Maintenance of Motors
Learning Objectives: When you complete this assignment you should understand basic maintenance of motors and methods of electrical troubleshooting of motors.
Troubles with the electrical motors used to drive the compressors of mechanical refrigeration systems fall into two classes—mechanical and electrical.
Some compressors are belt-driven from the electrical motor. For proper operation, both the belt tension and pulley alignment adjustments must be made. Belt tension should be adjusted so a l-pound force on the center of the belt, either up or down, does not depress it more than one-half inch. To adjust the alignment, loosen the setscrew on the motor pulley after tension adjustment is made. Be sure the pulley turns freely on the shaft; add a little oil if necessary.
Turn the flywheel forward and backward several times. When it is correctly aligned, the pulley does not move inward or outward on the motor shaft. Tighten the setscrew holding the pulley to the shaft before starting the motor.
Compressors may also be driven directly by a mechanical coupling between the motor and compressor shafts. Be sure the two shafts are positioned so they form a straight line with each other.
The coupling on direct drive units should be realigned after repair or replacement. Clamp a dial indicator to the motor half coupling with its pointer against the outer edge of the compressor half coupling. Rotate the motor shaft, and observe any fluctuations of the indicator. Move the motor or compressor until the indicator is stationary when revolving the shaft one full turn. Secure the hold-down bolts and then recheck.
Moisture in the System
When liquid refrigerant that contains moisture vaporizes, the moisture separates from the vapor. Because the vaporization of the refrigerant causes a cooling effect, the water that has separated can freeze. Most of the expansion and vaporization of the refrigerant occurs in the evaporator. However, a small amount of the liquid refrigerant vaporizes in the expansion valve, and the valve is cooled below the expansion valve and interfere with its operation. If the needle in the valve freezes in a slightly off-seat position, the valve cannot permit the passage of enough refrigerant. If the needle freezes in a position far from the seat, the valve feeds too much refrigerant. In either case, precautions must be observed to assure a moisture-free system.
A dehydrator is filled with a chemical known as a desiccant, which absorbs moisture from the refrigerant passing through the dehydrator. Dehydrators are installed in the liquid line to absorb moisture in the system after the original installation. An arrow on the dehydrator indicates the direction of flow. Desiccants are granular and are composed of silica gel, activated alumina, or calcium sulfate. Do not use calcium chloride or chemicals that form a nonfreezing solution. These solutions may react with moisture to form undesirable substances, such as gums, sludges, or waxes. Follow the manufacturer's instructions as to limitations of dehydrators, as well as operation, recharging, replacing, and servicing.
Loose Copper Tubing
In sealed units, loose copper tubing is usually detected by the sound of rattling or metallic vibration. Bending the tubing carefully to the position of least vibration usually eliminates the defect. Do not touch it against other tubing or parts at a point of free movement, and do not change the tubing pitch or the tubing diameter by careless bending.
In open units, lengths of tubing must be well supported by conduit straps or other devices attached to walls, ceilings, or fixtures. Use friction tape pads to protect the copper tubing from the metal of the strap. When two tubes are together in a parallel position, wrapping and binding them together with tape can prevent vibration. When two lines are placed in contact for heat exchange, they should be soldered to prevent rattling and to permit better heat transfer.
Doors and Hardware
When hinges must be replaced because of lack of lubrication or other reasons, the use of exact duplicates is preferable. Loose hinge pins must be securely braided. When thrust bearings are provided, they are held in place by a pin.
The latch or catch is usually adjusted for proper gasket compression. Shims or spacers may be added or removed for adjustment. Latch mechanisms should be lubricated and adjusted for easy operation. Latch rollers must not bind when operated. Be sure to provide sufficient clearance between the body of the latch and catch, so no contact is made. The only contact is made between the catch and the latch bolt or roller. These instructions also apply to safety door latches, when they are provided for opening the door from the inside, although it is locked from the outside. Warping of the door usually causes lack of complete gasket contact between the door overlap and the doorframe. Correct the condition by installing a long, tapered wooden shim or splicer rigidly in place under the door seal. If this does not tighten the door to the frame, remove the door and either reline or rebuild it.
