Machinedesign 2079 Severe Duty Motors995 0 0

What new motor standards mean to you

Sept. 1, 2000
About IEEE 841-1994: Looking for ways to improve motor safety, reliability, and efficiency? This newest specification from IEEE offers more than suggestions.

Improving motor operation is always high on an engineer’s list of priorities. Motors in tough industrial applications, though, such as those found in petroleum and chemical industries, have special requirements because of the environment they operate in. Thus, engineers from a cross-section of motor users and manufacturers, including members of the Petro-Chemical Industrial Committee of IEEE (PCIC), recently published a new standard for severe-duty motors: IEEE 841-1994.

This standard combines suggestions from the earlier recommended practice (RP) 1986 version, with additional requirements. Perhaps the biggest change between these two standards is the language. The “you should” from the 1986 version changes to “you shall” in the 1994 specification.

Compliance with this standard is still voluntary. However, customers now have a stronger tool when defining their wants to manufacturers. In fact, the number of customers calling for motors to comply with this specification is increasing.

Which motors shall comply

The motors affected are:
• Low and medium-voltage motors, with medium voltages ranging from 2,300 to 4,160 V.
•Totally enclosed fan-cooled (TEFC) squirrel cage induction motors.
•Motors to 500 hp, inclusive.
•Motors that operate in petroleum and chemical industry severeduty environments.

This includes Design B motors of all cast iron construction with corrosion-resistant hardware and paint. Of course these motors must have a stainless steel nameplate. They must also meet a service factor of at least 1.0, although common practice among tough-duty motor vendors is to have a service factor of 1.15.

Excluded are motors with sleeve bearings and other features required for explosion- proof motors.


The 1994 version specifies the following “shall haves:”

Nonhygroscopic insulation. Hygroscopic means the insulation materials will not absorb water. This type of insulation, around motor windings, is also chemical and humidity resistant.

To find the thermal classification of the insulation, manufacturers will use IEEE 117-1974 for random windings and IEEE 275-1992 for form windings. Motors for 2,300 V and 4,000 V will use vacuumpressure- impregnated form windings. Customers may also require form windings for line voltages of 600 V and below on motors rated above 200 hp.

Higher rated insulation. The RP 841 - 1986 specified a minimum thermal rating of Class B insulation, however it was general practice to use at least Class F in severe duty motors. The 1994 version now puts the minimal insulation rating at Class F.

To meet this classification, insulation must withstand temperatures as high as 155 C (311 F). However, engineering practice says motors should operate at temperatures much less than this.

Higher efficiency. Motors must now meet the new NEMA operating efficiency standard. (For a look at efficiency ratings of various horsepower motors, see PTD “High efficiency motors,” 5/95, p. 27.) Efficiently operating motors reduce energy consumption costs as well as help increase motor life.

Larger conduit box. The 1994 specification calls for a conduit box that is at least twice the size of the NEMA standard box. Many manufacturers already comply, as a part of good engineering practice. The increase in size ensures enough room for termination of cabling.

Lower vibration. The specification calls for motor vibration to be within 0.08 in./sec peak velocity. Low vibration in conjunction with a good installation, alignment, and base design provides a smooth operating system, as well as helps prevent premature bearing failure from mechanical stress.

Specifically, 2, 4, and 6 pole motors, at rated voltage and frequency, will not exceed vibration of 0.08 in./sec peak, measured in any direction on the bearing housing. For 8-pole motors, vibration will not exceed 0.06 in./sec peak velocity. When measured at a frequency that is twice the motor speed or twice the line frequency, vibration shall not exceed 0.05 in./sec peak. Axial vibration over a wide frequency range is not to exceed 0.06 in./sec peak on bearing housings. This limit does not apply to roller bearings.

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Motor manufacturers will test vibration at no load on an elastic mounting, or when the motor is bolted to a heavy, flat base.

Ground connection. Manufacturers must add a ground connection point on the motor frame, external to the terminal box. This drilled and tapped hole for a ground lug is in addition to the ground lug in the terminal box. Many manufacturers are already putting this in, because the connection of the terminal box to the steel frame does not always provide a good ground. So, this common practice is now part of the specification.

Lower bearing temperature. Bearing temperature must not rise higher than 45 C at full load for 4-pole, 1,800 rpm and slower motors. Temperature rise is limited to 50 C for 2-pole, 3,600 rpm motors. The result is cooler op erating bearings, longer lasting bearing lubricant, and less bearing wear.

Bearing lubrication. In some cases, manufacturers need to develop methods to regrease bearings without disassembling the fans or fan covers. (Vertical motors have special recommendations). To eliminate purged grease, bearings will have a reservoir with outlet plugs that extend beyond the fan cover.

Shaft seals. For protection from contaminants, severe-duty motors need bearing shaft seals. They must be noncontact to avoid losses due to friction. Some motor manufacturers offer motors that exceed this requirement. Some supply isolators on both ends of the motor and some supply seals for smaller frame sizes than called for in the specification.

Dust and water protection. Motors must also meet the International standards of IP55 for 320 frame sizes and larger. IP55 defines the degree of protection, Table 1. The first numeral of the designation (5 in this case) indicates the degree of protection from dust. The second numeral (also 5 in this case) indicates the degree of protection from water.

Bearings. For verticle flange-mounted motors, the manufacturer must note the use of thrust bearings other than the recommended angular contact ball (single or duplex type), Conrad deep-groove, or spherical roller thrust bearings.

“Soft” feet. Motors now must have flat feet. Motors larger than standard NEMA sizes generally meet this requirement. However, motors of all sizes drive critical processes.

If a foot is not flat against its mounting surface — and it is pulled down with a mounting bolt — the motor frame may experience twisting and springiness that could lead to vibration problems. Therefore, the new standard calls for foot planity to be within 0.005 inches at each bolt mounting hole. This practice offers another benefit in that it speeds up installation and alignment.

The specification also requires that the casting angle on top of the foot mounting holes not exceed 1.5 deg. Flat tops for motor feet reduce installation time and provide good bolt surface contact and good alignment by limiting movement as the mounting bolt is tightened.

Tighter shaft runout. The maximum shaft runout is now cut in half. For shafts with diameters from 0.1875 to 1.625 in., the allowed runout is 0.001 in. indicator reading. For shafts with diameters over 1.625 to 6.500 in., it will be 0.0015 in. indicator reading for ball bearings and over 0.002 in. for roller bearings. This practice speeds up motor installation and puts less mechanical stress on the bearings from misalignment.

Test results. Manufacturers will provide a test report with each motor. This report includes five vibration test points (horizontal and vertical — both ends and axial), along with winding resistance, noload current, voltage and speed. This information establishes a baseline for predictive maintenance programs. It also provides assurance that the motor was tested and meets the requirements of the specification.

Ed Swan is Senior Product Specialist for medium ac motors at Reliance Electric Co., Cleveland.

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