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All Textbook Solutions for Electrical Transformers and Rotating Machines

Is the north magnetic pole of the earth a north polarity or a south polarity?What were early natural magnets known as? The south pole of one magnet is brought close to the south pole of another magnet. Will the magnets repel or attract each other? How can the polarity of an electromagnet be determined if the direction of current flow is known? Define the following terms: Flux density Permeability Reluctance Saturation Coercive force Residual magnetism A force of 1 ounce is equal to how many dynes?What determines the polarity of magnetism when current flows through a conductor? What determines the strength of the magnetic field when current flows through a conductor? 3RQHow many lines of magnetic flux must be cut in 1 s to induce a voltage of 1 V?5RQInto how many time constants is an exponential curve divided? Each time constant of an exponential curve is equal to what percentage of the whole?An inductor has an inductance of 0.025 H and a wire resistance of 3 Ω. How long will it take the current to reach its full Ohm’s law value? 9RQWhat electronic component is often used to prevent large voltage spikes from being produced when the current flow through an inductor is suddenly terminated? How many degrees are the current and voltage out of phase with each other in a pure resistive circuit?How many degrees are the current and voltage out of phase with each other in a pure inductive circuit? To what is inductive reactance proportional?Four inductors, each having an inductance of 0.6 H, are connected in series. What is the total inductance of the circuit?Three inductors are connected in parallel. Inductor 1 has an inductance of 0.06 H; inductor 2 has an inductance of 0.05 H; and inductor 3 has an inductance of 0.1 H. What is the total inductance of this circuit?6RQAn inductor is connected to a 240-V, 1000-Hz line. The circuit current is 0.6 A. What is the inductance of the inductor?8RQ9RQAn inductor has an inductive reactance of 250 Ω when connected to a 60-Hz line. What will be the inductive reactance if the inductor is connected to a 400-Hz line? Fill in all the missing values. Refer to the following formulas. XL = 2fL L=XL2f f=XL2LWhat is a transformer?What are common efficiencies for transformers? What is an isolation transformer?All values of a transformer are proportional to its _________________.A transformer has a primary voltage of 480 volts and a secondary voltage of 20 volts. What is the turns ratio of the transformer? If the secondary of the transformer in question 5 supplies a current of 9.6 amperes to a load, what is the primary current? (Disregard excitation current.)Explain the difference between a step-up and a step-down transformer.A transformer has a primary voltage of 240 volts and a secondary voltage of 48 volts. What is the turns ratio of this transformer?A transformer has an output of 750 VA. The primary voltage is 120 V. What is the primary current?A transformer has a turns ratio of 1:6. The primary current is 18 amperes. What is the secondary current? 11RQ12RQ13RQThe primary leads of a transformer are labeled 1 and 2. The secondary leads are labeled 3 and 4. If polarity dots are placed beside leads 1 and 4, which secondary lead would be connected to terminal 2 to make the connection additive?The local utility company must provide service to a small business. The business is a wood working shop. What is the minimum standard size transformer that can be used to provide the following loads? The service is 240 volts single phase. 2Table saws rated at 12 amperes each. 2Drill presses rated at 4.5 amperes each. 2Drill presses rated at 6.8 amperes each. 1Air compressor rated at 24 amperes. 2Commercial routing machines rated at 10.6 amperes each. General lighting load rated at 4 kW. Air conditioning load rated at 12 kW. Gas heat with a blower motor rated at 10.2 amperes. (Note: The smaller of the heating or air conditioning load may be omitted because both do not operate at the same time.)Refer to Figure 4-79 to answer the following questions. Find all the missing values. Figure 4-79 practice problems 1 through 6. Refer to Figure 4-79 to answer the following questions. Find all the missing values. Figure 4-79 practice problems 1 through 6. Refer to Figure 4-79 to answer the following questions. Find all the missing values. Figure 4-79 practice problems 1 through 6.Refer to Figure 4-79 to answer the following questions. Find all the missing values. Figure 4-79 practice problems 1 through 6. Refer to Figure 4-79 to answer the following questions. Find all the missing values. Figure 4-79 practice problems 1 through 6.Refer to Figure 4-79 to answer the following questions. Find all the missing values. Figure 4-79 practice problems 1 through 6.7P8PAn AC power source is connected across 325 turns of an autotransformer, and the load is connected across 260 turns. What is the turns ratio of this transformer?Is the transformer in question 1 a step-up or step-down transformer? 