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All Textbook Solutions for Electric Motor Control

1SQ 2SQ To what does current limiting, or compensating time, acceleration refer?4SQ 5SQ 6SQ 7SQ 8SQ 9SQ 10SQ 11SQ 12SQ If the contacts on a manual starter cannot be closed immediately after a motor overload has tripped them open, what is the probable reason?If the handle of an installed motor starter is in the center position, what condition does this indicate?How may an automatic operation be achieved when a manual motor starter is used?What does the term trip-free mean as it relates to manual starters?5SQ 6SQ 7SQ 8SQ Select the best answer for each of the following. LVP is used mainly for a. fans. b. air compressors. c. pumps. d. protection of the operator.Select the best answer for each of the following. Thermal overload protection a. aids motor warm-up. b. prevents motors from freezing. c. protects motors and conductors from mechanical damage. d. provides overcurrent protection for motors.1SQ How many poles are required on motor starters for the following motors: (a) 240-volt, single-phase induction motor, (b) 440-volt, three-phase induction motor?If a motor starter is installed according to directions but will not start, what is a common cause for the failure to start?Using the time limit overload or the dashpot overload relay, how are the following achieved: time-delay characteristics; tripping current adjustments?What causes the AC hum or chatter in AC electromagnetic devices?What is the phase relationship between the flux in the main pole of a magnet and the flux in the shaded portion of the pole?In what AC and DC devices is the principle of the shaded pole used?What does the electrician look for to remedy the following conditions: a. loud or noisy hum? b. chatter?What type of protective enclosure is most commonly used and what is its NEMA number?What advantage is there in using combination starters?What safety feature does the combination starter provide that individual motor starting assemblies do not?12SQ How is the size of the overload heaters selected for a particular installation?What type of motor starter enclosure (include NEMA number and class) is recommended for an installation requiring safe electrical operation around an outside, inflammable paintfilling pump?15SQ 16SQ 17SQ 18SQ Select the best answer for each of the following. The purpose of overload protection on a motor is to protect the a. motor from sustained overcurrents. b. wire from high currents. c. motor from sustained overvoltage. d. motor from short circuits.20SQ 21SQ Select the best answer for each of the following. The purpose of a shading coil on an AC electromagnetic pole tip is to a. prevent overheating of the coil. b. limit the tripping current. c. limit the closing current. d. prevent chattering.23SQ Select the best answer for each of the following. Thermal overload relays react to a. high ambient temperatures and excessive heating due to overload currents. b. heavy mechanical loads. c. heavy electrical loads. d. rising starting currents.Select the best answer for each of the following. The thermal relay heating element is selected a. 15 percent under voltage. b. 10 percent over voltage. c. from the motors FLA and a manufacturers selection chart. d. by ambient temperature.Select the best answer for each of the following. When the reset button does not reestablish the control circuit after an overload, the probable cause is a. the overload heater is too small. b. the overload trip has not cooled sufficiently. c. the auxiliary contacts are defective. d. the overload heater is burned out.Select the best answer for each of the following. If an operator pushes a start button on a three-phase induction motor and the motor starts to hum, but not run, the probable trouble is a. one fuse is blown and the motor is single phasing. b. the overload trip needs resetting. c. the auxiliary contact is shorted. d. one phase is grounded.28SQ Select the best answer for each of the following. Vacuum contactors a. are quieter in operation than standard contactors. b. can control higher voltages in a smaller space. c. are more economical to purchase than conventional contactors. d. require the connection of a vacuum pump when in operation.30SQ 31SQ What is the difference between a grounded control system and a floating control system?A control transformer has a primary voltage of 480 volts and a secondary voltage of 120 volts. The transformer has a power rating of 0.5 kVA. What is the maximum output current of the transformer?Identify the following symbols.Electrical symbols usually conform to which standard?What do the following abbreviations stand for? a. SPST b. SPDT c. DPST d. DPDT e. NO f. NC Single-pole and double-pole switch symbols are shown as a. single break. b. double break. c. NO and NC. d. all of the above.The symbol shown is a a. polarized capacitor. b. normally closed switch. c. normally open held closed switch. d. normally open contact.The symbol shown is a a. normally closed float switch. b. normally open held closed float switch. c. normally open float switch. d. normally closed held open float switch.The symbol shown is a(n) a. iron core transformer. b. auto transformer. c. current transformer. d. air core transformer.The symbol shown is a a. normally open pressure switch. b. normally open flow switch. c. normally open float switch. d. normally open temperature switch.The symbol shown is a a. double-acting push button. b. two-position selector switch. c. three-position