This chapter delves into the core concepts of electric motors, examining their construction and how they operate.
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An electric motor is a mechanism that converts electrical energy into mechanical energy. It achieves this by generating a magnetic field through an electric current passing through wire coils, which creates torque on the motor shaft. Although an electrical generator is mechanically similar to an electric motor, it functions oppositely, turning mechanical energy into electrical energy.
Electric motors can be powered by direct current (DC) sources like batteries or rectifiers, or by alternating current (AC) sources such as power grids, generators, or inverters. They can be categorized based on power supply type, intended usage, construction, and output type. Motors may be DC or AC, either brushless or brushed, and can function on single-phase, two-phase, or three-phase power. Additionally, they can be designed for radial or axial flux and may utilize liquid or air cooling solutions.
Standard electric motors are prevalent in industrial applications, finding utility in equipment like blowers, pumps, industrial fans, machine tools, power tools, household appliances, disk drives, and vehicles. Smaller motors are employed in devices such as electric watches. Notably, in specific applications such as regenerative braking in traction motors, electric motors can act as generators to reclaim energy otherwise dissipated as heat and friction.
Electric motors comprise two main mechanical sections: the stator, which is stationary, and the rotor, which rotates. Key electrical components include the field magnets and the armature. One component attaches to the stator, while the other connects to the rotor, creating a magnetic circuit.
Field magnets, which can be permanent or electromagnets, produce a magnetic field interacting with the windings. Typically, these magnets are mounted on the stator, with the winding on the rotor, although this configuration can be reversed in certain motor designs.
Bearings support the rotor and facilitate its smooth rotation along its axis. The motor housing is responsible for supporting these bearings.
The rotor, which is the moving section, is crucial for delivering mechanical power. It generally has conductors that carry current, interacting with the stator's magnetic field to produce rotational force on the shaft. In some designs, the rotor might include permanent magnets, while conductors are positioned in the stator, leading to enhanced efficiency over a wide power and speed range.
The air gap between the rotor and stator allows for rotor movement. This gap's size significantly influences electrical performance, with a narrower gap generally enhancing efficiency and the power factor, while a wider gap increases energizing current and decreases the power factor. Conversely, if the gap is too narrow, it can cause mechanical issues, extra losses, and more noise.
The motor shaft continues through the bearings to the motor's exterior, where it accommodates the load, creating an overhung load due to the force applied beyond the furthest bearing.
The stator surrounds the rotor and typically holds the field magnets, which could be electromagnets with wound wire on an iron core or permanent magnets. These magnets generate the magnetic field that interacts with the rotor winding to produce force. The stator's iron core is made from thin, insulated metallic sheets called laminations to minimize energy loss.
Lamination helps reduce energy losses that are prevalent with a solid core. In resin-packed motors, frequently used in appliances like air conditioners or washing machines, plastic materials reduce vibrations and noise.
The armature comprises wire wound around a ferromagnetic core. When current flows through, it generates a magnetic field exerting a Lorentz force on the armature, turning the rotor to produce mechanical power. The windings, often wire coils, typically wrap around a soft, laminated iron core, creating magnetic poles as current is applied.
Motors can be either non-salient or salient-pole types. In salient-pole motors, both the rotor and stator have cores with protruding poles, each wrapped in wire forming north or south poles when current flows. Non-salient-pole motors (round-rotor or distributed field motors) maintain a cylindrical core with evenly distributed winding around it. The alternating current in these windings generates continuously rotating poles. Shaded-pole motors possess windings that partly cover each pole, causing a phase delay in the magnetic field for that pole.
A commutator, a rotary switch, provides either AC or DC to the rotor, reversing current in the rotor winding regularly as the shaft revolves. It consists of a cylinder with metal segments on the armature. Brushes, typically made from conductive materials like carbon, press against the commutator, ensuring sliding electrical contact and delivering current to the rotor.
The segments periodically reverse the rotor windings' current every 180°, maintaining the torque direction consistent. Without this, the torque direction changes each half-turn, stalling the rotor. Due to inefficiencies, commutator-based motors are being replaced by brushless DC motors, induction motors, and permanent magnet motors.
Electric motors transform electrical energy (AC or DC) into mechanical energy to create movement. This transformation involves interaction between current flowing through windings and a magnetic field. As the current's strength grows, so does the magnetic field's strength. From Ohm's Law (V = R*I), if resistance rises, the voltage must also increase to maintain equal current.
