Bill Finley
Technical Services Committee Member
Siemens Industry, Inc.
The recently published
IEEE 841-2021 has been improved and brought up to date with the latest standards. The standard defines the requirements for severe-duty totally enclosed squirrel cage polyphase induction motors (Figure 1) in the range from 0.75 kW to 370 kW (1 hp to 500 hp) up to 4000 volts. This is intended for applications where performance and reliability are critical. It reduces installation and operating costs and eliminates the need for end-users to develop their own internal specification. In addition to defining a reliable product, it supports the quick availability of these products in inventory.
The purpose of IEEE 841-2021 is to define specific requirements that improve the mechanical performance, electrical performance and reliability of the motor. This includes more robust electrical insulation systems and better corrosion protection along with more specific electrical and mechanical testing requirements. The requirements defined in this standard go beyond the basic requirements of the NEMA MG 1 Standard.
A new clause 14 was added to the standard, which now defines the core features of a fully compliant IEEE 841 motor that is permitted to be labeled IEEE Std. 841-2021. Since the last revision of IEEE 841, the IEEE Committee determined that there may at times be a need to produce motors with many but not all the features of an IEEE 841- 2021 motor or with additional features that are not standard requirements. As a result, the committee has included in clause 14 the options or features that may be included and still be labeled an IEEE Std. 841-2021 motor. For it to carry the IEEE 841-2021 label, it must meet without exception all the requirements defined in this standard. It also lists options that may be added to maintain the label as it will still have all the core features without exception. In addition, it defines what is an IEEE Std. 841-2021 features motor. An IEEE Std. 841-2021 features motor has options as listed that may not meet all the core requirements. The features motor has options for specific applications that may not need all the core features. To avoid complications, this must be communicated between the manufacturer and the end-user. The standard also clearly defines options that are not allowed when labeling either IEEE Std. 841-2021 or IEEE Std. 841-2021 features.
A new important addition to IEEE 841-2021 motors is the ability to drive a variable Torque (VT) application from zero speed up to rated speed and power while running on pulse-width modulation (PWM) adjustable speed drives (ASDs). This is now required for all IEEE 841-2021 motors rated less than 1000 Volts.
should not normally reduce the motor life. The maximum surface temperature must still not exceed 200°C.
There are a few additional critical changes that are worth mentioning. For one, the vibration level requirements have been simplified to be in harmony with NEMA MG 1 2021 grade B and IEC 60034-14 2017 grade B, which is defined for critical motors. The peak velocity limit (horizontal, vertical and axial) for all 2-, 4-, and 6-pole motors is now 2.03 mm/s (0.08 in/s). Eight pole limits remain unchanged at 1.52 mm/s (0.06 in/s).
Since the twice-line frequency vibration originates in the stator frame and has minimal effect on reliability, the limit will increase to 2.54 mm/s [0.10 in/s] if twice-line frequency vibration is dominant. Dominant twice-line frequency is now more simply defined as to when the twice-line frequency vibration is greater than 70% of the 0.08 in/s limit. This typically only occurs to two pole motors.
As always, the motor must be tested at rated voltage and frequency. The default mounting for test is on a resilient mount (elastic supports, springs or rubber). Suspending the motor from springs is no longer acceptable. When specified, or at the manufacturer’s discretion, the motor shall be tested on a rigid foundation per NEMA MG 1 Part 7.
Noise level requirements have also changed and as requested by the industry are now established as sound pressure limits. This change was made since this is how the measurements are taken and most of the industry is more comfortable with this type of limit. The sound pressure limits for motors less than or equal to 445 frame is now 85 dBA at three feet (1 meter). For motors greater than 445T frame the new limit is 92 dBA at three feet sound pressure. All the sound power limits have been removed. There has been no change to any of the testing requirements.
IEEE 841 and NEMA have always defined the usual service condition that would result in reliable service or what would be an unusual service condition that could adversely affect the motor’s performance. This understanding is critical and may not be obvious to anyone except the most knowledgeable experts. Therefore, it is important that it give examples to the end-user. It is not possible to list every service condition; therefore, it is only safe to assume that if the service condition is not specifically mentioned as usual it should be considered as an unusual service condition and the end-user should work with the manufacturer to understand any adverse performance effects. Added to Usual Service Conditions in IEEE 841 2021 is the ability to drive a variable torque (VT) load while running on a PWM ASD for all motors up to 1000 Volts. Removed from Unusual Service Conditions are speeds other than rated speed or exposure to flammable or explosive gases since this is likely in a Division 2 location. But some limits are still required on the ASD application; therefore, added to Unusual Service Conditions is operation at speeds above synchronous speed, constant torque application or constant horsepower operation since they could increase mechanical stresses and heating thereby damaging the stator, rotor or bearings.
