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They have found that it is critical to motor reliability to have the right connection design, materials, and manufacturing processes. Motor makers have conducted extensive studies on the impact of adverse environments on aluminum magnet wire. These studies have centered on some of the more challenging applications, such as domestic water pumps where the motor windings are subjected to mineral salts and pool pumps where the windings must withstand chlorinated water. Aluminum is well-suited to adverse environments and is used, along with copper for magnet wire, in many motors.
One last note on aluminum magnetic wire. This makes it almost impossible to distinguish the material used in the motor windings. Other than understanding the application and the limitations of aluminum, you typically have no way of knowing if you have a motor with aluminum or copper windings. The nearest wholesaler is 75 miles away, and they close at 5: The question you always face in emergency situations is: You do have one element in your favor when replacing motors in an emergency--NEMA provides you with a set of minimum standards for performance and mechanical interchangeability of motors.
Even with these standards, however, emergency replacements represent a particular challenge for the service technician. There are some motors built specifically for replacement purposes, and you should be familiar with these products and have them on your repair truck. You can use them with the confidence that they will do the job for an extended period of time in an emergency situation. In many cases, however, the advice I have provided is to get you through the emergency at hand.
Good practice dictates that you return to the job site with an exact replacement as quickly as possible. Among the many mysteries of life is the question of why the North American and Middle Eastern standard for line frequency is 60 hertz and why Europe uses 50 hertz. While you may not need this information every day, in our increasingly global economy, it may come in handy.
Actually, frequency is one of two potential issues that electric motor manufacturers face when selling to international customers. The second concern is voltage, but this is a relatively simple problem because most electrical equipment is designed to operate between plus and minus 10 percent of its rated voltage. To determine compatibility, you just need to know if the voltage source falls within the voltage range of the equipment in question. The issue of line frequency expressed in a unit called hertz can be a bit more perplexing, especially when magnetic devices such as motors, equipment with transformers, or equipment with magnetic ballasts fluorescent or vapor-type lamps come into play.
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One critical relationship between line frequency and magnetic devices is efficiency. The physics of electric circuits tells us that AC magnetic devices increase in efficiency as line frequency increases. Just build your power systems and devices to the highest frequency possible. Another physical characteristic to keep in mind is that current flow in a conductor tends to be closer to the surface of the conductor as frequency increases.
So as the frequency goes up, solid conductors begin to resemble hollow pipes as the electrons making up the current flow migrate to the outer surfaces of the conductors. At these higher frequencies, the energy of the electrons has a tendency to actually leave the surface of the conductor. A common example of this principle in action is radio transmission. As the frequencies get higher, all of the energy can be made to leave the conductor in a form of energy called radio waves.
This also helps explain why overhead power lines tend to interfere with radio reception the annoying cycle hum. What you are hearing is energy loss from the power lines becoming a radio wave that is intercepted by the radio receiver. Consequently, the designers of electrical devices must strike a balance: So the evolution of 50 and 60 hertz systems developed as a result of this need for balance, with additional influences coming from politics and geographic considerations.
North America and other regions struck the balance at 60 hertz, while Europe settled on 50 hertz. You probably realize by now that frequency is a parameter for motor selection and application. What you may not realize is that a motor designed to operate at the lower 50 hertz frequency may operate in a satisfactory manner at 60 hertz, at least from an efficiency and heat loss standpoint.
This is because a 50 hertz motor contains added material to make up for the modestly decreased efficiency found at the lower line frequency. On the other hand, applying a 60 hertz motor to a 50 hertz line frequency application is more problematic. A motor designed to operate efficiently at 60 hertz may not have enough active material copper and iron to sustain efficiency at 50 hertz. Heat loss and dissipation become issues. Another consideration is that some motors designed to operate on single-phase power may have an internal switch that is speed sensitive. At the lower frequency, the motor may never reach the normal switched operating speed.
The internal start switch will not open, and this will lead to burnout of the starting circuit. Anyone whose job involves servicing electric motors has encountered the problem of a missing nameplate. Other articles in this series have covered ways of determining the specifications of a motor lacking the nameplate, but what if you are trying to figure out how to wire that motor?
For some kinds of motors, principally motors with terminal-based connections, basic wiring is self evident. The terminal board itself usually has markings that indicate where line one and line two are to be connected. But what if you need to reverse that motor, use a different but available voltage setting, or have a motor that has nothing more than a bunch of color-coded or numbered leads coming out of it? The colors or numbers themselves are often a clue, but they alone may not provide sufficient information.
