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AC Motor Types
Introduction
AC motors can be divided into two major categories-asynchronous and synchronous. The induction motor is probably the most common type of asynchronous motor (meaning speed is dependent on slip). When reviewing induction motors, there are two rating designations-NEMA and IEC.
Another type of asynchronous motor is the wound rotor motor. This type of motor has controllable speed and torque because of the addition of a secondary resistance in the rotor circuit. A third type of popular asynchronous motor is the single-phase motor. The single-phase AC motor will not be covered because of their limited use in industrial applications when connected with variable-frequency drives.
The synchronous motor is inherently a constant-speed motor, when operated directly across the line. This type of motor operates in synchronism with the line frequency. Two types of synchronous motors are non-excited and DC-excited.
The basic principles of AC induction motors have been previously covered.
In this section, attention will be given to motor designations, ratings, and designs.
Standard AC Induction Motors (NEMA and IEC)
NEMA frame motors are in widespread use throughout U.S. industry. This motor design was developed before the 1950s and has well served many types of fixed-speed applications. In 1952 and 1964, NEMA evaluated standard frame sizes and re-rated the frame standards. The result was smaller diameter motor frames (e.g., original 326 frame prior to 1952, to a 284U frame in 1952, to a 256T frame in 1964). As the re-rating took place, the frame sizes (numbers) were reduced, as was the amount of iron in the stator. With less iron in the stator, less overload capability is realized com pared with the "U" frame or the original frame size.
However, with smaller-diameter frames comes more efficiency and faster response to changes in magnetic flux. FIG. 36 indicates the construction of a standard AC induction motor. All the major motor components are identified.
It should be noted that all standard motors include a small rectangular slot, cut lengthwise in the shaft, called a keyway or keyseat. This slot includes a tapered-cut rectangular piece of steel, call a key. The key is inserted into the keyway and pressure-fit snugly to mechanically connect the shaft and coupler or connection device, such as a pulley or gear.
As seen in FIG. 36, the induction motor is a fairly simple device. How ever, precision engineering is required to create small tolerances and air gaps that will allow maximum efficiency and torque generation.
FIG. 36. AC induction motor construction (ABB Motors)
The AC induction motor (polyphase induction motor) can be divided into five classifications, according to NEMA. The speed/torque characteristics for each classification have been presented in an earlier section. A brief description of each classification will be presented here, followed by a comparison to IEC frame motors.
• NEMA Design A: This type of motor has a high breakdown torque characteristic, compared with NEMA design B motors.
These motors are normally designed for specific use, with a slip characteristic usually less than 5%.
• NEMA design B: This type of motor is designed for general-purpose use and accounts for the largest share of induction motors sold. The typical slip for a design B motor is 3-5% or less.
• NEMA design C: This type of motor has a high starting torque, with a relatively normal starting current and low slip. The type of load applied to a design C is one where breakaway loads are high upon start. The loads, however, would be normally run at the rated point, with very little demand for overload.
• NEMA design D: This type of motor has a high starting torque, high slip, but also low full load speed. Because of its high slip (5- 13%), the speed can easily fluctuate because of changes in load.
• NEMA design E: This type of motor is known for high efficiency and is used mainly where the starting torque requirements are low.
Fans and centrifugal pumps make up the bulk of applications using this type of motor.
FIG. 37 indicates the NEMA designs and compares design with rated starting current and speed.
As shown in FIG. 37, though design E may have the highest efficiency, it also has the highest starting current-about 800%. This fact must be reviewed when sizing the proper overload heater elements. Most standard induction motors have closer to a 600% starting current rating.
FIG. 37. Current vs. speed for NEMA motor designs (ABB
Inc.)
Though NEMA motors are rated in horsepower, there are times when a motor is specified on the basis of its frame size. NEMA supplies standard frame designations, up to the 445T frame. Above that rating, motor manufacturers can supply their own standards and designate the motor rating as exceeding the NEMA ratings.
There are standard frame sizes of motors and are based on a given horse power or base speed. NEMA designates a foot to centerline dimension as an indication of the frame size. There is also a designator for frame diameter.
FIG. 38 indicates an AC induction motor, with an indication of frame size.
