PDA

View Full Version : VFD Question


porky
January 18th, 2007, 04:42 PM
Hi all

Can anyone tell me if it is possible to control the speed of a motor using its KW rating as a reference...maintaining its Kw rating by increasing its load, speed.

kittydog42
January 18th, 2007, 05:47 PM
I am not sure as to how well that it would work, but I don't see anything wrong with using kW as a process variable. You could implement this a couple of different ways.

You could set up the drive's internal PID control (if it has one) to use the drive's internal kW reading as a process variable. You would then enter a setpoint and see how it works. That is the easiest method since it does not require any additional parts.

You could also use an external kW sensor to transmit 0-10V or 4-20mA to the drive as a process variable for the drive's internal PID control. Again, enter a setpoint and observe. This is the next easiest method, assuming you can fit the kW sensor into the drive somehow.

If the drive is really basic, you can use an external kW sensor or the drive's internal kW reading transferred to an analog input in conjunction with an external PID control or PLC. Then give a 0-10V, 4-20mA, or fieldbus signal to control the drive speed.

bobwithdana
January 18th, 2007, 11:02 PM
Doesn't sound like a good idea to me. If there is a direct relationship between speed and current then it might work. Are you trying to get the highest speed possible at rated current or is there some other reasoning behind this?

porky
January 19th, 2007, 02:02 AM
The idea is to get the max out of a water cooling fan even when the when the cooling load drops...
I personally think its a dumb deal.... a waist of energy, and a poor control system..... placing a Temperature sensor and a PID loop I would be able to get the best out of the motor drive and my electricity bill..... but then again....who am I to decide these things lol... that is what my boss:reup: is paid for...all I can do is show him the water, but I cant make him drink it.:bonkhead:

504bloke
January 19th, 2007, 03:31 AM
The idea is to get the max out of a water cooling fan even when the when the cooling load drops...
I personally think its a dumb deal.... a waist of energy, and a poor control system..... placing a Temperature sensor and a PID loop I would be able to get the best out of the motor drive and my electricity bill..... but then again....who am I to decide these things lol... that is what my boss:reup: is paid for...all I can do is show him the water, but I cant make him drink it.:bonkhead:

When you show him the water and he's leaning over it, accidently give him a shove!

rsdoran
January 19th, 2007, 07:31 AM
I am confused on what you are trying to accomplish, if you want the max the fan can produce then set it to run at motor nameplate ratings. I am not sure about your application but the fan motors load will not change due to temperature, at least I do not think it will.

Another point, albeit I may be slightly off here, is that wattage or KW is the power consumed from current times voltage. If you are using 400v 50Hz then it will be 8v per hertz, current can be constant so when you run at 40Hz your supplying 320v which means your power consumption should decrease.

Just my thoughts but I would think you would use a VFD with a cooling fan to offer the ability to maintain the necessary air flow without having to run the system full speed all the time.

If working with fans and air movement much then you may want to check out http://cart.amca.org/publications/ These books are not free but there is a lot there that may help in designing control systems.

DickDV
January 19th, 2007, 08:12 AM
It seems to me that trying to regulate a motor to max KW would be equal to regulating it to max temperature. That is essentially what the overload calculation is doing.

If your drive has that capability, you could set up the calculated motor temp as a process feedback signal to the external PID loop. The setpoint would be the motor temperature limit.

With that scheme, the motor would run max speed until the motor temperature approached the limit, at which time the speed would start to pull back allowing the motor temp to settle at the setpoint.

I can see no useful purpose for controlling a motor this way but I suppose it can be done. And, of course, even at 61 years old, I'm still learning new things every day. Maybe this is today's lesson!

jraef
January 19th, 2007, 04:08 PM
I agree. In principal it can be done with the right drive, but in reality it may be more complicated than it appears on the surface, and I can't for the life of me think of what this would accomplish.

First off, forget trying to do it outside of the drive. You would need to calculate kW on the load side of the drive, and the PWM output messes with all but the most expensive of transducers, so by the time you are done with a design, it would have been far cheaper to buy a VFD with a kW output signal. So from here let's assume you have a VFD with that built-in capability.

No let's consider the why part of this. The idea is to get the max out of a water cooling fan even when the when the cooling load drops... So is this a cooling tower fan then? Putting VFDs on cooling tower fans is a well established practice that saves oodles of money, but nobody that I know of uses the motor kW as the process feedback. Everyone uses the return/sump water temperature. When you modulate the speed of the fan to maintain the water temperature, you are reducing the kW to only what is necessary, hence the savings.

