Can a VFD do magic with motors

[rogerhollingsworth]

VFD

Hi, can't help you there but just thought I would throw this in as an experience I have had.

VFD as with PLC's are often misused and over applied. One case I remember is in working for a food processing plant I was to document the number of drives used, their brand and all the specifics. To make a long story short after all the data was compiled I realized that 90% of the drives were operating at 60hz.

Go figure.

 

[Goody]

The meeting with all parties is scheduled for for either Thursday or Friday this week. I shall write up the results of this meeting here when it has taken place.

But boy am I glad that I got some information from here, that I did not know before hand.

 

[ian smith]

Goody
Theres a very good book about motors and drive systems in RS

Electric Motors And Drives by Austin Hughes
RS No. 912-076 £17

Very descriptive Not to much math covers alot of ground.
Possibly a little long in the tooth now (1993) But it's a classic text for colleges in the UK. There May be a later edition I'm not sure.

ian

 

[Goody]

The meeting finally took place today with all parties concerned.

This included me, the bosses of the company, the ABB specialist and the third party (who are motor rewinders)

The first thing that came out was that the ABB man said he had done his calculations on a 10% speed reduction. (thanks Dick for your correct input) This was the first time that this had been mentioned and was not documented anywhere.

The third party still claimed that savings would be made running at full speed but this was then discounted by the ABB man.

The third party and ABB came together as a package. Together they sell the energy saving devices.

The ABB man did not think that a 10% reduction in speed would stop the machine functioning properly.
He said that fans are usually over specc’ed and 10% usually made no difference. He has done similar projects elsewhere and massive savings have been made.

We then had a demonstration on the machine running at 90% speed and lo and behold the fabric came out wet.
Case proven M’lord, thank you very much.

However, there is a now a new suggested solution.
The four motors were 6 pole, 970 rpm, 11KW.
It has been suggested that we now install 4 pole, 1400 rpm, 11KW motors and reduce the speed via the VFD to the original speed of 970 rpm.
This will be a saving of more than 20%.

Is this a viable solution?

If this will work, then they have come out of this smelling of roses and have made another substantial sale.

What do we think?

 

[Steve Bailey]

Under the proposed new solution, you will have the same fan running at the same speed, pushing air through the same ductwork, therefore the same energy requirement as before. To achieve a 20 percent reduction in electrical energy usage, the motor/drive combination would have to be 20 percent more efficient in converting electrical energy to mechanical energy (unless the law of conservation of energy has been repealed).

If the key to achieving these energy savings is the new motors, then the client should be able to achieve even greater savings by converting to the 1400 RPM motors, installing a mechanical speed reduction between the motor and the fan, and eliminating the VFD. The efficiency of a timing belt drive between the motor and the fan is greater than the efficiency of the VFD.

 

[rsdoran]

I was thinking along the same lines as Steve when I read this earlier. I didnt have time to respond and actually I am glad I didnt.

Let me see if I can guess/understand this solution. EITHER motor (running at the same 3ph voltage) at rated rpm will pull the same rated current. I wasnt sure about your voltage so I used 415. 11000(KWx1000) divided by 415 (voltage)times pf (power factor: I didnt know so used 1) equals approximately 15 amps. Anyway if you take the 4 pole 11kw motor and run it at 70% (35hz) then it should run at approximately 970 rpm ...correct? Are the motors running at this frequency/current going to be able to handle the torque requirements?

By math this seems a viable solution if I havent missed something, which I am known to do.

What bothers me is that the cost factors werent given beforehand, no testing was done to see if the fans could operate at slower speeds, alot of this seems to be padding the lily a little.

I think if you look at the existing system, with the new inverter that you have installed there may be a possibility of changing some pulleys/belt sheaves (if used) to change the ratio to increase the fan speed at 50hz (I think thats what you use) then lower the inverter speed to accomodate. OF course load factors may have an effect...ie can the motor drive the fan faster and provide needed torque.

 

[Tom Jenkins]

Run, Goody, RUN! These guys don't have a clue, and are going to cause problems for everyone that comes in contact with this project!

Steve's analysis is exactly correct, and the first LAW of Thermodynamics rules: Energy can neither be created or destroyed (except for nuclear reactions - please let us know if these guys are selling THOSE!)

