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AMAZON multi-meters discounts AMAZON oscilloscope discounts 10.7 SERVICE CALLS Service Call 1 Application: Commercial refrigeration (walk-in cooler) Type of Equipment: Air-cooled condensing unit with evaporator Complaint: No refrigeration Service Procedure: 1. The technician reviews the work order from the dispatcher for avail able information. The work order information reveals that the walk-in cooler of the meat market is hot with rapid deterioration of stored product. The condensing unit's located outside behind the structure. 2. The technician informs the manager of his or her presence and obtains any additional information about the problem. 3. The manager has noticed that the compressor is starting and running only a short time before cutting off. The condenser fan motor is running normally. 4. The equipment is 240 V—1ø—60 Hz. With the condenser fan motor picking up power from the terminals of the compressor, the technician assumes that voltage is being applied to the compressor. The technician measures the current draw of the compressor while running. The current draw is seven times that of the normal running amps of the compressor. 5. The compressor overload breaks power to the compressor after only a few seconds of operation. 6. The technician turns the power off and locks or labels it when working with live circuits. 7. The technician checks the compressor motor and determines that the motor is in good condition. 8. The starting capacitor is hot to the touch. The technician checks to determine if the capacitor is being removed from the circuit by checking the amp reading of the capacitor; if the capacitor is reading a cur rent draw, then the capacitor is remaining in the circuit. The potential relay is checked to determine the condition of the relay contacts and coil. The technician reads the resistance between terminals 1 and 2 and 2 and 5. The reading between 1 and 2 reads 0 ohms. Terminals 1 and 2 are the normally closed contacts of the relay and should read 0 ohms. The reading between 2 and 5 reads infinite ohms. Terminals 2 and 5 are the relay coil terminals and should have a very high but measurable resistance. The potential relay coil is open, allowing the starting capacitor to remain in the circuit and causing the motor to overload. 9. The part number of the faulty relay is an RBM 128-122-1335CA. The relay must be replaced with a potential relay with the continuous coil voltage of 256 and a coil calibration of 300 to 320 volts. The technician finds the correct relay from the available supply on the service truck. 10. The technician replaces the relay and checks to make certain that the new relay is removing the capacitor from the circuit by observing the current draw of the capacitor. There should be a current draw through the capacitor while the motor is starting, but when the motor reaches a certain speed, the capacitor current draw should be 0 amps. 11. The technician checks the current draw of the compressor to make certain the compressor is operating properly. 12. The technician informs the manager of the problem and that it has been corrected. Service Call 2 Application: Domestic refrigeration Type of Equipment: Domestic chest-type freezer Complaint: Freezer defrosting Service Procedure: 1. The technician reviews the work order from the dispatcher for avail able information. The work order information reveals that the freezer is located in the utility room under the carport and will be unlocked. The customer has commented that the compressor tries to start but does not run. The freezer has been unplugged. 2. The technician plugs the freezer in and notices that the compressor tries to start but does not and the overload takes it out of the circuit. When checking a chest-type freezer the technician knows that only three or four components will need to be checked when the compressor is attempting to start: the compressor, relay, overload, and capacitor, if used. The overload is seldom the problem, so the technician will check that last. 3. The technician unplugs the freezer. 4. The technician reads the resistance of the compressor and determines that the compressor is electrically good. The relay is a current type relay and must be checked next. The relay has a normally open set of contacts and a magnetic coil that closes the contacts when the compressor tries to start. 5. The technician checks the resistance of the coil and reads infinity. The technician determines that the current-type relay coil is open and not energizing the starting winding, thus preventing the compressor from starting. The freezer motor is a split-phase motor and does not require a capacitor. 6. The technician obtains the correct relay from the supplier and replaces the faulty relay. 7. The technician starts the freezer and checks the current draw of the compressor to make certain the freezer is operating properly. 8. The technician leaves a note informing the homeowner of the service performed. Service Call 3 Application: Residential conditioned air system Type of Equipment: Packaged heat pump Complaint: No heat Service Procedure: 1. The technician reviews the work order from the dispatcher for avail able information. The work order reveals that the unit's a packaged heat pump. Someone will be home. 2. Upon entering the residence, the technician makes certain that no dirt or foreign materials are carried into the structure. The technician also takes care not to mar interior walls. The customer informs the technician that the auxiliary heat light has been on when the unit's operating. 3. This heat pump has a capacitor-start—capacitor-run compressor motor. With the indoor fan motor operating properly, the technician knows that voltage is available to the unit. 4. The technician removes the compressor access panel. The compressor is hot to the touch, indicating that it had been attempting to start and failed, resulting in an open internal overload. 