Efficiency Gains - Variable Speed Motors
We may view the system as very inefficient. We’re locked into whatever efficiencies the motor can provide, given a somewhat variable amount of loading. If the motor happened to be an alternating current (AC) motor, typically, the following would be true:
As seen, efficiencies vary as noted above.
If the conveyor motor happened to be 1 HP, we may expect to see only 70% efficiency, at 75-100% motor load. (% Efficiency = output power ÷ input power × 100). By strict definition, the 1-HP AC conveyor motor would be operating at a 30% loss at 75-100% motor load.
--- AC drive and motor efficiencies at various speeds.
--- an example of a 2-HP system. In this example, if we added a variable-speed AC drive, our efficiency of this constant torque (CT) system would be in the range of 80-90% when the conveyor is operated at 60% speed or higher. A conveyor is labeled a constant torque load and is indicated by a CT on the graphs.
It should be noted that the AC drive is an efficient means of varying the speed of an AC motor. Its 5-10% losses are attributed to thermal losses because of the alternating current's switching of power devices several thousand times per second. Variable-speed output from a drive has a direct impact on the total system efficiency. A manufacturer can operate the production equipment at the most efficient speed and load point-if drive and motor efficiencies are known.
--- AC drive and motor efficiencies: Efficiency (% of Rated Value) Motor Speed (% of Rated Speed) Motor Efficiency = cos phi = 0.83 (2 HP) VT drive CT System Drive and Motor Efficiency
Process Changes / Improvements
As previously indicated, in a fixed system there is no way to vary the speed of the conveyor. A fixed system won’t allow for changes in the process or production cycle. Some manufacturing circumstances may require a slow speed, others, a faster pace.
The same conveyor system is used in processes such as baking. --- the same type of conveyor, with the addition of an industrial oven.
Certain materials may require a longer baking cycle because of thickness.
If a fixed-speed motor is used, only one type of material could be processed in this system. To stay competitive, many companies require flexibility in manufacturing. A variable-speed system is often necessary to change production cycle times and increase capacity.
--- Industrial oven used in production: Production Industrial Oven
Warehouse Fixed Speed Motor
--- Manually controlled conveyor system: Production Industrial Oven Warehouse Fixed Speed Motor Operator Station
The system is typical of many manually operated processes. An operator turns on the system and turns it off for maintenance or at the completion of the production cycle. However, in an age of increased flexibility requirements, few processes are manually operated. Production cycles are constantly monitored by some type of computer system.
Computer systems will automatically oversee the process and correct for load fluctuations, material density, and size requirements. In industrial processes, the use of PLCs (programmable logic controllers) is typical. Programmable logic controllers are beyond the scope of this guide, but will be addressed at various points. --- a conveyor system that is manually operated by a control station.
Programmable logic controllers work effectively in place of the manually controlled operator station. Automatic control of the motor could there fore be accomplished, but only STOP and START control, in this case. Variable-speed drives would be effective in providing the flexibility and control needed by motors to meet almost any application requirements.
--- Generic variable-speed drive system: Power Source (Supplies power to the drive); Controller Drive Motor Coupler Machine (Generates and sends a reference to the drive) (Controls the speed, torque and direction of the motor) (Changes one form of energy to rotating energy) (Connects the motor to the machine) (Device that performs the work)
Drive Principles of Operation
At this point, we will look at a variable-speed drive system-from a generic standpoint. All drive systems, whether, electronic, mechanical, or fluid in nature.
To understand a simple drive system, we will start at the end of the system and move backward. We will devote individual sections of this guide to each of the basic components listed. For now, the intent is to develop a basic understanding of a drive system. A foundation will be built, which will allow more complex concepts to be discussed in later sections.
The essence of any drive system is the application, or machine. This is the heart of the system, since it ultimately needs to perform the work. Consider the machine-the application. It could be a conveyor, a press, a pack aging machine, or literally hundreds of applications that operate at variable speed.
The coupler is the device that connects the machine to the motor. Couplers come in all shapes and sizes. Its basic task is to make a solid connection between the motor and the machine. Couplers may accept one diameter of motor shaft and convert the output to another size shaft. In some cases, the coupler may actually be a device called a gearbox, which may include some type of speed-reducing or speed-increasing gears. Couplers could also be considered matching devices because of their ability to deliver power smoothly to the machine. To a certain extent, this device can also cushion shocks delivered by the motor to the machine.