There is always the trial and error method, but I don't recommend that because of the potential for destructive results. Instead, the Motor Doctor's suggestion is to equip yourself with an ohmmeter don't settle for just a continuity tester and learn to perform a few simple tests with it. The first thing you'll need to discover is whether you're dealing with a three-phase motor. You may already know this from the application, but another giveaway is that the lead wires of most three-phase motors are single colors, not multiple colors, and usually identified with numbers.
If, on the other hand, the motor diameter is less than seven inches and has a terminal board, it is most likely a single-phase motor. For wiring a single-phase motor, the most important objective is to distinguish the starting circuit from the main winding. These two circuits are isolated from one another electrically if the lead wires are separated and not in contact with each other.
Initially, the ohmmeter can be used to determine which wire belongs to which circuit as well as checking continuity between leads. You should be able to isolate into two groups any leads which have continuity with one another. The starting circuit is likely to isolate to two leads, the running circuit may have two or more leads that show continuity. If the running circuit has more than two leads, you will need to determine how those leads are to be used for voltage or speed changes.
You'll need to use the ohmmeter as an ohmmeter and not as a continuity checker for the next step in the procedure. You'll want to use the lowest ohm scale your meter offers, as the typical winding resistance in motors such as these is less than ohms. If the motor is a permanent split-capacitor motor, you're going to be looking for common and speed taps of the winding. Using the ohmmeter, find the pair of wires that has the highest resistence as measured in ohms.
This will give you your common and lowest speed tap. Using each of these two leads in turn, find the pair that gives you the the second-highest resistance. This should provide you the common and second-lowest speed tap and should also allow you to isolate which of the two leads from the first test is the common. In addition, note that the common lead in this type of motor is usually white or purple. If there are additional leads in the run widing group, continue to use the ohmmeter to test the now-identified common and additional leads.
Descending resistance will give you ascending speeds.
All is not lost if you don't have a diagram for a particular motor, at least not if you understand how to use and ohmmeter. As with any problem-solving exercise, the more tools you have at your disposal, the more effective you become in the field. Motor efficiency remains one of the top issues in our industry, but when you talk about efficiency, often you're talking about trade-offs. In other words, it is relatively easy to make a motor efficient, if money is no object.
But since cost is a factor, motor manufacturers keep seeking the right balance of increasing motor output without driving up the price of the product. Occasionally, a technician or service person will ask me, "why not just increase the output by increasing the voltage the current flow to the motor? To understand why, you need to become familiar with a physical characteristic called "hysteresis loss. Think of the atoms of magnetic material as an unruly herd of cattle.
Running electric current through the material will polarize these atoms, creating the magnetic field. But as I mentioned, this is an unruly herd, so it takes time for the current to bring all those atoms into formation. As you might suspect, when you reverse the current in an alternating current motor, it takes time for those atoms to get going in the opposite direction. And the amount of time is not necessarily the same as the time it took to get the herd moving properly in the first place. Without getting into a lengthy physics lecture, this process of reversing polarity produces heat or wasted energy.
This is known as hysteresis loss. And that helps explain why increasing the voltage into the motor will not necessarily increase the output. Instead, it can fight the resistance of magnetic materials to reverse polarity--and simply heat iron. For service technicians, this is also an explanation why a motor heats unexpectedly when the voltage supplied is higher than the device's nameplate voltage. One way to overcome this situation is by using "magnetically soft" material.
Magnetically soft material has atoms that readily reverse polarity a docile herd? Naturally, since the reversing process happens more quickly, there is less wasted energy. Here's where metallurgy comes into play. A motor rich in magnetically soft material will be more efficient, producing more work with less heat. And since the magnetic capacity of a motor also is influenced by the amount of active material more core, more laminations , the tendency might be to try to add as much magnetically soft material to your design as possible.
Magnetically soft materials, however, tend to be more expensive. The motor manufacturer must find that proper blend of just enough magnetically soft material to do the work required without putting too big a dent in the customer's wallet. It's important to keep this struggle between performance and cost in mind when you talk to customers about energy-efficient motor-driven equipment.
