Patent Application
20210223195
This invention relates to testing immersive coolants for electric motors, and more particularly to simulating motor dynamics for effects on such coolants.
Hybrid and battery electric vehicles are rapidly increasing in popularity for both light duty and heavy duty applications. There is a strong need for the electric motors in these vehicles to be compact and capable of delivering high power.
A concern with these electric motors is high internal temperatures, especially near the motor windings, which can spike up sharply. Some sort of cooling is needed to transfer heat away from the motor.
One approach to cooling electric motors is immersive cooling. Immersive cooling with an engineered dielectric coolant delivers far greater heat transfer efficiency, allowing motors to be designed with smaller size and for use at higher temperatures. These coolants may also have enhanced lubricity to extend the life of motor and pump bearings.
A problem with immersive cooling for electric motors is premature failure of the coolant. It is believed that electromagnetic field effects from the motor result in reduced viscosity over time.
A more complete understanding of the present embodiments and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which like reference numbers indicate like features, and wherein:
The following description is directed to testing immersive coolants for electric motors. By “immersive coolant” is meant that at least some parts of the motor are immersed in a liquid that provides a coolant and/or lubricative function. Typically, the immersed parts are the internals of the stator and parts of the rotor.
Immersive coolants provide a non-electrically conductive (dielectric) heat transfer fluid. The heat transfer may result from passive circulation from natural convection of the coolant or from active circulation by pumping the dielectric coolant around the motor windings and then transferring absorbed heat to a radiator or other heat rejection device.
Often, immersive coolants for electric motors are also designed to provide extra lubrication, such as oil-based coolants. Because the coolant must combine a high flashpoint with high dielectric strength, an immersive coolant medium may also be referred to as an “insulating oil” or a “lubricative coolant”. The test system and method described herein may be used to test any liquid in which a motor's internals are immersed, and such liquids are referred to herein collectively as “coolants”.
As is known, an electric motor produces rotation of its rotor from a repeated sequence of attraction followed by repulsion, which requires reversing polarity. As an external power source passes through the rotor field, it serves as an electromagnet that is attracted to the permanent magnet of the stator. For continued rotation, the electromagnet allows the rotor field to reverse the polarity of its magnetic field producing repulsion. The repulsive force between the poles repels the electromagnet along its path of motion.
These switching magnetic fields are emulated by test system 100. Thus, test system 100 is useful for testing immersive coolants used with any electric motor that operates with switching electromagnetic fields. Typically, such motors can be referred to as “coil-based” motors.
In operation, a sample of an immersive coolant to be tested is contained in a tank 10. Two coils 11a and 11b are immersed in the coolant. Each coil 11a and 11b comprises electrical windings wrapped around a core.
Coils 11a and 11b are designed to imitate motor winding patterns. The type of winding can be varied to best imitate the motor(s) with which a coolant of interest will be used. The distance between coils 11a and 11b can be varied to emulate distances between actual motor windings, and to match various motor sizes and types.
A DC (direct current) power source 12 provides DC power for test system 100. DC power source 12 is programmable, delivering a desired level of power, typically within a range of 0 to 300 kW.
As explained below, the DC power from source 12 is recirculated through junction box 13. This minimizes power consumed by test system 100. An example of a suitable range of voltage from junction box 13 is 0-700 volts
A first electrical path delivers current from junction box 13 to inverter 14, which converts the DC current to AC (alternating current) current. An example of a suitable range of outputs of inverter 14 is 0-480 Volts RMS with switching frequencies up to 30 kHz. This AC current is delivered to coil 11a.
The switching electromagnetic field from coil 11a is transferred inductively to coil 11b. As this switching occurs, the coolant is subjected to both electromagnetic and heat stress.
The induced current in coil 11b is delivered to rectifier 15, which converts the AC current to DC current. This current is fed back to junction box 13.
Because of this re-use of current induced in coil 11b, test system 100 is regenerative, generating minimal heat and consuming minimal power. It can be used as a universal test system for immersive coolants for electrical motors, with the electromagnetic field frequency and amplitude being variable to match any electric motor.
A controller 16 controls DC power supply 12 and the frequency of current provided by inverter 14. Controller 16 also acquires response signals from the coils and from a temperature sensor as indicated in
Controller 16 can be appropriately programmed to perform various tests on the coolant in-situ. For example, temperature and voltage variations can indicate changes in conductivity of the coolant. Alternatively, samples of the coolant can be removed and tested ex-situ.
Various metrics representing the condition of the coolant can be measured. One example of such a metric is dielectric breakdown voltage. Other coolant state of health metrics that can be indicated by appropriate measurement data are viscosity, change in coolant temperature as a function of number of hours of cycling, and change in the chemical composition of the coolant from extracted samples at various stages of cycling.
The test system 100 can be operated over time for a coolant of interest, thereby testing how the coolant's thermal performance and physical/chemical characteristics are affected by aging.
In some embodiments, it may be desired to use more than two coils. For example, aging effects or fluid breakdown due to electromagnetic fields can be accelerated with multiple coils. Also, the number of coils can match the number of windings in the stator to emulate distributed fields more closely.