Consider the following when selecting an electric motor:
Voltage – Decide if the motor will be powered from a wall socket or batteries. For wall sockets, the typical voltage may be 230VAC or more in industrial settings.
Frequency – Motors usually operate at 60Hz in the U.S. If used outside the country, you may need a 50Hz motor.
Speed – Assess the needed speed range. If adjustable speeds are critical, ensure the motor has suitable control capabilities.
Torque – Evaluate the starting torque needed and any variations during operation. Consider maximum necessary torque under adverse conditions.
Power – Specify if the motor will operate at maximum power capacity.
Duty Cycle – Recognize the motor’s duty cycle. For continuous operation, ensure it can handle extended use without overheating. For intermittent tasks, a smaller motor may suffice, assuming it can fully cool between cycles.
Lifetime – Applications with sporadic use might suit Universal or DC motors, which generally have shorter lifespans and higher maintenance needs. Operations requiring prolonged duration benefit from brushless DC or AC motors, known for longevity and reduced maintenance.
The different types of electric motors provide essential functionality for countless industrial, commercial, and residential applications. Understanding the distinctions between these motor types is crucial for anyone researching, specifying, or purchasing electric motors for projects varying from home appliances to large-scale manufacturing systems. Below, we explore the primary motor categories, highlighting how they operate and common use cases, while incorporating key search terms such as “AC motors,” “DC motors,” “industrial electric motors,” “servo motors,” and “stepper motors.”
A DC motor is a type of rotary electrical machine that converts electrical energy from direct current (DC) into mechanical energy. These motors operate based on the force generated by magnetic fields. Most DC motors incorporate internal mechanisms—either electronic or electromechanical—that periodically change the direction of the current in a part of the motor to facilitate its operation.
DC motors were the first kind of motor commonly utilized, as they can be powered from present direct current lighting energy distributing systems. DC motor speed may be controlled over a broad range, utilizing either a variable voltage supply or by altering the current strength in its field winding. DC motor speed controllers enable precise speed adjustment, making these motors ideal for applications needing variable speeds. Small DC motors are applied in appliances, toys, and tools. The universal DC motor can work on direct current but it is a light brushed motor utilized for portable appliances and power tools. Large DC motors are presently utilized in the propulsion of elevators and hoists, electric vehicles, and in drives for rolling mills for steel. With the arrival of power electronics, replacement of DC motors with AC motors has been made conceivable in many applications, offering increased efficiency and lower maintenance.
A 12V DC motor is compact and affordable, yet it provides sufficient power for a wide range of uses, including robotics, automotive systems, HVAC units, and hobby electronics. Its operating voltage is a notable feature, with lower voltages generally being preferable when using batteries, as fewer cells are needed to reach the desired voltage. Nonetheless, higher voltages often offer better efficiency in electronic motor drives. While DC motors can operate from as low as 1.5V up to 100V, the 12V, 6V, and 24V variants are among the most common and readily available for various applications. Important specifications for a 12V DC motor include its torque, speed (RPM), current consumption, and power output, which are essential for accurate selection and integration.
Typically, 12V DC motors are brushless, meaning they operate without the need for brushes to transfer electrical current. In contrast, brushed motors can encounter issues due to their more complex design and higher maintenance requirements. The brushless design of the 12V DC motor has resolved many of these problems, resulting in longer lifespan and improved reliability. Key components of a 12V brushless DC motor include an external rotor with permanent magnets and either a single coil or multiple coils (usually three-phase for optimal performance). Additional components might include integrated drive electronics and various feedback sensors.
Rather than using brushes, 12V DC brushless motors use sensors, such as Hall Effect sensors, to manage commutation and optimize current flow. Although some 12V DC motors might refer to AC types, this is rare compared to the predominance of 12V DC motors for portable, battery-operated, and low-voltage drive applications. Battery-powered robotics, electric actuators, and lightweight electric vehicles commonly utilize 12V brushless DC motors for their high efficiency and dependable, quiet operation.
A brushless motor, commonly known as a BL motor or BLDC motor, and sometimes referred to as an electronically commutated motor (ECM), is a type of DC motor. It may also be called a synchronous DC motor. This motor operates using direct current (DC) power and relies on an electronic controller to manage the current flowing through the windings. This process generates a magnetic field that rotates in space, causing the rotor with permanent magnets to follow suit. The electronic controller regulates the amplitude and phase of the DC current pulses to control the motor’s torque and speed. This system replaces the traditional brushes and mechanical commutators found in many conventional electric motors, greatly reducing friction and wear and increasing motor efficiency and lifespan.
