LM35 EMF interference

[AGS]

Has anybody found the answer to the problem of electrical interference when using an LM35 to monitor motor temperature.

I‘m using the white speed sensor wire for the output and having to use the 0v from the controller end. So it isn’t an ideal cable route.

The temperature signal is fed into a 3.5 digit 2 volt meter mounted on the cockpit and I also fitted a 75 ohm and 1uF damper at the meter end, which helps a bit, but it may be in the wrong place because the data sheet shows it should be fitted at the motor end. I didn’t want to fit the snubber in the motor in case it got tangled up when the motor is spinning, but thinking about it now, I’m sure there is enough room inside to keep it out of the way.

I have also fitted a 1k load resistor at the meter end which helps even more, but it’s not a perfect solution.

Another thing I haven’t done is to fit a bypass capacitor across the supply to the LM35 in the motor, because I didn’t think this was necessary at the time, because everything is derived from a dc supply.

So I have to stop peddling and get off the throttle to read the temperature accurately, which is not a problem and it may be the best I can do considering the high amount of interference when the motor is running and the sub optimal cable routing.

It reads accurately at temperatures above 75C now that I have fitted the load resistor and snubber, but not below that temperature with the motor running. So I can live with it and it’s certainly better than no temperature monitoring at all.

I have also fitted a dual comparator circuit and each one triggers a relay. One of them is set at 1.0v to sound a warning buzzer at 100C and the other is set at 1.1v to trip the brake input into the controller at 110C.

Motor is a Bafang SWX02 oil cooled running at 1500w. The oil cooling seems to work very well and I don’t see temperatures above 90C even after ragging the motor for extended periods of time. Before filling with oil I managed to get the motor up to 120C in around 10 minutes which started to discolour the windings.

Motor off

Motor running

 

[amberwolf]

If you can shield the sensor itself by insulating it electrically then wrapping it in foil that is grounded, and use either twisted-pair ground/signal wiring, or shielded ground/signal/power wiring to it (and the halls while you're at it), it should help a lot. (if you use a shielded cable, ground it only at the controller if possible, if not, ground it only at the sensor).

Bypass cap may help, too, at the sensor end, across the 5v/ground.

 

[AGS]

Thank you for your reply. I will try fitting a bypass capacitor and shielding the sensor. I’ll also move the snubber circuit to the motor end as well. It’s due an oil change soon, so that will be a good enough reason to open up the motor again.

I‘m not expecting amazing results because it sits in a very noisy environment, but it’s worth a try.