FIG. 38. 324 frame motor designation (Courtesy of ABB Motors)
Using a 324 T frame motor as an example, the motor designer designates the shaft centerline distance to the foot at 8 inches. To figure any shaft centerline distance to foot, divide the first two digits of the frame number by 4 (32 ÷ 4 = 8 inches). With this information, an application engineer can design a machine with the motor dimensions in mind. This also assists in comparing one motor with one from another manufacturer. All motor dimensions are standard.
Since motor dimensions are standard, so too are motor nameplate ratings.
As with DC motors, AC motor nameplates contain all the necessary information to effectively apply the motor. FIG. 39 is an example of a typical AC motor nameplate.
FIG. 39. AC motor nameplate (Marathon Electric)
• Frame: Indicating the frame rating per specific horsepower, given the rated voltage and frequency (example: 256T frame).
• HP: Available horsepower at the designated voltage and frequency ratings.
• Voltage, phase, and frequency: Designations for the rated volt age, phase, and frequency in hertz. Many industrial motors contain a dual-voltage rating. This means that they can be connected to two different voltage lines. The operating voltage is designated by either jumper strips or wire configurations that are completed in the conduit box. Typically, NEMA frame motors are rated for 60-Hz operation.
• FL Amps: Current rating of the motor, listed in amperes. Some nameplates indicate current rating as FLA (full load amps). This would indicate that the motor would draw the stated amps under rated voltage, frequency, and load. If the motor is a dual-voltage motor, two values of amps would be listed. The first value would coincide with the first value of voltage stated. The second value would coincide with the second voltage value listed. (Example: A 230/460V motor may indicate nameplate amps of 68/34 amps. The motor would draw twice the amps on the 230-V connection, com pared with the 460-V connection.)
• rpm: This is the motor speed in rpm at base speed. Base speed is indicated as rated voltage, frequency, and load. Due to less slip, an unloaded motor speed would rise from this speed to close to synchronous speed.
• Design and insulation class: The design class would indicate the NEMA designation for A, B, C, D, or E. Typically, the insulation class would indicate the temperature capability of the stator winding insulation. For example, a common designation of Class B insulation would allow for a maximum temperature rise of 130ºC (266ºF). A Class H insulation would allow for a maximum temperature rise of 180ºC (356ºF). Temperature rise means the amount of temperature increase, above the normal ambient rating of 40ºC (104ºF).
An additional classification is now being included with motors-that of the electrical strength of the stator winding insulation (referred to a dielectric strength). AC motors applied to variable-frequency drives run the risk of possible insulation damage from the power conversion technology in the drive. Voltage stress beyond the rating can cause microscopic pin holes in the insulation, which could result in an open phase and eventual motor failure. Motors designated as inverter duty have the electrical insulation strength to avoid failure due to drive technology issues.
NEMA MG-1, Part 31 standards indicate that motors operated on 600 V or less drives should be capable of withstanding peak voltage of 1600 V. Motor cable length and drive carrier (switch) frequency also play a part in the possible damage to a motor's insulation strength. Motors with a 1200 V or 1000-V insulation strength should not be applied to AC drives unless additional precautions are taken. Special drive output filters will reduce the effects of high-peak voltages and lower the risk of insulation failure.
The motor manufacturer should always be consulted when questions arise regarding the insulation strength of the windings. The manufacturer can make recommendations as to additional safeguards that may be needed to increase motor life when connected to a drive.
• Duty and S.F. (Service Factor): Most standard AC motors list duty as "continuous" or "intermittent." The service factor of the motor is the multiplier or additional safe power loading above the rating. Small fractional horsepower motors may carry a service factor of 1.4, while larger integral horsepower motors may list only 1.15 service factor. For example, a 1.15 S.F. would indicate a motor's capability of 15% additional horsepower output, above the rating. A 1.4 S.F. would indicate 40% additional horsepower output.
• Efficiency and Ambient: Many motors may list a designation of premium efficiency. In addition, an actual number may be referenced, such as 89.5. The efficiency is closely tied with the NEMA classification, such as design A, B, C, etc. The motor manufacturer will acquire the rating from an independent testing agency. The ambient temperature is the maximum normal operating tempera ture, below the amount indicated in the temperature insulation class.
