More about electric motors

Electric motors have two power ratings; Peak and Continuous.

Peak and Continuous power ratings for electric motors

In theory, the peak power is what the motor will produce if connected directly to the rated voltage. In practice, it is often the maximum power the controller can supply.

For racing, finding a higher power controller to improve peak power could be advantageous for autocross (or drag racing) especially if you are willing to stress the motor mechanically beyond what the manufacture suggests. For road racing, continuous power is more important.

 

The peak power rating is often expressed as the 5 or 10 second rating. Running too long at peak power will cause the motor to overheat. In our experience with electric motors that are under-rated for reliability on the street, we have found we can use peak power for full autocross runs or 2-3 qualifying laps on the track.

 

In our case, when the temperature sensors embedded in the windings reach 170 Celsius, the controller reduces the current to the motor. This allows the car to continue to run but at reduced power. The software allows the motor to continue to run at the highest power that will not further overheat the motor.

 

The continuous motor rating is the power the motor can steadily produce without overheating. This is controlled by how much waste heat the motor produces and how effectively it is able to dissipate the waste heat.

In our last race before we started this ground up rebuild, the battery current we could run and still finish a 30+ mile race was high enough to be on the edge of overheating the motors. For shorter races or to run any faster in the longer races, we need to get the motors to run cooler.

Cooling the motors  

Part of the goal in changing the gearing <see previous page> is to let the motors run in their more efficient rpm band. Less power lost in the motor means greater range or more power for a race distance. The other benefit is less of the energy is left in the motors as waste heat.

We also might be able to modify the motors to help them shed heat faster. Most of the heat is generated in the copper windings. Copper melts at around 1,084 Celsius, so the limiting factor seems to be the 'H" class insulation on the copper windings. It is rated for 180 degrees Celsius. The rule of thumb seems to be that for operating 10 degrees over the rated temperature will cut the motor's life in half.

If our budget were unlimited, we would look in to rewinding the motors with wires with higher temperature insulation. Polyimide-ML is rated at 240 Celsius. A motor running at a higher temperature would be at a greater temperature differential from the environment and would shed heat faster.  

It is useful to understand how well heat travels through various materials when thinking about cooling. Thermal conductivity is measured in watts per kelvin per metre (W·K?1·m?1).

Air 0.025
Epoxy (unfilled) 0.59
Water (liquid) 0.6
Thermal grease
0.7 - 3
Thermal epoxy 1 - 7
Aluminum 120-180 (alloys)
Copper 401

Information from http://en.wikipedia.org/wiki/Thermal_conductivity

Heat travels quickly through copper. Alumimium conducts heat but only ½ as well as copper. Water conducts heat only 1/668 as well as copper. Air is an insulator. Air conducts heat 1/16,040 as well as copper.

The other part of the equation is that heat travels faster when there is a greater difference between the temperatures. Cold water cools faster than hot water. If all else remains the same, water 2 degrees C cooler than the object will absorb heat 2 times faster compared to water 1 degree cooler than the object.  

The quickest way to pull heat from the copper windings is through the aluminum motor case around the windings. The heat first has to travel through the thin layer of resin insulation which conducts heat at around the same rate as water. Any air spaces between the insulation and the aluminum case will slow heat transfer substantially.

   

aluminum case around copper windings

   

Next, the heat needs to be pulled from the motor's aluminum case. The Siemens motor is liquid cooled. The water flows through galleries, absorbs heat and carries some away. The faster the water flows (up to the point of cavitation), the greater the difference between the temperature of the water and the metal case and the faster the heat will transfer from metal to water.


Another, perhaps more effective path for pulling the heat away is thermally bounding the motor case to the metal chassis of the car. The more area touching, the more heat transfers. The use of thermal grease to fill any air gaps will improve heat transfer. The entire car can become a heat sink.

It is also possible to blow air around and through the motor.

 

In the picture above, the copper windings at both ends of the motor are in a dead airspace and thus thermally insulated.

 
 

 

Improvement #1 Gearing
back to Lessons Learned menu
 Improvement #2 cooling the motors -Liquid

 


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