Brushless DC motors are manufactured in a manner akin to permanent magnet DC motors, but they can also be designed as asynchronous or induction motors, or as switched reluctance motors. They may incorporate neodymium magnets and come in various configurations, including out-runners (where the rotor encloses the stator), in-runners (where the stator encloses the rotor), or axial motors (where the stator and rotor are aligned parallel and flat).
Compared to brushed motors, brushless motors offer several advantages, such as higher speeds, improved power-to-weight ratios, near-instantaneous torque and speed adjustments (rpm), reduced maintenance requirements, and greater efficiency. They are commonly used in computer peripherals (like printers and disk drives), portable power tools, and a range of vehicles from cars to model aircraft. In modern washing machines, brushless motors have replaced traditional gearboxes and rubber belts with a direct-drive system.
Common search queries within the industry regarding brushless motors include: “brushless motor efficiency,” “brushless motor advantages over brushed motors,” and “applications of BLDC motors.” For buyers, key considerations include voltage range, power output, control systems, and compatibility with variable speed drives. Employing brushless electric motors is often the best choice for efficiency-driven applications or those requiring minimal maintenance and extended operational lifespans.
A stepper motor, also known as a stepping motor or step motor, is a type of brushless DC motor that divides a full rotation into discrete steps. Unlike traditional brushed DC motors, which rotate continuously when a direct voltage is applied, a stepper motor moves in precise increments based on input pulses. These motors can be controlled to move and maintain their position at each step without requiring feedback from a positional sensor, provided the motor is correctly matched to the application’s speed and torque requirements. Each pulse results in a specific rotational movement of the motor shaft, enabling highly accurate control for “open-loop” motion control systems.
Stepper motors consist of several toothed electromagnets arranged around a central rotor, which is typically a gear-shaped iron core. These electromagnets are activated by a microcontroller or an external driver circuit.
To initiate rotation, one electromagnet is energized, attracting the gear's teeth through magnetic force. As the teeth align with the activated electromagnet, they are slightly misaligned with the next electromagnet in sequence. When the next electromagnet is turned on and the initial one is deactivated, the rotor moves incrementally to align with the new electromagnet. This process is repeated continuously, with each incremental movement referred to as a "step." By repeating this sequence, the motor can achieve precise angular rotation, with a full rotation completed through a specific number of steps.
Common LSI search terms such as “stepper motor driver,” “microstepping,” and “stepper vs servo motor” are important for users researching motion control solutions. Stepper motors are widely used in CNC machines, 3D printers, and process automation systems, where consistent and repeatable positioning is crucial. When evaluating stepper motors for your application, assess parameters such as holding torque, step angle, voltage rating, and compatible controllers or drivers.
An AC motor operates on alternating current (AC) and consists of two main components: the stator and the rotor. The stator is the outer part of the motor and contains coils that are energized by AC to produce a rotating magnetic field. Inside, the rotor is connected to the shaft and generates its own magnetic field, which may be created by various means including reluctance saliency, permanent magnets, or AC/DC windings.
AC linear motors, though less common, function on principles similar to those of rotating AC motors. However, instead of creating rotational movement, AC linear motors are designed with their moving and stationary parts arranged in a linear configuration, resulting in linear motion rather than rotation. These are often found in automation lines and specialized conveyor systems.
The two primary kinds of AC motors are synchronous motors and induction motors. The induction or asynchronous motor always depends on a small variance in speed between the rotor shaft speed and the stator spinning magnetic field called slip which induces rotor current in the AC winding of the rotor. Therefore, the induction motor can’t create torque close to synchronous speed where slip (induction) ceases to exist or is irrelevant. In comparison, a synchronous motor doesn’t depend on induction of slip for functioning and utilizes either salient poles (projecting magnetic poles), permanent magnets, or an individually excited rotor winding. The synchronous motor creates its rated torque at exactly synchronous speed. The brushless wound-rotor double supplied synchronous motor mechanism has an individually excited rotor winding which does not depend on the rules of slip induced current. The brushless wound-rotor double supplied motor is a synchronous motor which can work exactly at the power source frequency. Other kinds of motors involve eddy current motors and DC and AC mechanically commutated machinery where speed relies on winding connection and voltage.
Popular queries for AC motors include “single phase vs three phase motor,” “induction motor applications,” and “ac motor vs dc motor efficiency.” When choosing AC motors, important factors include supply voltage, phase configuration, frequency, and whether variable speed motor control—such as with variable frequency drives (VFDs)—is required for your equipment or process.
