When the motor is working harder, it needs to rotate faster.

[jeremy480]

Hi all, long time reader first time poster here.

I have a DCBL blower motor that has a fixed wattage at each speed level. I noticed that when I increase pressure (say from 0" SP to .1" SP) the RPM increase and the CFM (airflow) decrease. I understand that CFM will decrease with increased pressure but why do RPM increase even though the wattage remains the same? I would think that in order to increase RPM you would need to make the motor work harder by increasing the wattage but that does not appear to be the case.

Is there a physics law that explains this?

 

[rcgldr]

Some electrical motors have a range of operation where near maximum power output is nearly constant where the output torque decreases linearly with an increase in rpm. If this were a fixed pitch prop, then increasing rpm would increase both the pressure and the cfm and also require more power.

 

[jeremy480]

Some electrical motors have a range of operation where near maximum power output is nearly constant where the output torque decreases linearly with an increase in rpm.

It's not clear to me how this blower increases pressure while decreasing flow. If this were a fixed pitch prop, then increasing rpm would increase both the pressure and the cfm and also require more power.

Thanks for your response. I should probably elaborate a bit more on how this is being used. The blower is installed in an HVAC environment with metal ducting. Increasing the duct length, turns or shrinking the diameter increases static pressure. The blower itself isn't increasing pressure on its own.

The PCB controlling the BLDC motor is a closed loop system that runs the motor at a specific wattage on each speed. The RPM and CFM fluctuate depending on the amount of pressure applied but the wattage always remains the same.

For example; On maximum speed the blower runs at 200W, 2000 RPM and outputs 750 CFM at 0" pressure. When it is installed in an environment with increased pressure it still runs at 200W but the RPM increase to 2350 and the CFM drop to 700.

Is there a physics law explaining how/why this happens?

 

[rcgldr]

The blower is installed in an HVAC environment with metal ducting. Increasing the duct length, turns or shrinking the diameter increases static pressure.

I updated my previous post too late, so moving the updated response here:

These blowers are rated based on an external resitance in the ductworks that the blower feeds that determines the amount of flow and static pressure. Since it's a fixed pitch squirrel cage or fan, the blower has to spin faster if the resistance is higher, in order to increase the pressure.

What occurs in the immediate vicinty of the blades of a fan is a pressure jump with little change in air speed. In this case you have ambient pressure air accelerating and decreasing in pressure as it approaches the intake side of the fan. Then the speed remains about the same while the pressure jumps as the air flows through the fan. After the air passes through the fan, it has higher pressure and again it accelerates and decreases in pressure, but in this case the pressure only decreases to the static pressure of the ductwork the fan outputs to.

The higher the static pressure in the ductwork, the less the pressure decreases aft of the fan and the less the amount of acceleration of air, and the velocity of the air when it's pressure decreases to that in the duct work is less if the pressure within the ductwork is higher, assuming that the power involved remains the same.

The counter intuitive part of this is that the higher pressure lower flow involves higher rpm but lower torque requirements from the motor so that power requirements can remain the same. update - I think this may be a property of centrifugal blowers.

 

[jeremy480]

I updated my previous post too late, so moving the updated response here:

The higher the static pressure in the ductwork, the less the pressure decreases aft of the fan and the less the amount of acceleration of air, and the velocity of the air when it's pressure decreases to that in the duct work is less if the pressure within the ductwork is higher, assuming that the power involved remains the same.

So when the pressure increases causing the RPM to increase what sort of disadvantages am I looking at (besides the blower being louder)? Do you think I will experience any additional wear and tear on the motor because the rotor is spinning faster or not really since the same amount of power is being applied to it regardless of the pressure applied?

With this closed loop system PCB I have the option of keeping the wattage fixed or the RPM fixed on each speed. The problem with keeping RPM fixed is with increased static pressure the RPM remains the same but the CFM takes a bigger hit and my goal is to maximize CFM output. It sounds like keeping the wattage fixed is my best bet to maximize CFM output.

I was just curious as to why the RPM increased as pressure increased even though wattage remained the same. I understand your example with the hair dryer and I'll continue reading through your reply until it sinks in a little more. I need to try simplifying the explanation a bit before I start explaining it to others. :P Thanks!

 

[rcgldr]

I was just curious as to why the RPM increased as pressure increased even though wattage remained the same.

That may be a property of a centrifugal type blower, such as a squirrel cage, where rpms increase as flow rate is restricted. A ducted fan would probably slow down under the same circumstances.

 

[russ_watters]

I have a DCBL blower motor that has a fixed wattage at each speed level.

What is a "speed level"? An rpm? A motor will not always have the same wattage for the same rpm. Or are you saying that the motor has some sort of controller that monitors wattage and adjusts rpm to hold it constant? I'm not sure I've ever heard of that.

I noticed that when I increase pressure (say from 0" SP to .1" SP) the RPM increase and the CFM (airflow) decrease. I understand that CFM will decrease with increased pressure but why do RPM increase even though the wattage remains the same? I would think that in order to increase RPM you would need to make the motor work harder by increasing the wattage but that does not appear to be the case.

Is there a physics law that explains this?

Fan affinity laws provide the answer (and they are based on Bernoulli's principle and conservation of energy): http://www.engineeringtoolbox.com/fan-affinity-laws-d_196.html

Wattage is proportional to static pressure, but proportional to the CFM squared. So if you increase the resistance while holding the rpm constant, static pressure goes up, CFM goes down and wattage goes down.

 

[rcgldr]

DCBL blower motor that has a fixed wattage at each speed level.

What is a "speed level"? An rpm? A motor will not always have the same wattage for the same rpm. Or are you saying that the motor has some sort of controller that monitors wattage and adjusts rpm to hold it constant?

The dc brushless motor and it's controller are keeping the wattage constant, but I assume that this is in a fairly narrow range of rpm, near 1/2 of the maximum unloaded speed of the motor, or that the motor is being operated well below it's maximum possible power output. Normally maximum power output occurs at 1/2 the maximum rpm of a motor and quadratically falls off as rpms increase or decrease from 1/2 max rpm.

I noticed that when I increase pressure (say from 0" SP to .1" SP) the RPM increase and the CFM (airflow) decrease. I understand that CFM will decrease with increased pressure but why do RPM increase even though the wattage remains the same?

Wattage is proportional to static pressure, but proportional to the CFM squared. So if you increase the resistance while holding the rpm constant, static pressure goes up, CFM goes down and wattage goes down.

So if wattage is kept constant, then as static pressure increases, CFM decreases by square root.

Assuming the OP data for the blower is correct, then when consuming a constant amount of power, the squirrel cage centrifugal fan runs at a higher rpm and lower torque when operating against a higher pressure but slower flow in a ductwork and vice versa.

update - I found this graph that includes a bit more info and seems to agree with the OP's data. Each curve is the operational static pressure versus CFM for a specific rpm. The blue area of the graph is for the 3 hp version of the blower, the red area for the 5 hp version of the blower. I assume the downwards sloping right edge of the blue area corresponds to the 3 hp limit, and the right edge of the red area corresponds to the 5 hp limit, so these downwards sloping edges correspond to a constant power situation. The pattern of those edges indicate that higher pressures and lower flow correspond to higher rpm at a given amount of (input) power. For example, at 3 hp, 16,400 rpm operates at 50 mBar pressure at 330 CFM, and 21,700 rpm operates at 180 mBar pressure at 175 CFM.

http://rcgldr.net/misc/blower.jpg