Not all AC motors contain every piece of data listed above. But all motor nameplates would indicate the most important information, such as volt age, frequency, amps, and rpm. This information is required by an AC drive, in order for the drive to match internal diagnostics with the motor data.
Some motor nameplates indicate a wiring diagram for the dual voltage windings; others have a sticker or label inside the conduit box, stating the wiring connections. Some of the new motors manufactured today indicate the dielectric strength of the insulation or mounting design.
IEC Ratings
At this point, it would be helpful to briefly review IEC motor ratings and then compare IEC with NEMA. The motor market today has become more global, with IEC rated motors on equipment exported from Europe.
IEC is the acronym for the International Electrotechnical Commission.
IEC, like NEMA, establishes and publishes mechanical and electrical standards for motors. Many IEC standards have been nationalized for a specific country, such as Germany, Great Britain, or France.
Though NEMA and IEC standards use different terms, they are essentially similar in ratings and in many cases are interchangeable. NEMA standards are probably more conservative, which allows for interpretations in design. IEC standards are more specific and categorized. They are typically more precise.
Both IEC and NEMA use letter codes to indicate mechanical dimensions.
They also use code letters to indicate general frame size. The NEMA and IEC dimension codes are not interchangeable, nor are the frame sizes (exception being the 56 frame, which is the same in NEMA and IEC).
As expected, NEMA designations are listed in inches and horsepower, whereas IEC designations are listed in millimeters and kilowatts. NEMA lists a handful of enclosure designations and descriptions, whereas IEC uses numbers.
IEC lists two numbers: the first number indicates protection against solid objects; the second number indicates protection against water entry. The enclosure letters "IP" indicate ingress protection. (Example: IP55. The first "5" indicates complete protection, including dust-tight, and the second "5" indicates protection from water sprayed from a nozzle from any direction.
This type of motor would be considered wash-down duty.) NEMA would list the enclosure type to indicate the particular cooling method employed in the motor. IEC, however, would use a letter and number code to designate how a motor is cooled. (Example: IC40. The "4" indicates frame cooling, while the "0" indicates convection cooling with no fan.) The temperature insulation class ratings are identical, whether NEMA or IEC.
IEC motors are listed as "50 Hz" rather than the NEMA "60 Hz." A 50-Hz IEC motor will normally operate satisfactorily on 60 Hz, as long as the voltage is increased by the same ratio as the frequency. (Example: 50 Hz at 380 V to 60 Hz at 460 V) The motor speed would be 1/6 higher than at 50 Hz. However, operating a 50-Hz motor at the lower U.S. voltage of 230 V may not operate satisfactorily without derating (requiring the motor to deliver 15 or 20% less torque at nameplate rating, due to motor heating).
When applying an IEC motor instead of a NEMA motor, it is always sound practice to consult a motor rating table for comparisons. NEMA ratings include a factor for overload, whereas IEC strictly rates motors with little to no overload capability.
Wound Rotor
The speed and torque characteristics of an AC induction motor are essentially defined by the design, number of poles, and line power applied. In contrast, the wound rotor version of an induction motor does have controllable speed and torque characteristics. Different values of resistance are inserted into the rotor circuit to obtain various performance options.
Wound rotor motors are normally started with a secondary resistance connected to the rotor circuit. The resistance is reduced to allow the motor to increase in speed. This type of motor can develop substantial torque, and at the same time, limit the amount of locked rotor current. The secondary resistance can be designed for continuous operation at reduced speeds.
Special consideration is required for heat dissipation at reduced speeds because of reduced cooling effects and high inertia loads. FIG. 40 indicates a wiring diagram of a wound rotor motor.
FIG. 40. Wound rotor motor diagram
The advantages of this type of motor include a lower starting current (less than 600%) with a high starting torque. This motor type also provides for smooth acceleration and easy control capability.
A disadvantage of this type of motor is that efficiency is low. The external resistance causes a large drop in rpm, based on a small change in load.
Speed can be reduced down to 50% of rated value. Another disadvantage is that the relative cost of this motor may be substantially higher than an equivalent three-phase induction motor.