This device changes one form of energy to rotating mechanical energy. It can be considered the prime mover because it takes power from the drive unit and translates it into motion. As we will see shortly, there are several types of motors using various forms of energy. In this guide, we will discuss mechanical, hydraulic, AC, and DC motors. The size of the motor usually dictates the amount of rotating motion it can generate from incoming power. We will see later that there are a few exceptions to this principle.
The drive can be considered the heart of the whole system. This section controls the speed, torque, direction, and resulting horsepower of the motor. The drive is very similar in nature to an automobile drive system.
The transmission and drive shaft controls the speed, direction, and power delivered to the wheels. Much of this guide will be devoted to AC and DC drives. However, we will take a brief look at other types of drive systems that exist in industry.
The drive must have a source of power to operate effectively. If the drive is electrical, it must have either single- or three-phase power available. The drive then accepts this power and modifies it to an output that is usable by the motor. If the drive is hydraulic, the power source could be considered the hydraulic-fluid reservoir, since it supplies the drive with the form of power it needs to accomplish the job.
The controller supplies a reference signal to the drive unit. Typically controllers are electronic and supply a small voltage or current signal to the drive. The larger the signal, the more power the drive generates, and the faster the motor rotates. In many cases, the controller is an automatic device such as a computer. The computer has the ability to take in signals from external devices such as switches or sensors. The controller then processes the signals, does calculations based on the sensor inputs, and generates a reference signal. This output reference signal is usually a speed signal to tell the drive how much power to generate. As we will see later, this is not always the case. The controller could generate an output signal to tell the drive how much power to generate in order to control motor torque or motor shaft position. The operator station can also be considered a controller. Instead of being an automatic device, the operator station provides a signal based on a manually operated switch or speed control set by a human operator.
Types of Drives, Features and Principles
In this section, we will briefly review the different types of variable-speed drives used in industry. For the most part, electronic AC and DC drives find their dominance in manufacturing and commercial HVAC applications of today. This brief look at drive technologies will assist you, should you encounter any of these types in the future. In addition, we will also review the benefits and limitations of each type. The types of drives we will consider are mechanical, hydraulic, and electrical/electronic (eddy current coupling, rotating DC, DC converters, and variable-frequency AC).
--- Mechanical variable-speed drive Constant Speed AC Motor Coupler Variable Speed Output Variable Diameter of the Pulleys Will Dictate Output Speed Mechanical
Mechanical variable-speed drives were probably the first type of drive to make their way into the industrial environment. ---A basic mechanical variable-speed drive.
As seen, the mechanical drive operates on the principle of variable-pitch pulleys. The pulleys are usually spring-loaded and can expand or contract in diameter by means of a hand crank (shown on the left side of the constant speed AC motor). The mechanical drive still gets its power source from an AC power supply-usually three-phase AC. Three phase AC is then fed to the fixed-speed AC motor. The ability to vary the diameter of one or both pulleys gives this drive unit the ability to change its output speed. The principle of variable speed is exactly the same as the gears of a 15-speed bicycle. Shifting gears causes the chain to slip into a wider- or narrower-diameter sprocket. When that happens, a faster or slower speed is achieved with basically the same input power.
Years ago, the benefits of this type of drive were low cost and the ability to easily service the unit. Many technicians liked to work on mechanical problems. The malfunction was rather obvious. However, the benefits of yesterday have turned into the limitations of today. Mechanical devices have a tendency to break down-requiring maintenance and downtime.
The efficiency of the unit can range from 90% down to 50% or lower. This is due to the eventual slipping of the belt on the pulleys (sometimes called sheaves). Sometimes the speed range can be a limitation because of fixed diameter settings, a characteristic of the mechanics of the device. Size can also be a limitation. Typically floor-mounted, this device sometimes stood 3-5 feet tall for general applications. Size and weight could prohibit the use of this device in areas that would be required for mounting a drive.
--- Hydraulic drive: Constant Speed AC Motor; Coupler; Pump Control Valve, Fluid, Reservoir, Hydraulic Motor, Variable Speed, Output
Hydraulic drives have been, and continue to be, the workhorse of many metals processing and manufacturing applications. The hydraulic motor's small size makes it ideal for situations where high power is needed in very tight locations. In fact, the hydraulic motor's size is 1/4-1/3 the size of an equivalent power electric motor. A hydraulic drive.