But using the kW output of the motor as the feedback into itself? How is that going to work? What is going to tell the VFD that the kW can be lower in the first place? I think he is closing the loop a little too much here.

porky
January 19th, 2007, 05:02 PM
I totally agree with you jraef......personally and like I said I would control the fan speed with the temperature of the return water....but as I said earlier... My boss thinks that his idea is a good one....lol... I do not, and honestly... I too, have never heard of this type of control, hence I placed the question on this site to see your reactions... I am sure glad to see that Im not the only one who thinks that it is a stupid idea....lol

Thanks to all

kittydog42
January 20th, 2007, 05:04 AM
There are ways you could make something like this work. Temperature may be the best and most established way, but they are undoubtedly other ways out there. I built a pump booster controlled using amps as the process variable, through a PLC which were programmed to take the operating curve into account to determine how much boost to provide. It actually worked surprisingly well. It was just a prototype design that we decided not to go with since there was too much job specific commissioning required. Maybe you could do something like this with kW.

brucechase
January 20th, 2007, 10:25 AM
I hung around and waited until I saw some answers, but I never did see what I thought I should have. The question was


Can anyone tell me if it is possible to control the speed of a motor using its KW rating as a reference...maintaining its Kw rating by increasing its load, speed.


How can a VFD increase the load of the motor to maintain its KW rating (which is pretty much the same thing). The load is based on the amount of work the motor is doing.

The idea is to get the max out of a water cooling fan even when the when the cooling load drops...


If you change a motor in a system from 1 HP (.75KW) to 2 HP (1.5KW) there won't be some magical extra load to fully load that motor. I want to keep explaining what I'm talking about, but I will wait for everyone to chime in.

Lancie1
January 20th, 2007, 11:43 AM
The idea is to get the max out of a water cooling fan even when the when the cooling load drops...Literally, to get the "max" out of a cooling fan, you simply run it at FLA (Full Load Amps), regardless of the load. So set your current limit on a VFD to the motor full load amps, and run it at 100%, 110%, or whatever speed it takes to get the "max" current, speed, air flow, cooling effect, power, horsepower, and kilowatts. All of these will be maximum at full load current and speed.

On the other hand, if you mean how do you get the maximum performance (by lowering the cost of running the motor), then you lower the fan speed as the water cools down (load decreases), until the fan is maintaining the water at some set workable temperature.

On the third hand (for those of you from Alpha Centuri), if you are trying to maximize the performance of the Cooling System, you would have to do some heat transfer and energy calculations to determine if cooling the water below the normal setpoint would be of any economic benefit. Probably not, as it is difficult to cool below the ******t air temperature with a fan and cooling tower.

EDIT: I wonder why it blocks out a-m-b-i-e-n-t ?

kittydog42
January 20th, 2007, 03:25 PM
The amp draw will vary on any motor that has a load attached, depending on the load. That is how the principal of using amps (or possibly kW) as a process variable works.

Gnook
January 20th, 2007, 03:27 PM
I would recommend using a drive with a built in pump and fan control algorhythm and possible flux optimisation. This would give the best energy performance and process performance.

rsdoran
January 20th, 2007, 05:26 PM
Lancie1, there is a drug with similar spelling, take off the t. It is commonly related to spam type messages.

Maybe I just do not understand the question since so many are stating methods but personally I do not see how or why this could be done.

The applications was stated to be a water cooling fan, no mention of pumps etc, just moving air. Fan systems are an entity unto theirself with numerous mechanical and desigh considerations involved.

Being from the US we think of motors in HP (horsePOWER) whereas other parts of the world think in KW, there is a direct relationship and it can be calculated:

Power (kW) = Power (HP) x 0.7457
Power (HP) = Power (kW) x 1.341
You have a fan system designed and know that at X air temperature it can move X cfm of air and cool X amout of water. To move that X cfm of air you know you will need a motor of X Kw or HP.

Let's say it is a 4KW (approximately 5 HP) 3PH motor at 415v 50Hz which will pull approximately 6 amps. In this situation the load should always be constant as long as bearings, fan blade, and mechanical aspects are maintained. So you build a panel with starter (with OL) add start and stop buttons and connect the motor. You should always be running at rated kW.

I may be all wrong, waiting to see if I am, but VFD's vary frequency which in turn varies the voltage and the speed. I know it gets a little more involved. I thought one of the reasons for using a VFD (in situations like this) was to be able to lower you power usage in an effort to save money on energy.

It seems to me in this situation the water temp would be the important factor, it may depend on whether the water has to be maintained at a constant temp or lower for its usage. If the temp can vary but never be allowed to reach boiling then a VFD with temp sensor may allow it to run at different (slower) speed(s) the majority of the time.

If I am wrong please let me know where I went awry or what I missed to make me go awry.

