Smart Cooling for Electric Motors

 

Motivation

It is generally accepted that the specific power of electromagnetic actuators is approximately five times lower than that of conventional electrohydraulic actuation. Why is this and can anything be done to increase the specific power of electric actuators? One possible improvement could be in thermal management. The insulation on most magnetic conductors in electric actuators is limited to operation below 120°C to 150°C. The heat in the actuator is related to the I²R losses in the coils. Subsequently, conventional current limits (and subsequently torque limits) on electromagnetic actuators are based upon the actuator’s thermal resistance, the ability of the actuator to remove the heat from the coils. In general, there are two maximum current ratings, the continuous and instantaneous. The ratio of these two values is generally around three, meaning that the actuator is actually capable of producing more than three times it’s actual continuous design torque, but only for short bursts of time. If the heat can be removed from the coils, it is possible to operate at higher torques for longer periods of time. Forced air cooling is available, but requires external pumps and control to regulate the air flow. This research is exploring the possibility of using the thermal magnetic properties of ferrofluids to cool electromagnetic motors.

Liquid Cooled Electric Motors

Our initial investigation focused on forced liquid cooling and showed an initial 87% increase in stall torque capacity for a 1 hp brushless DC motor. However, with the forced liquid cooling (and other approaches with forced air), there is a requirement for an additional pump for handling the fluid. In addition, the fluid must circulate under a “worst-case” scenario unless thermal feedback is included. These two requirements provide the motivation for replacing conventional fluids with ferrofluids. First, the electromagnetic actuator has the two key elements for a ferrofluid pump: an existing thermal and magnetic field. Second, the fluid is truly a smart material in this application. As the actuator heat up, the pressure gradient in the ferrofluid across the actuator increases, subsequently increasing the fluid flow and heat transfer out of the actuator. The natural behavior of the fluid serves as the control. Please see Ferrofluid Field Induced Flow for a description of ferrofluids, their synthesis, and the mechanism for field induced flow.

For publications related to ferrofluids, see the following link:

http://www.ornl.gov/sci/ees/mssed/res/ferrofluidinducedflow.htm

For further information, contact Dr. Lonnie J. Love.

 

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Last Updated:  June 24, 2009