Horsepower (HP) is a commonly used unit to measure the rate at which mechanical energy is used. One horsepower is approximately equivalent to 746 watts (W) or 0.746 kilowatts (kW). Although horsepower and kilowatts can be converted to comparable units, horsepower is predominantly used for measuring mechanical power and is rarely used for other forms of power measurement.
Electric motors typically have nameplates that show their output power, not the input power. This refers to the power delivered at the shaft, rather than the power required to operate the motor. Output power is usually measured in watts or kilowatts. In the United States, however, output power is often expressed in horsepower, with one horsepower defined as exactly 746 watts. Motors rated at 1 HP can be either AC or DC types and are commonly used in applications such as vehicles, electric boats, commercial hydraulic pumps, and agricultural equipment. When selecting a 1HP motor, consider its efficiency rating, starting torque, and compatibility with supply voltage.
Similar to a 1HP motor, a 2HP electric motor provides power directly at its shaft, equating to approximately 1.49 kW. It can be either a DC or AC motor, available in single-phase or three-phase configurations. A 2HP motor is commonly used for driving boat propellers, industrial cooling fans, or as an induction motor in various applications. These motors often meet the demand for higher output required in industrial production lines, material handling, and heavy-duty pumping systems. When evaluating 2HP electric motors, key considerations are rated speed (RPM), duty cycle, enclosure protection (TEFC vs. ODP), and energy usage.
Three-phase motors are a type of AC motor and are a specific example of induction motors, also known as asynchronous motors. These motors are made up of three primary components: the rotor, the stator, and the housing.
The stator consists of a series of steel laminations wrapped with wire to form induction coils, each corresponding to one of the three phases of the power supply. Each coil receives power from the three-phase electrical input, creating a rotating magnetic field essential for efficient operation in industrial environments.
The rotor also comprises metal bars and induction coils connected to make a circuit. The rotor surrounds the motor shaft and it is the motor part which rotates to make output mechanical energy of the motor.
The motor's enclosure houses the rotor and its shaft on a series of bearings to reduce friction during rotation. The enclosure features end caps that secure the bearing mounts and includes a fan connected to the shaft. As the shaft spins, the fan draws in external air and circulates it through the rotor and stator to cool the motor components and expel heat generated by the coils' resistance. Additionally, the enclosure often has external cooling fins to enhance heat dissipation. The end caps also provide access for the electrical connections required for the three-phase power supply.
Three-phase motors are widely used for heavy-duty industrial applications, including conveyors, compressors, water pumps, large fans, and machine tools. This popularity stems from their superior reliability, smooth power delivery, efficiency, and ability to handle high loads—making them the standard in most manufacturing and processing plants.
A single-phase motor is an electrical rotary device that transforms electrical energy into mechanical energy, powered by a single-phase electrical source. It operates using two types of wires: live and neutral. These motors can handle power up to 3kW, with input voltage varying accordingly. They operate with a single alternating voltage and involve a circuit with two wires where the AC current remains consistent. Typically, single-phase motors are compact and designed for applications requiring lower torque, such as residential pumps, washing machines, and small fans.
However, some single phase motors have a power of up to 10 HP which can operate with connections reaching up to 440V. They do not produce spinning magnetic fields, they can only produce an alternating field, which implies that for start-up they require a capacitor. As a result, many single-phase motors are classified as “capacitor start” or “capacitor run” designs, key LSI terms for this topic. They are simple to maintain and repair, as well as affordable. This type of motor is utilized mainly in offices, stores, homes, and non-industrial small companies. Their most popular uses include home and business HVAC, home appliances and other appliances like drills, air conditioning systems, and garage door closing and opening systems.
When comparing single-phase and three-phase motors, the latter are generally chosen for commercial and industrial environments where efficiency and capacity are critical. In contrast, single-phase motors are a reliable and economical choice for household and light-duty applications.
Industrial electric motors transform electrical energy into mechanical energy and are capable of generating either rotary or linear motion. While some industrial motors are powered by direct current (DC), it is more common for them to be driven by alternating current (AC) sources, such as the power grid or generators. These motors play a vital role in automation, assembly lines, and material handling systems across manufacturing sectors.
Industrial motors are comprised of several key components including the rotor (also known as the armature), stator, air gap, winding (or coil), and commutator. These motors come in various types such as DC synchronous, AC synchronous, and AC induction (asynchronous), among others. Other key terms and search phrases for this domain include “industrial automation motors,” “high torque motors,” and “energy-efficient industrial motors.”
