Running 7.5hp Motor Overfrequency with VFD to Maximize Fan Performance. 2

[MacMcMacmac]

Good Day Everyone,

We are trying to solve an issue with a large dehydration unit. We are finding that our reactivation blower which draws heated air through a Munters CargoCaire honeycomb dehydration wheel is not creating the proper pressure drop across the wheel. The wheel is not plugged or fouled in any way.


After some investigation, we found fan is turning approximately 4300rpm, 600rpm below its rated 4900rpm. It is connected to a 7.5hp 575V 3530rpm Marathon motor via a belt drive. On the VFD, at 60Hz, the motor is pulling slightly less than 5A, so it seems there is about 2.5hp left on the table. Not a large amount, but at 50% of present draw it would seem to be worth trying for.

My direct supervisor is recommending we up the AC frequency to the 7.5hp motor to achieve 4900 rpm at the fan. Using the formula Motor rpm = 120F/p where f = freqency and p = number of poles, I rearrange the formula and calculated a frequency of 67Hz to get us to the motor rpm required to max out the fan speed at 4900 rpm. This does not seem like a very large increase, which indicates to me the motor should be happy enough at that speed. Unfortunately, there is no free lunch, and further investigation shows that max motor torque will be about 80% at that rpm. Given the load is increasing with rpm, and torque is decreasing with the higher frequency, I am assuming (dangerous I know) that we would soon be operating into the service factor of this motor, which is 1.15. All in all, it seem like a lot of messing about, and what we gain on one hand, we lose on the other.

My solution is to put an adjustable sheave on the motor, run it at 60Hz, dial in the largest diameter that results in max fan rpm or motor amp draw, and call it a day.

The VFD is an Allen Bradley Powerflex 70 series. Reading through the manual has been a somewhat frustrating experience. I would not be the one manipulating the settings, so thankfully I can remain confused to no ill effect.

 

[Compositepro]

"and torque is decreasing with the higher frequency". Why do you say this? A pulley will cause this problem, not a vfd. Adjust your vfd frequency until you are at full load on the motor and be done with it. That may be more or less than 4900 rpm.

 

[rhatcher]

Compositepro, Mac is correct, the motor torque will be reduced to about 75% of the rated value when operated at 67hz because this is above the base speed and in the CHP speed range.

Mac, keep in mind that the fan is a variable torque load so a change from 4300 to 4900 rpm will require about 30% more torque and about 48% more power.

 

[Compositepro]

Only if the vfd is programmed to do so to keep the horsepower constant above base speed. The motor itself will maintain the same torque capability, and the heat rejection capability of the motor increases with speed. It is the current/torque of the motor that generates heat, not the speed of the motor.

 

[MacMcMacmac]

Well, my train of thought was if I am increasing the load on the motor by upping the fan rpm, I want as much torque as I can get. This is why the pulley option seemed to be the preferred route, plus I'm the dirty fingernails guy who goes for the mechanical solution first. It also avoids delving into the mysteries of VFD configuration. The system as a whole works well except during the very high humidity levels we will be experiencing within the next month, and up until October-ish. I am reluctant to begin changing VFD configurations when 90% of the time things are working well. Final air moisture has to be .07gr/lb, or -80F, so it is critical that our dehydrators work well at all times.

The 48% increase in power seems to dovetail nicely with the amount left to exploit.

This discussion will be a part of our weekly safety and maintenace meeting on Wednesday.

Thanks.

 

[jraef]

The Affinity Law for speed of a centrifugal machine: Power required by the load (pump or fan) varies by the cube of the speed change.

4900/4600 = 1.065x speed.
HP required by the load will then be 113% of what it is at 60Hz

Motor CAPACITY is 7.5HP at 60Hz and HP in the MOTOR remains constant above base speed. But given that your motor is not using it's full HP at full speed now, I think you have plenty of head room to handle only 13% more load.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[edison123]

Jeff, that would be about 21‰ more power needed?

Muthu

http://www.edison.co.in

 

[rhatcher]

Compositepro, the motor torque above base speed has nothing to do with the drive programming, it is a function of the motor’s volts/hz ratio. The voltage reaches its maximum value at base speed and as the frequency continues to increase the V/hz ratio decreases and reduces the motor torque output.

Jeff, LPS for for catching my mistake. I calculated the increase in torque and power based on the increase in the motor speed, not the increase in fan speed (which is what matters). The pulley ratio is the difference.

