Re: Cap at the Gecko, motor heating, fuse protection??

So one should look at the total power dissapated by the motor,
not just the current going through it, when trying to protect it.
This sounds like good reason not to go over the motors rated
voltage as some would advocate. Even with the current limiting
of the driver, the power that the motor may be able to dissapate
may be overstepped (when using a higher than rated voltage). I guess
one could figure out how much the motor can dissipate safely, and
lower the current limit of the driver- Get higher RPM's by using a
higher voltage, and in return set the current limit lower.
Does this sound right?


Scott

> The problem is that the gecko is a power IN = power OUT device. If a
> low-resistance motor (let's assume 1 Ohm)
> is stalled, and getting 20 Amps, the voltage drop is 20 V, so it is
> dissipating 40 W. This will cook the motor pretty
> fast, too. With a 70 V power supply, that will only draw 5.7 Amps.
> But, if you put a 5 Amp fuse there, and the
> motor needs to draw 15 Amps while spinning at high speed such that the
> back-EMF is 50 V, then the applied
> voltage at the motor will be 50 + 20 = 70 V, the current will be 15 A,
> so the power demand will be 1050 Watts,
> and the current will be 15 A. The fuse will blow, even though this

is a

> permitted condition. A fuse in the OUTPUT
> of the Gecko would prevent this, but that is a hazard to the Gecko, so
> you can't do this, either. One way would
> be to put a big .1 Ohm resistor in series with the Gecko motor wires,
> and then glue a PTC thermistor onto the
> resistor. This would provide a way to thermally integrate the RMS
> heating of the motor. It would take some
> serious fooling around to get the time constants right, etc. so as to
> have good protection without false trips.
>
> Jon