When selecting motors for industrial applications, consider characteristics such as duty cycle, power factor, starting/stopping frequency, and integration with modern variable frequency drives (VFDs) for enhanced control.
A servomotor or servo motor is a linear actuator or rotary actuator which enables precise control of linear or angular position, acceleration, and velocity. It comprises an appropriate motor coupled with a sensor for feedback of position. It also needs a relatively complex controller, usually a dedicated device designed especially for usage with servomotors.
Although servomotors do not constitute a specific category of motors, the term is commonly used to describe motors suited for closed-loop control systems. These motors are frequently used in applications such as CNC machinery, robotics, and automated manufacturing, where high accuracy and repeatability are required.
The specific type of motor is less critical in a servomotor, as various motor types can be employed. Typically, basic brushed DC motors (with permanent magnets) are used due to their affordability and simplicity. Smaller industrial servomotors often feature brushless, electronically commutated designs. In larger industrial applications, AC induction motors are commonly employed, sometimes paired with variable frequency drives (VFDs) for speed regulation. For high performance in a compact form factor, brushless AC motors with permanent magnets are used, resembling scaled-up versions of brushless DC motors.
Termed a staple in “precision motion control” and “automation systems,” servo motors enable modern assembly lines, medical devices, camera positioning systems, and aerospace mechanisms to achieve highly accurate and reliable movement. When searching for servo motor systems, users often compare specifications like response time, torque-to-inertia ratio, control compatibility, and integrated feedback features.
Contact us to discuss your requirements of Slip Motor. Our experienced sales team can help you identify the options that best suit your needs.
This section will cover the various uses and advantages of electric motors.
Electric motors are versatile and find applications in a wide range of devices such as fans, blowers, machine tools, turbines, pumps, power tools, compressors, alternators, rolling mills, movers, ships, and paper mills. They play a crucial role in high voltage AC heating systems, cooling and ventilation equipment, automobiles, and household appliances.
Maintaining electric motors involves the following considerations:
It's crucial for every organization to establish a cleaning routine for their motors. Regularly cleaning a motor enhances both its longevity and efficiency. Ensuring that a motor remains free from excessive dust, grease, and other debris is essential for optimal performance.
It’s possible to over lubricate an electric motor, which may lead to internal issues. However, a motor needs lubrication to work at maximum performance level. Every electric motor needs a different amount of lubrication. Lubricating a motor very early or very late may lead to premature tear and wear. Also, manufacturers generally recommend specific lubricants designed for their electric motor.
Motor bearings experience the most wear and tear, making them prone to issues over time. To prevent premature bearing failure, it's crucial to ensure proper motor alignment, as misalignment can place excessive stress on the bearings. Additionally, using the wrong type of lubricant can lead to early bearing wear. A common indicator of bearing problems is an overheating motor.
All motors produce some level of vibration, but excessive vibrations can lead to serious damage. If a motor begins to vibrate excessively, it should be shut down immediately. Common causes of abnormal vibrations include mechanical misalignment, damaged bearings, or excessive belt tension.
The rotor and stator are crucial components of the motor. It’s essential to check the gaps around these parts and measure the diameter clearance. The required clearance can vary depending on the specific motor and bearing design.
Tracking maintenance activities is vital for monitoring the motor’s lifespan and wear. Every inspection, bearing replacement, belt adjustment, and lubrication addition should be recorded. Proper documentation helps in forecasting future maintenance needs and managing related expenses more effectively.
Electric motors convert electrical energy to mechanical energy. Most of them work via the interaction of the motor magnetic field and electrical current in a wound wire to produce force in the manner of torque supplied on the motor shaft. The most important parts of a motor are the rotor and the stator. They may be energized by alternating or direct current. There are many types of electric motors including induction, servo, three phase, and industrial motors to name a few. They are used in electric vehicles, air conditioners, ships, and hydraulic machines.
Electric-powered irrigation pumps are widely used in the U.S. In , nearly 428,000 irrigation pumps were powered by electric motors in the U.S. (Farm and ranch irrigation survey, ). As such, there are significant opportunities for energy savings that can be achieved by improving the performance of irrigation pumping plants. Conducting energy audit studies helps in assessing the efficiency of these systems. One of the most important first steps is to accurately identify installed equipment. Original installation notes or manuals are often lost, leaving it up to the energy auditor to identify the make, model and serial numbers of pumping plant system components. In the case of electric motors and gear drive units, nameplates often remain intact and attached to the equipment, giving the auditor a wide variety of important information to accurately evaluate system efficiency.