3RQA load impedance of 52 Ω is connected to the secondary winding of the transformer in question 3. How much current will flow in the secondary? How much current will flow in the primary of the transformer in question 4? 6RQA 480-volt three-phase load is rated at 34.5 kVA. It is to be supplied with variable three-phase voltage. Because the load is continuous, the transformers ratings are to be increased by 1.25% of the rated load. Three variable autotransformers are used to supply this load. Refer to Table 4-2 and determine the standard size of each transformer that should be used to make this connection. If the load in question 7 was supplied by two autotransformers connected in open delta, what would be the minimum standard size transformers that could be used to supply this load?1P2PRefer to the transformer shown in Figure 5-13 to answer the following questions. What is the turns ratio of the winding between points B and E as compared to the winding between points D and E? Figure 5-13 Autotransformer practice problems. 4PRefer to the transformer shown in Figure 5-13 to answer the following questions. The transformer shown in Figure 5-13 is supplying 325 volt amperes to a load. The primary voltage is 240 volts. What is the primary current? Figure 5-13 Autotransformer practice problems. 1RQWhat is the standard current rating for the secondary winding of a CT?Why should the secondary winding of a CT never be disconnected from its load when there is current flow in the primary?A current transformer has a ratio of 600:5. If three loops of wire are wound through the transformer core, how much primary current is required to produce 5 amperes of current in the secondary winding?Assume that a primary current of 75 amperes flows through the windings of the transformer in question 4. How much current will flow in the secondary winding? What type of core is generally used in the construction of a CT? A current transformer has 4 turns of wire in its primary winding. How many turns of wire are needed in the secondary winding to produce a current of 2 amperes when a current of 60 amperes flows in the primary winding?8RQA CT has a current flow of 80 amperes in its primary winding and a current of 2 amperes in its secondary winding. What is the ratio of the CT?What is the most common use for a CT? 1RQ2RQA wye-connected load has a voltage of 480 V applied to it. What is the voltage dropped across each phase? A wye-connected load has a phase current of 25 A. How much current is flowing through the lines supplying the load?5RQA delta connection has 30 A of current flowing through each phase winding. How much current is flowing through each of the lines supplying power to the load?A three-phase load has a phase voltage of 240 V and a phase current of 18 A. What is the apparent power of this load? If the load in question 7 is connected in a wye, what would be the line voltage and line current supplying the load? An alternator with a line voltage of 2400 V supplies a delta-connected load. The line current supplied to the load is 40 A. Assuming the load is a balanced three-phase load, what is the impedance of each phase? 10RQ1P2P3P4PHow many transformers are needed to make an open-delta connection?Two transformers rated at 100 kVA each are connected in an open-delta connection. What is the total output power that can be supplied by this bank? 3RQAssume that a line voltage of 2400 volts is connected to the primary of the three-phase transformer and the line voltage of the secondary is 240 volts. What is the turns ratio of the transformer? 5RQHow much current is flowing through the secondary winding?How much current is flowing through the primary winding? 8RQ9RQHow much current is flowing in the secondary winding?How much current is flowing in the primary winding? What is the line current of the primary?1P2P3PRefer to the transformer connection shown in Figure 8-31 to fill in the following values. Figure 8-31 Practice problems circuit 2How does the National Electrical Code specify that the high leg of a four-wire delta connection be marked?An open-delta three-phase transformer system has one transformer center-tapped to provide a neutral for single-phase voltages. If the voltage from line to center tap is 277 volts, what is the high-leg voltage?If a single-phase load is connected across the two line conductors and neutral of the transformer in question #2, and one line has a current of 80 amps and the other line has a current of 68 amps, how much current is flowing in the neutral conductor?4RQA three-phase transformer connection has a delta-connected primary and a wye-connected secondary. The center tap of the wye is used as a neutral conductor. If the line-to-line voltage is 480 volts, what is the voltage between any one phase conductor and the neutral conductor?A three-phase transformer bank has the secondary connected in a wye configuration. The