Considerations in specifying large-frame motors

10/01/2001


 

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.

Key concepts

Efficiency helps determine whether a motor is repaired or replaced

Enclosures are available to meet most ambient needs

Bearing selection depends on how the motor is loaded

Sections:
Efficiency
Enclosures
Noise
Installation
Voltage
Bearings
Protection

Sidebars:
Motor enclosure acronyms
Issues to consider when a motor fails

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.

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.

Bearings

Above 200 hp, many manufacturers offer sleeve bearings instead of rolling element bearings. Sleeve bearings are most common on very large motors, above 2000 hp, and on 3600-rpm motors.

Sleeve bearings have limited capability for absorbing radial load, and should not normally be used on a belted load. Frequently, 3600-rpm motors are supplied with sleeve bearings because rolling element bearings are speed limited. Be wary of buying very large 3600-rpm motors with ball bearings.

In most applications between 200-2000 hp, rolling element bearings work well if properly maintained. Up to approximately 100 hp, there are few options regarding bearing selection.

Normally provided ball bearings work well in most applications.

Above 75 hp, ball bearings often do not have the required radial capacity to support belt drive applications. In those cases where bearing life is too short with ball bearings, roller bearings are normally required.

Roller bearings use cylindrical rollers instead of balls. Within the bearing, rollers have a line of contact rather than the point contact found in ball bearings. As a result, roller bearings are able to take much higher radial loads than ball bearings. Roller bearings should not be used in a direct-coupled application since the rollers need a radial load to prevent roller skidding and premature failure.

In cases where a standard roller bearing does not have enough radial capacity to support the load, there is a variety of other bearing types available from which to choose, generally at significantly higher cost.

When a roller bearing is required, it is usually a good idea to order a high-strength shaft as well. Shaft replacements can sometimes approach the cost of a new motor. It is advisable to specify an alloy shaft material - 4140 or 4340 - when ordering a roller bearing motor.

Protection

Protective devices fall into two primary categories: electrical and mechanical. Electrical devices protect against winding failures, while mechanical products prevent and detect bearing failures.

Electrical devices

Thermocouples and resistance temperature detectors (RTDs) provide the actual temperature of the winding (Fig. 3). A thermocouple consists of a bimetallic junction, which produces a voltage that varies with temperature. Temperature is measured only at the point where the two metals are in contact. An indicating instrument is required to interpret the voltage produced by the thermocouple.

Resistance temperature detectors are embedded in a stator slot. As the temperature of the stator winding changes, the electrical resistance of the wire in the RTD changes. Either two or three lead wires are connected to one end of the RTD.

Three-wire RTDs are more accurate and should normally be specified. Two-wire RTDs are less accurate because the resistance in the sensor leads cannot be differentiated from the resistance in the sensor element. This difference is particularly true for low resistance RTDs such as 10-ohm copper. Used properly, winding RTDs are the most accurate means of determining winding temperature.

Thermostats and thermistors are available on large and small motors. Thermostats simply provide a contact opening when a certain temperature is reached. Thermistors consist of a small bead of semiconductor material with two wires attached. The electrical resistance of the thermistor bead rises dramatically at a certain design temperature. A control module is necessary to interpret the resistance of the thermistor bead and provide a signal.

Thermistors, thermostats, and thermocouples are generally less expensive than winding RTDs, but RTDs provide the most useful and accurate information to the user and are recommended wherever possible.

Current transformers, surge capacitors, and lightning arresters are used along with temperature sensors. Current transformers can provide an indication of motor loading, which is valuable for maintenance and equipment troubleshooting. Surge capacitors and lightning arresters protect against transient voltage conditions associated with fast-acting circuit breakers. Normally supplied together, they are particularly useful where winding failures have been a problem and the cause cannot be traced.

Space heaters are one of the least expensive means of protection against winding failure. They are energized when the motor is de-energized. Their function is to increase the temperature of the windings above the dew point to prevent condensation, which leads to premature winding failure.

Space heaters are normally supplied with weather protected, open motors, but should be ordered with enclosed motors also.

Mechanical devices

Bearing failures are more common than winding failures, and are often easier to repair, especially if caught early. A bearing failure can cause lamination damage and destroy the windings. There are two primary methods to predict bearing failures: vibration and temperature sensing.

Vibration sensors work best on ball bearing motors. As bearings start to fail, high frequency vibration is generated by the imperfections on the bearing races. An accelerometer and vibration spectrum analyzer can detect this condition weeks or months before an impending failure, allowing time for a repair to be scheduled.

Online or continuous monitoring of motor vibration is very expensive, but well worth the cost when unplanned shutdowns cannot be tolerated.

Bearing temperature monitoring is a less expensive approach. Bearing temperatures rise rapidly just before failure. Bearing temperature detectors may not give enough warning to avoid an unplanned shutdown, but if connected to an automatic shutdown, they help prevent damage to the windings and laminations.

Bearing temperature monitors come in the same configurations as winding temperature detectors. There is no absolute value that provides the best indication of impending failure. The best way to set limits is to observe what the temperature rise of the bearing is once all the temperatures in the motor have stabilized - about 8 hr of running under full load.

Information should be charted over several weeks, noting the ambient temperature. Temperature limits can then be set, taking into account the highest ambient temperature in the area.

While the cost of a fully instrumented motor can be 50% or more above the base cost, the cost of a single unplanned shutdown can often justify the expense.

'Above NEMA' motors are substantially more complex than standard NEMA products, and should be carefully researched prior to purchase. Before that old 500-hp compressor motor fails for the last time, do the homework and fit the motor to the application rather than just replacing the motor.

- Edited by Jack Smith, Senior Editor, 630-320-7147, jsmith@cahners.com



Motor enclosure acronyms

ODP -Open drip proof enclosures are usually used indoors

TEFC -Totally enclosed fan cooled motors protect the windings from contamination because the design does not allow free air exchange between the interior and exterior.

TEAAC -Totally enclosed air-to-air cooled motors use air-to-air heat exchangers to dissipate heat.

TETC -Totally enclosed tube cooled is another name for a TEAAC motor.

WPI -Weather protected type I motors are similar to ODP motors except the WPI air passages are constructed to minimize the entrance of weather elements.

WPII - Weather protected type II are hooded motors that provide more protection than WPI motors and are usually used outdoors in mild-to-moderate climates.

Issues to consider when a motor fails

When faced with a large-motor decision, the following factors must be considered.

Repair vs replacement costs

Efficiency

Enclosure design

Noise

Voltage

Bearing type

Protective features



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