Erogo said:
LFP - a couple of the "sync"s in your post I presume should have read "sink", i.e. heat sink.
OK, I'm hoping I've still got a situation where an RC controller will survive. We'll see.
I'm gearing down 100:1, so top speed of about 3 or 4 km/h (2 or 3mph), for hillclimbing only. The plan (and instructions to the user) are to engage the drive while approaching the hill, so that the speed sits down into the drive's range, rather than having the drive bring the whole thing up to speed.
The other failsafe I have (admittedly very inelegant, like most of my ideas such as sacrificial FETs etc), is that the driven wheel will have only limited traction, as most of the load it is pushing against is not bearing down through it. So I imagine if things get too much it will just slip.
We shall see. Soon, hopefuly!
Eric
No, I mean sync, as in synchronization.
All sensorless drives (short of flux-vector type, which there are zero RC types that are able to do) just have to play a pre-defined series of pulses into the motor, called the starting algorythm, and hope they get rotor motion that generates BEMF that they controller can use to know rotor position. When this pre-defined series of pulses starts off, it's equally likely to be jerking the wheel backwards as it is forwards, but after that the average amount of jerks the rotor takes should be the desired direction of rotation. If the rotor doesn't rotate, then it has no way to know if it's applying torque in the right or wrong direction. If the rotor is stalled or not able to jerk into motion, then it has no way to continue to run the motor, and it just has to play the starting algorythm again and hope for the best. This is the nature of all sensorless controllers (excluding flux-vector drives).
So, this is how it gains sync to begin with, so it has an idea of rotor position to know which coil to energize next to continue making torque in the desired direction.
When you have a motor with a lot of poles spinning at a high speed, the pulses of BEMF it has to sense the peaks of (the transition between the rising and falling voltage in each phase) gets to be a very short amount of time, and motors with a BEMF that doesn't have a clear peak, or mostly rounded nose of the BEMF waveform can be very difficult to accurately position the rotor. When it looses rotor position and faults, this is called a loss of sync. Normally you get an awful high frequency squeal or scream from the motor, sometimes the motor tries to lock a given position, sometimes the controller just explodes into plasma, and most of the time you can just let off throttle, and gently re-apply and it finds sync again and continues to work for another bit.
The idea of fusing the phase leads is a definite fail, even if it's mosfets. The inductive spike flyback when the fuse pops will still kill the controller as the energy stored in the magnetic field in the motor tries to reach infinite voltage the instant the circuit opens.
Current sensors in the phase legs going to a fast uController that intercepts the throttle signal is the way to do it. Shunts in the phase legs with an isolated A to D on an isolated DC/DC and an opto-coupler to a uController is the other way to do it.