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Old 04-26-2015, 05:46 PM   #12 (permalink)
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Originally Posted by Grant-53 View Post
I don't want sound like a know-it all but I used to build slot car motors. The only thing that normally ruins permanent magnets is heat ... Good bearings and dynamically balancing the armature allow the maximum rpm ... Keeping the air gap between fields to a minimum is key. Air flow axially is helpful. Temperature raises resistance in wire and this reduces the amperes in the windings. Increasing the voltage does push more current for a given resistance ... So to get more total output amperes and voltage are needed. The controller may be the limiting factor.
I agree .. in context. So I'll make some side comments. Hopefully they help. BTW - I am *FAR* from a know-it-all. I share my opinions (type a lot)

Heat is bad - agreed - so I would put air flow axially as more important than 'helpful' .. you have access to the outside of the stator so you may be able to cool off the motor by adding heat sinks, getting more area to pass the air across, forced air cooling for low speed operation.

I'm not very mechanical, so I'd go with good bearings, rated for much more than the rpm you are planning to use.

Dynamically balancing the armature is out of my league - I'll take your word for that one. Same goes for the air gap. Magnetically, I read that the flux density is important so I don't think I'd mess with the air gap.

To run higher RPM, you need to maintain the current. The back EMF, as I understand it, goes up with speed. The back EMF, generated by the rotating magnetic field building and collapsing (my understanding) is subtracted from the applied voltage, so less current flows as you go faster. So more voltage is required to put through the same current in your motor. The motor insulation must be sufficient to 'stand' the applied voltage - otherwise the current 'leaks' through the insulation and gives you a ground fault. I am not familiar with scooter motors. The minimum insulation for wiring is normally good for 150V (stamped 300V).

The torque required to move air out of your way (wind resistance) is, I think, the part of the required power that increases exponentially. The power eaten up by rolling resistance and acceleration are linear (if I remember correctly). For cars, the point where wind resistance is much larger than the other 2 is between 30 and 45 mph. Where it would be for a scooter would be interesting for comparison.

You can surge above rated current (like to accelerate) but you have motor heating issues if you exceed rated current for too long. The more heat you can get out, the more current you can pump in (to get torque).

Jack, over at EVTV, says that a motor has infinite power as long as you can find a way to remove the heat. I think that is a highly simplified explanation, but it's useful because it gets you talking about heat flow, and air cooling versus water cooling.
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