Huanyang VFD current draw measurements.

[m_c]

It's a common thing that people don't understand the fundamentals of motor current and voltage.
Current = Torque. Voltage = Speed.
As motor speed increases, back EMF increases, so you need more voltage to push/drive the required current through the windings.
As load increases, due to the motor poles dragging further behind the magnetic fields, it takes more current to pull/push the rotor along.

If you were to run a motor at say 10% rated speed, but still apply rated voltage, you would burn out the windings, as due to the lack of back EMF to limit current, excess current would flow through the windings.
That's why VFDs reduce the voltage in proportion to commanded speed.

Then combine that with the glorified switch mode power supply that is a VFD, and the lines can get very blurred, but in nutshell, Power out = Power In.
So supply current = (motor voltage * motor current) / supply voltage.
That obviously doesn't allow for any inefficiency, but I'd guess that even cheap VFDs will still have efficiency ratings in the high 90s, as they don't generate any major amount of heat.


You could use the supply current to calculate spindle power, however without knowing the speed, it's relatively meaningless figure, as max spindle power is proportional to speed, so you end up with a percentage of a proportional power. And that's before you consider any inefficiency in the VFD.
That's why the VFD motor current figure is far more useful, as it corresponds directly to motor torque, and is measured after any VFD inefficiency, so you can quite easily convert to a percentage of available torque.

With the KFlop, you could pull that data from the VFD using Modbus, then show it in KMotionCNC.

[Edward]

With the KFlop, you could pull that data from the VFD using Modbus, then show it in KMotionCNC.

Thanks m_c. That would be quite handy, as the VFD will be some distance from the computer. However, learning all about Modbus is a little beyond me at this stage;)

I have my second Kflop now for the router I'm building, it took a little while to be familiarised with it sufficiently to get it fully going, with some simple C code for the probe and so on, but it's been very reliable.

Edward