Figure 1 shows a complete listing of the various parameters of interest for a gear drive and a typical three-phase AC induction motor. This is the most common motor found in irrigation systems and most of industry in general.
Figure 1. Common nameplate information found on gear-head housings and AC motors.
Basic knowledge of the terms listed on a nameplate allows the auditor to better understand the performance limits of the motor and gear drive, as well as their combined efficiency. The purpose of this Fact Sheet is to explain the meaning and purpose of nameplate information and show how to use nameplate and measured motor speed to calculate motor loading.
Nameplate information gives the auditor a snapshot understanding of several important operating limits. For example, if the motor’s nameplate Full Load Amps (FLA) is 45 and the auditor measures 50, then excessive Amp-pull or load is highly likely. Alternatively, if 15 Amps on a 45 FLA motor is measured, the motor is very under-loaded and operating inefficiently. Significant deviations in measured operating levels from nameplate information identify specific problem areas.
Not all motor manufacturers stamp all information given in Figure 1 on the nameplate. As federal energy efficiency guidelines for motors has increased (Energy Policy Act ), nameplate information has become more complete. Therefore, older motors may have only basic information; and just because a nameplate is still attached to equipment does not guarantee its legibility. In irrigation audits, the equipment can be old and weather-beaten due to constant exposure to the elements.
Normally, there are two ways to display information on a nameplate. The first is stamping a metal plate (Figure 2). This method normally prevents component information from fading over time. Sometimes the descriptive name where the stamped data is located cannot be read, but if one is familiar with the data fields it is easy to guess the category of data. A second way is the information is painted on a metal or plastic plate riveted to the component. This becomes problematic with older equipment because painted data fades under sunlight or is wiped off by solvents or abrasion. In this case, the auditor has little to start with.
Figure 2. Stamped metal nameplate.
An explanation of motor nameplate abbreviations and terms is given below:
Model Number and Serial Number
The model and serial number are usually a sequence of letters and numbers determined
by the manufacturer. Having just the model number can help the auditor track down
motor specifications even when all other information is missing.
Motor weight
Motor weight must specify pounds or kilograms. Larger electric motors (e.g., 100 HP)
used for irrigation can easily weigh 1,300 pounds.
Rating or AMB
AMB stands for ambient temperature. The rating or AMB is the maximum room temperature
or air space where the motor is located and time it can safely operate under those
conditions. The common rating of 40C-AMB-CONT means continuous operation at 40 C.
Motor life will be longer if ambient temperatures are less.
FLA, voltage and Hz
FLA is an abbreviation for the Full Load Amp rating. Motors are designed to operate
at 50 to 100 percent of their rated load. At FLA, the motor runs at 100 percent of
its rated load and the label specifies the current it will draw. Many electrical components
like wiring, circuit breaker and starter are sized based on FLA.
Most electric motors are designed to operate at a specific voltage. Motors can run safely at ±10 percent of the rated voltage. Exceeding the specified range can cause permanent damage. Some motors are designed to operate at dual voltages, i.e., 230V and 460V, depending on the selected wiring. For a dual voltage motor, the nameplate should have wiring information for the desired voltage at the bottom of the nameplate (Figure 3).
The abbreviation Hz is the Hertz or input voltage frequency of the motor. Motor speed is directly related to the line input voltage frequency. In the U.S., 60 Hz is the standard frequency while 50 Hz is common elsewhere.
Figure 3. Nameplate of a dual voltage motor with high and low voltage wiring diagrams.
HP, phase and RPM
Output horsepower, or HP, is the motor output at its rated load. It is dependent on the kilowatts, or KW, demanded by the motor along with efficiency, power factor and actual load. In energy audits conducted in central, northwest and Panhandle regions of Oklahoma, the horsepower of electric motor-driven irrigation pumps varied from 14 to 100. As the depth to water table (pumping depth) increases, higher motor HP is required. One can easily determine groundwater depth using a water level meter, then determine required motor horsepower. For more information on how to measure groundwater depth, please refer to Oklahoma Cooperative Extension Fact Sheet BAE-, “Measuring Depth to Groundwater in Irrigation Wells” (Frazier et al., ).
Generally, electric motors are either single phase or three phase. Motors larger than about 30 HP are usually three phase. Three-phase motors typically can be wired for different voltages and amperages described above.
RPM stands for revolutions per minute and is the shaft speed of the motor at the rated HP load. Depending upon the number of poles, frequency, design and motor slip (described below), the RPM will vary slightly for each manufacturer. For a four-pole motor operating at 60 Hz, the (no-load) RPM would be 1,800.
