# VFD parameters - torque/power



## John Valdes (May 17, 2007)

What is the question exactly? The settings for "motor type" are so the drive knows what to expect on the output side of the control. It has little to do with the waveform and everything to do with current.
VFD's are sized for constant torque and variable torque. With current being the barometer.
For example a drive running a variable torque application will be sized smaller than a drive for a constant torque application even though the same motor (constant torque) is used.
It is not unusual to see a 10 HP drive running a 15 HP motor in a variable torque application. 
The example Jraef made was exactly this. The slower you run a motor (below base speed) the less torque you produce. Same with over speed. Less torque. 
Full torque is achieved at base speed (nameplate).
The newer VFD's can do this job much better and some do produce full torque at zero speed. It seems most drives are like this now. When I was working with them there were few drives like this with the exception of the ABB. It was called an open loop vector and could operated much like a closed loop system.
The VFD will work in your situation. But you must ask "how slow will I run this motor"?
If you are reducing the speed considerably, you might want to consider other limiting factors not excluding a slower motor.
Like maybe using 6 or 8 pole motor. Engineering the application should be done now. Figure the speed related to the pressure and design the system accordingly.


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## JRaef (Mar 23, 2009)

acro said:


> I am installing a new pump/VFD combination and was doing some reading...
> 
> This partial response from another thread got me thinking.
> 
> ...


They do affect the torque capabilities a little bit. 

In the High Starting Torque setting, it tweaks the V/Hz ratio up higher at the lowest speeds so as to boost torque, but is essentially risking over heating the motor, so that is why they say high STARTING torque, not high RUNNING torque; they are assuming you are only going to need the higher torque for a short time as it accelerates from a stop.

The Pump and Fan setting will take ADVANTAGE of the fact that a _centrifugal_ pump or fan will not need as much torque at lower speeds, so the VFD will lower the voltage in the V/Hz ratio in order to lower the motor magnetization losses and squeeze a tiny little more efficiency out of the system. So the relationship between V and Hz becomes a parabolic curve (V/Hz squared) instead of linear. This also lowers the motor torque, but again, _this type of centrifugal load will not need it_. You should be fine with that, but if not, just change it to General Purpose.


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## varmit (Apr 19, 2009)

Is your pump centrifugal or a positive displacement? If the pump is centrifugal, your control idea COULD work. A PID loop with pressure feedback will probably control better. If you are driving a positive displacement pump, limiting pressure by limiting motor speed WILL NOT WORK. A PD pump, at reduced speed, will still attain it's maximum output pressure, only it will take a longer time to reach max output pressure. Also, as the output pressure of a PD pump increases, the motor torque required increases.


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## JRaef (Mar 23, 2009)

varmit said:


> Is your pump centrifugal or a positive displacement? If the pump is centrifugal, your control idea COULD work. A PID loop with pressure feedback will probably control better. If you are driving a positive displacement pump, limiting pressure by limiting motor speed WILL NOT WORK. A PD pump, at reduced speed, will still attain it's maximum output pressure, only it will take a longer time to reach max output pressure. Also, as the output pressure of a PD pump increases, the motor torque required increases.


Good point, but...


> ... My load is a 1-1/2 hp 3600 rpm 11stage pump ...


Not going to be a PD pump. Multi-stage is by definition a type of high pressure centrifugal pump.


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## varmit (Apr 19, 2009)

OOPS, I was in a hurry and didn't read the post fully. Sorry.

One possible issue: Depending on the system flow dynamics, if there is a pressure drop in the system, that the motor/drive can not compensate for, due to the motor speed limitation, will this possibly very low pressure be a problem?


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## acro (May 3, 2011)

Good info...


Well, I don't really have a specific question. Just wanting to know more about different operating characteristics. And I did the best I could engineering this for pump selection. Honestly, my ultimate demand for water is not quite known. I want to use a little as possible, but needed to make sure I have plenty of capacity should I need it.

The pump is an 11 stage Grundfos centrifugal. And the reason my demands are vague is that we are installing a dust control system on one of our crushers. I fabricated a spray bar system that currently has 12 spray nozzles, but there are places for up to 14 additional - should they be needed. Additionally, the nozzles I initially selected use the least amount of water, and should I need to , those could be replaced with ones with a higher demand.


A loss of pressure would only result in a diminished capacity to reduce the dust.


I have designed some controls to make it a little more fail safe and user-friendly. I can post some pics later. Right now, the cabinet is a little messy.


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## JRaef (Mar 23, 2009)

OK, now we know your goal. Should work fine.

What you'll have to do is determine, whith the current nozzle configuration you want, what pressure is required to get the desired effect. Easy enough to do once you have the VFD in place, just control the speed manually until you get what you want, then record the water pressure or better yet if you have already installed the required pressure transducer, just get the signal output at that level of performance. Then take that information into a PID control loop as the "Set Point" that you want to maintain. Every loop controller has various means of programming them, some provide "auto-tuning" that makes it simple. I'd look for one of those. Later when you change or add nozzles, just run it up manually to determine the new set point and re-enter that into the PID controller, done.

Some (most) VFDs provide an internal PID loop function, but here's my warning on that. If you have barely enough money to buy the VFD, then you might have to use it. But if you can afford another $200 or if you have a PLC being used for some other function, do the PID loop control OUTSIDE of the VFD. It's counter to what the VFD salesmen will tell you, but here's the reason. If the VFD craps out some day, you will have to START ALL OVER with programming the PID loop, or else you can buy EXACTLY THE SAME VFD. That is what the VFD salesmen are after of course, guaranteed return customers. If the PID loop is done outside of the VFD, then ANY other VFD can become a "universal donor" with a minimum of setup. I've done a LOT of crusher controls, waiting 2 days for a specific brand of VFD to arrive by FedEx as a replacement is generally unacceptable, so I have found myself reading manuals and learning some new VFD programming routine for their internal PID loop at night when I want to be at home getting ready for bed. Not fun. If the PID controller is separate, I can swap out ANY readily available off-the-shelf drive in 30 minutes and make everyone happy.


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## acro (May 3, 2011)

I started down the road considering a transducer that would connect to the drive for control of the speed, but I decided against it. I don't think my setup will require that type of accuracy and I wanted to simplify the best I could. I did install a high pressure switch that will shut off the pump and sound an alarm when the filter needs changed. Mostly to protect a filter housing that is downstream from the pump.


I think I will save the PID for another project.

But for safety, I hope to also limit the top speed of the pump using the parameters in the drive. But that brings up another question. Motor base rpm and motor max rpm.
My motor is 3600 rpm and I plan on determining the rpm needed to reach the pressure I want with new filters. I want to program the drive to not exceed that speed- whatever it is, nomatter what position the pot is turned to.

Now, my drive specs say that the maximum rpm setting cannot be set lower than the base rpm. But the base rpm is supposed to be the data on the namepate of the motor.

This may all become clear when I finish the wiring and fire it up, but should I be concerned with entering lower values for my motor base rpm?


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