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Discussion Starter · #1 ·
OK, so a few questions here. I've been working with VFDs for awhile but I had a thought that I can't answer. When checking the settings on a VFD controlling a 25 HP, 3540 RPM motor, I noticed that it had been programmed for 1725 RPM. Does anyone know if this will change the operation of the fan? I've since corrected the parameter, and am mostly curious.

Also, the same fan unit mentioned above supplies suction for one of our processes. The machine manufacturer is telling me that the fan speed may need to be increased up to 150%, or 90HZ. I've tried this but found that increasing it over about 62HZ runs the motor over it's FLA. Is there any way to use the measured current to control the fan speed? For example, the motor FLA is 32A, but at 60 HZ with material running through the system the FLA may only be 25A. I would like to have the drive attempt to keep the motor close to FLA, which would increase my airflow in this circumstance.

I am using an AB Powerflex 70.
 

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I am in a hurry for bed here, so just quick with the 1750 3400 stuff.

The drive was programmed for a 4 pole motor- 1750. You set it for a 2 pole 3400 rpm motor.

This messes with the hertz/speed reading if the wrong motor/or drive setting combination is used.
 

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I Used an Altivar 71 to solve a mechanical issue by setting the max output current to the Motor FLA. The drive would just slow the motor down once the current limit was reached. I would think you could do something the same in this situation. Make sure you set a reasonable maximum frequency.

Also 150% flow increase doesn't mean 90 hertz. Pump and Fan output is not linear to speed.
 

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OK, so a few questions here. I've been working with VFDs for awhile but I had a thought that I can't answer. When checking the settings on a VFD controlling a 25 HP, 3540 RPM motor, I noticed that it had been programmed for 1725 RPM. Does anyone know if this will change the operation of the fan? I've since corrected the parameter, and am mostly curious.
Shouldn't make a difference. If you are in V/Hz mode, that motor speed parameter is only used by the display if you want to display machine speed, as opposed to Hz. If you are in SVC mode, and you did an Autotune, it would see a mismatch and ignore that information. If you are in SVC mode and manually entered all of the necessary motor circuit parameters, then it might make a difference.

Also, the same fan unit mentioned above supplies suction for one of our processes. The machine manufacturer is telling me that the fan speed may need to be increased up to 150%, or 90HZ. I've tried this but found that increasing it over about 62HZ runs the motor over it's FLA. Is there any way to use the measured current to control the fan speed? For example, the motor FLA is 32A, but at 60 HZ with material running through the system the FLA may only be 25A. I would like to have the drive attempt to keep the motor close to FLA, which would increase my airflow in this circumstance.

I am using an AB Powerflex 70.
On a centrifugal fan, the fan power requirement changes as the CUBE of the speed change. So at half speed, it takes 1/8th power, which is why they save so much energy. But it works BOTH ways, so at 150% speed, the fan requires 337.5% more HP (1.5 cubed) than it does at full speed! Your motor power HP would increase with speed too, but only at the same ratio, so 150% HP in theory, still not enough. However, it's worse than that because once you are at full speed with a drive, you are also at full voltage. Then when you go higher with the speed, you have no more voltage to keep going, so your relationship between V and Hz begins to drop, and your motor LOSES torque and HP remains the same. So at 150% speed you have the SAME motor HP, but you now have a load that requires over 3X the motor HP. The motor overloads.

Now, CAN you set it up to maximize current? Yes, but to what end? Let's say your motor was over sized by 20%, a common number in the machine design world. So you had a 25HP motor for a 20HP load. You could theoretically tell the VFD to current limit at the VFD's maximum capacity of 34A (the PF70 rating for a 25HP fan load). You would tell the VFD to increase speed beyond full speed, but set the Current Limit at 34A, so the speed increase would cease once it hits that. What you will find is that will equate to about a little less than 110% speed, because if the motor draws 26A at full load, at 110% speed that is 133% power (and current), which will be around 34A, the rating of the VFD. So you are talking about, at best, going from 60Hz to 66Hz.
 
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Discussion Starter · #5 ·
Thanks JRaef for the detailed information! Fantastic!! Much of what you explained I found when experimenting with the VFD. I didn't understand why the current was increasing so much when I went over 60HZ, but after reading your post it now makes total sense. I'm starting to think that our machine manufacturer is using a different type of ventilation unit, since they're telling me I should be pushing the fan speed upwards of 140-150%. I've been able to push the fan to 65HZ, which gives me the 32 FLA of the motor. However, I don't think that the increased speed is producing any significant increase in airflow. I'm looking into this simply because management seems to think that our machine should be processing about 30% more material than it is. I think they're nuts.

Thanks to everyone for the info.
 

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Thanks JRaef for the detailed information! Fantastic!! Much of what you explained I found when experimenting with the VFD. I didn't understand why the current was increasing so much when I went over 60HZ, but after reading your post it now makes total sense. I'm starting to think that our machine manufacturer is using a different type of ventilation unit, since they're telling me I should be pushing the fan speed upwards of 140-150%. I've been able to push the fan to 65HZ, which gives me the 32 FLA of the motor. However, I don't think that the increased speed is producing any significant increase in airflow. I'm looking into this simply because management seems to think that our machine should be processing about 30% more material than it is. I think they're nuts.

Thanks to everyone for the info.
I see your problem right there.
ive never met any production manager who could think
hell most of them need video instructions on how to masturbate:laughing:
 

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... they're telling me I should be pushing the fan speed upwards of 140-150%. ... I think they're nuts ...
Technically, you might indeed be as to run the fan at 150% speed. Just not with that motor. I run into this sort of thing all the time; fan and pump guys who don't make that connection.
 
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