Design, frame and type
Design categorizes the motor’s starting torque using letters B, C and D to correspond
with normal, high and very high starting torque, respectively. Induction AC motors
experience high starting torques as they go from a standstill to FLA RPM. This is
related to Locked Rotor Amps, or LRA, where the starting current can be four to eight
times higher than FLA for a few seconds. LRA may be separately labeled on the nameplate.
The National Electrical Manufacturer’s Association, or NEMA, has defined frame sizes using a combination of numbers and letters. There are two categories of frame sizes based on whether it is a fractional- or integral-type motor. Fractional sizes include 42, 48 and 56; whereas, 140, 180 and larger are integral type motors. If a new motor’s frame size differs from the old motor, it might not properly fit.
“Type” refers to the category of motor enclosure protecting the windings, bearings, and other vulnerable parts. There are many types of enclosures listed by NEMA, but the most common ones are Open Drip Proof, or ODP, and Totally Enclosed Fan Cooled, or TEFC. ODP motors have an open enclosure, so air can freely enter, but liquids and solids cannot enter the motor from an angle of 0 to 15 degrees. An ODP enclosure is not waterproof and is better suited for indoor applications.
In contrast, a TEFC motor enclosure is totally enclosed and comes with an external cooling fan. Proper selection of the motor enclosure is very important because it must provide around-the-clock protection, regardless of the situation. TEFC motors are typically found on irrigation sites because they are designed to work outdoors. A TEFC enclosure also is required when explosive vapors are present.
Service factor
Service factor, or SF, is a number that indicates how much overloading a motor can
handle without causing permanent damage. For example, 1.15 SF means the motor can
be loaded 15 percent over its maximum rated load for a short time until its internal
temperature becomes excessive. This means a 100 HP motor with a SF of 1.15 can operate
at 115 HP load for some time before overheating. Continuously operating the motor
at its SF will adversely affect its efficiency and reduce useful life.
NEMA nominal efficiency and guaranteed efficiency
Motor efficiency is the ratio of output mechanical power produced to input electrical
power. NEMA nominal efficiency is the average motor efficiency obtained by testing
a representative group of motors. Minimum or guaranteed motor efficiency allows for
losses up to 20 percent more than nominal efficiency. It accounts for output variation
among the motors. Reduction in efficiency will increase pumping costs. Over time,
federal regulations have required newer motors to be more efficient. It is safe to
assume newer motors are more efficient due to a lack of degradation and lower past
efficiency standards. Economic analysis can help users decide if a newer motor will
pay for itself within its useful life.
PF and maximum KVAR
All inductive devices in an AC circuit have a Power Factor, or PF, rating. It is the
ratio of active or real power to total power (Figure 4) while “kVAR” is the amount
of reactive power that produces no practical work. A PF of one means reactive power
(kVAR) is zero and motor is using all delivered power.
Motor efficiency increases with PF because the motor better utilizes supplied power. Low PF (usually less than 0.80) can result in a utility company’s power factor penalty on the customer’s electric bill. An under-loaded motor can cause the PF to drop lower than the PF listed for nameplate rated load. Actual motor load can be calculated using the method described under Motor Load Calculation Using Nameplate information.
Figure 4. Power Factor triangle.
Ct and Vt
The label Ct stands for constant torque and Vt stands for variable torque. The presence
of these nameplate abbreviations on a motor nameplate indicates it is rated for a
variable speed drive. This is important for customers wishing to retrofit electronic
drives onto existing motors.
Duty, insulation and code
Duty is the duration of safe motor operation. Most motors operate continuously without
requiring a cooling period. Others operate intermittently, and require a cooling period
between on/off cycles. For larger motors, continuous duty is common.
The NEMA insulation class describes the motor’s ability to handle maximum allowable
operating temperature over time. Operating temperature is the sum of ambient temperature
and motor temperature rise. Common insulation class descriptors are B, F and H indicating
temperatures of 130 C, 155 C and 180 C, respectively, that the motor can withstand.
At full voltage, inrush current on startup is four to eight times greater than FLA. The NEMA code letter denotes the magnitude of inrush current. Additional information about the 15 NEMA code types are given at: https://www.engineeringtoolbox.com/locked-rotor-code-d_917.html
Gear drives play an important role in agricultural machinery. Prior to variable speed drives, changing drive-to-driven gear or pulley ratios was the only way to vary delivery shaft speed and torque. Gear drives are not only used to transmit and vary both, but also to alter power delivery orientation (angle). For example, a right-angle gear drive transposes power from a horizonally-mounted motor to the vertical driveshaft of a turbine pump.Gear drives have nameplates that are not as detailed as an electric motor. Figures 5 and 6 illustrate this fact.
