VFD Controlling HVAC Blower

[jim3394]

I am trying to understand why amps increase with no load(restriction of air), but amps decrease with a load(restriction of air). The VFD is running at 30 HZ. I am assumint the torque of the motor is constant. The thought seems counter intuitive to me. I have read several threads and don not really understand-can someone dumb it down for me.

Thanks

jim

 

[weressl]

I am trying to understand why amps increase with no load(restriction of air), but amps decrease with a load(restriction of air). The VFD is running at 30 HZ. I am assumint the torque of the motor is constant. The thought seems counter intuitive to me. I have read several threads and don not really understand-can someone dumb it down for me.

Thanks

jim

Could you explain better what you are trying to say? Or ask? The airflow would have two parameters, one is volume and the other is pressure that develops if the flow is impeeded by some means like long ductwork, elevation and outlet dampers. When there is no flow due to complete blockage there would be minimum 'work' required. As the dampers opened, the flow increases, therefore the A will rise. I also presume that you are talking about a centrifugal blower.

You can google flow affinity laws.

 

[jim3394]

Could you explain better what you are trying to say? Or ask? The airflow would have two parameters, one is volume and the other is pressure that develops if the flow is impeeded by some means like long ductwork, elevation and outlet dampers. When there is no flow due to complete blockage there would be minimum 'work' required. As the dampers opened, the flow increases, therefore the A will rise. I also presume that you are talking about a centrifugal blower.

You can google flow affinity laws.

I suppose if I understood better, I could ask my question better. In my mind-and I believe I am wrong, If a blower is running at a constant hz through a vfd (30 hz) the rpm would be contant. I would think that under restriction of flow there would be a resistance against the blower causing an increase in current likewise no restriction of flow would cause amps to decrease. I will look up flow affinity laws and hopefully that will help.

Thank you

jim

 

[Besoeker]

I am trying to understand why amps increase with no load(restriction of air), but amps decrease with a load(restriction of air). The VFD is running at 30 HZ. I am assumint the torque of the motor is constant. The thought seems counter intuitive to me. I have read several threads and don not really understand-can someone dumb it down for me.

Thanks

jim

Restricting the air flow reduces the load.

 

[Jraef]

The concept that runs counter to your thinking is based on what constitutes a "Load" on a centrifugal machine, such as many blower or pump designs. The definition of this kind of machine dictates that the load couples with the motor as a percentage of flow, not force, pressure or weight. So if you decrease flow, you decrease load on the motor and if you increase flow, you increase the load. Closing off a damper decreases flow, so load, and consequently amps, will decrease.

 

[jim3394]

Thanks for the insight, that helps. looked up info on Centrifugal fan /blower-I am understanding more.

Thanks for your help,

Jim

 

[mcclary's electrical]

I do not agree. Restricting air flow by closing a damper is going to cause the motor to work harder. Opening the flow will cause the motor to work less. I've seen it on hundreds of blowers. I'm not guessing at this

 

[topgone]

I do not agree. Restricting air flow by closing a damper is going to cause the motor to work harder. Opening the flow will cause the motor to work less. I've seen it on hundreds of blowers. I'm not guessing at this

As jraef said. Fan work is not the same as positive displacement work.

                  CFM x psi
Fan HP =       -------------------
              229 x fan efficiency

Decreasing damper opening will decrease the CFM although the psi (discharge pressure) will up a bit. The greater CFM change will lessen power required to run the fan. Even if the dampers are fully-closed, the impeller is still free to rotate inside its fan casing, but fan is lightly-loaded

 

[StephenSDH]

So 0 CFM would equal 0 HP? :wink: That is counter intuitive. I just have a hard time believing dead heading a fan would decrease amps, but I guess it depends on the fans design. I might have to do some personal research.

 

[Jraef]

A motor never uses 0 HP when rotating, there is still power involved in moving any mass. But the LOAD on the motor, or maybe better described as the WORK being done by the blower, is based on the flow going through it.

By the way, a Roots type blower is not a centrifugal machine, it is a PD (Positive Displacement) machine, so it will operate exactly the opposite. Maybe that's what people are thinking of when they think about load going up with restriction.

 

[LarryFine]

A typical blower fan, such as a blade fan or a squirrel-cage, will just make the air between the blades go 'round and 'round with it if there's no escape pathway.

Try it some time, like if you have a box fan, lay it down while it's running. It should speed up, not slow down.

 

[Besoeker]

I do not agree. Restricting air flow by closing a damper is going to cause the motor to work harder. Opening the flow will cause the motor to work less. I've seen it on hundreds of blowers. I'm not guessing at this

Turn on a vacuum cleaner. Then put your hand over the inlet pipe. The motor maybe sounds as if it is working harder but what actually happens is that the motor speeds up because it is more lightly loaded.
It is not uncommon on large fans in industry to start up with closed dampers to make them easier to start.

 

[StephenSDH]

Thanks. Good to know.

 

[don_resqcapt19]

I do not agree. Restricting air flow by closing a damper is going to cause the motor to work harder. Opening the flow will cause the motor to work less. I've seen it on hundreds of blowers. I'm not guessing at this

Exacty what kind of blowers have you seen this on?

 

[Rick Christopherson]

I was wondering why this thread kept appearing for so long. I wish I had stopped by sooner. I thought it was just about VFDs on the motor, not the change in loading.

There is a difference between restricting the inlet to a blower and restricting the outlet to a blower. When you restrict the inlet, you create a vacuum, and the impeller is under less load. This is why blocking the hose on a vacuum cleaner causes the motor to increase in speed. There is less air for the impeller to try to move. Consider the difference between swinging your arm around in a circle while in air versus in a swimming pool.