ASD operation at voltages greater than 1000 volts are still considered an unusual service condition since these are typically custom designed around specific applications. With these changes, load inertia or starting conditions limits now only apply to across-the-line operation.
In the latest revision of IEEE 841, many of the mechanical features have also changed. Such as for frames greater than 449 Frame, end shields, fan covers and terminal boxes may be cast iron or steel, but not aluminum, and rugged and resistant to corrosion. It is logical that a frame series has similar construction, which it now does. Previously, greater than 445 frame could be fabricated from steel plate.
Frames greater than 449T shall now be fabricated from cast iron, cast steel or steel plate. The minimum thickness of 2.8 mm (0.11 in) for steel plate remains unchanged.
Terminal box volume requirements have also changed. Now for motors greater than 445T and through 449T, less than or equal to 1000 volts must have minimum volumes as defined in 841 Table 1, must all be diagonally split, must have an NPT tapped conduit entrance and be rotatable in 90-degree increments. Drains are no longer required. For motors larger than a 449T but less than 1000 volts, the minimum required volume is now 19664 cm3 (1200 in3). For motors larger than a 449 frame but with voltages greater than 1000 volts, the terminal box dimensions remain unchanged, and drains are still required.
The standard will no longer include tables of efficiency, it will only point to the published standards in either NEMA MG1 or IEC 60034-30-1 where they are defined. Efficiency testing requirements may now be performed by any of the three commonly used tests known to provide similar results. These include IEEE 112 Method B, Canadian Standards Association (CSA) C390-10 or IEC 60034-2-1-1B. Thrust-bearing losses are still excluded, but methods for exclusion are now listed as follows:
- May be tested with standard 6000 series bearings.
- Thrust bearings installed but additional losses determined and removed by calculation or from previous test experience.
- If desired, may be tested with the thrust bearings installed with no adjustments.
In summary, the standard was brought up-to-date with the local and global standards, which are presently being used around the world. But at the same time, it was critical that the motors remained interchangeable with existing motors. It now clarifies what is permitted to be labeled an IEEE 841-2021 motor and what is not allowed to be labeled as an IEEE 841 motor. In addition, it added the definition of an IEEE 841 2021 features motor. The standard added the most common ASD application to be considered as a usual service condition thereby making these motors readily available. Highlighted here in this article are many of the major changes to the standard, but there are others that may also be critical to your application. It is important that you review IEEE Std. 841-2021 for additional changes that may not have been discussed here.
Though this may have been commonly done in the past, it was not defined by IEEE 841. It is critical that adding ASD operation to the IEEE 841 standard will not affect the reliability, manufacturability, safety or testing of the IEEE 841 motor, which may operate in a Class 1 Division 2 area. The steep voltage rise time of the ASD PWM requires a more robust insulation system that can achieve a long life. NEMA MG 1 part 31 defines the required insulation capability of this motor, and this requirement now applies to all IEEE 841 motors. This is not a major issue for the manufacturers, since typically they have been doing this for IEEE 841 and industrial motors for years. The IEEE 841 motor is still rated and tested on sine wave power. The IEEE 841 motor is still expected to be common stock for all applications. Since the harmonic content of an ASD does heat the motor more than a sine wave power source, the allowable maximum measured temperature on an ASD is raised to 95°C, knowing that this will normally only occur for short durations when running at maximum speed and should not normally reduce the motor life. The maximum surface temperature must still not exceed 200°C.
There are a few additional critical changes that are worth mentioning. For one, the vibration level requirements have been simplified to be in harmony with NEMA MG 1 2021 grade B and IEC 60034-14 2017 grade B, which is defined for critical motors. The peak velocity limit (horizontal, vertical and axial) for all 2-, 4-, and 6-pole motors is now 2.03 mm/s (0.08 in/s). Eight pole limits remain unchanged at 1.52 mm/s (0.06 in/s).