5. The technician must determine why the compressor isn't starting. The compressor and motor are found to be in good electrical condition. The starting components of the compressor must be checked to determine if they are good. 6. The technician uses an ohmmeter to check both starting and running capacitors and finds them good. The potential relay contacts must be checked. An ohm reading of infinity is read across terminals 1 and 2, indicating that the normally closed contacts are open and the start capacitor isn't being introduced to the motor starting circuits. 7. The technician turns the power supply off and locks it or labels it. 8. The technician replaces the faulty potential relay with the proper relay. 9. Power is restored to the unit and the technician checks the operation of the compressor, making certain that the capacitor is being removed from the circuit. 10. The technician reads the current of the motor and makes certain the compressor is operating properly. 11. The technician replaces the compressor access cover. 12. The technician informs the customer of the corrected problems. Service Call 4 Application: Commercial refrigeration (refrigerator) Type of Equipment: Air-cooled condensing unit mounted on top with evaporator Complaint: No refrigeration Service Procedure: 1. The technician reviews the work order from the dispatcher for avail able information. The work order information reveals that the temperature of the refrigerator is above a safe temperature for the product stored. 2. The technician informs the owner of the sandwich shop of his or her presence and obtains specifics about the operation of the refrigerator. 3. The technician makes every attempt not to disturb the activity of kitchen personnel. 4. The compressor is hot to the touch and not operating, but the con denser fan motor is operating, indicating that power is available to the compressor. 5. The technician turns the power supply off and locks or labels it. 6. The technician determines that the compressor and motor are in good condition. 7. The technician observes that the compressor attempts to start. The technician measures the current draw of the compressor; the current draw is extremely high. 8. The technician checks the current-type relay. The resistance reading for the coil is slightly above 0 ohms, indicating that the coil is good. The resistance of the relay contacts is infinite when the relay is inverted, indicating that the relay is bad. 9. The technician replaces the current-type relay. 10. The technician restores power and checks the operation of the compressor. 11. The technician informs the owner of the sandwich shop of the problem and that it has been corrected. Service Call 5 Application: Ventilation system Type of Equipment: Exhaust fan Complaint: Exhaust fan not running Service procedure: 1. The technician reviews the work order from the dispatcher for avail able information. The work order information reveals that the exhaust fan isn't operating, causing a stuffy condition in the room. 2. The technician informs the maintenance supervisor of his or her presence and obtains additional information about the problem. The supervisor informs the technician that the motor is operating, but the fan isn't. 3. The technician turns the power supply off and locks or labels it. 4. The technician visually inspects the exhaust fan and locates a broken belt. 5. The technician obtains the correct replacement and installs it, adjusting the tension. 6. The technician makes certain that the exhaust fan is operating properly. 7. The technician informs maintenance supervisor of the problem and that it has been corrected. SUMMARY Many types of electric motors require special electrical devices to keep the starting winding or starting component in the circuit until the motor has reached approximately 75% of full speed. Starting relays are used in most cases to drop the start windings and /or starting component from the electrical circuit once the motor has reached operating speed. It is essential that the starting components and /or starting windings be removed from the circuit at the correct time to prevent damage to the motor or starting components. There are four types of relays in common use in the industry today. The current-type relay operates on the principle that amperage flowing through a wire will produce a magnetic field. At its highest point, this magnetic field will close a set of contacts, thus putting the necessary starting components in the circuit and removing them at the correct time. The potential relay has a normally closed set of contacts that are opened and closed by the back electromotive force of the motor. This back emf corresponds to the motor speed, thus dropping out the starting components at the right time. Electronic relays used as starting relays are becoming increasingly popular in the industry. They have several advantages over the regular type of starting relay in that they can be used over a variety of sizes while incorporating other motor components if needed. The hot-wire relay is seldom used in the industry because better methods are available for dropping the starting components out of the circuit. All rotating heating, cooling, and refrigeration equipment must have bearings to operate efficiently and smoothly. Ball bearings and sleeve bearings are commonly used today for this purpose. Correct lubrication is essential to ensure long life and efficient service from the bearings. In most cases in the industry, electric motors drive some type of equipment. Direct-drive applications are common, as are V-belt applications. Direct-drive applications require very accurate fit with little vibration. The device being driven must turn at the same revolutions per minute (rpm) as the motor. V-belt connections have a certain amount of tolerance, but they must be correctly adjusted. They can be used to alter the revolutions per minute of the equipment by changing the pulleys.