Repair or replace missing, worn, warped, or loose door gaskets. If the gasket is tacked on, rustproof tacks or staples should be used. If the gasket is clamped or held in place by the doorframe or the door panel, an exact replacement is necessary. In either case, the gasket should be installed so when the door is closed a complete and uniformly tight seal results. If doors freeze closed due to condensation and subsequent freezing, apply a light coat of glycerine on the gaskets.
Cooling units in the 35°F to 45°F reach-in or walk-in refrigerators or cold storage rooms are generally defrosted automatically by setting the low-pressure control switch to a predetermined level. If this setting causes overload with consequent heavy frosting of the coil, manual defrosting is necessary.
Cooling units of 35°F and lower temperatures are defrosted manually. The most common method for manual defrosting is to spray water over the cooling coil, although warm air, electric heating, or hot gas refrigerant defrosts too. In any case, the fans must not be in operation during the defrosting. Defrost plate-type evaporator banks in below-freezing refrigerators when the ice has built up to a thickness of one-half inch or when the temperature of the fixtures or the suction pressure is affected by the buildup of ice. Before removing frost from the plates, place a tarpaulin on the floor or over the contents of the refrigerator to catch the frost under the bank.
The control systems for modern refrigeration systems are composed of many components that use or pass electrical power, including compressor drive motors, pressure switches, thermostats, and solenoid stop valves.
Figure 6-62 shows a simple refrigeration control system. You have learned the basics of electricity and how to use meters. Using this figure, you will put that knowledge to work. Remember one fact—if you are not sure what you are doing, call your supervisor or arrange for a Construction Electrician to assist you.
Figure 6-62.—Simple refrigeration control system.
An "open" is defined as the condition of a component that prevents it from passing current. It may be a broken wire, a burned or pitted relay contact, a blown fuse, a broken relay coil, or a burned-out coil winding. An open can be located in one of two ways.
For the components in series, such as the main disconnect switch, fuses, the wire from Point C to Point D (fig. 6-62), the relay contacts, and the wire from Point E to Point F, a voltmeter should be used.
Set up the voltmeter to measure the source voltage (120 volts ac, in this case). If the suspected component is open, the source will be measured across it. To check part of the main disconnect switch, close the switch and measure from Point A to Point B. If the meter reading is 0 volts, that part of the switch is good; if the voltage equals the source voltage, the switch is open.
To check the fuse F2, measure across it, Point B to Point C. Measuring across Points C and D or E and F will check the connecting wires for opens. One set of relay contacts can be checked by taking meter readings at Points D and E. These are just a few examples, but the rule of series components can always be applied.
Remember, the three sets of contacts of relay K1 will not close unless voltage is present across the relay coil; the coil cannot be open or shorted. When testing an electrical circuit, follow the safe practices you have been taught and use procedures outlined in equipment manuals.
Opens in components that are in parallel cannot easily be found with a voltmeter because, as you know, parallel components have voltage across them at all times when the circuit is energized. In figure 6-62, the branch with the motor relay K1 and the dual refrigerant pressure control are considered a parallel circuit.
Because when the main disconnect switch is closed and the fuses are good, there is voltage between Points C and H, regardless of whether the relay coil and pressure switch are open. To check for opens in these components, use an ohmmeter set at a low range.
Disconnect all power by opening (and locking out, if possible) the main disconnect switch. This action removes all power and ensures both personal and equipment safety. To check the motor relay K1 to see if its coil is open, put the ohmmeter leads on Points C and G. A reading near infinity (extremely high resistance) indicates an open. The contacts of the dual refrigerant pressure control can be tested by putting the ohmmeter leads from Point G to Point H. Again, a reading near infinity indicates open contacts. You may need to consult the manufacturer's manual for the physical location of Points G and H. Notice the contacts of the control are normally closed when neither the head pressure nor the suction pressure is above its set limits.
Shorts are just the opposite of opens. Instead of preventing the flow of current, they allow too much current to flow, often blowing fuses. The ohmmeter on its lowest range is used to locate shorts by measuring the resistance across suspected components. If the coil of the motor relay K1 is suspected of being shorted, put the leads on Points C and G. A lower than normal reading (usually almost zero) indicates a short. You may have to determine the normal reading by consulting the manufacturer’s manual or by measuring the resistance of the coil of a known good relay. If fuses F2 and F3 blow and you suspect a short between the middle and bottom lines (fig. 6-62), put the ohmmeter leads between Points C and H. Again, a low reading indicates a short.