Yes, efficiency is probably more important to homeowners now than ever, but that efficient operation comes at a price. And motor manufacturers will keep working to strike that balance between motor performance, efficiency, and cost. Electric motors, in essence, are conversion devices. They convert one form of energy electrical energy into another form mechanical energy. In the process, they consume power, and they do work.
It is easy to be imprecise about these terms as well as the units of measurement we use in connection with the terms, such as horsepower, watts, and amps. So, here are some precise definitions of terms. It may be the work involved when several stagehands move a piano or when a gas engine moves an automobile. Appropriate examples for this article include water being moved through a pump by an electric motor or a garage door being lifted by a motor-driven opener.
A bulldozer is capable of moving a hill of earth much faster than a garden tractor, therefore we say the bulldozer is more powerful than the tractor. Before bulldozers and garden tractors, horses performed much of the heavy work needed by humans. Energy is stored in such things as coal, gasoline, and the food we eat. For energy to be released, some chemical or mechanical action must be performed on whatever stores that energy. Coal is burned, gasoline is compressed and heated to make it explode in an internal combustion engine, and our bodies oxidize the food we eat.
Electrical energy is produced mechanically by a generator or chemically by a battery. As I said before, power includes a time factor. It takes more power to move the pound weight in our example 10 feet in one second than it would to move the same weight the same distance in two seconds. This power value is equivalent to one horsepower. Therefore, a four-horsepower electric motor would be able to move a 2,pound load 4 x a vertical distance of one foot in one second, or an 1,pound load two feet in one second.
One horsepower equals watts.
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Therefore, a horsepower motor also can be said to produce 7, watts of power. The input watts to this motor, however, will be higher because not all the electric power can be converted to mechanical power. Some of that input power is wasted in the form of heat. An electric motor does its work by turning a shaft.
Having the precise, scientific definitions of terms not only enhances your understanding, it also helps you to better see the relationship of concepts such as efficiency and torque when you look at electric motors and their applications in the field. Every service technician should have at least one multi-speed motor in his or her truck to help in making acceptable substitutions in the field.
Multi-speed motors come in two basic varieties. The first variety has an extra set of windings called a booster winding that behaves like a transformer. The second variety comes with two distinct separate sets of windings. So how do these motors work? Remember from last issue how you determine motor speed by the number of poles poles divided by the constant 7, gives you revolutions per minute.
If the load is constant, you can increase the slip by weakening the strength of the spinning magnetic field. One way is to decrease the voltage to the magnet wire that makes up the poles. You can decrease the voltage externally by using a speed control or internally through the use of the booster winding in a multi-speed motor.
In other words, the booster winding acts like a transformer, changing incoming line voltage to a lower voltage at the windings. The booster winding may come with taps that allow you to apply different voltages to the poles, creating different speeds in the motor. This is because slip occurs when the load works against the weakened magnetic field.
Consequently, this design is generally unsuitable for loads other than fans. The second type of multi-speed motor with two completely separate sets of windings allows you to use one or the other speed at a given time. Having two pole sets wound independently offers you more flexibility to produce constant horsepower in mechanical applications since you are energizing just one set of poles at a time.
Knowing this, you can begin to appreciate the versatility of multi-speed motors in the field. To achieve the correct results, simply select the correct tap and carefully insulate the two unused taps. The result would be a motor that produces the same performance, similar fan noise characteristics, and the same static pressure as the original single-speed model.
Multi-speed motors give the service technician another versatile tool in the field. That's why is always good to have some in stock for emergency substitutions. One way to determine that you are making good replacements in the field is to understand the concept of motor speed so that you can match the speed of one AC induction motor to another.
At the same time, you also need to become familiar with the concept of poles, since poles represent one key to a successful replacement. Every AC induction motor has poles, just like a magnet. Unlike a simple magnet, these poles are formed by bundles of magnet wire called windings wound together in slots of the stator core. In most cases, you can look inside the motor and count the number of poles in the winding: The number of poles, combined with the alternating current line frequency HZ , are all that determine the no-load revolutions per minute RPM of the motor.
So all four-pole motors will run at the same speed under no-load conditions, all six-pole motors will run at the same speed, and so on. The mathematical formula to remember in helping make these calculation is the number of cycles HZ times 60 for seconds in a minute times two for the positive and negative pulses in the cycle divided by the number of poles.