selector switch. d. maintained contact push button.If you were installing the circuit in Figure 412, what type of push button would you use for the silence button? a. Normally closed b. Normally open 11SQ Referring to the circuit in Figure 412, what circuit component controls the actions of the two CR contacts?13SQ When reading a schematic diagram, are the control components shown as they should be when the machine is turned off or de-energized, or are they shown as they should be when the machine is in operation?In which diagram do the physical locations of the wiring and the devices match the drawing?Which diagram conveys as much information as possible with the least amount of confusion?3SQ Which control wiring scheme can cause damage to industrial production or processing equipment with the sudden restart of machines on restoration of the power?In developing line or wiring diagrams, in which lines should the overload relays be placed for three-phase motor starters?In which control circuit wire are the overload relay contacts connected?The control circuit is connected to which lines for a three-phase motor starter?8SQ The contactor coil is connected to which line of a three-phase contactor?What does the X or 1 mean on a target table?11SQ 12SQ Draw an elementary line diagram of the control circuit from the wiring diagram shown in Figure 520. Exclude power or motor circuit wiring. FIG. 520 Draw a line diagram of the control circuit shown in Figure 521. (Exclude power or motor circuit wiring.) FIG. 521 What is meant by normally open contacts and normally closed contacts?Why is it that normally open and normally closed contacts cannot be closed simultaneously?How are colored pilot lights indicated in wiring diagrams?Select the best answer for each of the following. A single-break control is a a. heavy-duty selector switch. b. single-circuit push button. c. standard-duty selector switch. d. double-circuit push button.Select the best answer for each of the following. Control stations may contain a. push buttons. b. selector switches. c. indicating lights. d. all of these.Select the best answer for each of the following. A wobble stick is a. operated from any direction. b. knob controlled. c. palm controlled. d. glove operated.Select the best answer for each of the following. Common selector switches are a. one position. b. two positions. c. three positions. d. two and three positions.Select the best answer for each of the following. Most push buttons are a. momentary contact. b. single contact. c. double contact. d. a combination.Select the best answer for each of the following. Control station enclosures are a. general purpose. b. explosion proof. c. watertight. d. all of these.Select the best answer for each of the following. The diagram that illustrates a single-circuit, normally closed push button is:What are control relays?What are typical uses for control relays?What are the advantages of a mechanically held relay?Why are mechanically held relays energy-saving devices?Why does the coil (CR) in Figure 719 drop out when the thermostat touches the open position?Which device is used to connect the load to the line in a solid-state relay intended to control a DC load?Which device is used to connect the load to the line in a solid-state relay intended to control an AC load?What is optoisolation?What type of device is used to suppress voltage spikes in relay circuits?1SQ What is the principal difference between a contactor and a motor starter?What causes the arc to move upward in a blowout magnet?Why is it desirable to extinguish the arc as quickly as possible?5SQ Why will the blowout coil also operate on AC?7SQ Are ON control stations used in multiple locations connected in series or parallel for latched-in contactors?List several applications for a motor-driven timer.Which timer is the most commonly used in industrial applications?How is a pneumatic timer adjusted?List six factors to be considered when selecting a timing relay for a particular use.List several additional factors that will affect the selection of a timing relay.6SQ Connect these components in Figure 917: a. The start push button on the timer coil b. Load 1 to NOTC c. Load 2 to NCTO d. Show proper symbols FIG 917 Connect the following components in Figure 918: a. Start push button to the timer coil b. Load 1 to NCTC c. Load 2 to NOTO d. Show proper symbols FIG. 918 Describe how pressure switches are connected to start and stop (a) small motors and (b) large motors.What type of pressure switch is generally used to sense small changes in low- pressure systems?A pressure switch is set to cut in at a pressure of 375 PSI and cut out at 450 PSI. What is the pressure differential for this switch?A pressure switch is to be installed on a system with pressures that can range from 1500 PSI to 1800 PSI. What type of pressure switch should be used?A pressure switch is to be installed in a circuit that requires it to have three normally open contacts and one normally closed contact. The switch actually has one normally open contact. What must be done to permit this pressure switch to operate in this circuit?Refer to the circuit shown in Figure 105. If the pressure should become high enough for the pressure switch to close and stop the motor, is it possible to restart the motor before the pressure drops to a safe level?Refer to the circuit shown in Figure 105. Assume that the motor is running and an overload occurs, which causes the OL contact to open and disconnect coil M to stop the motor. What effect does the