VFDs

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billsnuff

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I have 3 machines equipped with 30 hp AC Mtrs and VFDs. (2 mitsubishi and 1 reliance)

machine 47 is set at 120 hertz and runs at 61 hz with a draw of 17 amps while running product at 1800 fpm

machine 48 is set at 60 hertz, runs with reostat maxed, draws 16.9 amps @ a speed of 1830 fpm

machine 49 is set at 70 hz (reliance) with a similiar amp draw at approx 1800 fpm.

I am trying to gain a 10% increase in speed (approx 2000 fpm).

What are the risks v. rewards in running this way?

Machines are run 24/5.

Looking for advice, opinions, experience.........
 

Jraef

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San Francisco Bay Area, CA, USA
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Electrical Engineer
I have 3 machines equipped with 30 hp AC Mtrs and VFDs. (2 mitsubishi and 1 reliance)
Same machines?

machine 47 is set at 120 hertz and runs at 61 hz with a draw of 17 amps while running product at 1800 fpm
What do you mean by "set at 120Hz running at 61..."? Are you meaning the MAX speed on the VFD is set for 120Hz, but you are running at 61 and if you increased the "rheostat" it would run up to 120Hz?

machine 48 is set at 60 hertz, runs with reostat maxed, draws 16.9 amps @ a speed of 1830 fpm
Curious that a 30HP motor is only pulling 16.9A

machine 49 is set at 70 hz (reliance) with a similar amp draw at approx 1800 fpm.
Curiouser and curiouser.

I am trying to gain a 10% increase in speed (approx 2000 fpm).

What are the risks v. rewards in running this way?

Machines are run 24/5.

Looking for advice, opinions, experience.........
The problem with giving advice and/or even assessing your risks is that we have no idea why the machine designer is doing things this way. But to help at all, we will need some added information.
  1. Motor nameplate data for each motor
  2. VFD nameplate data for each drive
  3. Line voltage
  4. Motor connection voltage (it may not necessarily match, I'll explain later).
  5. How does "fpm" relate to motor speed, i.e. gearbox / pulleys etc?
 

billsnuff

Senior Member
Same machines?
yes

What do you mean by "set at 120Hz running at 61..."? Are you meaning the MAX speed on the VFD is set for 120Hz, but you are running at 61 and if you increased the "rheostat" it would run up to 120Hz?
yes

Curious that a 30HP motor is only pulling 16.9A
Motor was designed for option equipment that we don't use.

Curiouser and curiouser.
Sorry, its running at 1800 fpm at 60 hz put drive set to run at 70 hz, to allow for more speed. Haven't tested product quality at the increased speed yet.

FREQROL K-400 60 or 120 Hz
FREQROL Z 200 UL 60 or 120 Hz
Reliance GP 2000 (Mfg has lock out on Hz at 80 max)

machines are currently running and I can't read nameplate thru guard.
30 hp - 1765 rpm - 34.1 FLA is all I got right now.

I do recall 25 years ago with the reliance tube type dc drive we used to run the drive motors at rated FLA + 10%.

We are just trying to push the motor past its designed speed a little. (10%)

Once upon a time these machine were called Folder-Gluers for the paperboard packaging industry. Now they have been converted to high speed flame sealers and the paperboard is coated with poly.

An eight 8hr shift will yield 1.2 to 1.3 million cartons. (x 3 shifts x 5 days x 52 weeks, is a lot of coin).
 

Jraef

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Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
When you run a motor over base speed, it is in "constant HP mode", meaning you cannot maintain the same V/Hz ratio because you are out of V once you get to 60Hz! So what happens is, your available motor torque begins to diminish above 60Hz. So for the same load, your motor will be working harder and the higher the level above base speed, the harder that harder gets (see chart). At the same time, if the motor relies on cooling fans, the fan efficiency may drop off at higher speeds so you increase the risk of over heating your motor (more current for the same load, less cooling). So generally it is a cautionary tale when recommending over speeding an AC motor with a VFD.


This chart is showing maximum (breakdown) torque.

In your case however, it appears as though your motors are seriously over sized for the current load. So as long as that is the case, should be no problem. Bearing speed rating can also be an issue, but 10% speed over base is not likely a problem for that either.
 
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