Bartleby Sitemap - Textbook Solutions
All Textbook Solutions for Delmar's Standard Textbook Of Electricity
A squirrel-cage induction motor is operating at 1175 RPM and producing a torque of 22 Ib-ft. What is the horsepower output of the motor?14RQ15RQ16RQName three factors that determine the amount of voltage induced in the rotor of a wound-rotor motor.Why will the rotor of a wound-rotor motor not turn if the rotor circuit is left open with no resistance connected to it?Why is the starting torque per A of starting current of a wound-rotor motor higher than that of a squirrel-cage motor although the starting current is less?When is a synchronous motor a synchronous condenser?What determines when a synchronous motor is at normal excitation?How can a synchronous motor be made to have a leading power factor?Is the excitation current of a synchronous motor AC or DC?24RQ25RQYou are working as a plant electrician. It is your job to install a 300-hp three-phase squirrel-cage induction motor. The supply voltage is 480 V. The power company has determined that the maximum amount of starting current that can be permitted by any motor in the plant is 3000 A. The motor nameplate is as follows: Phase: 3 FLA: 352 Volts: 480 RPM: 1755 Frame: XXX Code: L Will it be possible to start this motor across the line, or will it be necessary to use a reduced-voltage starter to reduce starting current?You have been given the task of connecting a nine-lead three-phase dual-voltage motor to a 240-V line. You discover that the nameplate has been painted and you cannot see the connection diagram. To make the proper connections, you must know if the motor stator winding is connected wye or delta. How could you determine this using an ohmmeter? Explain your answer.You are an electrician working in an industrial plant. A 30-hp three-phase squirrel- cage motor keeps tripping out on overload. The motor is connected to a 240-volt line. Voltage measurements indicate the following voltages between the different phases: A-B = 276 volts, B-C = 221 volts, and A-C = 267 volts. Determine the percentage of heat rise in the winding with the highest current.What are the three basic types of split-phase motors?The voltages of a two-phase system are how many degrees out of phase with each other?How are the start and run windings of a split-phase motor connected in relation to each other?In order to produce maximum starting torque in a split-phase motor, how many degrees out of phase should the start- and run-winding currents be with each other?What is the advantage of the capacitor-start induction-run motor over the resistance-start induction-run motor?On average, how many degrees out of phase with each other are the start- and run-winding currents in a resistance-start induction-run motor?What device is used to disconnect the start windings for the circuit in most nonhermetically sealed capacitor-start induction-run motors?Why does a split-phase motor continue to operate after the start windings have been disconnected from the circuit?How can the direction of rotation of a split-phase motor be reversed?If a dual-voltage split-phase motor is to be operated on high voltage, how are the run windings connected in relation to each other?When determining the direction of rotation for a split-phase motor, should you face the motor from the front or from the rear?What type of split-phase motor does not generally contain a centrifugal switch?What type of single-phase motor develops the highest starting torque?14RQ15RQWhen 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?17RQWhat causes the magnetic field to rotate in a shaded-pole induction motor?How can the direction of rotation of a shaded-pole induction motor be changed?20RQ21RQ22RQ23RQ24RQWhat is the principle of operation of a stepping motor?What does the term bifilar mean?27RQWhen a stepping motor is connected to AC power, how many phases must be applied to the motor?What is the synchronous speed of an eight-pole stepping motor when connected to a twophase, 60-Hz AC line?How can the holding torque of a stepping motor be increased?Why is the AC series motor often referred to as a universal motor?32RQHow is the direction of rotation of the universal motor reversed?34RQ35RQ36RQ1PAWhich NEC table is used to determine the full -load running current for single-phase motors?A 5-HP single-phase motor is connected to 208 volts. What is the full-load running current for this motor?A 10-HP single-phase motor is connected to 240 volts. The motor does not contain a NEMA code letter. What size conductors should be used to connect this motor? The conductors are to be copper, with THHN insulation.A 75-HP synchronous motor is connected to a 480-volt three-phase feeder. The motor has a NEMA Code letter C. The motor is to operate at 80% power factor. What size copper conductors would be used to connect this motor if they were insulated with THHN insulation?A squirrel-cage induction motor has a nameplate full-load current of 78 amperes and a marked temperature rise of 30C. The motor requires a NEMA size 4 starter. Using the overload heater chart in Figure 34-8, what size overload heaters should be used to protect this motor?A squirrel-cage motor has a nameplate current rating of 264 