Since the twice-line frequency vibration originates in the stator frame and has minimal effect on reliability, the limit will increase to 2.54 mm/s [0.10 in/s] if twice-line frequency vibration is dominant. Dominant twice-line frequency is now more simply defined as to when the twice-line frequency vibration is greater than 70% of the 0.08 in/s limit. This typically only occurs to two pole motors.
As always, the motor must be tested at rated voltage and frequency. The default mounting for test is on a resilient mount (elastic supports, springs or rubber). Suspending the motor from springs is no longer acceptable. When specified, or at the manufacturer’s discretion, the motor shall be tested on a rigid foundation per NEMA MG 1 Part 7.
Noise level requirements have also changed and as requested by the industry are now established as sound pressure limits. This change was made since this is how the measurements are taken and most of the industry is more comfortable with this type of limit. The sound pressure limits for motors less than or equal to 445 frame is now 85 dBA at three feet (1 meter). For motors greater than 445T frame the new limit is 92 dBA at three feet sound pressure. All the sound power limits have been removed. There has been no change to any of the testing requirements.
IEEE 841 and NEMA have always defined the usual service condition that would result in reliable service or what would be an unusual service condition that could adversely affect the motor’s performance. This understanding is critical and may not be obvious to anyone except the most knowledgeable experts. Therefore, it is important that it give examples to the end-user. It is not possible to list every service condition; therefore, it is only safe to assume that if the service condition is not specifically mentioned as usual it should be considered as an unusual service condition and the end-user should work with the manufacturer to understand any adverse performance effects. Added to Usual Service Conditions in IEEE 841 2021 is the ability to drive a variable torque (VT) load while running on a PWM ASD for all motors up to 1000 Volts. Removed from Unusual Service Conditions are speeds other than rated speed or exposure to flammable or explosive gases since this is likely in a Division 2 location. But some limits are still required on the ASD application; therefore, added to Unusual Service Conditions is operation at speeds above synchronous speed, constant torque application or constant horsepower operation since they could increase mechanical stresses and heating thereby damaging the stator, rotor or bearings.
ASD operation at voltages greater than 1000 volts are still considered an unusual service condition since these are typically custom designed around specific applications. With these changes, load inertia or starting conditions limits now only apply to across-the-line operation.
In the latest revision of IEEE 841, many of the mechanical features have also changed. Such as for frames greater than 449 Frame, end shields, fan covers and terminal boxes may be cast iron or steel, but not aluminum, and rugged and resistant to corrosion. It is logical that a frame series has similar construction, which it now does. Previously, greater than 445 frame could be fabricated from steel plate.
Frames greater than 449T shall now be fabricated from cast iron, cast steel or steel plate. The minimum thickness of 2.8 mm (0.11 in) for steel plate remains unchanged.
Terminal box volume requirements have also changed. Now for motors greater than 445T and through 449T, less than or equal to 1000 volts must have minimum volumes as defined in 841 Table 1, must all be diagonally split, must have an NPT tapped conduit entrance and be rotatable in 90-degree increments. Drains are no longer required. For motors larger than a 449T but less than 1000 volts, the minimum required volume is now 19664 cm3 (1200 in3). For motors larger than a 449 frame but with voltages greater than 1000 volts, the terminal box dimensions remain unchanged, and drains are still required.
The standard will no longer include tables of efficiency, it will only point to the published standards in either NEMA MG1 or IEC 60034-30-1 where they are defined. Efficiency testing requirements may now be performed by any of the three commonly used tests known to provide similar results. These include IEEE 112 Method B, Canadian Standards Association (CSA) C390-10 or IEC 60034-2-1-1B. Thrust-bearing losses are still excluded, but methods for exclusion are now listed as follows:
- May be tested with standard 6000 series bearings.
- Thrust bearings installed but additional losses determined and removed by calculation or from previous test experience.
- If desired, may be tested with the thrust bearings installed with no adjustments.
In summary, the standard was brought up-to-date with the local and global standards, which are presently being used around the world. But at the same time, it was critical that the motors remained interchangeable with existing motors. It now clarifies what is permitted to be labeled an IEEE 841-2021 motor and what is not allowed to be labeled as an IEEE 841 motor. In addition, it added the definition of an IEEE 841 2021 features motor. The standard added the most common ASD application to be considered as a usual service condition thereby making these motors readily available. Highlighted here in this article are many of the major changes to the standard, but there are others that may also be critical to your application. It is important that you review IEEE Std. 841-2021 for additional changes that may not have been discussed here.
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