Using this formula, you can see that a four-pole motor operating on the bench under no-load conditions runs at 1, RPM 7, divided by four poles. Note that when an AC motor is loaded, the spinning magnetic field in the stator does not change speed. So, going back to our spinning four-pole motor, it operates at 1, RPM under no-load conditions and approximately 1, RPM under load.
Motors of this speed are commonly found in belted applications such as blowers, fans, air-handling equipment, compressors, and some conveyors. Two-pole motors often are found in pump applications, such as sump pumps, swimming pool pumps, or water recirculating equipment. It is beneficial to become aware of the different speed-related sounds motors make.
They are often used for air-handling equipment, direct-drive applications, window fans, furnace blowers, room air conditioners, heat pumps, and other equipment where the relatively slower motor speed makes for quieter operation. All can come in either totally open, totally enclosed, or combination models, adding to their versatility. They are being used in applications where customers expect quieter operation, such as room air conditioners and outdoor heat pump applications. Less-common pole configurations include pole motors RPM that are used in applications requiring slow speeds, such as washing machines, and pole motors RPM unloaded , often found in ceiling fans.
When making replacements, there is one key thing to remember: Understand that nameplate speed is an approximation of the rotor speed under load. You can tolerate some variation here, since motors are designed to accommodate a range of loaded speeds. If the other characteristics match nameplate amps, etc.
Learning to understand the relationship between motor speed and poles will help you become a more knowledgeable, effective service technician in the field. The great copper windings versus aluminum windings debate goes on. It remains a topic of discussion today as engineers in a variety of industries question whether the quality and performance of aluminum windings can possibly compare with copper.
Some of us remember the s, when aluminum house wiring was the subject of much attention because of the apparent fire hazards it created. It turned out that the cause of the house fires was not the wire itself, but rather connection problems. The junctions would become so hot that the heat would transfer to the wire itself, eventually deteriorating the wire insulation.
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Consequently, all forms of aluminum wire received a bad rap. Aluminum is a good conductor and, when applied properly, performs quite well. There are, however, two factors to keep in mind. To compensate, aluminum magnet wire must have larger cross-sections than the equivalent copper wire to offer the same conductance.
This means windings wound with aluminum wire will likely have greater volume compared with an equivalent copper wire motor. The second consideration is properly connecting the ends of the aluminum magnet wire. The reason for this goes back to your high school chemistry course. You may remember that aluminum oxidizes much faster than other metals—in fact, if exposed to air, powdered aluminum will completely oxidize in a few days, forming a fine white powder.
But, when exposed to air, fabricated aluminum sheets, wire, etc. To make a proper connection that ensures conductivity, the oxide layer of the aluminum magnet wire must be completely pierced and yet pierced in such a way to prevent air from coming in any further contact with the aluminum.
Motor manufacturers have developed high-pressure, piercing crimp connectors to do the job. These improved connection methods have helped make motors with aluminum windings every bit as reliable as motors with copper windings. It must be pointed out that motor efficiency is a much trickier issue in the great copper versus aluminum debate. It is possible to match the power performance of a motor wound with aluminum to a motor wound with copper wire. In situations where efficiency and volume are not issues such as where the motor only has to work occasionally or for very short periods of time , aluminum magnet wires make an acceptable motor.
The bottom line is that, in terms of motor quality, reliability, and life span, aluminum windings can be every bit as good as copper-wound motors. Comparisons are fair as long as you keep efficiency issues apart when looking at copper versus aluminum. One of the most frequent challenges you'll face as a service technician is determining if a replacement motor that is not an exact duplicate of the original is suitable for an application. Often, you can use the motor's nameplate to help you in making the selection, but a nameplate doesn't always provide you with everything you need to know.
Take nameplate amps, for example. It is common practice to determine the power input of two motors by comparing nameplate amps of the original motor with those of the replacement. In other words, if the replacement motor's rated amps are at least as high as the original, you are reasonably safe in using it in the application. This practice works best when you're working with motor types whose efficiency varies little from one design to another. A good example would be three-phase motors. But, the nameplate may not tell the entire story, and as a superior service technician, you need to be aware of the chapters that are missing.
Motor nameplates typically do not include input watts as a power measurement, nor do they identify the motor's efficiency. I drew up a diagram using a guess for which type of switch you have. Here's the best shot I could do. Well, your dayton switch has a Furnas sticker in it Don't use the drawing I gave above, it won't work. Pretty sure you are going to use the split-phase diagram in the upper right. How bout a pic of the rotating contact?
