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Originally Posted by e*clipse
I was thinking that a 3 phase motor might be more reliable than a brushed dc motor (to power the pump) Would you find this true?
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Yes. The 3 phase induction motor is heavier for the same horsepower but it is less sensitive to high temperature. Neodymium magnets (popularly used in PM motors) lose their 'permanent' magnet status somewhere around 200C ... and bad things happen.
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Perhaps a really simple sensorless-type controller could be used? I suppose if a 3-phase motor is used, the controller could give an "everythings working" ok signal.
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A simple sensorless controller has 2 hall effect sensors and calculates the third (at least the ones I have taken apart). The 'all ok' signal is usually called 'run status'
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Also, could a **very rough** hall-effect type sensor be used for a minimal 3-phase control? I mean, theoretically, no control at all is needed, just a rotating magnetic field. If the hall effect sensor detected that the rotor wasn't turning even though the magnetic field was, then perhaps the pump is jammed?
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A simpler controller that does V/Hz only can be used without hall effect sensors, as well as a simple ramp up rate and a fixed operating setpoint. Power it up and it starts if it can. The run signal comes on when the pump is up to speed. If the pump is jammed, the current goes *WAY UP* - like 6X or 8X the regular current. If the controller has enough sensors to protect itself, it trips and gives a fault code. If not, it melts. Either way, the 'run status' turns off and you can safely shut down your system.
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Next to knowing if the pump is turning, a pressure gauge/switch would be pretty simple.
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So, perhaps with the right monitoring and a reliable power source, and external pump would not hurt the system reliability.
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The external pump, in my opinion, is not as reliable as the built-in pump. But with the measures listed, you're doing all that you can reasonably do to ensure cooling is available for the motor.