I just checked the Motenergy site, and I don't see any spec of 50rpm/volt. I have seen re-sellers include that kind of spec, but I believe that it's a carry over from their DC promotional verbiage. When I see that type of info for a PMAC motor, I start to question the reliability of the source.
This might be a little technical but here goes:
In all motors, the rotation frequency (RPM) is linked to the electrical frequency. If you have a commutator on the motor shaft (brushed motor), it takes care of this synchronizing the electrical and mechanical frequency. For a DC motor, this results in a set rpm/volt ratio.
With AC motors, IM, PMAC, BLDC, the commutation is done external to the motor with the power source, be it the AC mains (utility power) or an inverter (controller). The speed of the AC motor (RPM) is dictated by the applied electrical frequency. With synchronous motors, PMAC & BLDC, it is directly proportional. With induction motors (IM), which are asynchronous, there is a slip factor which subtracts from the direct proportionality of electrical to mechanical frequency.
However, although the RPM is dependent on electrical frequency, the AC motor must have the proper voltage applied for that particular frequency to function properly, meaning the efficient production of torque. This is why the inverter/controllers used to drive AC motors are considered Variable Voltage Variable Frequency, VVVF.
There is a base speed in all motors. This is where the knee in the Tq-Speed plot bends as the back-EMF matches the power input. In order to go faster, you have to spend energy to beat down the back-EMF. But in an AC motor, you can do this without increasing the system voltage, the controller allows you to exceed the base speed by manipulating the field phasing. Some people will talk about this base speed (the maximum without supressing the back-EMF) like the rpm/volt ratio of typical DC motors, they are results of the same physics. However, in DC motors it is a hard limit, in AC motors it is just a point within the operating range.
At least that is the way that I understand it. Perhaps I should have studied engineering instead of Business Management. :)
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, Michael Mccomb <mccomb.michael@...> wrote:
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> what does it mean in the ME0913 specs when it refers to 50rpm per volt..... is that some sort of fuzzy idealized number..... i know that i have read that somewhere
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