Most small motors are built to common dimensional and performance criteria provided in NEMA MG-1, and are readily interchangeable. Up to approximately 250 hp (the actual limit varies with enclosure type and synchronous rpm), NEMA provides the market with horsepower-to-frame assignments.
However, when that old 500-hp, medium-voltage motor used to drive the plant’s air compressor dies, the plant engineer is left with some difficult and generally expensive decisions to make. As manufacturers become increasingly efficient, new motor prices become more competitive compared to repair costs.
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Efficiency
Since larger motors are often run more hours per year than smaller models, consuming far more power, energy efficiency is important. Annual energy costs for a large motor often run 5-10 times the initial purchase price. Sometimes, the cost savings of repairs are offset by increased energy consumption of the old motor.
There is a limit to the number of times a motor can be repaired. Eventually, the laminations become worn, bent, and difficult to repair. If a bearing failure is accompanied by a winding failure, substantial lamination damage is common. If a winding burnout was not performed properly, stator core loss increases, which decreases motor efficiency. As a result, the amount of heat the motor must dissipate increases. Additional thermal load can lead to a shortened rewind life.
If a failed motor is to be fixed, don’t buy the repair on price alone. Use a vendor who has plenty of experience in larger motors. The cost of downtime and labor to remove and replace the unit are often more than the cost of the repair.
Enclosures
As motors become larger, it is more difficult to dissipate the heat generated inside them. As a result, large TEFC motors are often 2-3 times as expensive as their open drip proof (ODP) counterparts. As a general rule of thumb for enclosures, use ODP construction indoors and totally enclosed fan cooled (TEFC) construction outdoors.
TEFC design does not allow any free exchange of air between the interior and exterior of the motor (Fig. 1), which protects the windings from contamination. Due to the increased difficulty in dissipating heat, large TEFC motors are substantially bigger than their ODP counterparts (Fig. 2), accounting for their increased cost.
Fig. 1. Large TEFC motors protect the winding from contamination because the design does not allow air exchange between the interior and exterior.
When a TEFC motor is too large for traditional fin-cooling, an air-to-air heat exchanger is used to dissipate the generated heat. These motors are referred to as TEAAC or TETC. The main disadvantage to these motors is that the cooling tubes can become fouled with dirt or debris and degrade the ability of cooling the motor, which can lead to shortened winding life.
Fig. 2. Large ODP motors are not totally enclosed, which allows ambient air to circulate.
WPI motors are closest in design to ODP motors. They differ only because the air passages are constructed to minimize the entrance of rain, snow, and moisture. Manufacturers also protect the interior of WPI motors from corrosion by painting the stator and rotor. WPI motors are routinely provided with rodent screens since they are often used in lightly protected indoor areas where these creatures might chew on the wire insulation.
WPII motors provide a higher degree of protection than WPI models and are usually used outdoors in mild-to-moderate climates. WPII motors have a hood on top, which directs air through three, 90-deg changes in direction prior to entering the motor. Incoming air must either enter a low-velocity chamber, which allows particles to fall out, or be filtered. WPII motors are normally supplied with space heaters to prevent condensation on the windings during shut down periods.
Noise
Fan-cooled motors require large fans, which can generate a lot of noise. Differences of 5-10 dB are common between TEFC and equivalent open motors.
This difference may not seem like much, unless a person has to work next to one. Any attempts to reduce the noise generated by a TEFC motor must be weighed against any detrimental effects on cooling.
Installation
Since rating-to-frame assignment is not called out for ‘above NEMA’ motors, check whether the new unit fits the application. Since newer motors are often smaller than their older counterparts, all that is generally required is a set of adapter rails.
If it is decided to upgrade from an open motor to an enclosed one, be aware that enclosed models are generally much larger than their open counterparts.
Voltage
Smaller NEMA motors generally operate at 575 V or below, which is considered low voltage. Above 100 hp, motors are usually available in medium voltage. Medium voltage is defined in the National Electric Code as being between 601-6000 V, while 6001 V and higher is referred to as high voltage. While the supply voltage normally dictates the motor voltage, motors between 100-1000 hp are commonly available in either low or medium voltages.
Medium-voltage motors are substantially more expensive than their low-voltage counterparts, and are sometimes a little larger. To accommodate the higher voltage, it is necessary to increase the insulation thickness on the windings. While absolutely necessary for safety reasons, the added insulation does nothing to help make the shaft turn and is a significant impediment to heat transfer.
Medium-voltage motors are slightly less efficient than their low-voltage counterparts and often have to be somewhat larger to dissipate the increased amount of generated heat.