center tap is used as a neutral conductor. If the voltage across any phase conductor and neutral is 120 volts, how much voltage would be applied to a three-phase load connected to the secondary of this transformer bank? A three-phase transformer bank has the primary and secondary windings connected in a wye configuration. The secondary center tap is being used as a neutral to supply single-phase loads. Will connecting the center-tap connection of the secondary to the center-tap connection of the primary permit the secondary voltage to stay in balance when a single-phase load is added to the secondary?8RQ1RQWhat size overcurrent device should be used to protect the primary of a 480/120 volt 15 kVA transformer?3RQThe secondary of the transformer in question #3 has a rated voltage of 2400 volts. What size fuse should be used to provide branch circuit protection?A 10 kVA transformer has a primary voltage of 208 volts and a secondary voltage of 120 volts. What size circuit breaker should be used to protect the primary winding? A 10 kVA transformer has a primary voltage rating of 240 volts and a secondary voltage rating of 480 volts. The transformer is connected to a circuit protected by a 100 ampere circuit breaker. The secondary is protected by a 25-ampere fuse. Does the primary require separate overcurrent protection?What size conductor should be used to connect the primary of the transformer described in question #6? Assume the conductor to be copper with type THW insulation. What are the three main sources of heat in a transformer? What is one advantage of shell-type construction in small control transformers? How are larger control transformers supplying a motor control center usually cooled? What is a way to increase cooling on a large dry-type transformer used in a unit substation?What is one special problem with dry-type transformers used in unit substations?What are two types of cooling medium used in transformers?Name two functions of transformer oil.What type of coolant/insulator is banned in the USA?Name two ways of increasing the cooling of oil-cooled transformers. What hazard is involved in water-cooled transformers?What is the primary consideration for personnel performing transformer maintenance?What are the basic steps of transformer maintenance?Why is it important to keep accurate records of inspections and tests?What environmental conditions affect the life of a transformer?What factors govern the type of maintenance procedures and the maintenance intervals for transformers?What tests must be made before entering a transformer tank? What devices and procedures should be used to clean a dry-type transformer?What is a special hazard encountered in large industrial transformers used in unit substations? List some of the protective relays found with large transformers. Before opening manholes on large transformers, what material should be available?Why is it important to inventory tools and equipment before and after working inside a transformer? What effect will sludge deposits have on the internal parts of a transformer? What voltages indicate good oil in the standard dielectric test of oil?What amount of water in transformer oil is considered acceptable for units operating at less than 288 kV?1RQWhich of the following are considered triplen harmonics3rd, 6th, 9th, 12th, 15th, or 18th?3RQ4RQ5RQ1RQWhat type of voltage is produced in all rotating armatures?What are the three types of armature windings?What type of armature winding would be used for a machine intended for high-voltage, low-current operation? What are interpoles, and what is their purpose?How are interpoles connected in relation to the armature?What type of field winding is made with many turns of small wire?How is the series field connected in relation to the armature?How is the shunt field connected in relation to the armature?What is armature reaction?To what is armature reaction proportional?What are eddy currents?What condition characterizes overcompounding?14RQ15RQExplain the difference between cumulative- and differential-compounded connections.What three factors determine the amount of output voltage for a DC generator?To what is the voltage regulation of a DC generator proportional?To what is countertorque proportional?Of what is countertorque a measure?1RQ2RQ3RQ4RQ5RQWhat is counter-EMF? What three factors determine the amount of counter-EMF produced in the armature?8RQ9RQ10RQ11RQ12RQ13RQ14RQ15RQ16RQWho was the first person to establish a measurement for horsepower? One horsepower is equal to how many watts? A motor is operating a load that requires a torque of 750 lb–in and is turning at a speed of 1,575 RPM. How much horsepower is the motor producing? 