Cooling high-horsepower motors with heat exchangers

At first glance, a heat exchanger may seem like an unnecessary and unwieldy piece of equipment for an electric motor. However, the benefits of cooling large motors through a heat exchanger are numerous (Fig. 1.).Why cool motors?Thermal capacity is one of the primary application criteria of a variable speed ac or dc motor.

By Brad Menz, Reliance Electric/Rockwell Automation, Greenville, SC December 15, 2001
Key concepts
TENV motors rely on heat radiation for cooling.
They can be cooled by air-to-air or air-to-water heat exchangers.
Heat exchanger technology is advantageous when motors operate in harsh environmental conditions.
Why cool motors?
How it works
Which type of cooling?

At first glance, a heat exchanger may seem like an unnecessary and unwieldy piece of equipment for an electric motor. However, the benefits of cooling large motors through a heat exchanger are numerous

Fig. 1. There are many benefits to cooling large motors through heat exchangers. This photograph shows a totally enclosed air-to-air cooled motor.

Why cool motors?

Thermal capacity is one of the primary application criteria of a variable speed ac or dc motor. The most efficient way to cool variable speed motors is by blowing air through an open motor. This configuration is most often referred to as drip-proof force vent (DPFV). The advantages of using DPFV motors are size and cost.

When the environment requires the use of a totally enclosed motor, there are several options. One choice is to select a totally enclosed motor. Historically, totally enclosed nonvent (TENV), totally enclosed fan-cooled (TEFC), or totally enclosed blower-cooled (TEBC) motors have been applied in these applications. Each option has its shortfalls. In each case, a nonventilated product is much larger and more expensive than an open motor.

TENV motors rely on heat radiation for cooling. Because of this, they are the largest motors with relation to power rating. TEFC motors get additional cooling from a fan driven by the motor shaft. These motors are more efficient in size-per-horsepower than TENV motors. But the range of operating speeds is often limited because the volume of cooling air depends on motor shaft speed. This speed range limitation is improved by driving the cooling fan with a separate constant-speed motor. These TEBC motors have the broadest operating speed range. Typically they are the smallest totally enclosed motors for a given horsepower.

Historically, these methods of providing a totally enclosed motor have been used in all but the largest motors. When motors began to exceed 1000 hp, cooling requirements for the windings and bearings of these large motors made it impractical to supply bigger and bigger nonvent motors. Other methods of cooling were needed.

One method of cooling these very large machines is a heat exchanger. Large ac and dc motors operating in tough environments have been cooled with heat exchangers for years. This technology is now available in the sub-1000-hp range.

Without the use of a heat exchanger, a typical TEBC motor provides less than half of the horsepower of the equivalent frame in a DPFV motor. With a heat exchanger, this ratio is dramatically changed. Air-to-air cooling typically offers 70% and air-to-water cooling can provide up to 100% of the rating of a DPFV motor

If size is a concern, it may seem counter intuitive to choose a motor that requires the mounting of an additional cooling structure. However, the smaller size of these motors means that, even with the addition of the heat exchanger, the total package is still significantly smaller than a TEBC motor with an equivalent horsepower rating. In terms of length and width, the motors cooled by heat exchangers are approximately 33% smaller than a totally enclosed motor of equivalent horsepower.

Fig. 3. Motors with heat exchangers require less space than comparable NEMA TEFC motors.

The most persuasive feature of this package is the significant cost savings that accompany the use of a heat exchanger. As a cooling method, the heat exchanger is extremely efficient, providing the total enclosure and horsepower needed while maintaining a smaller, lighter, compact package. Lower rotor or armature inertia is also attained.

The DPFV motor has always been the densest motor package for the horsepower available. However, through the addition of a heat exchanger, this efficient motor can be used in applications requiring total motor enclosure, replacing more costly and cumbersome models.

How it works

The operation of heat exchangers with motors is relatively simple. Start with the DPFV motor, noted for its small size and high ratings. The heat exchanger is placed on to p of the motor. Recirculated motor air is cooled as it passes through the heat exchanger.

The cooling can take place in one of two ways: air-to-air cooling or air-to-water cooling. With air-to-air cooling, a blower on top of the heat exchanger pulls air through a filter at the bottom of the heat exchanger, then across cooling tubes. The recirculating motor air passes through and is cooled in these tubes. The second option is air-to-water cooling. Here, the recirculating airflow passes over the outside of cooling tubes. Water is circulated through the tubes in the air path to cool the air in the motor.

Which type of cooling?

Once the decision has been made to cool with a heat exchanger, the next step is to determine which kind of cooling is required.

Strictly in terms of efficiency, water cooling provides a 30% higher horsepower rating than air cooling. In addition, the totally enclosed water-to- air-cooled (TEWAC) heat exchanger is less expensive and smaller in size.

Fig. 4. A totally enclosed water-to-air-cooled heat exchanger.

The primary disadvantage to this method of cooling is its dependence on a water supply. Ideally, if water is used in some step of plant operation, the existing plumbing system can be used to supply water to the heat exchanger. If there is no water present, a chiller can keep cool water circulating through the heat exchanger.

When cooling water is unavailable, the totally enclosed air-to-air-cooled (TEAAC) heat exchanger can be used

Fig. 5. When cooling water is unavailable, the totally enclosed air-to-air colled heat exchanger can be used.

When cooling with air, no special plumbing is necessary. Ambient air works well. The major drawback with air cooling, in terms of convenience, is the need to clean or change the filter regularly to prevent clogging.

Heat exchangers are sized based on customer needs, environmental conditions, and motor specifications. The user does not have to determine the size of heat exchanger, because the motor manufacturer typically performs this task.

Heat exchanger technology is advantageous when motors are required to operate where environmental conditions are harsh. For dirty applications requiring extra-tough features, such as in paper, metals processing, mining, and many other applications, the ac or dc motor with heat exchanger is a high-horsepower solution that provides major savings in cost, space, and efficiency.

— Edited by Jack Smith, Senior Editor, 630-320-7147,

Frame size comparison

Hp@1750 RPM Standard TEFX A-C TEBC A-C DPFV
5 180 180
10 215 180
20 250 180 180
30 280 250 180
75 360 320 250
125 440 360 280
150 440 360 280
250 440 400 320
400 440 440 360
500 500 440 400
1000 580 440