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 compared 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. Figure 3-36 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 Figure 3-36, the induction motor is a fairly simple device. However, precision engineering is required to create small tolerances and air gaps that will allow maximum efficiency and torque generation.

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.

Figure 3-37 indicates the NEMA designs and compares design with rated starting current and speed.

As shown in Figure 3-37, 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.

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 horsepower 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. Figure 3-38 indicates an AC induction motor, with an indication of frame size.

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. Figure 3-39 is an example of a typical AC motor nameplate.

• 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 voltage, 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, compared 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 130°C (266°F). A Class H insulation would allow for a maximum temperature rise of 180°C (356°F). Temperature rise means the amount of temperature increase, above the normal ambient rating of 40°C (104°F).

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 inverterduty 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 temperature, 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 voltage, 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.

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...