DickDV
January 20th, 2007, 07:39 PM
Ron, your analysis would be exactly correct except that there are a couple of other variables. First, there is air temperature. Cold air pumps harder than warm air so constant speed on the fan will not always result in max hp or kw depending on the air temperature.

Second, there are changes in kw resulting from variations in backpressure in the air stream. This would not normally be a factor in a cooling tower but could be.

As I think about this, a possible third variable is the condition of the fan blades themselves. Icing, algae growth, or abrasive wear could all cause a variation in the speed/kw relationship.

That's why I don't think its as simple as setting the speed at the max kw point and just letting it wail away.

But, it still seems to me a useless control scheme. Almost sounds like a method to assure that the motor consumes maximum electricity regardless of process conditions! Does that make it seem ridiculous enough?

porky
January 20th, 2007, 07:57 PM
Here is the whole deal:-
Two cooling fans on a water cooling tower... in summer the return water temperature is about 36C hence there is more vapour.... thus due to the density being lower the fan motor does less work... The problem is that the cooling water tower has the ability to drop the water temp by 7C. At this rate the coolest the water will get is to 29C.. this is still warm, while the motor is not running at full load...
Please correct me if Im wrong... at least that is the way I think it works

jraef
January 21st, 2007, 12:43 AM
OK, what you are saying is that the motor is sized for maximum load, probably when the air is denser, i.e. colder. But when the air is warm / less dense, the motor is not being loaded to its full capacity. However, that time is exactly WHEN you want more work done by the cooling system!

Looking at it from the other side, you are thinking that if the motor kW is used as the process variable, you can determine when the air is less dense because the kW will drop. So you are thinking you can increase the fan speed to attempt to move more air, hence providing extra cooling capacity at the time when you need it the most, which just happens to correspond with the time when the lower air density allows for some extra head room in the motor's kW capacity.

Let me know if that's right before I go off half-cocked to think of the implications.

porky
January 21st, 2007, 05:02 AM
100 % correct

kittydog42
January 21st, 2007, 04:12 PM
Isn't there still going to be a maximum amount of air that the fan will be able to move, regardless of the speed? In other words, isn't it possible to be off of the operating curve, increase the kW, and then just be even further off of the operating curve? That is the case with pumps, which I usually work with. Once you are off of the operating curve, you are in 'no man's land' with regards to expected performance.

Lancie1
January 21st, 2007, 07:19 PM
Porky,
It seems easier to simply use the outside air temperature as an over-ride input, and adjust the operating curve, so that the fan runs faster in hot weather, than to use the KW as a parameter.

jraef
January 21st, 2007, 07:57 PM
Isn't there still going to be a maximum amount of air that the fan will be able to move, regardless of the speed? In other words, isn't it possible to be off of the operating curve, increase the kW, and then just be even further off of the operating curve? That is the case with pumps, which I usually work with. Once you are off of the operating curve, you are in 'no man's land' with regards to expected performance.

Porky,
What kittydog42 brings up is the main issue I was going to raise if this was your desire. What you can do however, is get a copy of the fan curve from the fan manufacturer. You may find that they left a little head room for you in the curve. if not, increasing the speed will not increase the airflow, the air simply starts slipping past the blades and in fact at some point your flow actually begins to decrease. If you get the curve and find that you can get more flow with increased speed, then you can set up your VFD to have a maximum speed limit that corresponds to the absolute top of the curve.

But here is the next problem down the line. Once you are at full speed and full voltage, there is no more HP available out of that motor. So as you increase the frequency without being able to increase the voltage, you begin to lose torque at a rapid rate. If your fan curve will move more air at the higher speed as mentioned above, you may end up not having enough HP left to do anything about it. While this of course is where we strarted because you mentioned that the kW load drops off with density, the dropoff of torque may be even faster.

Start with the fan curve first though, without that there is no point in going further with this.

porky
January 22nd, 2007, 02:09 PM
Thanks for all ur help & advise...

Tom Jenkins
January 22nd, 2007, 03:04 PM
I've designed systems to accomplish nearly the identical function.

At a constant speed fan power draw and air flow vary significantly with both discharge pressure and inlet temperature and pressure. I designed a system for an emergency gas scrubber where the inlet temp varied widely. To make sure that we were always moving the maximum air possible, thereby evacuating the gas as fast as possible, we wanted to get as much air moving through the fan as we could without overloading the motor, regardless of inet and discharge condition variations.

The answer is very simple - use motor amps instead of kW. Most drives have the inherrent ability to llimit motor speed if the current draw exceeds a pre-set value. Some call it speed clamping, some call it stall, some call it roll-back. Whatever the function is called, when motor current draw reaches or exceeds the set value the speed will be limited or decreased. No sensors or external logic or other devices required - just find the correct parameter in the drive configuration and set as required.