A constant-speed AC motor operates a hydraulic pump. The pump builds up the necessary operating pressure in the system to allow the hydraulic motor to develop its rated power. The speed control comes from the control valve. This valve operates like a water faucet-the more the valve is open, the more fluid passes through the system, and the faster the speed of the hydraulic motor. Note that this system uses a coupler to connect the AC motor to the pump.
The benefits of this type of drive system is the ability of the hydraulic motor to develop high torque (twisting motion of the shaft). In addition, it has a fairly simple control scheme (a valve), which operates at a wide speed range and has an extremely small size compare to most AC motors of the same power.
However, this type of system has several major limitations. The most limiting factor of this system is the need for hydraulic hoses, fittings, and fluid.
This system is inherently prone to leaks, leading to high maintenance costs. In addition, there is virtually no way to connect this system to an electronic controller. Automatic valve-type controls have been developed, but their use is limited in today's high-speed manufacturing environment.
--- Eddy-current drive system
Eddy-current drives have their roots in the heavy machinery part of industry. Grinding wheels are prime candidates for eddy-current drives.
This system uses an AC-to-DC power-conversion process, which allows variable shaft speeds, depending on the amount of power converted. --- indicates a simple eddy-current drive system.
As seen, an AC motor operates at a fixed speed. This causes the input drum to operate at the same speed. The function of the DC exciter is to convert AC power to DC power. This power is then fed to the coupling field. The coupling field generates a magnetic field based on how much DC power is being produced by the DC exciter. The more power produced, the more magnetic field is produced and the stronger the attraction of the coupling assembly to the input drum. How much power produced by the DC exciter is determined by the speed reference potentiometer (speed pot).
The benefits of an eddy-current system include initial cost and the simple control method (usually 1 speed pot). In addition, this type of system can produce regulated torque because of its ability to fairly accurately control the DC exciter.
However, several limitations dictate where and how this type of system is applied. Heat generation and power consumption are the major issues. For the coupling assembly to magnetically couple to the input drum, a large amount of power must be produced. When power is produced, heat is the by-product, and energy savings are not realized. Compared with other types of variable-speed drives, this type can be several times larger, thereby limiting the locations where it can be mounted. Size is also an issue when maintenance is required on the rotating machinery. Typically on-site repairs are required, which is more costly than shipping the unit back to the repair location.
--- Rotating DC variable-speed drive: Constant Speed, AC Motor Coupler, Field Exciter, Motor Field Exciter Gen. Speed Reference (Pot)
DC Motor Speed Reference (Pot)
DC Generator, DC Motor Gen.
Field Coils, Motor Field Coils
Rotating DC Drives
This system dates back to the mid 1940s. The system also gained the name M-G set, which stands for motor-generator set. As seen, that description is quite accurate.
As seen, the variable-speed system is more complicated than an eddy-current system. The constant-speed AC motor causes the DC generator to produce DC power. The amount of power produced by the generator is dependent on the magnetic strength of the field exciter of the generator. The field exciter strength is determined by the position of the speed pot. As will be shown later, the DC motor requires two circuits in order to operate properly. In this case, the DC generator feeds power to the main circuit of the DC motor (called the armature). The DC motor also needs another circuit called the field. The field magnetism interacts with the magnetism in the main circuit (armature) to produce rotation of the motor shaft. The strength of the field magnetism depends on how much power is produced by the motor field exciter. The field exciter strength is determined by the position of the DC-motor speed pot.
This system has several benefits. Years ago in the rotating machinery industry, this equipment was very traditional equipment. This system also had the ability to control speed accurately and had a wide speed range. It typically used motors and generator equipment that had a very large over load capacity, compared with modern-day motors.
Today, a system of this type, however, would carry several limitations.
Because of the need for three rotating units (AC motor, DC generator, and DC motor), this system is prone to maintenance issues. DC equipment uses devices called brushes, which transfer power from one circuit to the other. These devices need periodic replacement, meaning the machine needs to be shut down. This system is also larger than many of the other variable-speed units. In today's industrial environment, replacement parts are harder to find. The early units used a power conversion device called a vacuum tube (high-temperature electrical conduction), which is very difficult to acquire as a spare part. As to be expected, three rotating units increases the maintenance required on mechanical parts.
--- Electronic DC drive; DC Drive 3 Phase AC Speed Reference (Pot) DC Motor Field Coils Motor Field Exciter
Electronic Drives (DC)
DC drives have been the backbone of industry, dating back to the 1940s.
At that time, vacuum tubes provided the power conversion technology.
