Applications of Motors and Generators
Motors and generators are applied on the basis that they can carry the rated load and withstand the environmental conditions during their rated life
reliably. Because of the different requirements of various industries for motors, each industry has over the years developed different application criteria for motors. Therefore, a wide variety of motor insulation and construction classes are available (manufactured in accordance with NEMA MG1 standards) to satisfy these needs. The actual life and reliability of motors are determined by combining the experience of industry requirements and electrical characteristics. The reliability of a machine can be defined in terms of its electrical and mechanical integrity, which are explained next.
The electrical integrity of a machine can be stated in terms of the dielectric and load-time ratings.
Dielectric rating can be defined as the ability to correctly maintain the separation of the conducting and nonconducting parts from the power system supply voltage. To achieve the required dielectric integrity, various insulation systems are used, depending on the type of motor and service conditions. To provide the highest reliability, insulation materials must have a degree of thermal, voltage, mechanical, and environmental endur- ance. Therefore, insulation systems used for motors and generators are classified on the basis of their ability to withstand the total temperature during the life of the machine without deterioration. The insulation systems of motors and generators are discussed in Section 10.8.
Most equipment manufacturers have standardized on roughly the same set of dielectric strengths for their insulation systems given the operating conditions most frequently encountered, and the requirements of NEMA.
For installations where commercial or industrial power sources are used, the established practices have worked very well. Recently, with the introduction of variable frequency induction motor drives using pulse width modulated (PWM) switching technology, some standard 600 V class machines have experienced insulation failures. The reasons for such failures must be evaluated on case by case basis; however, the most common problem results from failure to consider the drives’ modulating frequency and the length of the motor leads between the drive’s output terminals and the motor itself. At frequencies in the several kilohertz to tens of kilohertz range, the supply cables to the motor no longer act as simple conductors as they do at 60 Hz. If the cables are too long, they tend to act as incorrectly tuned transmission lines, which lead to a significant reflected voltage wave each time the current is switched. This buildup tends to concentrate at the first few turns of the motor winding. Manufacturers are approaching these applications in several ways: (1) by specifying the maximum lead length for a given horsepower, voltage and modulating frequency and using a standard, NEMA design motor; (2) by applying snubber/RC-filter assemblies at key locations and keeping the standard, NEMA design motor; (3) by offering motors for PWM service whose dielectric designs have been enhanced in order to withstand the additional electrical stress produced by the PWM sources. In cases where the motor and drive are supplied by different manufacturers, each should be given the full nameplate information of the motor, the approximate length of cable between the controller and the motor, and the nature of the load to be driven. This will allow one or both suppliers to offer components that will work as a coordinated system.
The load rating relates to the ability of the machine to carry a load over a period of time. Load ratings can be classified as service-factor duty, short-time duty, and overload duty.
Service-factor duty is defined as the multiplier that is multiplied by the nameplate horsepower; the result is used for temperature testing with continuously applied load until the temperature equilibrium is reached. For example, a service factor of 1.0 means that the motor cannot carry an overload on a sustained basis without exceeding the insulation temperature. A service factor of 1.15 has been established as a standard for open and drip-proof general-purpose motors below 200 hp. Larger motors have a service factor of 1.0. A service factor of 1.15 usually translates into a 10°C higher temperature with 15% overload.
Short-time duty is defined by a motor operating at continuous load over a period of time less than that required to reach thermal equilibrium. The short-time duty ratings are usually 15, 30, or 60 min. Motors should be allowed to cool down to ambient temperature when they are operated in this manner before the next load cycle is applied; otherwise, the motor will overheat.
Overload duty is defined as the ability of the motor to continuously carry an overload for an extended period of time. A common rating is an overload of 25% for 2 h. It is expected that the motor will not reach thermal equilibrium for the time specified for the overload rating.
10.3.2 Mechanical Integrity
Mechanical integrity involves mechanical stresses, vibrational forces, and the ability to keep moving parts separate from stationary parts. The mechanical stresses imposed on motors are due to motor torques and loads. Motors that are switched frequently are more susceptible to these stresses. Vibrations are a result of incorrect alignment, incorrect mounting, and incorrect installation, which will tend to deteriorate the motor life performance.
However, the most critical mechanical part of the motor is the bearings, where stationary and rotating parts meet. To ensure optimum performance, the bearings should be correctly matched to the load, kept clean, lubricated, and aligned correctly. Motor bearings include babbitted, sleeve, and ball types. The fatigue life of ball bearings varies inversely with the cube of the load. The load usually imposes a side force on the shaft extension owing to driven-equipment connections such as belts, gears, or chains. The fatigue life of sleeve bearings is infinite due to the fact that the oil film supports the shaft; and if maintained correctly, they will last indefinitely.
Motors and generators conforming to NEMA standards are designed to operate in accordance with ratings under the usual service conditions as listed in NEMA MG1 standards. Motors and generators operated under service conditions other than those specified in NEMA MG1 standards, may involve some hazard. The severity of this hazard depends upon the degree of departure from the usual conditions. Hazards usually result from overheating, abnormal deterioration of the insulation, mechanical failure, corrosion, and fire. Therefore, the manufacturer should be consulted for further information concerning the usual service conditions.