I've got the same switch I think, but not in front of me. With a pic, I think we could give you a connection diagram. I've got exactly the same motor. I wired mine up for v using the original drum switch for my heavy If other posters don't help you clear it up in a day I can pull the cover tomorrow after work and snap some photos of how I wired mine. With that knowledge in mind, you may surmise that every four-pole AC induction motor runs at the same speed.
This is in fact the case, and you can determine this speed by using the following formula:. For 60 hertz electrical systems: For 50 hertz systems: Using the standard formula, you can determine that a four-pole motor operating under no-load conditions will run at 1, RPM 7, divided by four poles. Loaded, the motor will slip to between 1, and 1, RPM. Four pole designs are the most common pole configuration for AC induction motors and are typically found in belted applications such as blowers, fans, air-handling equipment, compressors, commercial garage door openers, and conveyors.
Such speeds are commonly found in pump applications, such as submersible pumps, sump pumps, pool or water recirculating equipment. Another typical application is small ventilating fans. To the untrained ear, two-pole motors appear to need servicing because they sound somewhat noisier when running. They are often used for air-handling equipment, direct-drive applications, window fans, furnace blowers, room air conditioners, heat pumps, residential garage door openers, and other equipment.
As you can imagine, lower mechanical speeds often result in quieter designs, which makes an eight-pole motor well-suited for many residential applications where noise is a factor. These motors operate at RPM unloaded between and RPM loaded and are being used more extensively today in room air conditioning, outdoor heat pump, and residential garage door openers.
A less common design is the pole motor. This motor, which runs at RPM unloaded, is used in washing machines and other equipment that require a slow cycle. Changing, say, from a four-pole to a six-pole design, the speed mismatch is likely to create significant problems. The original article gave a basic description of capacitor design and function, then moved on to some suggestions for diagnosing a defective capacitor in the field and how to deal with the problem.
I stated that it is an acceptable temporary fix to go up one rating size for example, from a 7. I stressed the importance of installing a capacitor of the correct size as soon as possible. There is a circumstance in which a run capacitor failure may indicate an application problem, in which case replacing the capacitor with one of higher voltage may worsen that problem. This leads to a decreased load on the motor and higher RPMs.
As the RPMs increase, the regenerated voltage increases and tends to stress the run capacitor.
The run capacitor, in air-moving applications, acts almost as a fuse, in that its failure could be an indication of a more significant problem. Fred goes on to add that he agrees about our prescription for emergency fixes, but stresses that nothing is as good as finding a replacement that exactly matches the specifications of the original. Larry Johnston, a knowledgeable service technician from Madison, MO, offers some additional insights into suggested capacitor testing procedures.
Larry suggests that when using an ohmmeter to troubleshoot capacitors, you should have a capacitor of similar value to compare how far the needle swings as it shows conformity on that particular test equipment. He points out that many defective capacitors will show some degree of continuity. Larry, in turn, was puzzled that a TV technician did not have more expertise in capacitor diagnosis. In his experience, motor start capacitors and certain round type run capacitors seem most prone to show continuity but still exhibit diminished capacitance.
Larry offers the following insights into testing capacitors: The needle swing test depends on quick connection of the probe. Note, I have repaired multiple motors in air conditioning equipment where the technician has discharged the capacitor with a screwdriver and the screwdriver brushed against the return leads. I have also seen situations where the capacitor has fallen down onto an evaporator, burning a hole in it. We welcome your letters and always appreciate your thoughts and comments. They help everyone expand their knowledge of issues in the field and the best way to service motors and keep them in operation.
Bearing system failures are one of the most common mechanical breakdowns in the field. As you can see from the illustration, the the oil flows from a reservoir to the feeder wick which rides right on the shaft. As the shaft rotates, the feeder wick pumps a microscopically thin layer of oil from the reservoir down onto the shaft. The oil flows down the shaft until it hits the "flinger" on the end which returns the oil to the reservoir. At the same time, the sleeve bearing circulates that oil to remove any contaminants from the lubricant.
Even under the best of circumstances, there are moments when the shaft is rotating that metal-to-metal contact occurs. So what causes mechanical breakdowns in what is essentially a closed lubricating system? The most common enemies of bearings are water that can interrupt the flow of oil to the shaft , solvents, or the wrong lubricant. As a rule of thumb, any type of oil that is labeled "motor oil" is okay to use as a lubricant. Avoid other lubricants—and tell your customers not to attempt to lubricate their motor with anything but motor oil.