Vacuum tubes led to solid-state devices in the 1960s. The power conversion device, called the silicon controlled rectifier (SCR), or thyristor, is now used in modern electronic DC drives. --- indicates the main components of a simple DC drive system.
As seen, the DC drive is basically a simple power converter.
It contains two separate power circuits, much like that of the rotating DC unit. Typically, three-phase AC power is fed to the drive unit. (Note: Some small horsepower DC drives will accept one-phase power.) The drive unit uses SCRs to convert AC power to DC power. The speed pot determines how much the SCRs will conduct power. The more the SCRs conduct power, the more magnetic field is generated in the main DC motor circuit, the armature.
In a DC-drive system, there is always a separate magnetic circuit, called a field. The strength of the magnetic field is determined by the separate motor field exciter, or a permanent magnet. The motor field is usually kept at full strength, although in some cases, the field will be weakened to pro duce a higher-than-normal speed. The interaction between the motor armature and field produces the turning of the motor shaft. We will go into further detail on DC-drive technology later in this guide.
There are some definite benefits to a variable-speed drive system of this type. This mature technology has been available for more than 60 years.
Because electronic technology is used, a wide variety of control options are available.
Monitors such as speed and load meters and operating data circuits can be connected to illustrate drive operation. A remote operator station, including an isolated speed reference and start/stop circuits, can also be connected to the drive. This type of remote control allows commands from distant locations in the building. The DC drive offers acceptable efficiency, when compared with other variable-speed technologies. In addition, DC drives offer a small size power unit and comparable low cost in relation to other electronic drive technologies. However, when comparing electronic DC-drive technology with AC technology, several limitations should be considered.
Probably the largest issue with DC-drive systems is the need for maintenance on the DC motor. As indicated in the rotating DC-drive section, DC motors need routine maintenance on brushes and the commutator bars.
Another issue that is critical to many manufacturing applications is the need for back-up capability. If the DC drive malfunctions, there is no way to provide motor operation, except through connection of another DC drive. In this day of efficient power usage, the DC drive's varying power factor must be considered when planning any installation. Total operational costs (maintenance, installation, and monthly operating costs) may be a limitation when comparing the DC system with the AC-drive system.
--- Variable-speed AC drive 3 Phase AC Speed Reference (Pot) AC Motor AC Drive
Electronic Drives (AC)
Basically, three types of AC drive technologies are currently available.
Though each type differs in the way power is converted, the end result is the use of a variable-speed AC induction motor. All AC drives take AC input, convert it to DC, and change DC to a variable AC output, using a device called an inverter (i.e., inverts DC back to AC voltage). For purposes of this section, we will confine our discussion to a generic AC drive.
--- a generic AC drive and its basic components.
The basic objective involved in an AC drive is to change a fixed incoming line voltage (V) and frequency (Hz) to a variable voltage and frequency output. The output frequency will determine how fast the motor rotates.
The combination of volts and Hertz will dictate the amount of torque the motor will generate.
When we look closer at the principles involved, we find that the AC drive essentially changes AC power to DC power. The DC power is then filtered and changed back to AC power but in a variable voltage and frequency format. The front end section consists of diodes. Diodes change AC power to DC power. A filter circuit then cleans up the DC waveform and sends it to the output section. The output section then inverts the DC power back to AC. This is accomplished through a series of transistors. These are special transistors that only turn on or turn off. The sequence and length in which these transistors turn on will determine the drive output and ultimately the speed of the motor.
With this type of variable-speed system, there are more benefits than limitations. When compared with DC drives, small-sized AC drives are equal to or lower in cost (5 HP or less). The efficiency of power conversion is comparable to that of DC drives. Also comparable is the ability to be con trolled remotely and to have various monitor devices connected. Because of modern transistor technology, the size of the AC drive is equal to or even smaller than that of an equal horsepower DC drive (125-150 HP or less). One major advantage of AC drives is the ability to operate an AC motor in bypass mode. This means that while the drive is not functioning, the motor can still be operating, essentially across line power. The motor will be operating at full speed because of the line power input. But the benefit would be that the system continues to operate with little or no downtime.
There may be a few limitations when considering AC drive technology.
With low horsepower units (above the 25- to 30-HP range), AC drives may carry a higher purchase price. However, the installation costs may be less because of less wiring (there is no separate field exciter). Some applications, such as printing and extrusion, lend themselves to DC technology.
