Mathis Menzel, Menzel Elektromotoren GmbH, Germany, discusses the advantages and disadvantages of different types of motor coolers.
Motor
Variants V ariants Introduction Unlike 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 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 the degree 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 necessary to dissipate this heat.
Reprinted from October 2008 World CemenT www.worldcement.com
Cooling methods With 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 feat ure of such mot ors 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.
Comparison In 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 fed into 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 opencircuit 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
Figure 1. IC 01 open-circuit ventilation cooler.
Figure 2. Workings of the IC 01 opencircuit ventilation cooler.
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.
System involves the highest procurement costs, comparatively speaking.
The frame size is larger than with the other cooling types due to the 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.
Disadvantages
Reprinted from World CemenT October 2008 www.worldcement.com
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.
factors, including deployment context, environmental aspects, the value-for-money ratio, and market forces._________________________________________________l
Figure 4. Workings of the IC 81W air-to-water heat exchanger.
Conclusion In 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
Figure 5. IC 611 air-to-air heat exchanger.
Figure 3. IC 81W air-to-water heat exchanger.
Figure 6. Workings of the IC 611 air-to-air heat exchanger.
Reprinted from October 2008 World CemenT www.worldcement.com