Their confusion, and yours, I think Ron, comes from the common confusion between motor size and motor load. Think of it this way - if I put a 500 hp motor on the vent fan in my bathroom, will it draw 500 hp? Certainly not! The power required by that fan is dictated by the energy to accellerate and compress a certain flow rate of air and by the eficiency of the fan. This is the power output of the motor, regardless of size. If the motor is too small it will overheat or fail to operate. Any motor capable of supplying the power draw of the fan will only output that power. The electrical power into the motor will be no more or less than the fan power divided by the efficiency of the motor at that particular load.

The VFD supplier's arithmetic is right, but the physics is wrong!

 

[rsdoran]

Wait a sec please, I am kind of confused but inverters were designed not just provide a means of speed control but a way to offer more efficiency at the same level of performance.

I see what you are saying but unless there is a torque problem (which I dont know enough to say one way or another) why couldnt a 4 pole 11kw 1400 rpm replace the existing motor and run at a lower speed? IF the 11kw 1400 rpm motor can produce the required torque at 970 rpm? That should provide an actually lower energy consumption ( not the 11kw that was used before or the 11kw that the new motor is rated at).

I dont see any reason for Goody to run, he is getting paid to do what they want/say...a win win pay situation. Install it with no headaches over whether it works or not because a company provided the specs and support.

I do NOT like the fact that they talked them into buying the inverters without testing this situation or at least doing a test run at a lower speed by using sheaves or similar.

Someone's padding their pocket on this deal...maybe it will be Goody when its all over.

 

[DickDV]

Tom Jenkins has it right, Goody. Run for your life and, while you're at it, hold your wallet too. The logic here is just berserk!

The six pole motor (sync speed 1000, loaded speed 950 or so) has available 11Kw of power if the fan needs it. If it doesn't need all 11Kw, it will efficiently run at part load if across the line or, if on an inverter, then you can program the drive to starve the motor for voltage until the available motor KW just matches the needed fan KW. At that point you will be putting into the system only what the fan needs plus a little for motor losses. Notice, I didn't say to slow down the motor, just reduce it's excitation to the point of maximum savings. Sadly, this is likely to be miniscule given the data that you posted earlier.

Now, if we install a four pole 11Kw motor (sync speed 1500, loaded speed 1450 or so) we will need to slow it down to 950 rpm to match the original fan speed. As a motor slows down, the available Kw also goes down in direct proportion to the reduction in speed. Let's see, that would be 11KW x 950/1450 = 7.21Kw. If the fan needs more than that to run at its full speed, this new motor won't have the muscle to run it and will stall on overcurrent somewhere slower in the accel ramp.

If the fan KW at full speed is less than 7.21, then you would be able to dial down the voltage a long way on the six pole motor--too far, in fact. You would have a more efficient system by removing the drives and substituting a smaller six pole motor more closely matched to the needed Kw. Even that wouldn't save enough to pay for the new motors for a long time.

The bottom line here is that the fan requires a fixed amount of power to run at full speed and modern induction motors are remarkably good at adjusting downward their output efficiently to match the underload. An inverter isn't going to help this if the fan remains turning at the same speed and therefore the same required Kw. And the four pole motor option likely will not work at all.

It's definitely time to call in a recognized expert in these matters to explain to everyone the facts of life here. It sounds to me like the ABB people are on the virge of eating some of their drives.

Glad I'm not them! Tasteless rascals, those inverters! And chewy, too! The heat sinks are bad enough but that modern plastic!!! Yuk!!!

 

[DickDV]

Goody, I just went back and reread this whole thread again. No where do you indicate how heavily loaded the existing motors are. The actual running amps for a motor at proper speed would give us some hint of how nearly the motors are matched to the fan load.

A second thought here, and a chance for you to be the hero. If the fans can tolerate a little overspeed (that's a big IF and you need to be sure before you proceed), the inverters may very well be able to pay for themselves by overspeeding the fans, increasing air flow to the process, and provide for increases in production. The payback can be very attractive if this can be done.

Can't hurt to ask the questions, don't you think? Sometimes only 5 or 10% can make a big difference.

 

[rogerhollingsworth]

VFD???