Comparable AC drives may need to be sized 1 or 2 HP frame sizes higher to accommodate the possible overload requirements. "Torque Control AC Drives" is devoted to flux vector and torque-con trolled AC drives. More detail is presented on the issue of overload, torque control, and AC/DC drive comparisons. Today's AC-drive technology can provide impressive response, filling the application needs that traditionally used DC drives.
There are various types of variable-speed drive systems. There are many reasons to use variable-speed drives, but basically they fall into three categories: efficiency gains, process changes and improvements, and system coordination. For example, efficiency of AC motors can be quite high, which reduces the overall monthly cost of operating the system. Variable speed drives also allow for changes in the process, as well as process improvements. Some processes operate at less than full speed, so optimum product quality can be achieved. System coordination is a major factor in today's industrial environment. AC- and DC-drive systems are typically applied in a manufacturing process. Computers control the entire process, from infeed rate to output of the machine. Today's electronic drives offer easy connection to many types of automated equipment.
A generic drive system includes the following components: machine, coupler, motor, drive, controller, and power source. No matter what type of system is discussed, these main components are involved.
Various types of variable-speed drives are available in industry. The basic categories are mechanical, hydraulic, and electrical/electronic. Electronic drives can be further divided into the following categories: eddy current, rotating DC, DC converters, and variable-frequency AC.
Each type of variable-speed drive system has its set of benefits and limitations. The trend today is moving away from mechanical and hydraulic types of variable-speed systems, and toward electronic systems. The reasons are again identified in the ability to control the process by computerized systems. This also allows for quick changes in the process to meet the rigorous demands of production schedules.
1. What is a drive?
2. What are three reasons why variable-speed drives are used?
3. Name three factors that cause the efficiency of an AC motor to improve.
4. Coordination of variable-speed drive systems in industry are typically controlled by what type of device?
5. Name the basic parts and functions of a variable-speed drive system.
6. Name the categories of variable-speed drives and their principles of operation.
7. What are the two separate electrical circuits in a DC-drive system?
8. What three principles are involved in the operation of an AC drive?
-- Answers --
1. A drive is a device that controls the speed, torque, direction, and resulting horsepower of a system.
2. Efficiency gains, process changes and improvements, and system coordination.
3. Motor load, the horsepower rating, and the speed of operation.
4. A programmable logic controller (PLC) or other computer device.
5. Machine-device that performs the work; Coupler-connects the motor to the machine; Motor-changes one form of energy to rotating; Drive-controls the speed, torque, and direction of the motor; Controller-generates and sends a reference to the drive. Power Source-supplies power to operate the drive.
6. Mechanical, hydraulic, and electric/electronic:
Mechanical-device that uses belts, chains, and pulleys to change the output speed through increasing and decreasing of pulley diameters.
Hydraulic-device that uses fluid and a pump to operate a hydraulic motor. Control of motor speed is done through a valve.
Electric/electronic-device that controls the speed of an electric motor by means of converting one form of energy to another. (An AC drive converts fixed power to a variable-frequency output. A DC drive converts AC power to variable-voltage DC power.)
7. The armature supply and the field supply.
8. Changing AC power to a fixed-voltage DC power, filtering the DC wave form, and inverting fixed DC voltage to a variable voltage and frequency AC output.
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It's HOT, and an article about keeping things cool is appropriate...
An electric motor’s insulation system separates electrical components from each other, preventing short circuits and, thus, winding burnout and failure. Insulation’s major enemy is heat, so it’s important to be sure to keep the motor within temperature limits. There is a rule of thumb that says a 10-degree Celsius (18-degree Fahrenheit) rise reduces the insulation’s useful life by half, while a 10° C (18° F) decrease doubles the insulation’s life. This implies that if you can keep a motor cool enough, the winding will last forever. However, that thinking ignores factors such as moisture, vibration, chemicals and abrasives in the air that also attack insulation systems.
The real issue is at what temperature the motor windings are designed to operate for a long and predictable insulation life — 20,000 hours or more. The National Electrical Manufacturers Association (NEMA) sets temperature standards based on thermal classes, the most common being A, B, F and H. The table below tin this article provides a summary.
Class B or Class F insulation systems are usually used in today’s industrial-duty NEMA “T frame” motors. Many manufacturers also design their motors to operate cooler than their thermal class might allow. For example, a motor might have Class F insulation but a Class B temperature rise. This gives an extra thermal margin. Class H insulation systems are seldom found in general-purpose motors but rather in special designs for very heavy-duty use, high ambient temperature or high altitudes.
