Direct Drive Fan Motor RPM Significantly Less Than Fan RPM

[BronYrAur]

I have a submittal for a vane axial direct-drive fan that has a fan speed of 1,654 RPM, but the motor speed is only 1,200 RPM. The vendor tells me this is "quite common" and that all you need to do it crank up the VFD past 60HZ until you reach the desired fan speed. Does this sound right? As I recall, torque takes a nosedive above 60Hz. I don't know how that translates back to the fan curve, but I am concerned with the motor selection.

Does anyone have experience with this?

 

[itsmoked]

You are correct a dive will occur. If you keep the V/Hz ratio the same you can speed up the motor while keeping the torque and actually have a higher HP motor. That would entail increasing the supply voltage to the drive. I'd have to think about it more. Likely you'd want a higher voltage drive and run it at a lower voltage as for the motor maybe you can increase the voltage successfully without insulation issues with a modern insulation system. Others here will know.

Alternatively you use a standard motor and a standard drive to run a 2 pole 3450 RPM motor down to your required speed. You'd need to do the math so that while running 'slow' the oversized motor and drive would still be providing the required HP e.g. the torque needed.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Artisi]

Keith: why would you use 2 pole and not 4 pole (1800 rpm), this only needs a slight twitch on the VFD to achieve the fan speed.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[itsmoked]

Yea gads! You're right Artisi. I slipped a cog thinking 1200 was 4 pole, it's 6 pole, so absolutely a 4 pole and yes, not much difference on the native motor's speed.

So BronYrAur it's a different motor (4 pole) and cranking down on the speed not up.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[EnergyProfessional]

first question when reviewing submittals, what was specified? If this was specified, you have no choice. if using a 4-pole (1750 rpm nominal motor) was specified, you should insist to get that. Motor also needs to match the hp-rating etc.

this over-speeding motors is brought up to me a lot, but i don't really like it. Most motors can run up to 90 Hz (150% speed) as long as you don't exceed hp-rating, so the fan needs to be matched. If a fan has 9 bhp at 1200 rpm, and you have a 10 hp motor all is fine, but if you run the same fan at 1600 rpm, it requires 20 hp and your 10 hp motor burns up.

Assuming your submittal is correct with motor power... consider this: you bearing only has a specific life at specific rpm and temperature. now you take this bearing designed for a 1,2000 rpm motor and run it at 1,645 rpm. It gets hotter, also has more friction. you may cut life in half or less easily. thsi still is acceptable to the motor and fan manufacturer since the motor will last through warranty....
when we started using VFD to operate motors below rated rpm (under 60 Hz), bearing failures almost disappeared. You judge what you think happen when you over-speed. Or imagine using an 10,000 motorcycle motor to pull a truck, and imagine how long it will last.

 

[QualityTime]

....has a fan speed of 1,654 RPM, but the motor speed is only 1,200 RPM. The vendor tells me this is "quite common" and that all you need to do it crank up the VFD past 60HZ.....

You are being sold a bill of goods. It is obvious that he has pulled this business of VFD overhertzing before on other inexperienced engineers and they believed what he said. You are asking for a lot of problems. Get him to supply the correct motor...1800 rpm.

They must have screwed up somewhere along the line. The funny thing is that a 1200 rpm motor is more expensive than a 1800 rpm motor. Something is not right. Must have been a left over from another job!

 

[willard3]

If it looks like poop and smells like poop, it's poop. Reject this and get what's specified.

 

[QualityTime]

...If it looks like poop and smells like poop, it's poop. Reject this and get what's specified...

Even if the motor rpm is not specified, hold firm and reject it. I have never heard of anybody trying to explain this away like this. This is a first for me. Maybe the vender is just plain stupid. Makes me wonder if he even has the right type of motor and if the vfd has the features that you should have. You need a vfd that prevents harmonics going into the motor (filters) and prevents harmonics backfeeding into the grid in the plant (line reactors). You need a motor rated for inverter duty


Merry Christmas!

