Quote:
Originally Posted by Brandon Holley
Was that calculated from the ~5500 free speed CIM RPM ? Or were you assuming some sort of amperage being pulled?
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Quote:
Originally Posted by Cory
The actual speed was approximately 80% of the no load 100% efficiency speed.
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No-load = free speed. That is, the speed the motor turns at when nothing at all is connected to it. Note, that this doesn't mean no amps are being pulled. There's still a small amount of friction to be overcome etc. Plus, the rotor coils are constantly changing orientations and making and breaking contacts. This means the magnetic fields are constantly collapsing and reforming, which means the back emf generated by the motor will vary over time. Since the voltage supply nominally stays the same and the coils have some resistance, current is flowing whenever the back emf doesn't exactly equal the input voltage. So all DC motors have a no-load current as well as a no-load speed.
Which brings me to another silly technical point. Since the CIM is drawing current it has some non-zero input power, even at no-load. However, since there isn't a load output from the shaft, it has zero output power. So the motor's efficiency at no-load is actually 0%, not 100%.