You would be amazed at the types of lubrication well-meaning but untrained people try to use in electric motors. One of our quality managers once received a failed motor from a health spa that smelled strongly of coconut. No one could figure out the source of failure until they began testing the lubrication. Turns out the customer had attempted to use analgesic cream coconut scented of course to lube the bearings. The solvents can attack and destroy the insulation in the windings leading to motor failure. Water or moisture will often get into the bearing system if the flinger is damaged or left out of the bearing.
Seriously, this little piece of plastic is essential in maintaining the controlled trap system and keeping oil in circulation in the bearing system. It may seem totally illogical, but a common form of field failure is from applying too much lubrication to a motor. Factory-fresh motors with sleeve bearing systems are always properly lubricated. Since it is essentially a closed system, there is no chance the oil will leak out during shipping or installation, so avoid the temptation to oil the motor when you put it in.
When you apply too much lubrication especially to the sleeve bearing systems in smaller electric motors , the oil by-passes the bearing flinger and dissipates throughout the motor. Too much lubrication causes oil circulation to exit the "closed loop" within the bearing system, and the bearing "freezes up. Another potential problem you may encounter in the field is bearing stress caused by excess tension on the belts used in belt-driven applications.
The excessive tension tends to pull the shaft in the direction of the bearing window. If the belt happens to cover the bearing window, it decreases the bearing surface area and its ability to circulate oil by up to two-thirds. The result is increased risk of overload. In addition to decreasing the belt tension, you can also rotate the motor to keep the bearing window away from the belt.
Remember that heat kills motors, and sleeve bearings help to reduce the amount of heat the motor generates. One of the things about motors that I find interesting is the wide range of work that they do. Depending on the application, a motor may be required to run continuously or start and stop frequently.
They may drive mechanical loads or move air. The challenge is to design a motor that is well-suited to the specific requirements of the application. This often involves spending a lot of time learning about related devices, such as capacitors, connectors, and the topic of this particular article, switches. Many single phase motors used in hard-to-start applications, such as conveyor belts, oil burner pumps, carbonated beverage pumps, belted fans and blowers, and commercial garage door openers, use a set of parts called a rotating governor and stationary switch assembly.
Motors in these applications must have a starting circuit that produces high starting torque while at the same time limiting a high starting current. The rotating governor and stationary switch assembly enable the starting circuit to energize for a brief period of time typically a fraction of a second to quickly get the motor up to running speed, limiting the starting circuit to a short "burst.
Rotating governors and stationary switch assemblies have been around for decades. Manufacturers continue to make improvements to these components, developing easier-to-assemble designs with better functionality. Some of this evolution has come as the result of testing the devices under conditions peculiar to certain applications. Depending upon the end use of the product, manufacturers evaluate component performance under very low temperatures or rapid fluctuations in temperatures.
In other instances, they may create a test where the switch reverses after each operating cycle. Typically, the engineers will run the switch to failure under these extreme conditions while monitoring every step in the process using high-speed photography combined with other recording instruments. Their objective is to capture the exact moment and hopefully the cause of failure. From there, the research team will analyze the problem going through a cycle of failure analysis, design improvement, and process improvement.
New components or design changes are tested once again, often by restarting the failure analysis process. The results are numerous improvements not always perceptible to the casual observer or even the trained service technician. Companies take these improvements seriously, however, and often they are kept as trade secrets of the manufacturers.
Here are some examples of how this exhaustive process yields improvements in switch design. One discovery made about cold-temperature operation is that most switch designs are not affected by just the cold. Repeated exposure to humidity while the part is cold can build up damaging layers of frost and ice on components. One such application would be an overhead garage door opener, where the repeated opening and closing subjects the opener and the switch to a wide range of temperature variations.
Manufacturers have created designs that clear offending ice from the switch without causing malfunction. Another factor, discovered through the use of high-speed photography, is that the switch needs to make and break contact "cleanly. This bouncing could cause the start circuit to arc unnecessarily and fail prematurely in the application. But consistency and reliability—especially in the face of unusual operating conditions—call for a lot of engineering know-how. A critical element of motor servicing technique is being able to determine whether or not a replacement motor that is not an exact duplicate of the original is suitable for the application.
