Tom Bishop. P.E.
EASA Senior Technical Support Specialist

What are the normal conditions for which a motor is designed? This is a question that does not often come up except when there is an issue with a motor application.   

The NEMA MG1 motor and generator standards provide details on this subject by defining usual and unusual service conditions. The IEC 60034-1 standard, “Rotating Electrical Machines, Part 1 Ratings and Performance,” also addresses some application conditions in clause 6, though not to the extent given in MG1. Our focus here will be on MG1 since it provides greater detail than IEC 60034-1.

A first step in understanding what NEMA considers usual is to view MG1 clause 1.6, which provides definitions for a wide variety of motors and generators. These include General-Purpose Alternating-Current Motor, General-Purpose Direct-Current Small Motor, General-Purpose Generator, Industrial Direct-Current Medium Motor and Industrial Direct-Current Generator.   

A General-Purpose Alternating-Current Motor is “designed in standard ratings with standard operating characteristics and mechanical construction for use under usual service conditions without restriction to a particular application or type of application.”   

The definitions for the four other motor categories have the following description in common: a mechanical construction suitable for general/industrial use under usual service conditions.  

The common factor is a mechanical construction suitable for use under usual service conditions. The mechanical construction of the motor is designed by the manufacturer for a specific type of motor, such as a General-Purpose Alternating-Current Motor. Thus, the variables that could affect the successful operation of the motor are the usual service conditions. 

Usual Conditions

NEMA MG1 clause 14.2 outlines usual service conditions, and clause 14.3 outlines unusual service conditions. The environmental conditions that are usual (see Figure 1) as given in 14.2 are:   

• Exposure to ambient temperature in the range of -15°C to 40°C or, when water cooling is used, an ambient temperature range of 5°C (to prevent freezing of water) to 40°C, except for machines rated less than 3/4 hp and all machines other than water-cooled having commutator or sleeve bearings for which the minimum ambient temperature is 0°C  
• Exposure to an altitude which does not exceed 3,300 feet (1,000 meters)  
• Installation on a rigid mounting surface  
• Installation in areas or supplementary enclosures which do not seriously interfere with the ventilation of the machine.   

Discussion of the first bullet item: The motor nameplate frequently indicates the maximum ambient rating of 40°C, but almost never indicates the lower ambient temperature limit. Caution: Note that the ambient rating on the nameplate applies to what we often term “room temperature.” The temperature rise, or maximum winding temperature, is rarely provided on a motor nameplate. For more details about temperature rise, see the article “Understanding Motor Temperature Rise Limits” in the November 2003 edition of Currents.  

Since the usual ambient limits are given, the correct inference is that operation below the minimum or above the maximum is not normally permissible. It is best to check with the manufacturer regarding the use outside of the usual ambient temperature range. At low temperatures, bearings and lubrication may be the primary concerns. At high temperatures, the winding, as well as the bearings and lubrication, may be primary concerns.  

Discussion of the second bullet item: Similarly, operation above 3,300 feet (1,000 meters) is not normally permissible without derating the motor power rating. Temperature Rise provides guidance for operation at altitudes above 3,300 feet (1,000 meters) without derating the motor power rating. 

Discussion of the third bullet item: Since most motors are mounted on a rigid base, that is not often a concern. However, as an example, when a motor is mounted on something such as a cantilevered baseplate relying on belt tension to support the motor, the motor manufacturer should be consulted for guidance.  

Discussion of the fourth bullet item: This indicates that the normal condition for a motor is when the ventilation system is not compromised. An example of a condition that is outside this rule would be an air compressor motor in an enclosure, such as a cabinet, with the motor, controls and compressor all inside. 

Unusual Conditions

Clause 14.3 in MG1 states that the manufacturer should be consulted if any unusual service conditions exist that may affect the construction or operation of the motor. These unusual conditions (see Figure 2) include exposure to: 

• Combustible, explosive, abrasive or conducting dusts  
• Very dirty operating conditions where dirt accumulation may interfere with normal ventilation  
• Chemical fumes, flammable or explosive gases  
• Steam, salt-laden air or oil vapor  
• Damp locations, radiant heat, vermin infestation or atmospheres conducive to the growth of fungus  
• Abnormal shock, vibration, or mechanical loading from external sources.  

The above is not the complete list from clause 14.3, nor is clause 14.3 intended to be an exhaustive or complete list. That would be too voluminous to be of practical use. To better appreciate what is an unusual exposure condition, we can consider the opposite view.  

From that perspective the exposure should be to clean dry (but not too dry) air and not disturbed by other mechanical or physical factors, such as the conditions to be expected when the new motor is tested in the motor manufacturer factory. 

Unusual conditions also include operation where the machine is subject to: 

• Excessive departure from rated voltage or frequency, or both 
• An alternating-current supply voltage that is unbalanced by more than one percent  
• Operation at speeds above the rated speed (see Table 1)  
• Operation in a poorly ventilated room, in a pit or in an inclined position  
• Torsional impact loads  
• Repetitive abnormal overloads  
• Reversing or electric braking  
• Frequent starting 

As with exposure conditions, the unusual operating condition list is also not the complete list from clause 14.3 and thus not intended to be an exhaustive or complete list. The first two bullets in this category address the electrical supply, which should have voltages that are +/-10 percent of motor rated voltage, and frequency within +/-5 percent (+3/-5 percent per IEC 60034-1, clause 7.3) of rated. Operation above rated speed could be a concern if the motor is powered by a variable frequency drive (VFD). Table 1 provides overspeed limits for induction motors.

The ventilation issue in the fourth bullet is very similar to that of the exposure issues already mentioned. Likewise, the operation in a pit suggests potential issues with dampness or possibly submergence. Although small and medium motors are typically capable of operating in an inclined position, even vertical, that orientation is classified as an unusual condition. Potential issues include grease lubricant paths that may not reach the bearings when the orientation is changed and lubricant leakage from oil-lubricated motors.  

Excessive mechanical forces, such as overload and torsional impact, can lead to premature shaft failure, making it obvious that they are unusual conditions. Frequent starting or reversing can also result in excess mechanical load. This can also result in the stator and rotor overheating because of the high ratio of starting to full load current (typically from 5:1 to 8:1). Similarly, electric braking can result in above-rated current during braking and rapid-heating if the braking power is still applied after the motor is at rest.

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Categories: Technical topics, AC motors, Design/theory/application, Standards

Tags: Motor design

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