When you block the outlet, then the load on the impeller increases because there will be a higher back pressure on the outlet side of the blower, even though no air is leaving the system. What some people are forgetting here is that air is still moving across the blades of the blower, even though the blockage prevents it from leaving the system. That blockage is relatively speaking, far isolated from the impeller. The impeller is still creating and maintaining the pressure differential from the inlet side to the outlet side. The impeller doesn't know that the ducting is blocked; it just knows that it has to work harder to move a molecule of air from the low pressure inlet side of its blades to the high pressure outlet side of its blades.

When people have said that no work was being done because the outlet was blocked, they were correct, but only from the system-wide standpoint and not from the blower's standpoint. If you applied the same concepts, but limited them to the area directly adjacent to the impeller blades, you would see that a large amount was work was being performed because the pressure differential was high.

P.S. If you block the exhaust of a vacuum cleaner, you will hear the motor slow down because it is under greater load. This is the reason why most shop vacs have bypass ports on the exhaust even when they have hose connections for blowing.

 

[Besoeker]

I There is a difference between restricting the inlet to a blower and restricting the outlet to a blower. When you restrict the inlet, you create a vacuum, and the impeller is under less load. This is why blocking the hose on a vacuum cleaner causes the motor to increase in speed. There is less air for the impeller to try to move..

There is less air to move whether you restrict the input or the output.

 

[mpoulton]

There is less air to move whether you restrict the input or the output.

Correct. The work done by a blower takes two forms. The first is work done to push air from a low pressure zone (inlet) to a higher pressure zone (outlet), which is essentially a force*distance calculation, and thus increases linearly with the product of pressure and flow rate. The second form of work done is to accelerate fluid from not moving (in the inlet zone) to moving (in the outlet duct). This work is unrelated to pressure, but proportional to the third power of flow rate (based on 1/2MV^2, since M and V are related by constant density and duct cross-section). Thus the total power done by the blower is proportional to (pressure*flow)+(flow^3). As can be seen, any action which reduces flow, even if it increases pressure proportionally, will decrease the load on the blower since the load is much more dependent on flow than pressure. Thus, restricting the output will decrease the load. This applies to all non-positive-displacement pumping devices, from centrifugal pumps to fans and blowers. The power and pressure vs flow curves in this document verify these relationships:
http://www1.eere.energy.gov/buildin...fs/furnaces_boilers/furnace_boiler_app7_5.pdf

 

[steve66]

So the power depends on the pressure and the flow. More flow means more power. More pressure means more power.

But when you block a fan, the pressure goes up and the flow goes down.

Sounds like the power could go either up or down, depending on a bunch of details like: how much leakage there is through the blockage, exactly where the blockage is, what points on the fan curve you started and end at, the exact fan and motor design, air pressure, etc, etc, etc.

 

[iwire]

When you block the outlet, then the load on the impeller increases because there will be a higher back pressure on the outlet side of the blower, even though no air is leaving the system.

If it was a positive displacement pump, for instance like your super charger then I would agree. But if we are talking about centrifugal blowers, fans and pumps I have to disagree.

If you block either side of a typical squire cage fan used in HVAC equipment you will see the motor load drop.

 

[Rick Christopherson]

There is less air to move whether you restrict the input or the output.

No. When you block the inlet, there is less makeup air available to be passed from the low to high pressure differential. You also reduce the overall pressure differential across the impeller.

The blades of an impeller are comparable to the wings of an airplane. An airplane will have significantly more lift at sea level than it will at 40,000 feet, which is why all airplanes have ceilings for which they cannot exceed. Above the plane's particular ceiling, the wings can no longer achieve enough upward lift (force) to overcome the plane's weight.

An impeller with a blocked inlet is similar, in that there is less air available to be moved across the blades, and therefore, less force required to move that small amount of air.

Unlike other types of compressors/pumps, such as a piston pump, a fan-type air-pump cannot have a pressure differential without air flow. We know the pressure differential exists because we can feel it with our hands. So even though there is no external air flow with either inlet or outlet blocked, there will always be internal air flow across the pump itself. The only time you will see a zero pressure differential across a fan-type air pump is when it is immersed in an absolute vacuum. The more molecules available to be moved across the differential, the higher the differential will be. This goes back to the airplane analogy.

Yes it is true that a blower will be working harder when it is pushing a large volume of air. It isn't solely the volume of air that is causing this work to be performed, but the pressure differential that the volume of air must cross.

The fan curves that were posted while I was still writing this response are somewhat misleading. First off, they imply that there is maximum airflow with zero pressure. That is referring to the backpressure of the system, not the pressure differential across the blower. This is comparable to the resistance of a wire, where the lower the resistance,the higher the current flow. These graphs are not taking into account the pressure differential across the blower itself, which is what this discussion is about.

The point is that this curve might lead some to believe that maximum air flow will occur with zero pressure differential. Air cannot flow if there is no pressure differential. Just as electricity cannot flow without a voltage differential.

The other thing you will notice about the curves is that they never go down to zero airflow. The power required by the blower is dependent on both flow and pressure-differential. When the air flow is relatively large, the change in pressure differential is very small, and therefore, the less significant of the two components. But when the volume is very low, the pressure-differential component becomes more significant, because it changes more rapidly.

Within normal operating range, the blower works harder as the volume increases (due to a reduction in the backbpressure of the lines). However, below the normal operating range, you will see a slight increase in power required because the pressure increase. This is why there is a difference between blocking the inlet versus the outlet of a blower.