Evo Electric wrote:
The voltage constant for the AF-140-4 is 1.44Vs/rad, measured in Vpk, line to line. Hence, your back emf is about 150Vpk/1000rpm. Thus, to spin it up to 1000rpm you need a bit more than this in terms of dc voltage from the battery, let’s say 160Vdc. From this point of view you are right that you would need about 800Vdc to spin the machine up to 5000rpm. However, what you would typically do is employ field weakening, which effectively uses current to supress your bemf. Hence, you will find that it is perfectly possible to spin the machine to 5000rpm on a 600Vdc or even lower dc bus. If you dc voltage is about 640Vdc, as I understand from your email, you will be perfectly fine running the AF-140-4 machine.
You are also right that you could potentially use a AF-140-3 machine instead. This would mitigate the need for field weakening but would have two big drawbacks: Firstly, the inductance of the machine is much lower, which translates into a choppier ac waveform and, consequently, magnets which will overheat when operated at higher speeds for a longer period of time. This is difficult to quantify but the risk is very real. Once demagnetised, you will see an irreversible loss in performance, i.e. torque and power will drop significantly. Second, since you have a lower voltage constant on the AF-140-3, you also have a lower torque constant, which means that you need more current to produce torque. For example, the AF-140-4 needs about 350Arms to produce 600Nm whereas the AF-140-3 needs 465Arms to achieve 600Nm.
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