I am no VFD expert by any means but I don't see how any VFD is going to give you any savings. The VFD is not going to do any magic, it is only going to give an quick/easy way of adjustment to match the motors most efficient operation to the desired load. Tom hit it on the head, after all the dust clears and all the smoke is gone you will find energy can't be created or distroyed. Unless that ABB VFD can create energy you are stuck with the laws of physics and the best you are going to do is pick the right size motor to deliver the required output. Go just a little larger and tune down. Speed reducers and pully/belts can do it, it just takes longer and is less flexable.

That is my 2 cents.
Good luck.
P.S.
If these are very large motors you may want to look into some capacitors. VARs / Power Factor

 

[Goody]

Thanks once again for the replies, its now clear as mud again.
I am sure you know it is not a money thing for me. I am trying to do my job for the customer to the best of my ability.

I shall act on some of these suggestions and I will give more details of loadings etc for DickDV (and the rest) to digest.

I dont know how this is going to end, its a good thriller isnt it.

 

[rogerhollingsworth]

Check this out

Vector drives are the latest technology available to run induction motors, and can dramatically improve system performance. Vector drives provide optimal motor performance over the widest speed range, and have three major advantages over older “variable frequency” drives.
1) rated torque of the motor is available down to zero speed,
2) speed control is very accurate,
3) there is no possibility of “stalling” the motor and not knowing it happened.

Vector drives achieve the above performance by precisely controlling the current running through each of the three “phases” of the motor wiring. This current generates a magnetic field which has a vector direction and magnitude that are modulated precisely to generate the required torque in the motor. Since the vector is controlled according to the rotation of the motor, a device like an encoder is required to determine the rotation. The encoder lets the drive “know” exactly what speed the motor is running, and thus can provide very precise speed control down to zero speed. This same function also allows the vector drive to avoid the problem of a stalled motor by always detecting and responding to the exact motor speed. Haas Automation uses vector drives in almost all of their machines. In addition, because the vector drive is closely matched to the motor, the Haas drive can provide 10 minutes of 150% motor load and 3 minutes of 200%.

Worth looking into.

 

[Steve Bailey]

Roger,

Your points regarding vector drives vs volts/hertz drives are well taken, but:

Goody's client has purchased 30 drives on the basis of claims of reduced electricity consumption made by the drives vendor. It now appears that the projected energy savings were based on the false assumption that the speed of the fans could be reduced. There does not appear to be a need for anything other than on/off control of these motors, at least not for the first 4. The other 26 may be on different applications.

Faced with the fact that the motor needs to rotate at approximately 1000 RPM, the drives vendor is now promoting new motors as a means to achieve the energy savings.

It seems to me that the vendor is saying, "I sold you 30 drives that can't deliver what I promised, but if you buy 30 motors from me, everything will be all right". Tom is correct in his warning not to trust this vendor.

When the new motors don't deliver the energy savings, is the vendor going to propose new transformers? Would you buy a used car from this salesman?

 

[Tom Jenkins]

When it hits the fan ...........

Ron you said: "I dont see any reason for Goody to run, he is getting paid to do what they want/say...a win win pay situation."

Now, on a logical basis you are, of course, correct. However, in this old boy's experience when the s**t hits the fan, there is enough to cover the guilty parties with a lot left over for for all innocent bystanders in the neighborhood. The effectiveness of Teflon coating derived from innocence is an inverse square law relative to the proximity to the source of power (i.e. money). The bosses of the company are in a position to make sure they aren't to blame. "Those darned ABB guys lied to us, that Goody feller overcharged for installation, the dang things don't work, and we are poor innocent victims." Meantime, everybody else remotely connected with the system is going to walk away brown and smelly, whether they deserve it or not!

Goody's best bet is to tell the bosses "This isn't going to work unless some significant engineering, by guys that know what they are doing, can show energy savings without performance decrease. I want no part of this project, since it is doomed to failure and I am an honest man." The bosses may appreciate it later, or they may not, but Goody will cut his losses either way.

Goody, to claculate the power for a fan (or blower) you can use the formulas on this page: http://home.earthlink.net/~escor/technical/aeration.html

For some motor power consumption formulae, see: http://home.earthlink.net/~escor/technical/power.html Note that centrifugal fans and blowers follow the "pump" and fan laws. ALL OTHER THINGS BEING EQUAL, flow drops as a direct ratio with speed, pressure drops as a ratio of the square of the speed, and power drops as ratio of the cube of the speed.

These guys have to show an analysis based on formulas and assumptions like these, and verify their calculations against the process requirements.