Water-cooled electric motors – In which cases do they make sense? Things to consider

Susanne Reinhard, Baumüller Nürnberg GmbH

Read min.

Electric motors have a high efficiency of over 90 percent. In other words, a large portion of the electrical energy that is supplied is converted into mechanical output. The remaining approximately 10 percent of the electrical output is lost in the form of heat.

Function and advantages of water cooling

Index

To prevent the motor from overheating, heat must be dissipated

To prevent the motor from overheating, this heat must be dissipated. For this, a variety of options exist. Uncooled motors dissipate the heat into the machine housing to which they are mounted and into the surroundings via convection. Air-cooled motors have a fan that blows air onto the motor housing, thereby cooling it. Motors with fluid cooling are cooled with water or oil that flows through cooling channels in the motor.

Functionality of water cooling

The fluid cooling of electric motors can be converted in different ways. Basically, there are always ducts in the stator of the motor through which mainly water is conducted because water has the best heat coefficient. For that reason, we refer, both as a rule and also in this text, to water-cooled electric motors. The cooling pipes can either be housed in a special cover or, as in the latest water-cooled servo motors from Baumüller, directly in the motor casing. The version with the cover cooling has the result that the motor needs more space and when realized directly in the housing the dimensions of the water-cooled motors are identical to those of the uncooled motors. In principle, the water is then simply pumped through the motor and repeatedly cooled down again by a cooling unit. Then the motor does not get too hot and can deliver optimal output. The general rule: the more current is fed to the motor, the more the motor heats up and the more the cooling has to do.

Selecting the right cooling method is not simple. Many factors go into the decision and the reason for the selection of water-cooled electric motors can be varied.

How to video

In the video you will learn what makes water-cooled servomotors special and how you can use them successfully.

To the how to video

10 challenges where the use of water-cooled electric motors makes sense.

Dynamics

Dynamics refers to the way the process requires a rapid start-stop operation with few pauses. The greater the dynamics, the more current has to be supplied, and the drive heats up faster. Water-cooled electric motors are better at dissipating this heat than air-cooled motors, and thus more dynamic processes are possible. Because of the better cooling performance, the motor output is also up to 50% higher than with uncooled motors. This means that in many cases the motors can be smaller in dimensions. Smaller motors have a lower mass inertia and thus also allow for greater dynamics. These good properties of water-cooled motors with respect to dynamics make it possible to reduce cycle times and increase the productivity of machines.

High torque

The level of nominal and stall torques of a motor is contingent on the energy that can be supplied. The water cooling dissipates the heat in the system especially effectively so that a large quantity of energy can be supplied, thereby achieving high torque levels without overheating of the motor.

Is water cooling more expensive than air cooling? Can hydraulic oil be used as a cooling medium instead of water? You will find answers to these and other questions in our FAQ.

FAQs on water cooling

High torque PLUS high dynamics

Usually it is necessary to strike a compromise between the requirements of dynamics and those of torque. Smaller motors mean better dynamics but less torque. Larger motors deliver high torque, but allow for less dynamics. When using water cooling, both requirements can be met: small size for high dynamics plus high torque thanks to optimal cooling.

Impurities in the air

If there are air impurities at the machine location, e.g. from cotton fibers in the textile industry, these can cause fans of air-cooled motors to become dirty. Dirty fans prevent effective motor cooling. Regular cleaning is required and that incurs high service costs and downtimes. Water cooling does not require cleaning and thus incurs minimal service costs. An additional problem with textile fibers in the environment is their flammability. Cotton fibers can ignite at temperature as low as 80 °C. Since the heat is dissipated directly on the motor from the water cooling, high temperatures don't even occur in the system and the motor surface remains comparatively cooler. For that reason, there are a few advantages in using water-cooled motors in the textile industry.

Water-cooled servo drives

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High ambient temperature

When the ambient temperature is especially high at the production location, air-cooled and uncooled electric motors can no longer achieve optimal output because the air can no longer absorb the heat sufficiently. The result is a derating. The effectiveness of the water cooling is not contingent on the ambient temperature, and there is no loss in performance.