Synchronous Motors
The three-phase AC synchronous motor is a unique and specialized type of motor. Without complex electronic control, this motor type is inherently a fixed-speed motor. This type of motor is used in applications where constant speed is critical. It is also in cases where power factor correction is desired, since it can operate at leading or unity power factor. The synchro nous motor is a highly efficient means of converting AC electrical power to mechanical power.
The synchronous motor could be considered a three-phase alternator, operated backward. Direct current is applied directly to the rotor to pro duce a rotating electromagnetic field. The stator windings are connected in either a wye or delta configuration. FIG. 41 indicates a diagram of the synchronous motor.
It should be noted that the synchronous motor has a "wound" rotor that is connected to a brush assembly system connected to DC power. In reality, synchronous motors have little to no starting torque. An external device must be used for the initial start of the motor.
FIG. 41. AC synchronous motor diagram
Devices such as an auxiliary DC motor/generator or damper windings are typically used to initially start the synchronous motor. The motor is constructed such that it will rotate at the same speed as the rotating stator field. At synchronous speed, rotor and stator speed are equal, and there fore, the motor has no slip. With a load on the shaft, slip increases and the motor responds with more torque, which increases the speed back to "synchronism." Synchronous motors in sub-fractional ratings are usually self-excited using damper windings. High-horsepower synchronous motors are usually DC excited using an external DC motor/generator.
Multiple Pole Motors
Multiple pole motors could be considered multiple-speed motors. As stated earlier, the speed is a direct result of the number of pole pairs. At 60 Hz, a four-pole motor would have a synchronous speed of 1800 rpm. At the same 60 Hz, a two-pole motor would have twice the synchronous speed- 3600 rpm. Typically, an AC induction motor has only one set of pole pairs-2, 4, 6, or 8 poles, or more. However, specially designed multiple speed motors would be wound for two different pole pair connections.
Most of the multiple pole motors would be dual-speed or two-speed design motors. Essentially, the conduit box would contain two sets of wiring configurations: one for the low-speed and one for the high-speed windings. The windings would be engaged by a two-position switch or electrical contacts. The switch or contacts would connect either the low- or high-speed winding to the three-phase power source.
This type of motor configuration provides a certain amount of flexibility in manufacturing. Perhaps the low-speed winding would be used for a production process taking place on a feed conveyor. Once the process is complete and a limit switch is closed, that same conveyor would move the product at high speed to the packaging and labeling section. There are many other industrial, packaging, food processing, and HVAC applications where two-speed motors could be an advantage. The possible disadvantage of this type of motor is the additional cost of some type of external switch control.
Specialty Motors
General Principles of Operation-Stepper
A step or stepper motor is one in which electrical pulses are converted into mechanical movements. A standard DC motor, for example, rotates continuously; but a stepper motor rotates in fixed increments whenever it is pulsed on. A standard DC motor would be considered an analog device, while a stepper motor would be considered digital.
The size of the step, or the step angle, is determined by the motor construction or by the type of controller connected. (Note: The step angle is determined in fractions of 360º, which is one complete shaft rotation.) For example, the step resolution of 90º would be four steps per rev (revolution). A 15º resolution would indicate 12 steps per rev, and 1.8º would indicate 200 steps per rev. Microstep motors are capable of thousands of steps per rev.
Because of their exactness of rotation, stepper motors are used, "open loop" in control systems where position is critical. In many high-accuracy applications, an encoder or position feedback device is used to confirm the actual position of the motor shaft.
Stepper motors require a drive package with an electronic controller, power supply, and feedback device, if needed. FIG. 42 indicates the principle of stepper motor design.
The stepper motor is a two-phase type of motor. The indexer provides step and direction pulses to the drive controller (amplifier). The amount of cur rent for each phase is determined by the controller, which is then used as an output to the stepper motor. The stepper motor is operated by pulses, which determine the "steps" of the motor shaft. The frequency of these steps determines the speed of the motor.
The most common types of stepper motors are probably the permanent magnet (PM) and the variable reluctance (VR). The diagram in Figure 3 42 is one type of a PM stepper motor. Is could be considered a design similar to the synchronous induction motor.