 

[ChasBean1]

This gives you an 83 Hz motor. I’ve dug through this before and haven’t found a manufacturer adverse to the higher speed. However, my opinion, this should have been a nominal 4 pole motor with a 56 Hz speed rather than a 6 pole motor with an 83 Hz speed.

 

[QualityTime]

You know what is even better and simpler? Have a 1200 rpm motor and put a damper on the discharge of the fan and lock the set screw of the damper once the correct air flow is achieved. Just like they did it in the old days before VFD's came about. Having a VFD for a constant air flow situation is ludricous

 

[EnergyProfessional]

quality time:
his fan is designed to provide the required flow and pressure at 1,654 rpm, so the 1,200 rpm motor won't perform well. to use a 1,200 rpm (without over-speeding) he would need a larger fan etc.

And adding a damper instead of VFD wastes pressure and energy. And you don't take advantage of less wear due to reduced rpm.
It is like driving a car at full throttle and adjusting speed by hitting the brakes.

 

[QualityTime]

I am sorry it is an 1800 rpm motor that should be specified as I said in my previous posts.

Too many engineers specify the use of VFD's without knowing the side effects. They are not as efficient as you are led to beleive. They create harmonics issues upstream of the vfd and that causes problems. Furthermore these VFDs will probably last 10-15 years at most. Interim parts are virtually impossible to get. These are throw away VFDs. They are a hassle to program them because no one knows how is an issue. The maintenance staff are most likely not trained in programming or trouble shooting a VFD. Therefore there is an added cost for bringing in someone to replace the VFD and program it. Maintenance staff however know how to change out a motor or a fan belt. It is a no brainer. VFD controllers are inefficient and give off 3-5% heat. That fact is not usually put forth in the "energy savings" calculations. That alone is a strike against them. I beleive that if the designer worked out the numbers comparing the total energy used and the added maintenance costs versus just throttling of the damper, the latter is the way to go.

On one job I was asked to QA design concepts at a sewage treatment plant in Toronto for a plant retrofit. There must have been 40 fans/HVAC units in the project, 16 of which were in a process building.... All of the fans/HVAC units were on VFDs...LOL....even a roof exhauster...lol. The consultant used the same arguments that you used. At the end of the day the City's maintenance staff agreed with my assessment on getting rid of the VFD's for the exact reasons I listed above. Saner heads from the consultant finally agreed. The fact is the consultant never did any "energy" savings calculations to justify his position

I agree that VFDs have a purpose in certain applications. They however should not be used willy nilly. They can create more problems than they are worth. A lot of the hype around the use of VFDs were created by the VFD manufacturerers in an attempt to create sales. How do they create the hype? They always fall back on energy savings and we suck it up lock, stock and barrel. If you look closely at how they do the calculations they are not comprehensive.

 

[Compositepro]

"It is like driving a car at full throttle and adjusting speed by hitting the brakes."
This analogy is a completely misleading and wrong in many ways. I don't have much time to detail it all now, but perhaps others will chime in.

It comes from thinking of a fan/blower as a positive displacement compressor, which they are not. Axial and centrifugal fans have significantly different performance characteristics. Axial fans do use more power when when throttled, but not by much. Centrifugal fans use less power when throttled. In this case we are talking about an axial fan but centrifugals are more common as air movers in HVAC. Centrifugal blowers will consume very little power when the damper is completely closed.

A car's throttle controls engine power not speed, directly. An electric induction motor tries to run at a fixed speed and will run at maximum speed when there is no load on it. Motor power is controlled by the mechanical load on the motor. Your analogy implies that an electric motor is always running at full power and the only way to make it use less is to slow it down with a VFD.

 

[QualityTime]

It is so interesting to see how brainwashed people have become when it comes to VFDs

 

[EnergyProfessional]

A balancing damper is a throttle and always will waste energy. If that is more than the VFD losses depends on the situation.
Running motor at lower rpm also saves bearing life (less friction, less heat). And for any start you have a soft-start instead of a hard-start.
I don't know about this specific applications, but many applications benefit from variable volume operation. Once you can apply variable-volume operation, the energy savings are tremendous depending on the application. check fan and pump laws and imagine less reheat etc.

Once you have a VFD, you can adjust speed at will. Even if you have a constant volume application, you can make up for filter dirt etc.
And maybe the design engineer was conservative in sizing the fan and in real installed system the required airflow could be achieved with 1,400 rpm? then your balancing damper would waste even more.
Harmonics are not really an issue with appropriate design and filters if needed. it all comes down to quality, if you use really cheap VFD, yes you can have problems.

Programming them also should not be a problem, if the controls contractor doesn't know how to program them i would be nervous about all the other controls....
If you have a constant volume application you just set a fixed frequency anyway. And you save the cost of sensors for motor failure, fan status etc. you also can read out current, voltage, kW.... if you add all those sensors for building automation system you don't end up saving much.

If you think VFD are wasteful, go to any modern HVAC system and just put them all in bypass and see how energy consumption (and noise, and other nuisances) go up.

 

[QualityTime]

Programming after the fact is a problem. Nobody knows what the fan speed should be after the consultant finishes the job. And nobody is going to play around with the fan speed if you have a dirty filter...LOL. Don't expect the maintenance staff to know. How about just changing out the dirty filter? If you think programming contractors know HVAC you are in for a rude suprise. If you think big name building automation manufacturers/contractors know...you are in for a big suprise. There are many BAS contractors out there that program things incorrectly, intentionally or not, just to get a service contract. The money is not is the capital cost of the equipment. The money is in the after market service contract

Prediction of what the rpm should be is very easy. Your example of going to 1654 rpm to 1400 rpm "as an added benefit for using a VFD just in case...the engineer screwed up" is misleading. If this occurs it is just a plain blunder.

The HVAC industry, both consulting, equipment manufacturers, BAS contractors, is very competitive. The equipment is generally of poor quality and there are generally not enough fees for proper engineering and inspection.

Getting back to the application. It appears to be a constant volume application. The need to use a VFD is not required. Use a pulley or gear reducer and let the damper do the throttling. The increase in static pressure can be determined. There is nothing simpler than to change a motor, belt or gear reducer. A VFD is another piece of equipment where another layer of complexity has been added and another trade has to be brought in to trouble shoot problems

The morrow of the story is to do the proper calculations to determine what the power savings actually are. Then determine if a VFD is required. Don't use VFDs willy nilly. A lot of engineers think they know about harmonics. If you scratch the surface they know very little

Putting a VFD in bypass mode to prove your "energy savings" point won't prove a thing because the true way to compare it properly is to have a discharge damper throttled to obtain the same air flow as when the VFD is controlling the fan. That way you are comparing apples with apples. If the VFD running at 100% speed is still giving off heat (which I think it still does) that has to be taken into account in the calculations

Anyhow I do not want to talk about this subject any further

 

[EnergyProfessional]

I think this is direct-drive, which is why the rep suggested over-speeding. This wouldn't be necessary with a belt/pulley setup.

Going from direct-drive to belt drive is not a way to save maintenance cost.

You don't need to play around with fan speed when filters get dirty, the flow station controls the fan. Obviously yes, at some point they should be changed. But in a critical application where accurate airflow is important, you can't change filters all the time. We have a constant volume gun range where the filters get replaced by a Hazmat contractor due to lead, so one needs to adjust the fan speed in between filter changes. If you don't have a VFD, you would lose airflow over time.

 

[QualityTime]

One issue with VFD direct drives is that the footprint of the HVAC unit changes. There are redesign costs and space issues when trying to go to a VFD driven retrofit in existing belt and pulley installations. I believe the standard of measure to help justify things is if there is a payback in 4-5 years. If you are comfortable with the use of VFDs...great. Happy New Year