Class A insulation, while not used on today’s industrial-duty motors, was standard on industrial “U frame” motors built in the 1960s and earlier. Because Class A insulation has such a low temperature rating, older motors were required to have far lower maximum temperatures. This accounts for the perception among many long-time motor users that modern motors “run hot.” In fact, they do when compared with older motors, but modern insulation systems are so much better that the reliability and durability of new motors are equal to or better than older-design motors. Plus, better insulation systems allow motor manufacturers to put more horsepower in a smaller package.
Insulation Classes and Their Thermal Ratings
Maximum Winding Temp.
*Most common classes for industrial-duty motors
Table shows highest allowable stator winding temperatures for long insulation life. Temperatures are total, starting with a maximum ambient of 40° C (104° F).
Determining Correct Operation
Though many people believe they can judge a motor’s operating characteristics by feeling its surface, this isn’t a very effective method. Design ratings for temperature apply to the hottest spot within the motor’s windings, not how much of that heat is transferred to the motor’s surface. Unless you have intimate knowledge of a specific motor model’s design — including benchmark lab readings of heat runs that show “normal” surface temperatures for that specific model in exact locations on the frame — a motor’s “skin temperature” provides little, if any, evidence of what’s going on inside. This is true even if temperature measurement methods far more sophisticated than the human touch are used. In addition, for safety reasons, it’s unwise to touch operating motors anyway.
Specifying motors with inherent overload protectors, thermostats or resistive temperature devices, or installing similar protection in motor controls, can help ensure that a motor is taken off-line before winding damage occurs. Motor protection of one sort or another is advisable in almost any application. A common and reliable field test for motor heating involves checking the motor’s amperage draw with a clamp-style ammeter. Use this to confirm that actual amps are less than or equal to the nameplate rating. A precise test for winding temperature is the resistance method. This involves careful measurements with sensitive equipment, calculations and several hours of time. Procedures to conduct such tests can be found in technical manuals. Or, contact your motor manufacturer.
Sometimes a motor overheats because of a manufacturing or design defect. But far more often, overheating can be traced to misapplication. Overloading is the leading cause. This could take the form of using an undersized motor, a situation that may become more common as concern for energy efficiency puts the emphasis on eliminating oversized motors. Use an 80 percent loading as your guide. Most electric motors reach their peak efficiency at that load, and a comfortable overload margin remains.
Other common causes of overloading include a load seizing up or misalignment of power transmission linkages. Plus, unanticipated changes in environment, aging of equipment, misuse and other factors can subject the motor to stresses for which it was not intended.
Environmental conditions that can result in motor overheating include high ambients (especially look at the near vicinity of the motor for any heat-generating device) and high altitudes (above 3,300 feet or 1,005.84 meters, where the thin air has less cooling potential). You might have to derate a motor under these conditions, probably choosing the next size up. Another environmental concern is dirt and fibers, which can clog ventilation openings, coat heat-dissipating surfaces and cause a variety of mechanical problems.
Power supply problems are another overheating cause. Low voltage will result in the motor drawing higher current to deliver the same horsepower, and the higher current means higher winding temperatures. A 10 percent drop in voltage could cause nearly that much rise in temperature. Excessive or sustained high voltage will saturate a motor’s core and lead to overheating, as well. In three-phase motors, phase imbalances can result in high currents and excessive heat, the extreme being the complete loss of voltage in one phase (called single phasing), which, if correct protection is not in place, will result in motor burnout.
Often overlooked as a cause of overheating is the number of start/stop cycles. It’s not uncommon for a motor at starting to draw five times or more the current it does while running. This accelerates heating dramatically. Though various provisions are made relative to loading and off-time, NEMA essentially limits a three-phase continuous-duty motor to two starts in succession before allowing sufficient time for the motor to stabilize to its maximum continuous operating temperature rating. This is highly dependent on application, so it’s best to check with your motor manufacturer if you’re facing a high-cycle application.
Finally, pay special attention when applying adjustable-speed inverter drives, especially if you are introducing an inverter in a system of older motors. Some additional heating to the motor windings will inevitably occur because of the inverter’s “synthesized” AC waveform. A greater cooling concern involves operating for an extended time at low motor speed, which reduces the flow of cooling air. Modern inverter-duty motors have higher insulation ratings to help alleviate this concern, and the robust insulation systems used in most of today’s general-purpose industrial motors are also adequate for many applications. In extreme cases, though, a secondary cooling source may be required.