The serial number (Figure 5) of a gear drive is often expressed using a combination of letters and numbers. These will vary from company to company, depending on the type of the gear drive. Letters S, SH and SL denote three different types of gear drives: standard hollow shaft drive with standard thrust capacity, standard hollow shaft drive with heavy thrust capacity and standard hollow shaft drive with opposed thrust capacity, respectively.
Figure 5. A gear drive nameplate.
Ratio
The listed ratio represents the ratio of the input speed to output speed of the gear
drive. A 1:1 ratio means motor and pump shaft speed are identical (Figure 6). A 1:1.5
ratio for a motor running at 1,770 rpm means pump drive speed will be: 1,780 rpm (1/1.5)
= 1,190 rpm. Output speed is important when determining suitability of a particular
pump for a given set of depth, flow and pressure conditions.
Figure 6. Gear Drive nameplate for a 1:1 (non-reduction) drive ratio.
Oil specifications
Oil and lubrication requirements are often specified on the gear drive nameplate.
Only use the recommended oil type and grade. Oil flow rate recommended by the manufacturer
should be followed. Normally, a drip oiling system supplies needed lubrication. Following
the manufacturer’s recommendations will prevent over-oiling that will contaminate
groundwater or under-oiling that leads to premature wear and tear.
RPM
RPM stands for recommended revolutions per minute of the gear drive. The rpm of a
gear drive unit is proportional to the rpm of the attached motor. The input rpm of
the gear drive should match the output rpm of the motor. Mismatched motor/gear drive
rpm (e.g., 1,800 vs 3,600 rpm) could lead to premature failure of the gear drive.
Some of the additional specifications that are unique to the manufacturer and general requirement like oil capacity may often be found on the nameplate.
This section gives an example of how to use nameplate information to determine motor loading or slip, also known as Slip Calculation. Actual efficiency of an AC induction motor depends on motor load. Maximum efficiency is realized when the motor is operating near 75 percent or more of its maximum rated load. In contrast, actual efficiency significantly decreases when load drops below 50 percent of the maximum rated load (Challenge, M., ). One of the easiest ways to determine load is to calculate “slip.” Full or design load slip is the difference between full load speed and no-load speed. Full load speed is motor rpm at its rated voltage at maximum rated load. No load speed is higher than full load speed because there is minimal resistance to movement. No-load synchronous speeds of 3,600; 1,800; 1,200 and 900 RPM correspond with 2-, 4-, 6- and 8-pole motors, respectively. No load speed is inversely related to the number of poles of the motor. A greater number of poles proportionately decreases rpm. Information about full load rpm and design horsepower can be found on the nameplate. Sometimes the no-load rpm is not listed. However, the full load rpm will be close to 1,800 or 3,600 rpm. Actual load is the ratio of true slip to design slip. True slip is the difference between synchronous and measured rpm. A tachometer is used to measure the actual speed. An example on how to calculate load is below.
Given:
Full Load rpm (FLRPM) = 1,770
No Load rpm (NLRPM) = 1,800
Measured rpm = 1,780
Design hp = 60
Required: Calculate a) design slip, b) true slip, c) percent load and d) true load. Solution:
In this particular case, the motor will operate efficiently as it is loaded above 50 percent.
Acknowledgment
This material is based on work supported by the Natural Resources Conservation Service
(NRCS), U.S. Department of Agriculture (USDA), under number 69-3A75-16-013. Funding
was also provided by the Agricultural Research Service, USDA, under number --011-47S.
The authors are thankful to Dr. Don Sternitzke, the Water Management Specialist with
NRCS Oklahoma State Office for his valuable comments.
Challenge, M. (). Determining Electric Motor Load and Efficiency. Program of the US Department of Energy.
Energy Policy Act (). Electric Motor Regulations.
Farm and Ranch Irrigation Survey (). Accessed on February
Frazier, R.S., Taghvaeian, S., Handa, D. () Measuring Depth to Groundwater in
Irrigation Wells. Oklahoma Cooperative Extension Fact Sheet BAE-.
Divya Handa
Graduate Research Assistant
Saleh Taghvaeian
Assistant Professor and Extension Specialist, Water Resources
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