Recognize service conditions for motors and generators
It is important to understand the differences between the usual service conditions most motor designs assume and unusual service conditions that lead to unreliable operation and costly shutdowns
When selecting a motor for a new application or solving a problem with an existing one, it’s important to verify the motor will operate normally in the conditions the application presents. Although these situations may not occur often, it’s helpful to understand the differences between the usual service conditions most motor designs assume and the unusual service conditions that can lead to unreliable operation and costly shutdowns.
Good starting points for this discussion are the definitions of usual and unusual service conditions in the National Electrical Manufacturers Association (NEMA) Standard MG 1: Motors and Generators (MG 1). The International Electrotechnical Commission (IEC) Standard 60034-1: Rotating Electrical Machines, Part 1: Ratings and Performance, also addresses application conditions (see Clause 6) but to a lesser extent, so the focus here is on MG 1.
MG 1, 1.6 defines usual service conditions for a wide variety of motors and generators. These include general-purpose alternating-current motors, general-purpose direct-current small motors, general-purpose generators, industrial direct-current medium motors and industrial direct-current generators.
According to MG 1, general-purpose ac motor designs have “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.” Its definitions of the four other motor categories share this characteristics: mechanical construction suitable for use under usual service conditions. Since the manufacturer designs the mechanical construction for a specific type of motor (e.g., a general-purpose ac motor), the variables that could affect successful operation are the usual service conditions.
According to MG 1, 14.2, the usual environmental/service conditions include (see Figure 1):
- Exposure to ambient temperature in the range of -15°C to 40°C, or 5°C to 40°C for water-cooled machines (to prevent water from freezing). For machines rated less than 3/4 hp and all machines (except water-cooled) that have a commutator or sleeve bearings, the minimum ambient temperature is 0°C.
- Exposure to an altitude of 3,300 feet (1,000 meters) or less
- Installation on a rigid mounting surface
- Installation in areas or supplementary enclosures that do not seriously interfere with the ventilation of the machine.
Each of these items deserves fuller explanation.
Ambient temperature. Motor nameplates frequently indicate the maximum ambient rating of 40°C but rarely state the lower ambient temperature limit. Nevertheless, operation below the minimum or above the maximum ambient temperature normally is not permissible.
A best practice is to check with the motor manufacturer regarding operation 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 bearings and lubrication, may be the main issues.
Caution. The ambient rating on nameplates applies to what we often term “room temperature.” The temperature rise, or maximum winding temperature, rarely appears on motor nameplates and is beyond the scope of this article.
Altitude. Operation above 3,300 feet (1,000 meters) is not normally permissible without derating the motor power rating. Temperature rise may provide guidance for operation at such altitudes without derating the motor’s power rating, but as mentioned earlier, that is beyond the scope of this article.
Rigid mounting. Most motors mount on a rigid base, so this is seldom a concern. Consult the motor manufacturer about other mounting arrangements (e.g., a cantilevered base supported by belt tension).
Ventilation. An uncompromised ventilation system is the normal condition for a motor. An example that violates this rule would be to place a compressor motor, air compressor and controls inside an enclosure such as a cabinet.
MG 1, 14.3 provides a list of unusual service conditions and recommends consulting the manufacturer if any of them may affect motor construction or operation (see Figure 2), including exposure to:
- Combustible, explosive, abrasive or conducting dusts
- Accumulated dirt and debris that may interfere with normal ventilation
- Chemical fumes; flammable or explosive gases
- Steam, salt-laden air, oil vapor
- Damp locations, radiant heat, vermin infestation, atmospheres conducive to the growth of fungus
- Abnormal shock, vibration or mechanical loading from external sources.
The list of unusual exposure conditions in MG 1, 14.3 is not intended to be exhaustive or complete, because that would be too voluminous for practical use. To better appreciate what constitutes unusual service conditions, consider their opposites. From that perspective, exposure should be to clean, dry (but not too dry) air with no mechanical or physical disturbances, e.g., conditions that would be expected for factory testing new motors.
As with exposure conditions, the list of unusual operating conditions in MG 1, 14.3 is not intended to be exhaustive or complete. Among the items it covers are:
- Electrical supply voltage and frequency
- Excessive departure from rated voltage or frequency, or both
- The ac supply voltage more than 1% unbalanced
- Operation at above rated speed (see Table 1)
- Operation in a poorly ventilated room or a pit
- Operation in an inclined position
- Excessive mechanical forces
- Repetitive abnormal overloads
- Torsional impact
- Frequent starting or reversing
- Electric braking.
As with the list of usual conditions, each of these unusual conditions merits more discussion.
Electrical supply voltage and frequency. Unusual service conditions exist if voltages exceed ±10% of motor rated voltage, frequency exceeds ±5% of rated (+3%/-5% per IEC 60034-1, 7.3), or both. Voltage unbalance greater than 1% among phases also is an unusual service condition.
Operation at above rated speed. This could be a concern for a motor powered by a variable frequency drive (VFD). Table 1 provides overspeed limits for induction motors.
Poor ventilation and pit operation. Poor ventilation resembles unusual exposure conditions already mentioned. Likewise, operation in a pit may lead to problems with dampness or possibly submergence.
Operation in an inclined position. Small and medium horizontal motors typically can operate in an inclined or even a vertical position, but those orientations are classified as unusual service conditions. Changing the motor’s orientation could affect its lubricant pathways, preventing grease from reaching the bearings. On oil-lubricated motors, it could cause leaks.
Excessive mechanical forces. Excessive mechanical forces, such as overload and torsional impact, are obviously unusual service conditions because they can lead to premature shaft failure. Frequent starting or reversing also can result in excess mechanical load, as well as stator and rotor overheating due to the high ratio of starting to full-load current (typically from 5:1 to 8:1).
Similarly, electric braking can cause above-rated current during braking, and rapid heating if the braking power is still applied after the motor is at rest.
Knowing the differences between usual and unusual motor service conditions may not be something you’ll use often. But this information will be invaluable when it’s time to choose a motor for a new application or to troubleshoot a problem or failure.
Thomas H. Bishop, PE is a senior technical support specialist at EASA Inc., a CFE Media content partner.