Industrial Motor Service is an authorized re-seller for LEESON Electric Corporation, a manufacturer of motors, gearmotors and drives. To learn more, visit www.IndustrialMotorService.com or call 864-226-2893.
FIVE IMPORTANT TIPS FOR YOUR PREVENTATIVE MAINTENANCE CHECKLIST!
It is important to ensure electric motors perform well because they have a massive impact on a business' productivity and profit. Although operating these motors may seem straightforward and simple, their condition should not be overlooked. This is why it is essential to perform preventive maintenance (PM) checks on electric motors as a part of managing facility assets.
By preparing a checklist for PM program, facilities can make sure that every motor is properly examined and monitored. This also provides managers with an opportunity to detect potential issues and address these ahead of time. By doing so, costly repairs or unplanned expenses can be prevented in case there is a need to replace motors completely.
These five components are essential for a PM program and must be implemented regularly by a business owner.
1. Perform visual inspections on the motor
There are so many things to discover by just conducting a visual check on an electric motor. Take a good look at its physical condition and be sure to record any pieces of information. If the motor has been operating in a rugged environment, it is possible to find signs of corrosion or dirt buildup on its individual components. These all present a potential internal problem since any debris can limit the performance of the equipment.
Make it a point to observe the motor windings and look for a burned odor from overheating. The contacts and relay should also be free from dirt and rust, which are detrimental to the life of the motors. Situate the equipment in an environment without exposure to dirt, moisture, toxic elements, and harsh conditions.
2. Maintenance checks on the commutator and brush
Do not wait until the electric motors stop working or experience inconsistencies in performance. As a part of the PM schedule, users should take a closer look at the brush and commutators. Make sure there are no signs of wear and tear. An excessive wear in the brush can lead to commutation problems with the motor. This is why the brush will need to be changed to regain the integrity of the equipment's function.
In the same way, the commutator needs to be kept in check. Its natural condition is smooth and polished. It should also have no dents, scratches, or grooves since any rough spot suggests brush sparking. Make a thorough inspection of the motor mount, stator, rotor, and the belts. Replace any worn components, which no longer serve their purpose.
3. Conduct a motor winding test
After the different machine components have been inspected, the next thing to do is test the motor windings. This will give the user a better idea on existing anomalies or failures in the motor windings. Moreover, if burn marks and odors, as well as cracks in the windings have been discovered, motor winding tests are mandatory.
To prepare for the test, be sure to disassemble the motor. This will help determine any abnormalities that the motor has been undergoing. In case the windings have experienced overheating, then there is a high chance that a serious damage is present. Rewinding the motor is a crucial part of this test, along with the testing of the wind insulation that reveals information on the resistance level.
4. Check the bearings
Inspect the bearings if there is any vibration or noise. These are signs of potential problems including dirt buildup, poor lubrication, or wear and tear. The bearing housing may also end up too hot to the touch. This could signal issues such as an insufficient amount of grease or overheating of the motor.
Depending on the bearing type, a specific PM task might be necessary. Other factors include the motor application and the environment where the equipment is situated. There are some motors with a low horsepower that no longer need lubrication as these have sealed bearings. Managers have to be aware of the type of bearing and the kind of repair it requires.
5. Keep records
Each time PM schedules occur, users should document the tests performed, and the results gathered for the purpose of establishing trends. Record all repairs or replacements made on every motor component. This creates a better understanding of each piece of equipment, which includes issues addressed or parts replaced. This will be handy for future inspections.
Industrial Motor Service can assist you with this important aspect of Plant Operations and Facilities Management. Call on us to assist or to execute your Preventative Maintenance Program to avoid costly shutdowns or repairs!
A person who has had the pleasure of seeing the inner workings of a pool pump (on their own workbench or looking over the shoulder of a technician) may have noticed it looks simpler than expected. Considering the pump is this piece of machinery that can pump 80 gallons or more a minute, leaves one to expect a few SECRETS hidden away inside the casing. But in practice the pool pump can be broken down into two categories of parts; the drivetrain and the outer structure. Or, the parts that push and pull the water and the ones that keep it from leaking.
This is an informal guide to identifying pool pump parts, so a "Do-it-yourselfer" can either fix it themselves or at least understand the terminology when talking to a tech. Let’s break down the parts of a pump and see how they contribute to the overall functionality.
