Why use constant torque ECMs?

[JimtheHVACguy]

There are three types of ECM motors: constant cfm, constant rpm, and constant torque. I think it is obvious why constant cfm ECMs are used, besides being more energy efficient than psc motors, but I can't think of a great reason to use constant torque ECMs over these constant cfm ECMs. Constant torque ECMs are more energy efficient than psc motors, of course, but why use them (the constant torque ECMs) instead of the constant cfm ECMs? There must be a reason why the X13 was made, and being used by oems, instead of the 2.3 or 3.0 versions, which are constant cfm motors. What am I missing here?

[cjpwalker]

They're a lot cheaper. That little bit of extra efficiency over psc pushes several models and combinations into that next seer level without the premium price and extra controls required for variable speed.

[jbarron4]

If you think about it the constant torque is very similar to the constant CFM, except that the constant torque on the fan blower wheel will move the same MASS of air while the constant CFM moves the same volume of air which differs at different temperatures, Hotter air is less dense and it would take more volume to move the same mass of air. We are usually concerned rith the mass of air to transfer heat and the flow rate actually less so. Genteq has a discussion on the history from GE to Regal - Beloit to Genteq and the development of the different series of motors.
Which is better I couldn't be sure, but I would think that more constant mass would be perhaps better for control.
Either could be better for a particular application.
I have found that better control of the air has improved the air quality quite a bit and ofter less is more to give the "dwell" time an increase at lower power cost.
Also the power factor should be better since the electronically commutated motor (ECM) has lower inductive reactance and gives the benefits od the DC motor without the maintenance of commutating brushes to go bad while improving the power factor of the motor.
Nowadays both the air handler motor and the condenser motors are ECM leaving only the compressor as a squirrel cage induction motor and we wre headed towards that as well.
I have a question for the experts here and that is the protection of these ECM from electrical spiked form the powerline. We have lost several eventhough they are rated 4 -6 KV suoge protection. I have thought about adding additional surge protection for our smaller units here at the steel plant. These are 5 - 10 ton units. I don't think the 10 ton units have ECMs, but the 3-5 ton units of the office areas do. Any experience with these little units? Our larger unita are 480 3 phase and are not applicable. Of course the big units are all three phase 480 and if we did anything for them we would install VFD, but we have not identified the savings if any we could generater here in the Mobile AL. area.

[hvacrmedic]

If you think about it the constant torque is very similar to the constant CFM, except that the constant torque on the fan blower wheel will move the same MASS of air

Constant torque ECMs like the X13 are not constant mass blowers.

[jbarron4]

If you have a constant torque motor the only thing that affects the torque is the amount of torque that it takes to turn the wheel. The only thing that affects the amount of torque is (ignoring bearing losses) id the weight (mass) of the air being moved. The mass changes with tenperature (density). While I don't spend much time thinking about ECM and mass of air I do spend lots of time working (sometimes thinking) about larger VFDs for induced draft fans in large applications. Wneh you are takling about 6000+ horsepower and a temperature that changes form 80 degrees F to 1200 degrees F ther is a very noticable difference in the torque (and therefore horse power at a given RPM) required to move the air.
Constant torque by definition would mean constant mass of air or weight per unit volume.
The CFM would change with the temperature.
Constant RPM would vary required power to keep the speed constant with differing density of air.
Constant CFM requires the manufacturer to calibrate the motor controls to the equipment as is really required for these motors.

[hvacrmedic]

If you have a constant torque motor the only thing that affects the torque is the amount of torque that it takes to turn the wheel. The only thing that affects the amount of torque is (ignoring bearing losses) id the weight (mass) of the air being moved. The mass changes with tenperature (density). While I don't spend much time thinking about ECM and mass of air I do spend lots of time working (sometimes thinking) about larger VFDs for induced draft fans in large applications. Wneh you are takling about 6000+ horsepower and a temperature that changes form 80 degrees F to 1200 degrees F ther is a very noticable difference in the torque (and therefore horse power at a given RPM) required to move the air.
Constant torque by definition would mean constant mass of air or weight per unit volume.
The CFM would change with the temperature.
Constant RPM would vary required power to keep the speed constant with differing density of air.
Constant CFM requires the manufacturer to calibrate the motor controls to the equipment as is really required for these motors.

Ok, so a particular X13 motor is moving 660 CFM at a TESP of 0.6" and 924 CFM at a TESP of 0.1". Same speed tap and same unit. This is directly out of a blower performance table for a Carrier FX4D air handler. Are you sure that the air density decreases as TESP drops, because that's what your constant air mass flow theory requires. Not only that, but the drop in air density in this particular example would be quite dramatic don't you think? For reference

Mass flow = Density x CFM

[bhahvac]

The way the manufacturer intended. To get the seer bump without using a more expensive full blown 3.0 or higher think tank motor. So for an extra 100 bucks or so, you now have an furnace that, when rated with the AC, gives and extra .5 or 1.0 seer to the whole system.

