Condenser VFD

[dtbrown]

I have an application whereby i wish to control a large 12 fan condenser using a VFD - to try reduce energy usage.

The problem is that I beleive the motors can only run down to a min speed of 30% (15Hz). The means my min running capacity would be quite high (compared to what it is now) - and would result in the continuous pulsing on / off all fans at the slowest speed when the demand is low - which does'nt sound very efficient to me.. any ideas?

[Brian_UK]

Don't try to control all 12 fans at once.

Try stepping some of the fans using simple pressure switches and perhaps using VFDs on a few.

[US Iceman]

Or, use a combination of two inverters to control separate groups of fans with a deadband between the two control inputs to the inverters.

[SteinarN]

Its not any problem to run the fans down to zero Hz. The fans must however be 3-phase.

12 fans on one VFD is mayby a bit out of the normal. I would have used say 3 VFD's with 4 fans each. It is several ways to control the VFD/fan speed. If you have a PLC with HP transducer, you can change the program in the PLC and take out a say 4-20mA signal to control the VFD's. If you dont have a PLC, then conect a HP transducer to one of the VFD's, configure it to run the fans to maintain desired HP, take out a signal from that first PLC telling what Hz it is running, feed that signal into the other VFD's, configure them to follow the Hz to the first VFD.

You then have a system where all fans run at the same Hz from Zero to full speed.

[walden]

Some UK based supermarkets are using electronically commutated (EC) motors for condenser fans. These motors have a conventional 3 phase power supply and need a 0/10V DC speed regulation signal from an electronic controller which is monitoring the discharge pressure. The EC motors are more expensive than conventional induction motors but are more efficient and have been shown to give worthwhile energy savings. The projects I am working on will regulate the speed of all the fans in parallel. There are condensers with 5, 8 and 12 fan motors.

[750 Valve]

You are correct, you cannot usually run fans down to 0hz, I have a statement from zeihl abegg somewhere letting me know the mins that the particular motor we were using can run at, I would check with your manufacturer of the motors - in our instance 7hz was the minimum but this was only for that particular model motor.

There can become an issue with heat disappation from within the motors when running at very low hertz and will likely void warranty if you run lower than the minimums and burn one out.

[US Iceman]

There can become an issue with heat dissipation from within the motors when running at very low hertz and will likely void warranty if you run lower than the minimums and burn one out.


This raises a good point. You also need to be aware of the motor construction. Some motors have an internal fan for cooling the rotor and stator. Therefore, as the motor speed decreases the volume of air delivered by the fan also decreases.

However, I think we also have to be aware of a simple fact: When you need head pressure control the air temperature is usually a lot lower than when you do not need head pressure control.

The colder air temperatures can help to provide some of the motor cooling!

A second comment on using multiple VFD's on a single condenser. My opinion is; if the condenser does NOT have a partition inside you should not use multiple VFD's. I forgot to include this in my previous post.

A fan partition is that simple piece of sheet metal inside the condenser which is located on top of the condenser coil and fills in the space all the way up to the top of the condenser.

Without the fan partition, the fans on one VFD may rotate backward when the fans controlled by the other VFD are operating.

It's kind of fun to watch.... (for a little while:D).

[SteinarN]

You are correct, you cannot usually run fans down to 0hz, I have a statement from zeihl abegg somewhere letting me know the mins that the particular motor we were using can run at, I would check with your manufacturer of the motors - in our instance 7hz was the minimum but this was only for that particular model motor.

There can become an issue with heat disappation from within the motors when running at very low hertz and will likely void warranty if you run lower than the minimums and burn one out.

I have som experience with VFD condenser fans running down to zero Hz at low ambients. The oldest system has been running without fault for 5 years. If you are using the correct torque setup in the VFD, that is variable torque, the motor normaly gets cooler as the Hz decreases. A fan motor running at 15% speed (7,5 Hz at a 50 Hz system) has a teoretical power consumption of 0.15³ of the nominal power, that is a motor consuming 500 W at 50 Hz is consuming only 1,7 W at 7,5 Hz! In reality the power consumption at low Hz will be somewhat higher, but it is no problem regarding heat development in the motor to run down to zero Hz if the VFD is set up correctly. If the VFD is set up incorrectly, then the motor can easily get overheated and burn at low Hz.
It is also highly preferable to run all fans at the same speed. As US Iceman notes, if some fans is running at high or full speed, other fans may turn backwards. If those fans turning backwards is controlled by a VFD trying to run the fans at a very slow speed then maybe problems with excessive heat could arise.

[SteinarN]

A second comment on using multiple VFD's on a single condenser. My opinion is; if the condenser does NOT have a partition inside you should not use multiple VFD's. I forgot to include this in my previous post.


