Power Consumption on VFDs 3

[nightfox1925]

For calculating the power consumption on VFD motors, is this formula valid for use?

HPnew = HPrated [(RPMnew]^3 / (RPMold^3) ]

Since change in motor speed is proportional to motor HP then this may be applicable to get the approximate HP consumption. Does the Load factor be considered?

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[sreid]

Only if the torque requirement goes up by the square of the RPM (fan).

 

[nightfox1925]

Thanks sreid. Is there a situation were the Torque requirement does go up by the square of the RPM? I would appreciate much if can you elaborate further...I am not that familiar with this.

I remember that HP as a function of torque and speed is:

HP = (Torque x Speed)/ 5252

Therefore, for decreasing speed at the same torque requirement, the actual torque increases. Is this applicable to VFD drives?



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[sreid]

Fans and centrifugal pumps typically have a torque curve that is the square of the RPM.

As you correctly state the HP formula, HP is a function of RPM and Torque. For, say, a constant torque load, the HP would rise lineary with RPM.

 

[nightfox1925]

Sreid, is there a case of application were the motor torque does not increase as the square of the RPM? Just curious though.

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[gepman]

nightfox1925 in his second post appears to be describing a constant horsepower load where the torque increases while the speed decreases. This type of load may exist but I don't know of any. He states "for decreasing speed at the same torque requirement, the actual torque increases" which doesn't quite make sense because if it had the same torque requirement the torque wouldn't increase.

Usually with VFDs one talks of constant torque loads (such as positive displacement pumps, conveyors, etc.) where the torque requirements are constant and when the speed is varied the horsepower varies linearly as sreid states. Then there are the variable torque loads as stated again by sreid (usually centifugal pumps and fans) which vary with the square of the speed (because the pressure output of the centrifugal device varies with the square of the speed).

 

[sreid]

Constant HP requires some thought. An example might be; a variable speed conveyor belt and the motor load is caused by friction from, say, the weight of the sand on the belt. Now feed the conveyor from another conveyor running at constant speed with a constant sand load. The rate of the sand being transfered between belts is constant but the weight of the sand on the variable speed belt will decrease with the speed of the belt. So the motor torque will decrease with speed and the HP requirement will remain constant (which makes some sense since the rate of product delivery is constant).

 

[CJCPE]

The constant horsepower application examples that are usually cited are center driven winders and machine tools.

When a center driven winder is taking material from a process at constant tension and constant speed, the center speed of the empty spool is high and the torque is low. As the spool fills up, the center speed is reduced to keep the surface speed constant. The spool torque is material tension times the radius of the spool at the surface of the material.

Machine tools are often operated at a constant rate of material removal. A large diameter cutting head or piece on a lathe is operated at a slow speed and a small diameter cut is taken at a high speed. The force at the cutting edge is the same, so the torque is low for small diameter, high speed cuts and high for large diameter low speed cuts.

 

[nightfox1925]

Thank you for all the contributions. I appreciate that.

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[nightfox1925]

Sreid, does this mean that "Affinity Laws" will not be applicable to Constant Torque motor applications?

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[gepman]

Those are interesting examples of constant horsepower loads by sreid and CJPE.

The affinity laws definitely do NOT apply to constant torque applications. As sreid said previously the horsepower is proportional to speed in constant torque applications. I recently advised someone to replace an eddy current drive (remember those!) that was used in a constant torque application since at half speed it was wasting half the energy (the eddy current coupling was cooled with chilled water). Eddy current drives were really only useful prior to VFDs and only good on variable torque applications.

 

[nightfox1925]

Thank you very much technical folks!!!

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[nightfox1925]

By the way, for assessing the energy savings obtained from the VFDs, are we going to consider the mechanical HP or the Motor Rated HP?

HPnew = HPrated [(RPMnew]^3 / (RPMold^3) ]


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[gepman]

To assess the energy savings due to using VFDs you must use the actual mechanical horsepower (and then further include the motor efficiency and then the VFD efficiency). You can't save energy you don't use. The best calculator that I have seen for this is by ABB. You can download them (one for fans and one for pumps) at

http://www.abb.us/product/ap/seitp322/24b03100d005c31ac1256e040043f4c1.aspx

 

[nightfox1925]

Thank you gepman...this is very helpful.

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