opening of the overload contact have on the pressure switch circuit?Refer to the circuit shown in Figure 106. Assume that when the thermostat contact closes nothing happens. Which of the following could NOT cause this condition? a. The high-pressure switch is open. b. The CCOL contact is open. c. The low-pressure switch is open. d. he coil of the compressor contactor is open. Fig. 106 Pressure switches are often used in air-conditioning circuits.Refer to the circuit shown in Figure 106. Assume that when the thermostat contact closes the compressor does not start, but the condenser fan and evaporator fan do start. When troubleshooting the circuit, a voltmeter connected across the compressor contactor coil CC indicates 0 volts. A voltmeter connected across the high-pressure switch indicates 0 volts, and a voltmeter placed across the thermostat contact indicates 0 volts. When the voltmeter is connected across the low-pressure switch it indicates 24 volts. Which of the following conditions could cause this problem? a. The refrigerant pressure is too high. b. The compressor contactor coil is open. c. The refrigerant pressure is too low. d. The thermostat contact is open. Fig. 106 Pressure switches are often used in air-conditioning circuits.Refer to Figure 106. When the thermostat contact closes, the compressor and evaporator fan start but the condenser fan does not start. Which of the following could NOT cause this problem? a. The condenser fan motor is burned out. b. The coil of the CF contactor is open. c. One of the CF load contacts is defective. d. The CFOL contact is open. Fig. 106 Pressure switches are often used in air-conditioning circuits.1SQ 2SQ 3SQ Refer to the circuit in Figure 1110. What is the purpose of control relay CR in this circuit?In Figure 1110, the reset button is a. single-acting normally closed. b. single-acting normally open. c. double-acting normally closed. d. double-acting normally open.In Figure 1110, assume that both motors are running. What would be the action of the circuit if the overload contact connected in series with coil 1M should open? a. Both motors will continue to run. b. Both motors will stop running. c. Pump motor 1 will continue to operate and pump motor 2 will stop running. d. Pump motor 1 will stop running and pump motor 2 will continue to operate.7SQ 1SQ Draw a line diagram to show that a green light will glow when liquid flow occurs.Draw a one-line diagram showing a bell that will ring in the absence of flow. Include a switch to turn off the bell manually.To answer the following questions refer to Figure 1210. When the thermostat contacts close the blower motor starts and the heating elements turn on normally. When the thermostat contacts open, the heating elements turn off and the blower motor stops running immediately. Which of the following could cause the problem? a. The thermostat contacts did not open. b. The flow switch contacts did not open. c. The fan thermostat contacts did not close. d. BMR coil is open. FIG. 1210 Air flow must be present before heating elements can be energized.When the thermostat contacts close, the blower motor starts and heating element #1 turns on. After a time delay heating element #3 turns on, but heating element #2 never turns on. Which of the following could cause this condition? a. Timed contact T1 did not close. b. Heating element #2 is open. c. CR relay coil is open. d. The flow switch is defective and did not close its contacts. FIG. 1210 Air flow must be present before heating elements can be energized.When the thermostat contacts close, the blower motor does not start. During troubleshooting of the unit, a voltmeter placed across the bottom terminals of the circuit breaker indicates a voltage of 240 volts. When the voltmeter is placed across the secondary of the transformer it indicates a voltage of 24 volts. When placed across the thermostat contacts it indicates 0 volts. When the voltmeter is placed across the coil of CR relay it indicates 24 volts. When the voltmeter is placed across the flow switch contacts it indicates 0 volts. When placed across the coil of HR contactor coil it indicates 0 volts, and when placed across the coil of BMR contactor it indicates 0 volts. Which of the following is the problem? a. CR coil is open. b. BMR coil is open. c. The thermostat is defective. d. The flow switch is defective.When the thermostat contacts close the blower motor starts but the heating elements never turn on. Which of the following could cause this problem? a. The flow switch is defective. b. The fan thermostat is defective. c. CR relay coil is open. d. Timed contact T2 is defective. FIG. 1210 Air flow must be present before heating elements can be energized.Draw a simple circuit showing how a red pilot light is energized when a limit switch is operated by a moving object.2SQ What is the advantage of a photodiode in a photodetector?What type of photodetector is often used in burglar alarm systems? Why?What do Hall effect sensors detect?Hall effect sensors do not depend on electromagnetic induction to operate. Explain how they can sense the presence of a magnetic field if induction is not involved.7SQ 8SQ 9SQ How is it possible for the miscroswitch to change contact position with such a small amount of movement of the activating arm?What is the purpose of phase failure relays?What are the hazards of phase failure and phase reversal?Why is it important to understand the purpose and action of the total operational system when working with controls?2SQ 3SQ What are some