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From the thread thread237-489913 I 1

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itsmoked

itsmoked

Electrical
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From the thread thread237-489913 I got the extruder fired up.

We have 83kW available. Motor is 123kW. I preheated the extruder chain using 35kW. Once the old stuff was heated up enough and 90% of the heaters were off, started the dual screw extruder at 5RPM. It ran! We successfully ran all the old flexible material out of the chain.

Current at 5rpm was ~40A
Current at 10rpm was ~50A
Current at 30rpm was ~58A
Current at 40rpm was ~60A

We're trying to understand why an empty extruder running at 40 out of 800rpm is drawing 60A/480V = 50kW with no product in it.

OR, is the amp value on the Snider VFD screen, the Real Power amps or does it include reactive/excitation current too?


20211025_170330_s5tf74.jpg




Keith Cress
kcress -
 
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itsmoked said:
...is the amp value on the Snider VFD screen, the Real Power amps or does it include reactive/excitation current too?
Click to expand...
It is the output current going from the VFD through the transistors, there are (typically) hall effect transducers ahead of each transistor pair. So it is inclusive of the reactive/excitation current going to the motor.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden
 
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Today mulling over the logic I realized that it was going to be ALL the current. What you'd see on the motor plate at the stated frequency.

If we get the VFD to cough-up the power that will be the Real power.

Thanks for the confirmation Jeff.

Keith Cress
kcress -
 
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Reading the motor current on the display, 60A no load varies from about right to being on the low side for a 116A-203A FLA motor, and will be nowhere near 60kW of power, since you didn't account for both the much lower output voltage of the VFD or the poor power factor of the unloaded motor.

The VFD varies the output frequency so that means it runs the motor at full speed all the time. The available shaft power varies directly proportional to the percentage of rated frequency you apply.

The VFD is a power conversion device. The input current would have been MUCH less than 60A. I'd bet you could have started it with the heaters on. I have no idea what you programmed in the VFD or what was running at 5rpm, but assuming 5RPM out of 1780rpm for a 110kW motor the VFD would have been only drawing a few kW off the source to run the motor at rated output torque.
 
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Thanks Hutz; The picture you paint makes way more sense to what we were seeing. The VFD is all pre-programmed in this monstrosity. Ah yes, the voltage was much lower at that lowered speed, totally missed that. Indeed, I should've put a clamp-on on the supply to the machine instead of reading the PLC's interpretation of what the VFD was giving it, (which we noticed didn't agree exactly). Apparently with extruders operators must always have torque feedback since most of the torque is going into shearing the material and the shear is often where the 'processing' is happening.

I'll be changing the VFD's display to show kW instead of the rather useless -in this situation- amps.

Thanks.

Keith Cress
kcress -
 
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So, I went back today and switched the Altivar 61 110kW VFD display to show Motor Power. Unfortunately it doesn't show Power but rather Percentage of Full Power.

I heated up the extruder chain then ran the augurs at 10rpm. 50A but it never got off 0.0% power. I then ran it up to in steps to 40rpm. Still 0.0% power. So I clamped an ammeter onto the lines feeding the VFD while at 40rpm. 1.5A!! Probably why it's still showing 0.0% when FLA is like 205A.

Thanks LH you were right-on.

Keith Cress
kcress -
 
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People often don't believe it until they actually see it. 40rpm / 1780rpm x 110kw = 2.5kW at full torque or full 40rpm power. Unloaded is much less, just the power required to keep it turning.
 
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