REVIEW QUESTIONS 1. What is the purpose of a starting relay? a. to start an electric motor b. to remove the starting winding or component from the circuit c. to protect the motor from starting overloads d. to prevent the motor from starting under heavy loads 2. Explain the operating principle of a potential relay. 3. Explain the operating principle of a current relay. 4. What is the difference between a current and potential relay? 5. The contacts of the current relay are 6. The controlling factor for a potential relay is ____ a. back electromotive force b. line voltage c. voltage drop d. none of the above 7. True or False: The contacts of the potential relay are normally open. 8. What is the approximate speed at which the starting windings or components should be removed from the motor electrical circuit? a. 50% b. 75% c. 90% d. 100% 9. As a single-phase motor’s speed is increased from a stationary position, the current draw _____ a. increases b. decreases 10. As a single-phase motor’s speed is increased from a stationary position, the back electromotive force_______ a. increases b. decreases 11. Which of the following statements reflects the correct terminal identification of a potential relay? a. 1 and 2 coil, 2 and 4 contacts b. 2 and 4 coil, 3 and 6 contacts c. 1 and 5 coil, 2 and 5 contacts d. 2 and 5 coil, 1 and 2 contacts 12. A G.E. potential relay with the number 3ARR3-D4F6 would have a continuous coil voltage of _____. a. 214 b. 332 c. 420 d. 502 13. An RBM potential relay with the number 128-122-2324K would have a continuous coil voltage of: a. 130 b. 170 c. 256 d. 336 14. A G.E. potential relay with a number 3ARR3-C3A1 could be replaced with which of the following RBM potential relays? a. 128-212-1167AB b. 128-112-1161AB c. 128-151-1111CD d. 128-212-1147AB 15. An RBM potential relay with a number 128-122-1161BC could be replaced with which of the following G.E. potential relays? a. 3ARR3-C4B2 b. 3ARR3-C9D2 c. 3ARR3-C6B2 d. 3ARR3-C2B2 16. What are the two types of bearings used in the industry? 17. Which of the following isn't an advantage of the ball bearing? a. more efficient than a sleeve bearing b. requires less maintenance than a sleeve bearing c. cheaper than a sleeve bearing d. longer life than a sleeve bearing 18. What is the main reason that sleeve bearings are used in the industry? a. cost b. maintenance c. life d. efficiency 19. What is a direct-drive application? 20. How is the direct-drive application used in the industry? 21. Why are V-belts popular in the industry? 22. What is the correct tension on a V-belt? 23. True or False: If the starting relay is working properly, there will be no ampere draw through the starting circuit after the motor reaches full speed. 24. What is the purpose of a hard-start kit in conjunction with a hermetic compressor? 25. What electrical components make up a hard-start kit? 26. Name the three ways in which ball bearings may be lubricated. 27. True or False: Over-lubrication of a motor and its bearings is as damaging as underlubrication. 28. Name three ways in which sleeve bearings may be lubricated. 29. What are the common sizes of V-belts and what are their applications? 30. What is the compressor speed of an open-type compressor if the motor rpm is 1750, the motor pulley diameter is 4 inches, and the pulley diameter of the compressor is 16 inches? a. 438 b. 468 c. 498 d. 528 31. What size motor pulley would be required if the motor turns 1750 rpm, the fan pulley is 8 inches, and the desired rpm of the fan is 650 rpm? a. 3 b. 4 c. 5 d. 6 32. What is one situation in which a ball bearing cannot be used? 33. Explain the operation of a solid-state PTC starting relay. 34. What advantages does the solid-state starting relay have over the conventional current-type relay? PRACTICE SERVICE CALLS Determine the problem and recommend a solution for the following ser vice calls. (Be specific; don't list components as good or bad.) Practice Service Call 1 Application: Commercial refrigeration Type of Equipment: Frozen food display with air-cooled condensing unit (240 V—1Ø— 60 Hz) Complaint: No refrigeration Symptoms: 1. Condenser fan motor is operating normally. 2. Evaporator fan motor is operating properly. 3. Internal overload is cycling compressor on and off. 4. All starting components are in good condition. 5. Compressor motor is in good condition. Practice Service Call 2 Application: Domestic refrigeration Type of Equipment: Frost-free domestic refrigerator (compressor has split-phase motor) Complaint: No refrigeration Symptoms: 1. Compressor isn't operating. 2. Correct voltage is available to compressor. 3. Contacts of current-type relay are good. 4. Compressor is cycled on and off by compressor overload. 5. Compressor draws locked rotor current when start is attempted. Practice Service Call 3 Application: Residential conditioned air system Type of Equipment: Air conditioner using air handling unit with air-cooled condensing unit (240 V—1Ø —60 Hz and CSR motor with internal overload) Complaint: No cooling Symptoms: 1. Indoor electrical components are operating normally. 2. Condenser fan motor is operating properly. 3. 240 V—1ø—60 Hz is available to compressor. 4. Compressor starts and runs for a short period of time. 5. Compressor and motor are in good condition. Practice Service Call 4 Application: Commercial refrigeration Type of Equipment: Glass door refrigerator (240 V—1Ø --60 Hz and CSR motor with external overload) Complaint: No refrigeration Symptoms: 1. Evaporator fan motor is operating normally. 2. Proper voltage is available to compressor. 3. Compressor tries to start, but does not. 4. Compressor and motor are in good condition. 5. Start and run capacitor are good. 6. Condenser fan motor is operating properly. Practice Service Call 5 Application: Residential conditioned air system Type of Equipment: Oil-fired furnace with a belt-drive blower motor Complaint: No heat Symptoms: 1. Blower motor isn't operating. 2. Correct voltage is available to blower motor. 3. Blower motor is drawing locked rotor current. 4. Blower can’t be turned by hand. Practice Service Call 6 Application: Commercial refrigeration Type of Equipment: Ice machine (120V - 1Ø - 60 Hz with CS motor) Complaint: No ice production Symptoms: 1. Correct voltage is available to compressor. 2. Compressor attempts to start but is cut off by external overload. 3. Current-type starting relay is good. 4. Compressor and motor are in good condition. |