As a technician, you must consider a number of factors, but for this article, I'd like to focus on one of the more important issues: One typical way that technicians determine whether the replacement motor has sufficient power output is to compare the nameplate amps of the original motor with the replacement model. If the replacement motor's amp rating is at least as high as the original, you can consider the replacement suitable.
In many cases, this comparison simply confirms what other factors, such as nameplate horsepower and rated voltage, tell us. This practice is most satisfactory when there is little or no variation in the efficiency from the original motor to the replacement. This method works well, for example, with most three-phase motors.
In other cases, however, comparing amps may be misleading. The comparison process tends to break down when the motors in question are single-phase models where there is a wide range of efficiencies common for a single design. This category includes permanent-split-capacitor motors, shaded-pole, and some types of split-phase and capacitor-start motors. Since nameplate amps reflect the total current consumption of the motor which includes both the current converted to output power and the current lost to heat due to design inefficiency , higher nameplate amps can just as likely mean poor efficiency as higher power output.
As motor manufacturers become increasingly sensitive to the energy efficiency issue, they work hard to develop motors that deliver higher power while consuming the same or fewer watts.
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That efficiency may or may not be reflected in the amp rating of the motor. For the service technician, this generally means placing more importance on comparing the horsepower of certain motors rather than comparing amps of the replacement to the original. Since there are no efficiency standards for most single-phase motors, there is one good way for the technician to verify that a replacement motor of the same horsepower but higher or lower amps is a satisfactory replacement. The method is to measure the actual amps delivered to the replacement motor in its normal operating state under normal load and compare that measurement only to the nameplate amps for that particular replacement motor.
In summary, using just an amp comparison is not sufficient with certain types of motors. You need to be aware of variations in efficiency and take this factor into account when determining a successful replacement. How often do you encounter this problem in the field? The motor in question continually nuisance trips. You look at the motor and the application: Many fractional horsepower motors come equipped with an internal overload device that is sensitive to both current and temperature.
In situations like this, where there appears to be no mechanical or electrical problem, you may be tempted to blame the thermal device itself. Remember, thermos are extremely reliable, and their job is to alert you to an unseen, but potentially catastrophic problem with the application. For example, you may have inadvertently substituted the wrong motor in the application.
Have you matched the motor to the operating condition? Do the voltages of the replacement motor and the original correspond? A related consideration is determining that you have the right load for the motor. Has the load changed from the original equipment load?
You may find that a larger diameter fan has been applied to the motor, or a larger blower wheel. Or the pulley ratio is different from the original specification. Once again, a clamp-on ammeter can help you reach a correct diagnosis. If a motor in an older application suddenly begins to nuisance trip, you may want to look for blockages in air flow caused by airborne debris. Other air flow problems include applying a motor with excessive horsepower for the load in an air-over application.
Too much horsepower often provides inadequate air flow to dissipate heat—even if the load is light. You may find that added or re-positioned baffles or filters have re-directed air flow or decreased the amount of air flow. Another condition that may cause unexpected thermal tripping is excessive ambient temperature. Almost all motors are designed to produce their nameplate-rated output up to a specific temperature. If the environmental temperature is higher than the nameplate rating, the motor is at risk, even if everything else about the load and power supply fall within normal ranges.
If a motor begins to nuisance trip, consider the environmental temperature issue. With motor-driven devices located in mechanical rooms, check to see if additional heat-generating equipment has been installed in the room. Or perhaps room ventilation has changed, either due to construction or ventilating equipment failure. Consider the effects of foreign material or contaminants. Material can build up on the surfaces of motors, even enclosed designs that prevent foreign materials from entering the motor.
Oily vapors caused by cooking oil or other chemicals can condense on the motor. Dust and lint adhere easily to these oily films, creating a very effective insulation. The results are often higher internal temperatures and thermal tripping. One last item to check is the power supply leading into the motor. Overvoltage and undervoltage can cause the motor to overheat, resulting in nuisance tripping.
To check for this condition, use a volt meter to measure the power supply while the motor and the other equipment on the same circuit are running. Listen to what the thermo is trying to tell you and always take the time to search for the root cause of the problem. In other articles, I have discussed thermal protection, cycling, and preventing mineral build-up in electric motors. In this column, I would like to focus on a related topic: Many motor applications that involve moving air from one place to another, either by fan or by blower, rely all or in part on application air flow to dissipate motor heat.