It is no problem using multiple VFD's on one multi fan condenser. Just set up the system and configure the VFD's so all VFD's run at the same Hz.

[US Iceman]

It is no problem using multiple VFD's on one multi fan condenser. Just set up the system and configure the VFD's so all VFD's run at the same Hz.

Hi SteinarN.

What you say is true. However, by setting the individual VFD's to operate simultaneously you could loose part of the total capacity control on the condenser.

For those situations where additional capacity control is required having multiple VFD's operating simultaneously at the same speed would not offer an increase range of capacity regulation.

And, if some fans were running at a much lower speed than others adjacent to these the air can take the path of least resistance. I've seen fans run backwards before because of this. That's the reason behind the fan partition(s).

[SteinarN]

Hi SteinarN.

What you say is true. However, by setting the individual VFD's to operate simultaneously you could loose part of the total capacity control on the condenser.

For those situations where additional capacity control is required having multiple VFD's operating simultaneously at the same speed would not offer an increase range of capacity regulation.

And, if some fans were running at a much lower speed than others adjacent to these the air can take the path of least resistance. I've seen fans run backwards before because of this. That's the reason behind the fan partition(s).Hi US Iceman

What I ment is that all fans/VFD's is controlled simultaneously, in paralell, between zero Hz and full Hz. The capasity control will be from zero to full capasity, the same as one VFD drives one fan on a one fan condenser.

Well, not quite zero as the condenser has some capasity with the fans stopped also.

[750 Valve]

I have to disagree SteinarN, I also have experience with VSD's and find it very hard to believe a motor manufacturer will warrant a fan in an application running at 1hz. Have you discussed the warranty implications with any manufacturers of fans and/or air cooled condensers?

[SteinarN]

I have to disagree SteinarN, I also have experience with VSD's and find it very hard to believe a motor manufacturer will warrant a fan in an application running at 1hz. Have you discussed the warranty implications with any manufacturers of fans and/or air cooled condensers?

No, I havent discussed it with the manufacturers.

I have done tests on 500 mm Ziehl condenser fans before I installed the first system. I have used Danfoss VLT 2800 series VFD. I monitored the motor temperature at low Hz in a test run in my workshop before first install. When configuring the VFD correctly, the motor hardly gets any warmer than ambient at low Hz.
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I was just out at the nearest systems I have and checked the numbers on a two fan/one VFD system:

The output wattage from the VFD is indicating 0.00 kW at speed below 11 Hz for two 500 mm Ziehl 1440 RPM fans with 500 W motors each. @20 Hz- 0.08 kW, @32 Hz- 0.26 kW, @38 Hz- 0.36kW, @50 Hz- 1.06 kW. The torque of the fans is miniscule at very low Hz. It is easy to stop the fan with your hand at Hz below say 4 Hz. I run this particular system with a max fan speed of 38 Hz. That is 17 kW heat rejected at a DT of 9-10K with a fan power consumption of 360 W.

This shows how the power consumption of the fan motors decreases with decreased fan speed. If the power consumption is this low at very low Hz, then overheating of the motors is clearly impossible.

This is how I have done it and what my experience on this subject is.

[Lowrider]

If indeed there are separators in the condensor it's also possible to run one or two off the frequency drive, 3rd and perhaps 4th in y/d and the rest in high speed only. If set up correct the FD will start. If pressure gets to high and FD can't handle the next will start in low speed. If dead band is chosen correct the FD will turn down before fan goes off. If pressure drops more then fan in low speed will stop.
And so on.

I think this is a very good way to control and prevents fans turning at very low speed and stopping and starting too much.

We have a couple of the systems running and offset is very small!

[750 Valve]

I also think a problem occurs when starting a fan at 1hz consistently, I believe if there is any resistance (ie wind thru condenser) then issues will arise, especially if your control system is calling for min speed and your fans aren't spinning due to resistance - yet power is still applied to them - this situation may continue for a lengthy amount of time in low ambients - if the fans aren't spinning then additional heat will be generated.

If you cannot get warranty on a motor then why use it out of its application? I also don't see a massive difference between the operation of the condenser if the hz are limited to the mfg specs, in our case 7hz versus running down to 1hz - just cycle them off and start them again at min hz knowing full well you are within the mfg's design specs for the motor.

[US Iceman]

A very good discussion gentleman.

[SteinarN]

I also think a problem occurs when starting a fan at 1hz consistently, I believe if there is any resistance (ie wind thru condenser) then issues will arise, especially if your control system is calling for min speed and your fans aren't spinning due to resistance - yet power is still applied to them - this situation may continue for a lengthy amount of time in low ambients - if the fans aren't spinning then additional heat will be generated.