applications of temperature-actuated switches?How is the average electrician involved with electronic temperature controls?Should a metal be heated or cooled to make it expand?4SQ 5SQ Why should electrical contacts never be permitted to open or close slowly?7SQ 8SQ Why are thermistors often used as solid-state starting relays?What type of chemical is used to cause a pressure change in a bellows-type thermostat?1SQ How many wires are required for a two-wire control device?In the conduit diagram shown in Figure 173, determine how many wires are in each conduit between each piece of equipment. Identify each wire terminal connection and the quantity of wires above the disconnect switch, as shown in the example.Draw an elementary diagram of a pressure switch controlling a motor starter only.What is an intrinsically safe system?6SQ Draw a control circuit with no-voltage protection. Describe how this method of wiring protects the machine operator.2SQ 1SQ Determine the minimum number of wires in each conduit shown in Figure 1911. FIG. 1911 3SQ Refer to the chart in Figure 195. When the switch is set in the off position, are contacts 5 and 6 open or closed? Fig. 195 Selector switch with multiple double-break contacts.5SQ Refer to the chart in Figure 195. At what position must the HOA switch be set in order for contacts 7 and 8 to be closed? Fig. 195 Selector switch with multiple double-break contacts.Using the schematic diagram in Figure 2023, determine the number of control wires needed to control the three-phase motor. Use the terminal and wire identification shown in the schematic as well as proper power circuit identification. FIG. 2023 How is a change in the direction of rotation of a three-phase motor accomplished?2SQ If a reversing control circuit contains push-button interlocking, what would happen if both the forward and reverse buttons were pushed at the same time?How is auxiliary contact interlocking achieved on a reversing starter?After the forward coil has been energized, is the auxiliary forward interlocking contact open or closed?If a mechanical interlock is the only means of interlocking used, what procedure must be followed to reverse the direction of the motor? Assume the motor to be running forward.If pilot lights are to indicate which coil is energized, where should they be connected? Additional auxiliary contacts are not to be used.What is the sequence of the operations if the limit switches, shown in Figure 216, are used. (Limit switches were covered in Unit 13.) Fig. 216 Elementary diagram of the reversing starter shown in Figure 211. The mechanical, push-button, and auxiliary contact interlocks are indicated.In place of the push buttons in Figure 214, draw a heavy-duty selector switch for forward-reverse-stop control. Show the target table for this selector switch. FIG. 214 Mechanical interlock between the coils prevents the starter from closing all contacts simultaneously. Only one contactor can close at a time.From the elementary drawing in Figure 2110, determine the number and terminal identification of the wiring in each conduit in the conduit layout. Indicate your solutions in the same manner as the example given below the disconnect switch. FIG. 2110 Convert the control circuit only, Figure 2111, from the wiring diagram to an elementary diagram. Include the limit switches (RLS, FLS) as operating in the control circuit.13SQ Describe what is meant by sequence control.2SQ 3SQ What is the sequence of operation in Figure 222? Fig. 222 Auxiliary contacts (or interlocks) used for automatic sequence control. Contact (M1) energizes coil (M2); contact (M2) energizes coil (M3).5SQ Draw a symbol for a solid-state logic element AND.What is the safety feature of a lock-stop push button?In Part (A) of Figure 233, what will happen if both the run and jog push buttons are closed?What will happen if the start and jog push buttons of the circuit shown in Figure 234 are pushed at the same time?In Figure 236, what will happen if both jog push buttons are pushed momentarily?5SQ In the event of line voltage fluctuations or failure, how is the starter maintained in a closed position?2SQ Under what condition will the motor restart automatically after a voltage failure?Explain in detail the purpose of a time-delay, low-voltage release relay assembly installed in a motor control circuit.1SQ When can full-voltage starting be used?Name the simplest, most rugged, and most reliable type of AC motor. Describe how it operates.Define the term slip.When the voltage is reduced to start a motor, what happens to the a. torque? b. current?Why is it more advisable to start some machines at reduced torque?7SQ 8SQ 9SQ 10SQ What is the purpose of inserting resistance in the stator circuit during starting?Why is the power not interrupted when the motor makes the transition from start to run?If the starter is to function properly and a timing relay is used, where must the coil of the relay be connected?How may additional steps of acceleration be added?What is meant by the low or poor starting economy of a primary resistor starter?6SQ Why is it desirable to remove the autotransformers from the line when the motor reaches its rated speed?What is meant by an open transition from start to run? Why is this condition objectionable at times when used with large horsepower motors?Which of the following applies to an autotransformer starter with a five-pole starting contactor: open transition or closed transition? Locate in Figure 273.How are reduced voltages obtained from autotransformer starters?5SQ What is a disadvantage of starting with autotransformer coils rather than with resistors?What is one advantage of using an autotransformer starter?1SQ Which voltage may be applied to a 230/460-volt motor if used with a part winding starter? Why?If the nameplate of a dual-voltage motor connected to a part winding starter reads 15/30 amperes, which amperage value is used as the basis for the selection of the overload heating elements?If a timing relay fails to cut in the run starter, what percentage of the full-load current can the motor carry safely?5SQ 6SQ Who determines the compliance of an installation to applicable electrical codes with regard to adequate overload or starting and short-circuit protection?8SQ Refer to the circuit shown in Figure 284. When the start button is pressed the motor starts immediately with the resistors connected in series with the first half of the motor winding. After a delay of 2 seconds full voltage is applied to the first half of the motor winding. After a delay of 10 seconds the motor stops running. After troubleshooting the circuit it is discovered that the motor was stopped when the OL-S contact opened. Which of the following could NOT have caused this problem? a. TR1 timer coil is open. b. TR2 timer coil is open. c. TR2 timed contact did not close. d. R contactor coil is open.Refer to the circuit shown in Figure 283. It is assumed that timer TR is set to a delay of 4 seconds. When the start button is pressed, the motor does not start. Holding the start button down for over 4 seconds does not cause the motor to start. Which of the following could NOT have caused this problem? a. Contactor coil S is open. b. The control transformer fuse is blown. c. OL-S contact is open. d. OL-R contact is open.Indicate the correct terminal markings for a star-delta motor on Figure 299. FIG. 299 What is the principal reason for using star-delta motors?3SQ The closed transition contactor is energized only on transfer from star to delta. How is this accomplished?If a delta-connected, six-lead motor nameplate reads Full Load Current 170 Amperes, on what current rating should the overload relay setting or the selection of the heater elements be based?List some advantages of solid-state control.What element is the heart of the solid-state starter?3SQ 4SQ 5SQ Excluding slip, at what speed (r/min) will an eight-pole, 60-hertz, 240-volt motor run?What is meant by the description separate winding two-speed motor?3SQ 4SQ In Figure 314, what does the broken line between both time-delay contacts (TR) mean?When the high button is depressed in Figure 314, what prevents the high-speed starter from closing immediately?Which coils are energized immediately when the high push button is closed in Figure 314?At what speed can a four-pole, 240-volt, 5-hp, 50-hertz motor operate?What is meant by the compelling action produced by an accelerating relay?10SQ 1SQ 2SQ What will happen if terminals T5 and T6 in Figure 323 are interchanged on high speed?Is the motor operating at high speed or low speed when it is connected in series delta? In parallel wye?Describe the operation of the line diagram in Figure 325 (B) by adding jumper wire D only.Describe the operation of Figure 325 (B) by adding jumper wire A to the original circuit.In Figure 327, the motor cannot be started in high speed. Why?8SQ 1SQ 2SQ 3SQ What is the primary reason for using a compelling relay?What is the purpose of an accelerating relay?When is it most important to use decelerating relays for multispeed installations?Why are different motor connections shown for what appears to be the same motor in the various diagrams in Figure 332? FIG. 332 Typical arrangements for four-speed, three-phase, two-winding motors.How many windings are required for three-speed motors?What characteristic of wound rotor motors led to their wide use for industrial applications?2SQ What other name is given to the wound rotor motor?Does increased resistance in the rotor circuit produce low or high speed?5SQ What is the difference between a manual faceplate starter and a manual faceplate regulator?Refer to the basic control schematic in Figure 344. The limit switch is shown as a. normally open. b. normally closed. c. normally open held closed. d. normally closed held open. Fig. 344 Wiring diagram of manual speed regulator interlocked with magnetic starter for control of slip ring motor (three-wire control).1SQ 2SQ 3SQ 4SQ Do all contactors remain energized when the motor is in high speed?6SQ 7SQ Are the secondary resistors connected in three uniform wye or delta sections?Do secondary resistors on starters with three or more steps of acceleration have more or less current inrush than those with two steps of acceleration?Does reversing the secondary rotor leads mean that the direction of rotation will reverse?If one of the secondary resistor contacts (S2) fails in Figure 361, what will happen? Fig. 361 Typical control circuit for accelerating a wound rotor motor with three steps of speed.In Figure 362, how many different interlocking conditions exist? Name them. Fig. 362 Typical elementary diagram of a starter with two points of acceleration for a reversing wound rotor motor.Referring to Figure 364, why is it not possible to remove all of the resistance from the secondary circuit? Fig. 364 Automatic acceleration of wound rotor motor using a simplified frequency relay system.7SQ If there is a locked rotor in the secondary circuit, what will be the value of the frequency?

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