In other words, the motor and fan or blower must operate as a system. Given the importance of application air in these instances, let's look at some of the impediments to air flow that could make the difference between a short motor life and a long-lasting application. The first thing a service technician needs to consider is whether and where the original motor is located in the air stream.
The replacement motor should be positioned as closely as possible to the location of the original motor. This positioning can be critical depending upon whether the motor is belly-band mounted or cradle-mounted, since these mounting methods will affect freedom of movement. If the motor is driving a fan located in a venturi, pay particular attention to positioning the fan blade with respect to the venturi.
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Single-phase, split-phase, and capacitor-start motors usually incorporate some system of self-cooling, such as an internal or external cooling fan. This makes these motors somewhat less sensitive to positioning when compared with permanent-split-capacitor or shaded-pole motors. Do be aware, however, that some enclosed motors that are split phase or capacitor start, particularly those designated as totally enclosed non-ventilating TENV , also rely on application air for cooling.
A second point to remember is that dirt and chemical deposits-over time-can deteriorate the motor's ability to radiate heat and thus compromise its temperature limits. In open motors, such materials can clog not just the visible vent openings of the motor, but also internal air passages that are often part of the rotor core. Short of burnout, nuisance tripping of the motor's thermal protection may indicate clogging of this nature.
If you notice this type of material build-up, take care to examine and clean the internal air passages as well as clearing any clogged vents. Even totally enclosed motors are subject to deteriorating heat dissipation capacity if dirt and chemical residues accumulate on the exterior of the motor. These materials may act as an insulating blanket, preventing the necessary heat radiation that is part of the motor's cooling design. Simply cleaning this material off the exterior will help restore cooling capacity in these applications.
One often-overlooked consideration is application underload. Underload can cause excessive heat in two ways:. First, underloading a motor designed to move air may not produce enough cooling air to dissipate motor heat;. Second, a motor not operating at its designed load may be operating at less than peak efficiency. This will cause a larger percentage of input power to be turned into heat rather than moving force. We've talked before about the importance of considering nameplate amps when selecting a replacement motor.
This is another instance where it may not be good practice to oversize a motor, since the larger motor may be underloaded. Bigger is not necessarily better in air-moving applications-in fact it could lead to premature failure of the motor. Always be cautious when dealing with replacing motors in air-moving applications. Remember, application air flow is one of those factors that can reduce a motor's life expectancy.
Understanding the characteristics of air flow and how they affect a motor's performance will enable you to select the right design and provide proper motor maintenance in the field. In a perfect world, each electric motor would be percent efficient. In other words, percent of the power input into the motor watts would be converted into work horsepower.
Alas, the world we live in is far from perfect, and that imperfection extends to the motor as well. As a result, whenever you energize a motor, you will get two outputs: That can be a real issue in many cases. For example, many motors used in single-phase applications such as shaded pole motors , barely rise above the 50 percent mark in efficiency. So you know these types of motors will use almost as many input watts to produce heat as produce work.
Equipped with this knowledge, you can understand why one of the criteria you must consider when selecting a motor for an application is the effect of operating temperatures on that motor. A number of universal factors come into play when you deal with operating temperatures, no matter what the application. One of the most fundamental design criteria relating to motor lifespan is the selection of materials used to insulate the electrical parts of the motor and the capacity of those materials to withstand heat.
Insulation is critical to the safe and consistent operation of the motor. If the insulation system fails, the electrical parts become short circuited which causes the winding to break down. The result is motor failure. To help you identify which system is right for a given application, insulating materials are grouped into classes designated with letters that identify the maximum temperature capability of the materials in that class. These identifying letters are virtually universal among motor manufacturers because they are specified by the trade organization, NEMA.
For example, Class A insulation materials are designed to withstand a maximum temperature of 95 degrees Centigrade approximately degrees Fahrenheit in most motor applications. Class B insulating materials must be capable of withstanding maximum temperatures of degrees C or about degrees F. These are the two most common classes of insulation for general-purpose motors. Other classes for example, Class F, Class H exist for unusual, high-temperature applications. Consider a motor that is operating normally.
The temperature of its insulation will be the sum of two components. The first is the ambient temperature in other words, the temperature of the environment surrounding the motor when it is at rest. If that motor is operating in a room, the ambient temperature would be room temperature. The second component is the temperature rise that motor experiences when it converts some of its input power to heat rather than work.