would speed up and try to reach infinite speed. The machine would eventually be damaged. What would happen if the load torque is removed in a shunt DC motor after it has been started up? The machine would operate at no-load speed. Does the presence of armature reaction have any effect on the operation of a DC shunt motor? If so, briefly explain what the effect is and why. If not, briefly explain why not. Armature reaction makes the motor operate at higher speeds than it would without it. This is because it results in flux weakening and DC shunt motors operate at higher speeds as the flux is reduced. State the two most common methods by which the speed of DC motor can be controlled? I. Adjusting the field resistance (and thus the field flux) 2. Adjusting the terminal voltage applied to the armature. When should field resistance control be used to control the speed of a DC motor? Briefly explain why. Field resistance control should be used when the machine is operated above rated (or base) speed. If field resistance control is used below rated speed, then the current in the field winding might exceed the winding’s rating. When should armature voltage control be used to control the speed of a DC motor? Briefly explain why. Armature voltage control should be used when the machine is operated below rated (or base) speed. If armature voltage control is used above rated speed, then the voltage might exceed the voltage rating of the armature circuit. For what applications are series DC motors used? They are used in applications requiring very high torques. What would happen if the load of a series DC motor is removed? (a) The motor would try to speed up to infinite speed and be damaged. or (b) The motor would speed up to its no load speed and would operate at that speed. (a) The motor would try to speed up to infinite speed and be damaged. Sketch a graph showing the power (or torque) limits of a separately excited/shunt DC motor vs the speed. On the graph, show the regions where armature voltage and field resistance control are used and indicate what the maximum torque τmax and the maximum power Pmax are? What is compounded DC motor? A compounded DC motor is a motor with both a shunt and a series field. What are the two main type of compounded DC motors? 1. Cumulatively compounded DC motor. 2. Differentially compounded DC motor. What is the best feature of a cumulatively compounded DC motor? It combines the best features of both the shunt and the series motors. Like a series motor, it has extra torque for starting; like a shunt motor, it does not overspeed at no load. What is the main drawback of a differentially compounded DC motor? The machine speeds up and becomes unstable as its load is increased. Can a compounded DC machine be used as part of a motor/generator set in which power is expected to flow in both directions? Briefly explain. Although the machine can be used as a cumulatively compounded DC motor or generator, it cannot be used as a differentially compounded DC machine. If the machine operates as cumulatively compounded DC motor or generator and the polarity of the shunt and series field windings are the same, then the two field winding polarities will be different when the direction of power is changed and the machine will try to operate as a differentially compounded DC machine. Permanent magnet DC motors are smaller, cheaper and more efficient than conventional DC motors. List at least two disadvantages that they have. They cannot produce as strong a field as conventional DC motors and thus cannot operate with as high a torque. Their poles can become demagnetized over time, thus rendering them useless. Compensating windings cannot be added to reduce armature reaction. What kind of material is best suited for the poles of a permanent magnet DC motor? Ferromagnetic material with high residual flux. Answer this multiple choice question: A brushless DC motor is (a) a conventional DC motor without brushes and with no need for a power electronics interface. (b) a synchronous motor with a power electronic interface. (c) an induction motor with a power electronic interface. (d) a conventional DC motor without brushes and with a power electronic interface. (b) a synchronous motor with a power electronic interface. List two problems associated with the brushes of a conventional DC motor. mechanical wear sparking What is typically the main difference between the physical construction of a brushless DC motor and that of a stepper motor? The stepper motor typically has more poles. What is meant by the term "four-quadrant operation"? An electric machine's ability to motor and brake in two opposite directions. Draw a graph of speed vs torque labeling the four regions of four-quadrant operation of an electric machine. (Note: Positive speed means forward direction, negative speed means reverse direction, positive torque means positive force, and negative torque means negative force. If the speed and the torque are in the same direction, the machine is motoring. If speed and torque are in the opposite direction, the machine is braking.)