Constant torgue helps a little with less than perfect duct systems .....does not fix them, just a LITTLE better airflow....

IAQ / Constant fan properties ... Low wattage constant fan operation (at 200 watts I think) for 24/7 airflow...uv lights. media filters.

Is it the "poor man's variable speed?" maybe. It does not dehumidify, and uses slightly more power than the true VS. But it's $100's less.

[jbarron4]

It would depend on how Carrier programmed the motor.Genteq delivers the basic motor to the OEM and they program the system to produce the fan curve response they want.
If the motor is truly constant torque it will do exactly what I said.
The fan is a constant volume macnine at a given static pressure and temp and the cfm varies directly with rpm
1. CFM varies as RPM
2. SP varies as (RPM)2
3. BHP varies as (RPM)3
4. HP = (torque X RPM) / 5252
5. torque = (HP * 5252) / RPM

A good discussion on the fan laws is found here:
http://www.nyb.com/Catalog/Letters/EL-02.pdf

If static pressure goes up then yes the density increases and vice versa.
If the temp goes down then the density increases.
If we consider that the static pressure were remain relatively constant then the power or torque to move colder air (denser) will increase.
Constant torque would mean that the CFM would decrease, but the mass of air moved would remain the same.
If the static pressure increased then the density of the air would also increase and while the CFM would decrease, but since the exhausted air will be denser the mass would also be the same.
Since we are talking constant torque to maintain that constant torque the RPM must increase if the density or static pressure decreases. If the RPM increases and the torque remains constant the horsepower will increase by the same amount and the CFM will increase due to that higher RPM, but the total mass of the air moved will remain the same.
If the constant torque motor actually provides constant torque or not I can't say, but the fan laws should still be correct.

[hvacrmedic]

It would depend on how Carrier programmed the motor.Genteq delivers the basic motor to the OEM and they program the system to produce the fan curve response they want.
If the motor is truly constant torque it will do exactly what I said.
The fan is a constant volume macnine at a given static pressure and temp and the cfm varies directly with rpm
1. CFM varies as RPM
2. SP varies as (RPM)2
3. BHP varies as (RPM)3
4. HP = (torque X RPM) / 5252
5. torque = (HP * 5252) / RPM

A good discussion on the fan laws is found here:
http://www.nyb.com/Catalog/Letters/EL-02.pdf

If static pressure goes up then yes the density increases and vice versa.
If the temp goes down then the density increases.
If we consider that the static pressure were remain relatively constant then the power or torque to move colder air (denser) will increase.
Constant torque would mean that the CFM would decrease, but the mass of air moved would remain the same.
If the static pressure increased then the density of the air would also increase and while the CFM would decrease, but since the exhausted air will be denser the mass would also be the same.
Since we are talking constant torque to maintain that constant torque the RPM must increase if the density or static pressure decreases. If the RPM increases and the torque remains constant the horsepower will increase by the same amount and the CFM will increase due to that higher RPM, but the total mass of the air moved will remain the same.
If the constant torque motor actually provides constant torque or not I can't say, but the fan laws should still be correct.

I'm familiar with the fan laws. FWIW the simplified version that you posted assumes constant air density. Maybe you should look into it a bit further.

Better yet, calculate the change in air density at the fan inlet for yourself with a drop in return air static of 0.5". You'll find that the air density decreases by about 2%. The CFM in my FX4D air handler example drops about 28%, so I'm estimating based upon the formula I posted earlier that the mass flow must drop by about 30% in this situation. In summary, the X13 blower in that air handler is very definitely not a constant mass flow blower.

The truth is that the X13 only provides one notable advantage over a PSC blower, and that's it's lower energy consumption. Will it provide more airflow on marginal duct systems? No. The PSC motor has speed taps just like the X13. If you have it on high already and still aren't getting proper air flow, then an X13 isn't going to help the situation. The X13 will also not provide proper airflow on that system because it can't take as high of a static pressure as the PSC, and a higher static is what you'll get if you try to get more airflow than you already have on that marginal duct system. It's a lie being spread in the industry, and X13's are failing daily because contractors are under the impression that with an X13 they won't have to modify marginal duct systems because the X13 can compensate for them. That's just BS.

