Estimating temperature rise via armature resistance variation.

[MedievalMan]

1) relative cost of temperature sensor on armature of brushed dc motor.
2) Is Ra always proportional to temperature /temperature rise (besides, for instance, varying , slightly with other factors, such as brush resistance variation during commutation, and slow brush resistance wear over time).
I know the equation ? = ?0(1 + ?(T ? T0)) where ? is the resistivity.

The reason I’m asking this is I’m wondering if it would be interesting to see if the temperature could be estimated from the drifting rise in Ra (identified by ex. Parameter estimation process); at least in a rough sense that a certain threshold could be reached and the motor shutoff to protect it from damage. I suppose in this sense you could also use the time total of the power dissipated (based on I2R) and compare that to a thermal model, which seems to be more complicated.

Note: this application is an EPS (electric power steering motor). I believe the motors are designed such that they supply a lot of mechanical power for their size, thus this overheating condition could probably be reached rather quickly.

I searched google and found an interesting patent (abstract):

Electric power steering apparatus
The atmosphere temperature calculator calculates an estimated atmosphere temperature value by subtracting a temperature rise amount of a temperature sensor which is calculated based on a motor drive detected current value from a detected temperature value of the temperature sensor. The overheating prevention calculator calculates estimated temperature values of a plurality of protection objects to be protected from overheating based on the estimated atmosphere temperature value and the motor drive detected current value and determines an upper limit of target current of driving a motor based on such estimated temperature values. The temperature compensation calculator calculates a temperature compensation current value to be added to a target current value in order to compensate a temperature characteristic of a predetermined portion based on the estimated atmosphere temperature value.

I think they are saying they are estimating the temperature based on current, and that it uses a thermal model to compare and change the reference current to the motor control loop (a more elegant solution than simply shutting off the motor when a limit is hit.). Not sure, though, it’s worded so nice. ?

 

[waross]

The varying voltage drop across the brushes will make it almost impossible to measure the armature resistance accurately through the brushes.
An electro-mechanical overload relay is a fairly dependable mechanical "Thermal Model".
respectfully

 

[aolalde]

For copper R1/R2 =(234.5+T1)/(234.5+T2)
R in Ohms, T in degrees C.
Since the armature circuit includes the brushes resistance in series, that is not a accurate point of reference.

Try using the resistance change on the interpoles winding.

 

[IRstuff]

The big issue is that the temperature of the winding is unlikely to be uniform. Additionally, a short will most likely produce a localized heating effect that will be diluted by the remainder of the series resistance that is not at that temperature.


TTFN