Overheat protection circuit.

[mr.electric]

I love my BMC V3 bike. I want to use it frequently but I worry about overheating it. I started thinking about a simple warning circuit. It would be a 3 color LED. Green - OK. Orange - Overheat. Red - Severe Overheat. I have seen those simple temperature calibrated switches that are very small and cheap. I think I would install one for the orange light and a second sensor calibrated at a higher temp for the red light. Both sensors could be stuffed into the windings and wires brought out through the axle tube. I guess you would have to be able to fit three or four fine wires which could be challenging on a BMC. If you wanted to go a step further the Orange light could connect the current limit and speed limit jumpers on the controller and the red light could connect the brake signal. Does anyone think this circuit would be effective?

 

[dogman dan]

You are thinking right to want to know the temp inside the motor unless all you do is ride 5 miles at a time. Just put the sensor for a remote thermometer in the hub if you can fit two narrow wires through the axle hole with the other wires. If that's impossible, then mount the sensor on the axle stub, and then add 30-40 F to the reading.

I've been using cheap indoor outdoor thermometers from the car accesory section. Mounted on the axle they work perfect, mounted inside the hub, they read to 160F, and then just say high temp after that. Since I consider 180F max temp, I don't mind if I start slowing down or stop to cool the motor at 160F, I don't want to reach 180 at all anyway. More expensive thermometers can read higher, but I found the ten buck cheap one tells me all I need to know, I'm not drag racing the bike, just trying to climb a hill without the smoke pouring out. I found it really handy to know not only the temp of the motor, but just how fast the temp is climbing. With this tool, I was able to figure out what works and what doesn't work to cool the motor, or to ride so it gets less hot. When it's really hot, like over 100F, and I want to ride a long ways, I can figure out what speed is sustainable for 30 miles or more of riding. The trick is to ride at a speed where the motor is at it's max temp, but does not climb beyond 160F. So if 20 mph results in a temp of 170, slowing down to 18 mph may result in a max temp of 160.

 

[fechter]

I stuffed a temperature sensor inside my Puma motor (similar construction) that runs off the hall sensor power and sends a signal back out on a single wire. The off the shelf digital thermometers can be done with a single wire and share the ground wire with the hall sensors. Stuffing even a single wire through the axle is challenging.

Using the motor winding temperature to start cutting back on the controller current limit would be the ideal setup, so as the motor approaches max temperature, the power gradually tapers off and prevents any damage. I think this should be a standard feature of any motor controller.

A really cheap setup would be to use a thermostatic switch to kill the hall sensor power if it goes overtemp. This could be a backup safety to the other system. The nice part about this would be you don't need any more wires through the axle. I think it would work best if the hall power was disconnected on overtemp.

I guess what would be really nice for a retrofit would be a wireless temperature sender. I have an outdoor thermometer that uses a wireless sensor. The battery has lasted several years so far. I don't think it updates very fast, but for motor temperature, I think every 5 seconds would be fast enough. It could be powered by the hall sensors.

 

[amberwolf]

As long as you can get the signal out of the metal motor case, that'd work.

 

[mr.electric]

So you could use one thermostatic switch with to run an "idiot light" another to kill hall sensor power. I am guessing it would be wired like this:
Thermostatic switch one for idiot light is normally open, set to 160 degrees. one wire to hall power other wire out of the axle tube to an led then to battery ground. or to a relay that connects speed limit and current limit jumpers if available on the controller being used.
Thermostatic switch two to kill power is normally closed, set to 180-or higher. wired in line with hall power. Or would it be better for thermostatic switch two to be normally normally open and short hall power and ground.

 

[fechter]

Either shorting or opening the hall power line will stop the motor. I was just thinking that the supply for feeding the halls might be happier if it wasn't shorted, as this is usually tied to the processor. In most cases it is current limited, so shorting shouldn't blow up anything.

Radio waves will follow along the wires going out through the axle (like a wired-wireless system). It will be no problem. You could even couple the antenna to one wire to make sure.

 

[mr.electric]

What if you connected a high temperature (180-190 degree) N.O. thermostatic switch to the hall ground inside the hub and ran the wire out of the hub to the throttle signal wire. When you hit 180-190 the bike stops. A second lower temp thermostatic switch could connect the hall ground to throttle with a resistor in the circuit so that it would limit the throttle position signal to something lower than the actual throttle position.
With this strategy you could still disconnect the wire coming from the thermostatic switches inside the hub for trouble shooting purposes motor/controller issues.

 

[fechter]

Yes, that should work. The hall ground is the same as the throttle ground.
Putting a resistor in series with the line so that the throttle does not totally die but gets reduced to something like half power so you don't completely die in the middle of an intersection or something like that sounds like a good idea.

 

[dogman dan]

The heinzmann 24v motors on the EV Global bikes had the thermistor switches in them, that were wired to the controller on off circuit on the keyswitch. I kinda hated it, since the cutoff was sudden and unpredictable. But having it saved my second heinzmann, the first one I smoked the first afternoon with the circuit disconnected. I first tried the remote thermometer on the heinzmann, and now it's a crucial part of my dashboard on all bikes.

I much prefer to simply know the temp inside more or less, so I can see if a particular speed is a temp going up speed, or a the motor is as hot as it can get in this weather speed. So many variables with motor cooling, Speed, temp, humidity, tailwind, headwind, crosswind. A motor can run all day on one 90F day, and get all hot on another 90F day. Knowing the temp of the motor, AND how fast the temp is climbing is vital to me in summertime. Even in winter, serious hills need temp monitoring if you climb long enough.

 

[mr.electric]

Yes a gauge is more informative but I have a nasty habit of forgetting to look at my gauges on vehicles. Also I keep my CA in my front bicycle bag to keep my handlebars free of extra stuff. My BMC V3 bike looks like a regular bike with all wires and components hidden.

 

[dogman dan]

You'll have no trouble remembering to watch guages after you second melted motor. But yeah, nothing wrong with the idea of a 170F temp switch in the hub. Ebikes CA sells em. I'd rather have a warning than a cutoff though. In my climate, riding in 105F up long hills everyday all summer, I needed to learn to ride cooler. The temp guage taught me what worked and what didn't. How you climb hills matters.

Quick way to cool a hot hubmotor, take a rag or hankie, wet it, and drape it over the hub. Actually stopping cools a motor much better than just slowing down. But in desert low humidity, it takes forever without some water involved. I've considered riding with a mister on the forks.

 

[mr.electric]

The blower fans Fechter installed on old zappy scooter motors worked amazingly well. A manifold that allows pressurized air to be pushed into a spinning hub would be hard to make but allow for much better cooling. One guy that worked at Zap in Sebastapol had a roller drive bike that had a windshield washer pump connected to the motor can. He ran a the 12v bike motor on 48v with no current limiting device. That motor got very hot very fast. It made steam when he hit the water squirter. I think he called it the flying chicken. He worked at Zap so he got new motors cheap or free so it was no big deal to fry a motor occasionally. Now that we are using $1000 motors frying a motor is no laughing matter.

 

[dogman dan]

Yep, if the motors had the space, a quarter inch air hose would cool it a lot. The way they are designed, you could never fit one in there, unless you hollowed out the other end of the axle. Hmmm.. Rv's use 12v air compressors in their water systems. Hmmm.

So far though, I'm doing ok just by knowing when to stop.