If you cannot get warranty on a motor then why use it out of its application? I also don't see a massive difference between the operation of the condenser if the hz are limited to the mfg specs, in our case 7hz versus running down to 1hz - just cycle them off and start them again at min hz knowing full well you are within the mfg's design specs for the motor.

It is correct that wind could be an issue. The fans may not start at say 1 Hz or even turn backwards slowly. My point is however that it is possible to configure the VFD so that the power applied at low Hz is miniscule. The heat development in the fan motor can't be higher than the power applied to the fan for obvious reasons. If The VFD is applying 5 W power to the fan, then the heat development in the fan motor can't possibly be more than 5 W. As I said in my previous post I just checked the power consumption on different Hz. Two 500 mm, 500 W each fans had a combined power consumption of less than 10W at speed below 11 Hz. This is from the display of the VFD. I don't know how accurate it is. But nevertheless it clearly shows the power applied and consequently the maximum possible heat development in the fan motors is miniscule at low Hz. When I did the test run in my workshop before I installed the first system I also checked the result of blocking a fan at low Hz. The VFD didn't really notice the fan wasnt turning and didn't apply more power. At say 5 to 6 Hz the torque increases ever more and the fan start to turn whatever the wind might be. An other point is that we cant install a condenser in a very windy location and expect the system operating without problems in winter temperatures of say -15*C. Wind shielding would be necessary then.

To cycle both fans on-off at 7 Hz at -15*C ambient, chilled system, low load condition isn't ideal either. The pressure variations would be large with eratic outflow of liquid from the receiver. To cut off one of the fans continous at low ambient would be possible but that would require more wiring and equipment.

This makes a simple and very efficiently regulated system. I feel confident that this system is reliable and not suspectible to premature fan failure due to motor overheating at low Hz. I initially considered the posibility of fan failure in the one year warranty period. If failure was to arise, then one fan isn't that much to loose.

I clearly can see others have problems with this solution as it probably is outside of manufacturers specifications. But I am a person which like to push limits when I feel confident on a subject as I do in this case. What my experience has told me so far is that no problems what so ever has arised on these systems, at least so far.

I'm more than happy to discuss this subject further :)

[750 Valve]

I understand where you are coming from re: pushing the boundaries to achieve better operation, and I can't say i have any experience with ambients such as -15C, to me 0C is bloody cold and its about as low as we ever see where i am at.

In our applications we also see fans running at min hz on even the highest of ambients (even at design max) due to the series heat reclaim setup we commonly run in aus (includes a split cond valve) where the air onto the reclaim coil is often 20 to 23C while the ambients are sitting at 35C, one particular customer dedicates one rack to heating air which is delivered down the rear of all the caes and spills into the aisles via the kickplates. This sytem runs all year round to offset the spill air from the open cases, it is in this scenario that I would especially adhere to the mfg specs. Those fans are baking in direct sunlight on a high ambient with metal casings.... they get hot I can tell you even without any electrical input

[US Iceman]

Those fans are baking in direct sunlight on a high ambient with metal casings.... they get hot I can tell you even without any electrical input


This is an example of the sort of issue that can cause problems when you are doing this....


]
But I am a person which like to push limits when I feel confident on a subject as I do in this case.


I do what SteinarN is talking about on a lot of different projects (pushing the limits). However, in a lot of instances the confidence is only gained by investigating all of the little details such as mentioned by 750 Valve.

This is where one gets outside of the customary procedures of "that's how we have always done it" where a good record of operation and knowing what to expect are experienced. However, when you start to do something outside of the norm you have to look at any and all little details to understand their interaction or dependency on what you are trying to accomplish.

The more time you spend on investigating and understanding the little details the more successful you will be when trying new things.

Food for thought...<!-- / message --><!-- sig -->

[SteinarN]

However, in a lot of instances the confidence is only gained by investigating all of the little details such as mentioned by 750 Valve.

<!-- / message --><!-- sig --> The more time you spend on investigating and understanding the little details the more successful you will be when trying new things.

I generally agree with this, US Iceman.

But dependent on what the challenge or problem is, it is at least two different ways to investigate a potential problem.

In say a complicated system with a lot of different valves controlled by different operational parameters, you might have to consider or investigate a large numbers of possible mode of operation, what the pressure and temperature in different part of the system will be, as well as other possible unintended consequenses of those different mode of operations.