FIG. 42. PM stepper motor diagram
The rotor moves in step with the stator windings when the windings are energized. If the windings are continuously energized from the two-phase supply, then the motor would essentially act as a low-speed synchronous motor. As seen in FIG. 42, the PM rotor is surrounded by the two phase stator. The rotor sections are offset by 1/2 tooth pitch (180º) from each other. As the voltage rotates clockwise, from phase A to phase B, a set of rotor magnets will align themselves with the stator magnetic field. The rotor will therefore turn one step. If for some reason, both phases are energized simultaneously, the rotor will establish a location midway between the stator poles. If that were to happen, the motor would be considered half stepping.
The VR type stepper motor is basically constructed the same way as a PM motor. The difference is that the VR type does not have magnets in the rotor. It would contain, however, 2-, 3- or 4-phase stator windings. The motor would operate similar to an induction motor, with the rotor aligning itself with a stator pole that is energized.
The stepper motor is essentially a brushless motor. It can deliver high torque at zero speed, with no drifting of the shaft position. The direction of the motor can be reversed by reversing the direction of the pulses from the controller. The device has low inertia, similar to a servomotor, a result of the windings in the stator and a permanent magnet rotor.
There are several application considerations that come with stepper motors. Periodically, possibly at low speeds, this type of motor exhibits oscillations with every step. This is caused by poles in the rotor seeking the next available magnetic field. Many times, the magnetic fields of the rotor and stator do not match up, typically upon power-up. Also, the motor, controller, and load must be somewhat matched to minimize the oscillations. Stepper motors tend to run hotter than standard induction motors.
This is due to the pulse waveform from the controller, especially at low speed, with high current levels present (a product of high torque response at low speed).
AC Vector Motors
This type of motor is a specific type that would be applied to an AC vector or flux vector drive. Principles of operation for this motor are basically identical to the standard AC induction motor. Because this motor is operated from a flux vector drive, special design characteristics are required.
Vector control basically means the requirement of full torque at zero speed. In applications such as elevators, hoists, and ski lifts, the motor usually is started while under rated load. If the device is an elevator car, the position of the device cannot change when the motor is started. If a standard induction motor were used, the motor would have to "slip back" for torque to be developed. During the process of developing "motor slip," the elevator car may have dropped several feet before the motor could develop enough torque to move it upward. The vector motor is specially designed to operate at extremely low slip and be able to handle the heat generated by providing full torque at zero speed.
The general principle of operation lies with analyzing the motor in terms of voltage and flux vectors. The rotor is divided into 360º of rotation, which is one complete rotation. A vector would be the direction and amount of a certain quantity in the motor circuit-in this case, rotor flux or stator flux. The relationship between rotor and stator flux is indicated in FIG. 43.
FIG. 43. Vector motor relationships - stator and rotor flux (ABB Inc.)
The torque in an induction motor is developed by the relationship of rotor and stator flux. The physical torque developed is a byproduct of the magnitude of the stator and rotor flux vectors. Stator flux is a function of the input voltage to the motor. (The voltage vectors are indicated by U1 to U6 in the figure.) We could consider the dashed curve set the torque span developed in the motor. The device that would control the amount of stator and rotor flux generated would be considered a vector or flux vector AC drive.
The vector motor, in most cases, must have provisions for the mounting of a feedback device on the shaft end. The feedback device (encoder or resolver) sends information back to the drive control, indicating exactly where the rotor position is located. The drive control needs this information to calculate and generate V/Hz. The V/Hz waveform is then used by the motor to generate the flux vectors shown in the figure.
Vector control, drive control, and feedback devices will be discussed in Section 4 (AC Drives section) and Section 5 (Closed Loop Control section). This type of technology is definitely in high demand throughout industry today. The use of motor vector control (torque control) allows manufacturing systems to increase accuracy and productivity. The basic design of the AC induction motor has not changed much in the last few decades. Magnetism is magnetism. However, the ratings are now more precise than they were a few decades ago. The efficiencies are definitely higher than a few decades ago. There are AC drive manufacturers that require a flux vector drive and motor combination--a matched set. The direction of industry, however, is to be able to use a combination of vendor equipment to achieve the desired results.