amperes. Current transformers with a ratio of 350:5 are to be used to reduce the current to the overload heaters. The motor service factor is 1.10. Using the overload chart shown in Figure 34-7, determine the size overload heaters needed to protect this motor.A 100-HP three-phase squirrel-cage induction motor is connected to 480 volts. The conductors supplying the motor are copper with THW2 insulation. The nameplate lists a full-load running current of 118 amperes, NEMA Code B, and a service factor of 1.00. Current transformers with a ratio of 200:5 are used to reduce the current supplied to the overload heaters. The motor is protected with dual-element time delay fuses. Determine the conductor size, overload heater size using the chart in Figure 34-7, NEMA and IEC starter sizes, and fuse size for this motor.A 60-HP synchronous motor is connected to 240 volts. The motor is to be operated at 90% PF. The motor is a NEMA Code D. What size copper conductors with type THW2 insulation should be used to connect this motor?9RQBoth the motors and the feeder in Question 9 are protected by inverse time circuit breakers. What size breaker should be used to protect each motor, and what size circuit breaker should be used to protect the feeder circuit?A 30-HP squirrel-cage induction motor is connected to a 240-volt three-phase feeder. The motor is to be replaced with a 50-HP squirrel cage induction motor connected to 480 volts. The motor has a NEMA Code B, a full-load current of 49.8 amps, and an SF of 1.10. The present conductors are 3 AWG copper with type THWN insulation. Are these conductors sufficient to connect the 50-HP motor, or will they have to be replaced?Refer to the preceding Practical Application. When the 30-HP motor is replaced with the 50-HP, will the existing motor starter have to be replaced? If so, what size starter should be selected? A NEMA size 3 starter was used for the 30-HP motor.Refer to the first Practical Application in this set. When the 50-HP motor replaces the 30-HP motor, the overload heaters will have to be changed. Using the chart in Figure 34-8, what size overload heaters should be used to protect the 50-HP motor?A 10-HP single-phase motor is connected to 240 volts. The motor nameplate indicates a NEMA Code B, a full-load running current of 47.5 amperes, and a temperature rise of 40C. What size copper conductors with THWN insulation should be used to connect this motor?A 500-HP synchronous motor is connected to 2300 volts. The motor is to operate at 90% power factor. What running current would be used to compute the conductor size and fuse size?A 30-HP three-phase squirrel-cage induction motor is connected to 240 volts. The motor is to be protected by an inverse time circuit breaker. What size circuit breaker would be used to protect this motor?A 75-HP synchronous motor is connected to a 480-volt three-phase feeder. The conductors are to be copper with type THW2 insulation. The nameplate lists a running current of 74 amperes, a NEMA code letter B, and a service factor of 1.00. The motor is to operate at unity, or 100% power factor. The motor circuit is protected by dual-element time delay fuses. Determine the conductor size, overload heater size using the chart shown in Figure 34-8, NEMA and IEC starter sizes, and the fuse size for this motor installation.5PPA 350-HP squirrel-cage induction motor is connected to a 480-volts three-phase feeder. The nameplate lists the following: HP 350, Service factor 1.10, Full-load amps 408, NEMA Code B. The conductors are to be copper with THWN insulation, and 600:5 current transformers are used to reduce the current supplied to the overload heaters. The motor circuit is protected with dual-element time delay fuses. Determine the conductor size, overload heater size using the chart shown in Figure 34-7, the NEMA and IEC starter sizes, and the fuse size for this installation.Three squirrel-cage motors are connected to a 480-volt three-phase feeder. The nameplates list the following: Motor 1: HP 10, SF 1.25, Full-load amps 13.1, NEMA Code B Motor 2: HP 25, SF 1.10, Full-load amps 32.8, NEMA Code B Motor 3: HP 15, SF 1.00, Full-load amps 19.6, NEMA Code B The main feeder conductors and conductors to each motor are to be copper, with THWN insulation. The main feeder and each of the motors are protected by dual-element time delay fuses. Determine the conductor size for each motor and the conductor size for the feeder supplying all motors, the overload heater size of each motor using the chart in Figure 34-8, the NEMA and IEC starter sizes for each motor, the fuse size to protect the circuit for each motor, and the fuse size to protect the main feeder.What is the frequency of the second harmonic?Which of the following are considered triplen harmonics: 3rd, 6th, 9th, 12th, 15th, and 18 th?Would a positive-rotating harmonic or a negative-rotating harmonic be more harmful to an induction motor? Explain your answer.What instrument should be used to determine what harmonics are present in a power system?A 22.5-kVA single-phase transformer is tested with a true-RMS ammeter and an ammeter that indicates the peak value. The true-RMS reading is 94 A. The peak reading is 204 A. Should this transformer be derated? If so, by how much?