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Thanks Allen. So the Io value adequately allows for friction, wind age and other losses that contribute to the overall efficiency?
you want to get as much current into the small space as possible and you want to get the best magnetic field you can. The first you achieve by using lots of turns of wire
The problem I have is with understanding the motor power curve. From what little I understand, and so far as I can see your explanations confirm this, the torque should drop as a straight line from 0 to max speed (cos the back-EMF is proportional to speed), and that this should result in max power at half of max speed, at least in theory. So why am I getting max power at around 80% of free running speed?
The problem I have is with understanding the motor power curve. From what little I understand, and so far as I can see your explanations confirm this, the torque should drop as a straight line from 0 to max speed (cos the back-EMF is proportional to speed),
No, because this sort of defined power curve and torque curve is a feature of IC engines, not electric motors. The forces are the result of the interaction between the magnetic fields and the current in the windings (not the voltage). The current varies with load and so the torque depnds on the torque demans, so comparing it to the power curve of an IC engine is a chalk & cheese thing.If your load is only drawing three amps and is not turning fast enough then the only way you're going to turn that load faster is to increase the voltage. Try doubling it.
Ah, OK - I didn't realise you meant with a constant voltage. Always keep in mind that a DC motor has a fixed characteristic "Torque per amp" (Kt), but the relationship to speed depends on the voltage. When it comes to relating torque (or power) to RPM it can only be defined for a particular motor & voltage source system (not for the motor in isolation).AS