With the Baumüller DSD2 servo motors, the output of a water-cooled servo motor in the size 45 is equivalent to that of an uncooled motor of size 71 The dimensions can thus be two sizes smaller

Limited footprint

Space-saving installation

Compared to other cooling methods, the output of the electric motors is up to 50% higher with water-cooled versions. This means that in many cases the motors can be dimensioned smaller, resulting in less of a footprint. Water-cooled electric motors can also be installed close together, as there is no risk that the motors might cause each other to overheat. Space can also be saved on the fans.

Temperature-sensitive product

The direct cooling on the motor results in less heat intake in the machine and this means that temperature-sensitive products can also be processed.

Water and oil are already in the system

When water and oil are already in use, as e.g. in hybrid servo-hydraulic applications or perhaps in metal processing, then the high initial costs for the cooling unit also fall. The system thus becomes significantly more economical and advantages such as high output and compactness become more important.

Strict requirements for hygiene

In foodstuff production and pharmaceuticals, requirements for hygiene are particularly stringent. The fans in air cooled motors produce swirling air, which is conducive to the spread of spores and germs. This problem can be remedied with water-cooled motors.

Hygienic Design Motors

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Noise emissions must be reduced

Without the fan motors, there is significantly less of a noise load when using water cooling.

FAQs on water cooling:

1. Is water cooling more expensive than air cooling?

Yes, water-cooled motors are more cost-intensive to acquire than uncooled or air-cooled electric motors, but they have a better €/Nm ratio. The higher purchase price of the motors reflects the costs of the cooling unit with accessories. If there is already water and oil in the system that can be used for cooling the motors, then these costs fall. But often the higher initial costs are outweighed by the advantages, such as lower service costs or higher output, such that the higher costs quickly pay for themselves. An individual calculation must be made for each application as to which cooling method makes sense and is economical.

2. Can hydraulic oil be used as a cooling medium instead of water?

Yes, it will work, but it will result in a derating. When hydraulic oil is used in accordance with the standard HLP 46, then there is a derating of between 10 and 20 percent depending on the motor type. The reason for this is the higher viscosity of the oil, which causes the medium to flow more slowly through the ducts, resulting in less cooling effect. In addition, the heat capacity of water is greater than that of oil, i.e. water absorbs the heat better than oil and can thus dissipate it better.

3. What requirements does the cooling medium have to meet?

Clear water that is free of suspended matter and dirt is to be used as a coolant, which must meet certain specifications, for example with regard to pH value, water hardness, sulfate content, etc.. However, complex treatment of the cooling water is not necessary for Baumüller electric motors, as the cooling lines are made of the corrosion-resistant materials stainless steel or KTL-coated aluminium. The inlet temperature of the cooling medium (oil or water) is ideally around 25 °C, but can be up to 40 °C, but then derating has to be accepted. However, if the inlet temperature is very low when the outside temperature and humidity are high, then condensation will form. In this case, a small derating at a higher inlet temperature can be accepted in order to avoid the formation of condensation.

4. What about the electronics?

If the motors are water-cooled, then water cooling is also an option for the servo drives. The electronics can be cooled with the same cooling system as the motors, meaning that in this case the cooling unit will be doubled. When it is possible to downsize by using the water-cooled electric motors, then the servo drives will also be smaller as will the installation surface of the control cabinets.

5. What else do I need if I did not previously have water on the machine?

For a machine with fluid-cooled motors, a cooling unit is needed that cools down the cooling medium to the optimal temperature. This cooling unit is not included in the delivery scope of Baumüller drive systems. We will be happy to recommend reliable partners

6. What service tasks is the machine operator responsible for?

No additional service costs are associated with the water cooling itself. The cooling system is a closed circuit that does not require frequent service orders.

The motor series from Baumüller are available with water cooling:

Motor	Properties	Installation sizes and water cooling
DS2	Three-phase synchronous motor	100-200
DA1	Three-phase asynchronous motor	100-280
DSD2	Dynamic servo motor for maximum requirements for acceleration capacity and start-stop qualities	45-132
DSC1	Compact servo motor with high torque density	45-100
DSP1	High-speed servo motor with good acceleration and overload capability and increased speed and output range	45-100
DST2	Multipole torque motor as direct drive with very high torque	135-560

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Markus Keila
Product Manager Motors

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