20RQWhat conditions must be met before two alternators can be paralleled together?How can the phase rotation of one alternator be changed in relationship to the other alternator? What is the function of the synchronizing lamps?What is a synchroscope?Assume that alternator A is supplying power to a load and that alternator B is to be paralleled to A. After the paralleling has been completed, what must be done to permit alternator B to share the load with alternator A?What two factors determine the output frequency of an alternator?At what speed must a six-pole alternator turn to produce 60 Hz? What three factors determine the output voltage of an alternator? What are slip rings used for on a rotating field type alternator?Is the rotor excitation current AC or DC?When a brushless exciter is used, what converts the alternating current produced in the armature winding into direct current before it is supplied to the field winding? 12RQ1RQ2RQWhat is synchronous speed?What two factors determine synchronous speed?Name three factors that cause the magnetic field to rotate.Name three factors that determine the torque produced by an induction motor.Is the synchronous motor an induction motor?What is the amortisseur winding?9RQ10RQ11RQWhy can an induction motor never operate at synchronous speed?13RQ14RQ15RQ16RQ17RQ18RQ19RQ20RQHow is the speed of a consequent pole squirrel-cage motor changed?A three-phase squirrel-cage motor is connected to a 60-Hz line. The full-load speed is 870 RPM. How many poles per phase does the stator have?1RQ2RQ3RQ4RQWhat is the advantage of the capacitor-start induction-run motor over the resistance-start induction-run motor?6RQ7RQ8RQ9RQ10RQ11RQ12RQWhat type of single-phase motor develops the highest starting torque?What is the principle of operation of a repulsion motor?What type of commutator is used with a brush-lifting repulsion-start induction-run motor?When a repulsion-start induction-run motor reaches about 75% of rated full-load speed, it stops operating as a repulsion motor and starts operating as a squirrel-cage motor. What must be done to cause the motor to begin operating as a squirrel-cage motor?17RQ18RQ19RQHow is the speed of a consequent pole motor changed? Why can a multispeed fan motor be operated at lower speed than most induction motors without harm to the motor windings? What is the speed of operation of the Warren motor? What is the speed of operation of the Holtz motor?24RQ25RQWhat does the term bifilar mean? Why do stepping motors have teeth machined in the stator poles and rotor?28RQ29RQ30RQ31RQWhat is the function of the compensating winding? 33RQ34RQExplain how to set the neutral plane position of the brushes by using the series field.Explain how to set the neutral plane position by using the compensating winding. Why is it necessary to keep lubrication records for motors?Why is it necessary to pick between the commutator bars of DC machines?What are the effects of improper brush tension in a DC machine?What instrument is used to test the insulation of motor windings? How do you check for an open armature winding? What instrument is generally used to test an armature for a shorted winding?7RQ8RQ9RQAssume that a nine-lead dual-voltage motor is being tested with an ohmmeter. The meter indicates resistance values as follows: T7–T8 = 6Ω, T7–T9 = 6Ω, and T8–T9 = 10Ω. What would these readings indicate? 1RQ2RQ3RQA motor nameplate indicates an insulation temperature rating of F. What is the temperature rating of this insulation? What is the most common NEMA design code found on squirrel- cagetype motors?A motor develops 225 ft-lb of torque at 860 RPM. What is the horsepower output of the motor?The nameplate of a motor lists the frame as 256T. What is the distance from the base of the motor to the center of the shaft?A motor develops 189 ft-lb of torque at 1745 RPM. What is the horse-power output of the motor? 9RQA motor nameplate lists the voltage as 208–230/460. The nameplate also lists the full-load amps as 34.2–31.6/15.8. If the motor is operated at 240 volts, what should be the full-load current? 1RQ2RQ3RQ4RQA 125-hp synchronous motor is connected to a 230-volt three-phase AC line. The motor is intended to operate at 80% power factor. What is the full-load running current of this motor? What are the minimum size NEMA and IEC starters that should be used to connect this motor to the line?What is the full-load running current of a three-phase, 50-hp motor connected to a 560-volt line? What are the minimum size NEMA and IEC starters that should be used to connect this motor to the line?7RQ8RQ9RQ10RQThe short-circuit protective device supplying the motors in question #10 is an inverse-time circuit breaker. What size circuit breaker should be used?12RQIf dual-element time-delay fuses are to be used as the short-circuit protective device, what size fuses should be used to protect the circuit in question #12? A 75-hp, three-phase squirrel-cage induction motor is connected to 480 volts. The motor has a NEMA code D. What is the starting current for this motor? A 20-hp, three-phase squirrel-cage induction motor has a NEMA code E. The motor is connected to 208 volts. What is the starting current for this motor?
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