Its normal for RTDs to come already installed in motors, and its normal for controller to have an input for them. your controller doesn't have an input?

 

The controller does have a thermistor input. I think I have said this three times now... My problem is that I can't wire BOTH motors into it, and expect it to behave like I want it to. I'm beginning to think that parallel thermistors mightn't be so bad, but a couple of small current clamps will also serve as a good warning.

 

The controller does have a thermistor input. I think I have said this three times now... My problem is that I can't wire BOTH motors into it, and expect it to behave like I want it to. I'm beginning to think that parallel thermistors mightn't be so bad, but a couple of small current clamps will also serve as a good warning.

You only said it once actually, but you did say it and I did miss it. sorry for that; I just came on the scene. you could do this easily with a microcontroller. connect your 2 RTDs to the the microcontroller and have it output a voltage (or current, might be a little more complicated if the motor controller is looking for current) proportional to the input of the lowest RTD.

 

You only said it once actually, but you did say it and I did miss it. sorry for that; I just came on the scene. you could do this easily with a microcontroller. connect your 2 RTDs to the the microcontroller and have it output a voltage (or current, might be a little more complicated if the motor controller is looking for current) proportional to the input of the lowest RTD.

That's OK

For interest, this is the controller I'm using,
http://kellycontroller.com/racing-controller-kdh14121e24-144v1200a-pm-with-regen-p-601.html
and the manual is here:
http://kellycontroller.com/mot/downloads/KellyKDHEUserManual.pdf

In the wiring diagram towards the end (page 12) you will see one of the leads goes to the thermistor and return via the throttle ground.

 

That's OK

For interest, this is the controller I'm using,
http://kellycontroller.com/racing-controller-kdh14121e24-144v1200a-pm-with-regen-p-601.html
and the manual is here:
http://kellycontroller.com/mot/downloads/KellyKDHEUserManual.pdf

In the wiring diagram towards the end (page 12) you will see one of the leads goes to the thermistor and return via the throttle ground.

well the good news is, with the way they show that 1KΩ resistor, it creates a voltage divider, so the controller is looking at voltage and not current. You could probably make a simple circuit with a comparator to compare which thermistor is lower in value and transmit the voltage to the the controller. or use a microcontroller.

 

... MOSFETS don't behave properly at voltages above ~110 V...

We learn new things every day!

But perhaps it may not be so advisable to make sweeping statements on subjects which are not our speciality, as they may be taken at face value by others with limited knowledge. Whatever may be the practical limit for the type of MOSFET used in your present motorbike design, MOSFETS are available for operation at many hundreds of volts. Here is an example:
http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00222838.pdf

 

More or less, conventional HV MOSFETs are long and narrow generally giving them higher rds(on). More or less, conventional high current MOSFETs are stubby and wide generally giving them lower rds(on) but higher gate capacitance.
Silicon Carbide MOSFETs, like the CREE Z-FET work at 1200v with only 0.160 Ω rds(on). These are hogging into IGBT territory.

 

Quite. I did not even seek to challenge the OP in asserting that he would have a problem with the operation of his particular MOSFET arrangement above 110V. The bald statement that MOSFETs don't behave properly above ~110V had to be challenged though, given the number of applications using these things at considerably higher voltages.

 

Wow, I didn't think that the MOSFTS could go that high... That's really cool. Still, there must be some reason why they don't have DC motor controllers that go much higher than 144 V. Otherwise, we'd all be running much higher voltages to keep the currents down, and minimise losses. I know IBGTs are used in the high voltage AC motor controllers quite extensively.

With respect to the thermistors, I spoke to some of the teams who are using them, and they resigned to the fact that while the brush housing might have a temperature of 90°C, the armature has temperatures exceeding 250°C, but it can't be measured cause it's busy spinning at 5000 rpm. So, in their experience, the thermistors didn't help them anyway

A friend is developing a water mist spraying device which squirts water onto the armature. As the water will evaporate quickly and the nature of the motor sort of expels air from the radius, it should be fairly effective for Australian race conditions >30°C air temperature in winter!

Thanks for your help guys!

CHRIS

 

What you really require is an output temperature gauge mounted on your bike that correlates to the temperature of the the thermistor (motor operating temperature).

This works the same way that some digital voltmeters have temperature probes that come with them.

The correct operating voltage of your custom circuit supply is an equation based on

1 - The resistance of the thermistor at normal operating temperature.
2 - The temperature of the thermistor at crisis point
3 - The resistance of the coil of an analog meter

The end result will give you an exact voltage reading to drive the circuit that gives you very close temperature readings.

With these 3 factors a custom circuit can be designed for your temperature meter gauge that will be accurate to the point of warning and crisis operation temperatures.

Since you have a visual feedback I would throw in an audible alert at the warning stage in conjunction with a flashing LED to ensure you look at your temperature gauge. The warning stage temperature can be set based on your thermistor readings and your knowledge of what the temperature should be for the "warning" stage.

You can always plot a thermistor resistance graph on 4 known temperatures:

1 - Ice at 32 degrees F.
2 - Room temperature at 71 degrees F.
3 - Body temperature at 98.6 degrees F.
4 - Steam at 212 degrees F.

If you are using higher industrial temperatures then you are 100% reliant on the manufacturer's Temperature/Resistance curve.

Please submit the 2 temperature resistance readings that you are concerned with:

1 - Early Warning
2 - Shutdown

And also the resistance reading of a high impedance analog meter coil with with the voltage to get 50% of the scale reading.

With this data it is not hard to custom design the analog meter circuit that you require.

If you can post the manufacturer's model or Temperature/Resistance curve or an online reference that would be great.

We're all here to assist you.

Danny