Inside a Pool Pump Housing
All of the pump’s inner workings – the impeller, diffuser, seals, and motor – fit in or onto this outer casing. The standard housing material is currently the high impact plastic composite called Noryl. This resilient, lightweight material is rustproof and holds up extremely well under duress from heat, rain and water pressure.
Older pumps (early 80’s and back) were made of brass or bronze; these materials proved to be extremely durable, but costly to maintain. A plastic injection molded impeller is going to be much less expensive than a brass version that has to be smelted and forged. Also, the switch to plastic allowed major weight reduction as those metal pumps weigh upwards of 100 pounds. Metal pumps were really durable workhorses and can still be found on pools today.
The strainer lid is the pump’s main inspection point from which a tech or homeowner can determine a system’s health. If we find large air bubbles or no water at all in the strainer lid while the pump is running, that could be a sign of an air leak along the suction side. Because of its necessity as an inspection point, the lid is made of a clear or tinted Lexan glass. If you are unfamiliar with Lexan, it is the material used in bulletproof windows and other glass-like materials.
The strainer basket collects debris before it reaches the important parts of the drivetrain. This basket can save you a bundle just by preventing hard debris like pebbles from surging into the pump and chipping the impeller.
Gaskets & Seals
Pool Pump Gaskets
The gaskets and seals are what keep your equipment pad dry and your water moving. A bad set of gaskets can have your equipment area flood or a pump filled with air rather than water. There are four main gaskets and seals on a pool pump:
Lid Gasket – located on or under the strainer lid.
Diffuser Gasket – Found on the cone tip of the diffuser.
Housing Gasket – Largest of the gaskets and found in the seam between the main housing on the motor seal plate. This gasket is also called seal plate gasket.
Shaft Seal – The two sided seal that sits underneath the impeller on the shaft of the motor. This is the most important seal as it prevents water from surging into the motor and causing fatal damage.
The seal plate is a motor’s mounting flange that allows it to be secured to the pump housing. The seal plate is named so because it houses the shaft seal that encloses around the shaft to prevent water leaks into the motor. This is made of the same Noryl plastic as the housing, making it lightweight and super strong.
The Drive Train
The driving force behind the pump is its motor which creates the churning force necessary to prime the pump and circulate water. The standard is single speed induction motors, but they are slowly losing ground to dual speed and variable speed motors. Variable speed and dual speed have grown into popularity due to their energy efficiency and have also been bolstered by electric companies offering rebates for homeowners who choose to upgrade.
Most single speed motors run a dual voltage setup which allows them to run on either 230 or 115 voltage (user must adjust voltage before installing). Variable speed and most dual speed run strictly on 230 voltage; there are some exceptions in the dual speed category that run on 115.
The impeller is what makes all the magic happen; magic meaning, transforming the spinning shaft of the motor into the pulling force for pumping. An impeller is essentially two discs glued together to sandwiching fan blades (also called veins). The front disc has an opening with a porthole to focus the churning power of the impeller towards the suction pipe to the pool. The water is pulled through the impeller face and then expelled through the impeller’s slotted sides – it’s H2O’s version of a merry-go-round. The water is then rushed back to the pool.
We call this an impeller accessory and a very important one at that. A diffuser amplifies the pull of the impeller by creating a tightly enclosed vacuum lock to the front housing to maximize its power. The diffuser resembles a funnel or a cone that shrouds the impeller; its tip butts up to the front housing, sealed by the diffuser gasket. As the impeller spins, the diffuser shroud concentrates the turbulent energy of the impeller towards the suction pipe which makes the pump prime.
The impeller ring, aka the wear ring, is a plastic ring that fits to the tip of the impeller. The purpose of a wear ring is to act as an extension of the impeller tip to ensure a seal between the impeller and diffuser. The centrifugal force of the impeller forces the wear ring to affix to the diffuser to ensure an even tighter vacuum for priming and pumping. Wear rings are not found on every pump style but are usually seen on high pressure and high head style pumps, i.e. Hayward Super II or Pentair Challenger.
The impeller screw is meant to secure an impeller to internal threaded shaft motors. The impeller screw is quickly becoming obsolete as most manufacturers switched to external thread motor shafts. This change to external threads allows for the impeller to be screwed directly to the motor shaft without the need for extra securing. The thread type of the new impellers also means the impeller is being spun in the direction that keeps it tight to the motor to prevent any slippage.
If you’re still in need of more information, please do not hesitate in calling us at 864.226.2893 at Industrial Motor Service. We’re here to help!
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