Remember, in all operations using an ohmmeter, it is imperative that all power be removed from the circuit for equipment and personal safety. Don't fail to do this!
A ground is an accidental connection between a part of an electrical circuit and ground, due perhaps, to physical contact through wearing of insulation or movement. To locate a ground, follow the same procedure you used to locate a short. The earth itself, a cold-water pipe, or the frame of a machine are all examples of ground points. To see whether a component is shorted to ground, put one ohmmeter lead on ground and the other on the point suspected to be grounded and follow the rules for locating a short.
Be sure to turn off all power to the unit. It may even be wise to check for the presence of voltage first. Use a voltmeter set to the range suitable for measuring source voltage. If power does not exist, then use the ohmmeter.
The limited amount of instruction presented here is not enough to qualify you as an electrician, but it should enable you to find such troubles as blown fuses, poor electrical connections, and the like. If the trouble appears more complicated than this, call your supervisor or ask for assistance from a Construction Electrician.
Testing the Motor
As a refrigeration technician, you should be able to make voltage measurements in a refrigeration system to ensure the proper voltage is applied to the drive motor, as shown on the rating plate of the motor. If the proper voltage is applied (within 10 percent) to the terminals of the motor and yet it does not run, you must decide what to do. If it is an open system (not hermetically sealed), it is the electrician's job to repair the motor. If it is a hermetically sealed unit, however, you must use special test equipment to complete further tests and perhaps make the unit operational again.
If the unit doesn't run, it may be because the motor rotor or compressor crankshaft is stuck (remember, in a hermetically sealed unit, they are one and the same). If you apply electrical power to try and move the motor in the correct direction first and then reverse the power, you may be able to rock it free and not have to replace the unit. This is one of the purposes of the hermetic unit analyzer (fig. 6-63).
Figure 6-63.—Hermetic unit analyzer.
To rock the rotor of an hermetically sealed unit, follow these steps:
- Determine from the manufacturer's manual whether the motor is a split-phase or a capacitor-start type.
- Remove any external wiring from the motor terminals.
- Place the analyzer plugs in the jacks of the same color. If a split-phase motor is used, put the red plug in jack No. 3; if the capacitor-start motor is used, put the red plug in jack No. 4; and select a capacity value close to the old one with the toggle switches.
- Connect the test clips as follows: White to common Black to the running winding Red to the starting winding
- Hold the push-to-start button down and at the same time move the handle of the rocker switch from normal to reverse. The frequency of rocking should not exceed five times within a 15-second period. If the motor starts, be certain that the rocker switch is in the normal position before releasing the push-to-start button.
- More tests can be made with the hermetic unit analyzer, such as testing for continuity of windings and for grounded windings. Procedures for these tests are provided in the manual that comes with the analyzer. Generally, if the rocking procedure does not result in a free and running motor, the unit must be replaced.
Troubleshooting Refrigeration Equipment
Troubleshooting of any type of refrigeration unit depends, in part, on your ability to compare normal operation with that obtained from the unit being operated. Obviously for you to detect these abnormal operations, you must first know what normal operation is. Climate affects running time. A refrigeration unit generally operates more efficiently in a dry climate. In an ambient temperature of 75°F, the running period usually approximates 2 to 4 minutes, and the off period, 12 to 20 minutes.
It is beyond the scope of this text to cover all of the troubles you may encounter in working with refrigeration equipment. If you apply yourself, you can acquire a lot of additional information through on-the-job training and experience and studying the manufacturer's instruction manuals.
First and foremost, safety must be stressed and safe operating practices followed before and while doing any troubleshooting or service work. All local and national codes concerning safety must be observed. Some of the more important safety steps that are often overlooked are as follows:
- Protective equipment, such as eye protection, gloves, hard hats, and so forth, must be available and worn.
- Fire extinguishers must be readily available, in good working order, and adequate for the situation.
- Safety tags with such notations as "Danger," "Hands Off," "Do Not Operate," and "Do Not Throw Switch" should be attached to valves, switches, and at other strategic locations when servicing or making repairs.
- Install machinery guards properly before operating machinery.