It would run faster if it had sufficient torque at the VFD's limiting voltage. This would likely be the case if the pump/system is set up so the maximum power isn't required. Not the point on its curve that has maximum flow and pressure.
Being submersible it would likely not overheat easily.
You say the MAX frequency. It depends first on if other settings and inputs allow the motor to reach max frequency. After that see Keiths response. Either run faster/pump more, little change, or burnout. It's hard to tell exactly what most submersibles are doing.
Bill
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"Why not the best?"
Jimmy Carter
The VFD would allow it but the power requirements of the pump may not.
If it is a centrifugal pump then the theoretical power requirement will be cubed. As a quick rule:
72/60=1.2 1.2x1.2x1.2=1.728 multiply 1.728 by your rated HP and that will show your typical power requiremnt at 72Hz. Example; a 10Hp motor/pump would demand 17.28Hp at 72Hz.
Typically before you got there, assuming everything is sized correctly, the VFD would hit current limit.
I've run a centrifugal pump at 70Hz for over ten years with no problems. But not continuously. So it may work but you certainly need to consider ozmosis's point.
My clauses are the words "may" "typically" and "assuming".
Most loads are not operating at their rated capacity so you will have an amount of leeway. Simple thing to do is to increase the Hz step by step and watch the current on the VFD display. As long as you are not pulling more amps than the motor is rated for, then it is typically ok. Mechanically you should be ok to 72Hz.
When a VFD takes a motor over base speed, it cannot continue to increase the output voltage with the frequency, so the motor runs in what is called "Constant HP" mode. That means, in Oz's example, the 10HP motor tops out at 10HP even though the LOAD requirement increases to 17.28HP. So even if you over sized the VFD to provide additional current, you will still overload the motor. It just wasn't built to do the extra work.
But in the case of itsmoked's example, if the motor were significantly over sized to begin with it could very well work out. For example in his case, that 2Hz difference in output speed means he only needed 158.8% HP instead of 172.8% as in the 72Hz example. So if the motor was over sized by 20%, which is common, AND the motor had a 1.15SF, which is also common, then the effective motor overload is down to 113%. Plot that on an I2t OL curve and you can get away with it for a good amount of time on occasion, just as itsmoked said. But it is not something to be flirted with carelessly.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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Given the variation in both static and dynamic head that a submersible pump may experience from one application to the next, it is possible that this application moved the pump so far down the pump curve that an installer drove it up to 72 Hz. to get the maximum safe output.
In one hole 72 Hz may be the best setting and in another hole it may be a disaster.
Bill
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"Why not the best?"
Jimmy Carter
