method of cooling

7/27/2019 Method of Cooling

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IntroductionUnlike internal combustion

engines, electric motors are generally

characterised by very high efficiency. In fact,

ongoing engineering efforts have pushed the

degree of efficiency of modern motors to within a

few percentage points of 100%, depending on the size

of a given motor. Today, an efficiency of 96% has virtually

become the standard, and even 97% is not infrequently

achieved. At the same time, soaring energy costs have

become a powerful stimulant that drives redevelopments and

the engineering progress toward further improvements in thedegree of efficiency, the idea being to save on resources

and to lower the operating overhead.

Notwithstanding these efforts, the operation

of electrical motors continues to involve

certain energy losses in the form

of friction losses, core losses,

hysteresis losses, all of which

manifest themselves as

waste heat. In order to

prevent an impermissible

heating of the motor

and specifically of the

winding, it is necessaryto dissipate this

heat.

Mathis Menzel, Menzel

Elektromotoren GmbH,

Germany, discusses

the advantages and

disadvantages of different

types of motor coolers.

Motor

Variants

Reprinted from October 2008World CemenTwww.worldcement.com


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Cooling methodsWith smaller motors (depending on the type and

size, generally up to somewhere near frame size

560 mm, which translates into a performance

envelope of about 2 MW), the standard cooling

method is surface cooling (usually using ribbed

housings). In the case of larger motors, the type

most commonly used is a modular construction. In

fact, slip ring motors in the medium voltage segment(nominal currents above 1000 V) are manufactured

almost exclusively in the form of modular motors.

A characteristic feature of such motors is that the

electric part is identical for any of the various cooling

types, so that the differences are mainly associated

with the added cooling system.

ComparisonIn principle, motors of the cooling types IC 01

and IC 81W are identical except for the respective

heat exchanger, as the cooling performance of an

ambient-air system practically matches that of a

water-cooled system (assuming that the air fedinto the motor does not exceed a temperature of

40 C). By contrast, the cooling effect of air-to-air

heat exchangers (where the internal circuit and the

external circuit both use air) is markedly inferior,

resulting in a performance loss of about 30%

depending on frame size, even though the electrical

construction is identical. The difference is also

reflected in the decidedly higher procurement costs

for a motor of the IC 611 cooling type, the power

output being the same.

Today, motors of the IC 611 cooling type

equipped with a mounted air-to-air cooler are

predominantly used in the cement industry (serving

as drives for mills, crushers, and fans), among other

applications. Few motors, and least of all large mill

drives, follow the IC 01 cooling type: i.e. the open-

circuit cooled design.

In the cement industry, for one, the IC 81

cooling type is rarely found. Then again, its specific

advantages cause it to be the motor of choice in

a variety of other sectors, such as power plant

technology or the paper and steel industry, where it

accounts for an overwhelming share of the electric

motors deployed. It is unlikely that the IC 81W

cooling type will ever gain a substantially higher

Table 1. Advantages and disadvantages of different cooling systems

Cooling type IC 611 air-to-air heat exchanger IC 81 W air-to-water heat

exchanger

IC 01 open-circuit ventilation

Advantages Winding is protected against

environmental impact.

System is independent from any

cooling water circuit.

Winding is protected against

environmental impact.

Cooling pipes and motor winding

remain clean from deposits.

The ambient temperature has a

negligible impact.

System is marked by a superior,

constant cooling effect.

System involves lowest

procurement costs.

Cooling pipes, in which deposits

might collect, are not required.

The cooling method is very

effective.

Disadvantages System involves the highest

procurement costs, comparativelyspeaking.

The frame size is larger than with

the other cooling types due tothe less effective cooling method.

System requires an increased

cleaning effort.

A cooling water system is

required.

System is more expensive than

IC 01 type motors.

Ambient, airborne pollutants are

drawn into the motor.

Figure 2. Workings of the IC 01 open-

circuit ventilation cooler.

Figure 1. IC 01 open-circuit ventilation cooler.

Reprinted fromWorld CemenT October 2008www.worldcement.com


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share of the market in this sector than it has today,

not least because the use of water in any given

process is avoided as much as possible in the

cement industry.

By contrast, the deployment of water-cooled

motors is particularly sensible wherever water-cooled

drives are already part of a given process. This is

often the case with water-cooled gear units, in which

the use of a shared cooling circuit seems the obvious

answer.In terms of its basic design, the IC 611 cooling

type with air-to-air heat exchanger representing the

established standard in the cement industry has

the one major advantage in that it requires neither

water nor a clean environment. Then again, a motor

of this type does require regular monitoring and

visual inspection to make sure that the cooling ducts

are not clogged by deposits that would prevent the

dissipation of heat into the environment. Failure

to dissipate the heat would eventually cause the

motor to overheat and as is to be hoped trigger

an alarm by the temperature control, assuming a

thermostat is properly installed and set.

ConclusionIn closing, it should be added that it is impossible

to make an unqualified recommendation for any one

cooling type over all others. Here as everywhere,

the specific conditions and requirements of a given

facility and location always need to be taken into

account. At the same time, it is safe to diagnose

a tendency in the cement industry to favour

open-circuit cooled motors of the IC 01 cooling

type. This is explained by the fact that, on the one

hand, the required output and with it the size of

the motors keeps increasing, while, on the other

hand, the environmental conditions of newly raised

cement plants continue to improve, causing the

advantages of lower acquisition costs to outweigh

the disadvantages of pollution. In short, the choice of

cooling type is ultimately subject to a combination of

factors, including deployment context, environmental

aspects, the value-for-money ratio, and market

forces._________________________________________________l

Figure 3. IC 81W air-to-water heat exchanger.

Figure 4. Workings of the IC 81W

air-to-water heat exchanger.

Figure 5. IC 611 air-to-air heat exchanger.

Figure 6. Workings of the IC 611 air-to-air heat exchanger.

Reprinted from October 2008World CemenTwww.worldcement.com

method of cooling

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