But in other instances i feel it is more helpful to minimize the problem. This is what I have done in this instance regarding the posibility of overheating of the fan motor at low Hz. I have concluded the posibility of overheating at low Hz simply is not present du to the fact of the low power supplied at low Hz, less than 10 W, probably less than 5 W each motor under 10 Hz. It cant possibly develop any overheating of the motor regardless of wheter it is turning or not with that low power supply. If my motor don't start turning as the Hz increases, then 750 Valve's motor wont start turning either as the Hz passes 7 and we will both have a problem. ;)

[Billy Ray]

SteinarN

what pressure transducer do you use to connect to the Danfoss VLT 2800 series VFD?

Is there any other items you need to configure the setup?

Is there much programming of the VFD?

Will a 'novice' of VLT's be able to do this?

Billy Ray

[SteinarN]

SteinarN

what pressure transducer do you use to connect to the Danfoss VLT 2800 series VFD?

Is there any other items you need to configure the setup?

Is there much programming of the VFD?

Will a 'novice' of VLT's be able to do this?

Billy Ray

I use 4-20mA pressure transducers from Danfoss. They are expensive, but any other brand will do the trick also.

Some configuring is necessary, but it's not that hard. It is maybe 15 different parameters that must be configured on a VLT 2800 series for condenser fan operation. All done from the operator panel on the VFD. I have software on my laptop which I use when setting up drives, especially VLT 5000 series for compressors, but it's not any problem doing it manually. If you are asking for some help here when you are ready you will get it configured without problems.

The only other "item" you need is a run signal. You run a wire from the VFD through a relay, which closes when the VFD is to be running the fans, and back to the VFD again. Thats about all.

One advice is to study the manual for the VFD. Each parameter is given a good explanation and there is several examples on different setups. The manual is downloadable from Danfoss website.

[Billy Ray]

SteinarN

would the AKS32R suffice?

Billy Ray

[SteinarN]

SteinarN

would the AKS32R suffice?

Billy Ray

No, the 32R is a radiometric transducer, that is it has power supplied, say 5V and the transducer put out a percentage of that voltage, say exactly 50% at 3 bar. That will be 2,5V at 3 bar. But the supply voltage can vary, if it is 5,5V then the voltage output at 3 bar will be 2,75V. The VFD has no possibility to read this signal in a meaningful way. The AKS32R is for the Adap Cool system only like the AKC 114A-116A regulators.
You need a 4-20mA AKS 33 transducer or alternatively an AKS 32 DC transducer. I advice the AKS 33 0-34 bar, 1/4 flare, number 060G2051 for condenser fan/HP operation.

[US Iceman]

But dependent on what the challenge or problem is, it is at least two different ways to investigate a potential problem.

In say a complicated system with a lot of different valves controlled by different operational parameters, you might have to consider or investigate a large numbers of possible mode of operation, what the pressure and temperature in different part of the system will be, as well as other possible unintended consequenses of those different mode of operations.


I fully agree with you. The best solutions I have found are those that are based on the simplest method that addresses all perceived operating conditions in a cost effective manner.

[750 Valve]

I guess we will have to agree to disagree here SteinarN, I just would rather not lose the ability for a warranty claim on a $1000 motor, especially if the manufacturer states it should not operate under such conditions.

It is also not all about low ambients and overheating a motor, there are also other brands of drives other than Danfoss, these quite often vary in their performance and design, many have their own intracacies, all of these are factors which one should consider before setting up any system outside of its recommended design parameters. Just because one indidvidual feels at home with one particular brand and its applications doesn't necessarily mean a novice to drives will be able to achieve the same results and system longevity.

I honestly believe that the original poster, given their lack of experience with drives should consult the motor manufacturer and heed their recommendations... after all if it all goes pear shaped then they will be the one giving him warranty.

I fully understand pushing the envelope and am guilty of it myself - i think most of us on this forum are or we wouldn't be so passionate about the industry we are all part of. After all nothing innovative ever came from sticking with the norm.

[SteinarN]

Just because one indidvidual feels at home with one particular brand and its applications doesn't necessarily mean a novice to drives will be able to achieve the same results and system longevity.

I can agree completely with this.
I haven't experience with other brands VFD than Danfoss, neither many different sizes and brands of fans. I understand there can be cases where different considerations comes into play and make it difficult to push the envelope to far. And in any case is the correct configuring of the VFD of great importance.

However, I will still install systems like this as I have very satisfactory experiences with this operating range here in northern Norway.

[SteinarN]

I just checked the original poster. He hasn't been on since he made his only post in this thread. Seems like we have been discussing in vain. :D

[750 Valve]

I just checked the original poster. He hasn't been on since he made his only post in this thread. Seems like we have been discussing in vain. :D

Oh well - his loss mate, at least we had a good old discussion about it:D

[US Iceman]

The good points are now recorded for posterity and the younger people coming into the industry. It is never a waste to have a good discussion.

Afterall, this is how we convince someone to change their mind.;)