Load Meter, current transducer

[Karl_T]

I'm looking for a sensor to input spindle load into my Camsoft CNC control. I'm using a VFD and would like to sense the load or current and hook up to an analog input.

I'm finding a bewildering selection from $20 to $800. At this point, I'm leaning toward purchase of a current transducer in McMaster Carr catalog #110 page 657. Part 6583K32 ($114) provides a 0 - 10 volt DC output.

Good choice? Is there a better option?

Karl

[Al_The_Man]

Check first to see if your VFD has a analogue output that can be scaled for current out, many do, cost $0.00
Al.

[Karl_T]

That was plan "A". After reading the VFD manual for quite some time, all I have is a setable output current alarm. Not what I want. The VFD readout has load, but not an anaolog out for it.

karl

[Al_The_Man]

What model VFD did you go with as your previous link for the Hitachi's show they have that feature, a cheaper alternative might be to use the one of the Amveco at this link, with a suitable output resistor to give 1v/amp and then put a small bridge after it for dc analogue signal. http://dkc3.digikey.com/PDF/T053/1605.pdf
Al.

[Karl_T]

The Hitachi you reccomended has that feature, the SJ300. It also has a sensor to not allow single phase input So, I downgraded to the SJ100. They don't reccomend 1 phase input, but it works. I have a 10 hp VFD powering a 7 /12 hp motor where a 2 hp. motor was original.

Karl

[Karl_T]

Follow Up. The Hitachi SJ100 VFD drive has a PWM output for load. Needs a much higher impedance than the Galil 1900 provides. So a good freind desinged and built a buffer amp. There are other PWM functions on the VFD drive that this could be used on.

[murphy625]

Maybe I am way off here but here goes..

Wouldnt you have to put the current shunt on the input to the VFD to measure its output current??

Correct me if I am wrong (its been a while) but current is measured accross a shunt by measuring voltage drop. Voltage accross that shunt would be very hard to measure continuously because of the constantly chaning frequency from the VFD.

Am I off ????

The way I figure it, you would have to measure the input current to the drive and subtract for losses.

Or am I an idiot?? <<<Been there before

[Al_The_Man]

Originally Posted by murphy625 Wouldnt you have to put the current shunt on the input to the VFD to measure its output current??

Murphy, I think you may be getting confused with monitoring the outgoing phase to the motor over monitoring the incoming three phase.
These drives have micrprocessors that monitor the drive condition and supply either analog or PWM outputs that can be configured to indicate various VFD states.
It look like this circuit that Karl had made is a Buffer with an integrator input, this, together with the op-amp converts the PWM to an analogue signal that can be used to supply a meter etc.
At least thats the way I am seeing it.
Al.