Severe-duty motors must have a non-contact shaft seal. Inpro Co. Inc. offers the Inpro Seal TM, a non-contact labyrinth seal.

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.

Changes

The 1994 version specifies the following “shall haves:”

In IPXX, the numerals designate varous levels of protection. This chart briefly lists the types of protection associated with a particular numeral.
Select figure to enlarge.

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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