[jbarron4]

Perhaps we are talking around the issue.
I am certain you are more familiar with the pure HVAC applications while I am generally dealing with very large fans where densities and power are much more of a problem.
In the case of the actual rplacement motors (The X13s are replacement for a particular manufacturers part number) The evergreen IM motors have two models for basic replacement/
One is for 1/5 to 1/2 hp and the other is 1/2 to 1 hp.
The settings select the max hp which with constant torque changes with rpm.
A DC motor (which these are Electronically commutated DC motors) exhibits constant torque with constant current to the motor. If the constant torque (or current) is held the rpm will change based on density, static pressure, or temp. When the motor speeds up while maintaining constant torque or current the voltage must increase and the actual power will also increase porportionally. HP=(torque X RPM)/5252. The only thing that can change the torque load at a given RPM is density as reflected from temp, static head, input suction etc.
If the static head increases the fan will slow down and the power will also decrease. By the same token if the air is denser (colder) the power will decrease and the RPM and power will also decrease and the CFM will aslo decrease tending to flow constant mass and not constant CFM.
Is this a good thing? I don't know, but usually what we want is to control the mass, but it is easier to measure the CFM and TESP. The heat transfer which we are usually most concerned with is really based on the mass of the air moved. Comfort and air exchanges may depend more on cfm, I don't know. I usually deal with industrial applications and not comfort control ieth the various system I work with.
The settings on the Evergreem IM set the max allowable hp for a particular application in replacement to keep the CFM within the original specification. I don't know if this motor is constant current, constant RPM , but probably not constant CFM since it lacks the calibration for a particular application that is made by the manufacutrer of the air handler to meet their needs.
I am seriously looking into the Evergreen IM for my own system and the plant offices not so much for power savings, but control of the airflow for dehumidification.
My thoughts are that proper selection of the output from the dehumification contacts of the Honeywell Prestige thermostat can do a better job than the PSC.
Humid air is also denser than dry air. While the RPM and CFM may change with the load (Static pressure should remain relatively constant assuming filter maintenance) perhaps this will prove to be the case.

Thanks and best regards,

J.D.B.

[mark beiser]

Perhaps we are talking around the issue.
I am certain you are more familiar with the pure HVAC applications while I am generally dealing with very large fans where densities and power are much more of a problem.
In the case of the actual rplacement motors (The X13s are replacement for a particular manufacturers part number) The evergreen IM motors have two models for basic replacement/
One is for 1/5 to 1/2 hp and the other is 1/2 to 1 hp.
The settings select the max hp which with constant torque changes with rpm.
A DC motor (which these are Electronically commutated DC motors) exhibits constant torque with constant current to the motor. If the constant torque (or current) is held the rpm will change based on density, static pressure, or temp. When the motor speeds up while maintaining constant torque or current the voltage must increase and the actual power will also increase porportionally. HP=(torque X RPM)/5252. The only thing that can change the torque load at a given RPM is density as reflected from temp, static head, input suction etc.
If the static head increases the fan will slow down and the power will also decrease. By the same token if the air is denser (colder) the power will decrease and the RPM and power will also decrease and the CFM will aslo decrease tending to flow constant mass and not constant CFM.
Is this a good thing? I don't know, but usually what we want is to control the mass, but it is easier to measure the CFM and TESP. The heat transfer which we are usually most concerned with is really based on the mass of the air moved. Comfort and air exchanges may depend more on cfm, I don't know. I usually deal with industrial applications and not comfort control ieth the various system I work with.
The settings on the Evergreem IM set the max allowable hp for a particular application in replacement to keep the CFM within the original specification. I don't know if this motor is constant current, constant RPM , but probably not constant CFM since it lacks the calibration for a particular application that is made by the manufacutrer of the air handler to meet their needs.
I am seriously looking into the Evergreen IM for my own system and the plant offices not so much for power savings, but control of the airflow for dehumidification.
My thoughts are that proper selection of the output from the dehumification contacts of the Honeywell Prestige thermostat can do a better job than the PSC.
Humid air is also denser than dry air. While the RPM and CFM may change with the load (Static pressure should remain relatively constant assuming filter maintenance) perhaps this will prove to be the case.

Thanks and best regards,

J.D.B.

I think you may be over analyzing things, and missing an important point.

None of the controls in residential equipment are able to measure static pressure or CFM.
The module for the variable speed "constant CFM" motors only see the RPM and current draw of the motor, so in reality is only able to control mass flow.
The equipment manufacturer tests the blower system in a particular piece of equipment and comes up with a table for the CFM values vs the RPM and current draw, and programs the module with it.
As long as the air density is close to what it was when the manufacturer developed the table, it will act like a constant CFM motor, but at lower air density, like you would find at higher altitudes, the delivered CFM will be higher.

The constant torque motors, like the X13 and Evergreen motors just maintain the rotational torque each "speed" tap is programed for, so the CFM and mass flow will vary with the air density and TESP on the blower.

[jbarron4]

Mark,

I believe you are precisely correct and that is what I think I was trying to convey.
If your motor was constant selected RPM the CFM at a particular speed would be constant, but mass would change with density.
Constant torque the mass would remain constant, but the CFM would change.
Since CFM is average velocity x cross section area the velocity would change since the area remains constant.

[mark beiser]

Just to give a clear answer to the OP's question.

Constant torque ECM motors, like the X13, are used because they use less energy, and have a flatter airflow vs static pressure performance curve than a PSC blower, but cost a lot less than a fully variable speed ECM motor.

[ksefan]

On the trane or am. std. variable speed system there is a switch between constant torque or CFM. It seems torque might be better for certain high static applications but draws more power.