EdTesla, see below the quote of your post for possible solutions.
For those reading that don't know about this problem, it is that the faster a motor is spun, the higher the voltage it generates. A motor wound for a very slow top speed under power makes a higher voltage per RPM (kV) than one wound for a higher top speed. If the slow motor is forced to spin fast enough to generate a voltage higher than that which the controller's FETs or other battery-side components can handle, those components will fail (usually dramatically).
The motor itself can generally easily handle this, but the controller (and the battery's BMS, if it has one) cannot. And sometimes, when the controller FETs fail, they fail in a way that shorts motor phases together, and at that point the motor becomes a brake, generating significant amounts of heat. If the conditions persist, the motor can be overheated and destroyed.
EdTesla said:
We had a problem with our build at high speeds. A person who had never driven the vehicle before became very fast downhill. The top speed was three times as much the speed that the vehicle reaches on level ground with the slow motor.
When the person stopped, the display of the slow motor was destroyed. Since we have very good mechanical brakes, we do not use recuperation. The current would be too much for our battery. The only problem is that then the voltage increases too much with speed. Due to the body diodes of the mosfets reducing throttle to zero has no effect.
Is that a known problem? I have unfortunately no model name of our controller and our display. If I open the case, I might be able to tell what kind of controller it is.
With this specific problem, then you would indeed need a way to either limit the speed of the system to the max "tolerable" to the slower motor's controller, or to disconnect it's phase wires from the controller, or ensure you use a controller/display system for it that can handle the highest voltage it would output.
The last method is the simplest, if the voltage is low enough for an affordable controller. To find out this highest voltage, you can measure it, with a voltmeter between two phases, set on AC Volts, while riding the bike at the highest speed it will ever reach. If the meter has a Peak Hold feature, use that to make it easier, as you won't need to keep an eye on the display while riding. Then you can use the simple math (that I don't remember) to convert three-phase AC Volts to DC Volts, and then get a controller/display system that can handle *at least* that much, *but* that has either an adjustable LVC, or an LVC low enough to accomodate the lowest voltage you want to be able to run at when that controller's battery is empty.
The battery will also need a BMS that can handle that voltage, if it remains connected to the controller when in this high-voltage state. If both controllers share the same battery, then the other controller *also* has to be able to handle this same high voltage.
Now, if you do want to disconnect the phase wires, you can use regular relays, but there is a small chance that contact chattering can occur under severe vibration. If it happens while current is flowing (using the motor), then the contacts can be damaged over time, or even weld together (so that they don't disconnect when commanded).
SSRs don't have that problem, but like FETs they do have a voltage limit, and a current limit, and internal resistance, and they heat up during current flow. So you need to get ones that can handle all of the requirements of your system, meaning you still need to know the max voltage they'll see, and in this case it's the actual AC phase voltage, not the DC battery voltage. You'll also have to use SSRs taht can handle the highest phase currents the system could see while using the motor as a motor, and like the speed-generated phase voltages, the phase currents are significantly higher than battery voltages. Then you'll need to be sure the SSRs stay cool (which probably won't be an issue, but should be considered when packaging the disconnect unit).
Alternately, you can use a large 3PDT switch, and manually disconnect or connect it. This switch will *also* have to handle the max voltage and current, and might be a bit on the large side. I don't recommend using three separate switches in this case, as turning them on or off at different times might cause issues with the controller.
If you use SSRs or relays, you can setup a voltage monitor on that motor, and when the voltage exceeds whatever limit you set, it autodisconnects. When the voltage drops below whatever other limit you set, it reconnects. Or you can use speed for this. Or do it manually, if you prefer...but then you have to remember to do it, and if you don't....
However, since you don't use regen anyway, but you really want to keep two different speed motors, then what I would really recommend doing is to either use a geared hubmotor with internal freewheeling clutch for the slow motor, or move the slow motor to a middrive thru the pedal drivetrain (which also lets you shift gears for it to use it for a wider speed and torque range). In either case, it is never backdriven. This is a much simpler solution than any of the above, electrically, with less risk of dramatic failure in operation.