Mac, as indicated, jraef’s torque and power requirements are the correct ones, my apologies for the misleading information.

 

[jraef]

Muthu,
Yes, my math was lacking... (squared instead of cubed). It is indeed 121%.

Probably still OK. Personally I would try it since everything is already owned and in place.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[rhatcher]

Wow, I accepted Jeff’s written explanation that the power is the cube of the speed change as being correct, which it is, without checking his math. My bad again, Muthu is correct.

 

[edison123]

Jeff - Yes, since OP says the present current draw is for 5 HP, the motor still has 50% HP to deliver at higher RPM. But what about the torque requirement at the higher speed of the fan? Affinity laws talk about volume, head and power but not the torque.

Muthu

http://www.edison.co.in

 

[itsmoked]

Mac. Easy and fun.

Break out the manual. (yes it sucks but others suck worse)

Figure out how to change parameters. Takes a couple of minutes but it becomes obvious shortly.

Hunt down the parameter for MAX FREQ (param 55)
See what it is and write it down so you can return if you ever want to.
Set it to 68Hz.

Then you want parameter 3 which shows you the motor current.

Once you can see the current you can probably use the front panel button directly to up the frequency and hence the speed while monitoring the motor current draw.

You can essentially speed the motor up without damaging it as long as the speed doesn't exceed rational levels and more importantly you don't exceed the FLA motor amperage listed on its plate.

At the very least you can turn it up until you see FLA.

That could be actually faster than the wheel wants to see so check the frequency as you mess with this.

I agree you will probably reach your goal simply by pushing buttons.


Keith Cress
kcress -

http://www.flaminsystems.com

 

[MacMcMacmac]

I looked at parameter 55 and it was set to 130Hz, which I believe is one of the presets for the maximum frequency the drive will allow, not the max frequency set by the user.

Would it not be parameter 82 Maximum Speed? This was set to 60Hz. Default is 50Hz or 60Hz depending on the mains frequency within the area the VFD has been deployed.

 

[waross]

Given the relationship between Power Factor and a partially loaded motor, you are probably running less than 5 HP at 5 Amps.
You may have a little extra capacity available.
I like Keith's idea.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[MacMcMacmac]

Well, no luck. Changed the 55 and 82 parameters and the VFD maxed out at 60Hz. Obviously there is something else going on.

 

[itsmoked]

Mac; We need a little more info.

Often a drive is set to only allow its speed to be controlled by, say, a digital input or an analog signal. If yours is set that way it simply won't care about you trying to change its speed with its front panel.

A quick test of this is 'can you slow it down from the front panel'? If you can then indeed it is a set limit parameter that's holding things up. If you can't even slow it from the front panel, then it's instead a control options setting that needs modification.

Can you slow it?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[MacMcMacmac]

We think the VFD is probably getting a 4-20 feedback from the Munters control system. It ramps down to the 40Hz minimum programmed into the machine, but never up past 60Hz despite what we put into the control panel.

At any rate, we have found a motor sheave that is within .010" of what is needed for the extra rpm and we are going to go that route. It is more urgent to get this done during our brief window of inactivity than spend a lot of time figuring out how the two systems are interacting.

Thank you for your guidance.

 

[itsmoked]

OK that works.

Yep that's probably the hitch, it won't pay attention to your HMI key entry.

The solution would've been to recalibrate what the 20mA means, but the sheave should do it too.

Let us know how it works out for you.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[edison123]

If the system (motor, VFD, pulley and fan) is designed for 4900 RPM, how did the fan speed drop to 4300 RPM now? Are you measuring the fan speed directly and correctly?

Muthu

http://www.edison.co.in

 

[MacMcMacmac]

The pulleys are not the correct ratio for the full 4900rpm @ 60Hz, which is the fan max rating.

It seems the manufacturer has set up this fan very conservatively.

After manipulating a few other parameters, we have had success.

We modified the analog in values which had pegged the 20mA feedback to 60Hz operation, and upped it to 68Hz.

FLA comes out to 7.2, which is comfortably within the spec of the motor. We actually achieved 5120rpm, so we will pull it back to 67Hz and see what's what.

Parameter 91 Speed Ref A Hi, required modification to the desired frequency.

Early results are encouraging, as the regeneration temperature out from the dehydrator seems to be achieving setpoint. Inlet humidity is ~90Gr/lb, which isn't the highest we'll see, but not too far off. We get well over 100gr/lb on the most humid days.