# Freq drive= Energy Savings?



## Bob Badger (Apr 19, 2009)

In your friends application I would expect to see a savings with the installation of a VFD.




> When I was learning about VFD's in class we had a motor to run a conveyor belt. When the motor was at full speed we metered it at around 5 amps, when it was ramped down to almost a crawl we metered it at the exact same current draw... 5 amps.


Not the same, had you had the motor set for full speed and tried to slow it down with a brake you would see the power used skyrocket.

Essentially that is what is going on with the pump, it is running full speed but the valve is working much like a brake.


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## sparky970 (Mar 19, 2008)

Bob Badger said:


> In your friends application I would expect to see a savings with the installation of a VFD.
> 
> 
> 
> ...


If that's true, then why does the motor current increase the more the valve is opened?


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## Toronto Sparky (Apr 12, 2009)

sparky970 said:


> If that's true, then why does the motor current increase the more the valve is opened?


Have you proved this? Makes no sense.. If the flow is restricted the motor would be under more stress.. unless the valve is on the intake of the pump.


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## sparky970 (Mar 19, 2008)

Toronto Sparky said:


> Have you proved this? Makes no sense.. If the flow is restricted the motor would be under more stress.. unless the valve is on the intake of the pump.


Try for yourself. As the flow rate increases, the motor is working more, thus the increased current. Deadheading a pump is not the same as locked rotor current.


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## professor poptart (Mar 2, 2010)

If it is a chilled water system, you are talking about a closed loop. So with the valve, basically what you are doing is reducing the load on the pump. You can accomplish the same thing with a VFD, but there sre some things to consider. Heating for one. When you slow a motor with a VFD, you are also slowing the motors cooling fan. (unless it has a seperatly powered fan). 
Harmonic distortion and power factor can also be issues but with a single motor installation this probably isn't big problem. 

A better installation could be to install a flow control valve and a pressure transmitter to keep the system at the pressure of you choosing via the valve. 

Motor speed and cost of two systems need to be looked at. If the motor will be operating at anything above 60 or 70 percert of rated speed, its probably just a cost issue. 

I am not an engineer, and you really should have one look into this. I've seen situations where owners wanted to reduce the speed of a motor for one reason or another and ended up with motor problems due to excessive heating.


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## micromind (Aug 11, 2007)

The HP needed to drive a centrifugal pump is directly related to the flow through it. Nothing else. 

If the flow is reduced by increasing the pressure across it, it'll take less HP. 

The opposite is true of any positive displacement pump, like a hydraulic pump. This type of pump will maintain a flow regardless of pressure, and HP is related to pressure. 

The cost savings of using a VFD depends on a lot of factors;

1) If the motor is loaded to less than rated HP, and the utility bill includes a charge for VAR usage, then a VFD will save a lot.

2) There are heat losses in the VFD. Always more power in than power out, but this is a small loss.

3) A VFD will distort the waveform of the power system, (less at the source, more further downstream), and thus cause transformers and other motors running across the lines to be SLIGHTLY less efficient. this is so small that it really doesn't count though.

4) Initial cost recovery. Sometimes, it just isn't worth it.

5) Reliability. VFDs are pretty reliable these days, but not as reliable as an across-the-lines starter. One must also look at downtime, both for initial installation as well as possible failure. If it burns up, how long will it be until it can be fixed or replaced.

6) Type of motor. If it's not wound with spike resistant wire, a VFD will reduce the life of the motor. This is especially true if the motor is some distance from the VFD. 

7) Phase balance of the system. If the phase imbalance is more than about 2%, a motor running across the lines will lose efficiency. The phase balance on the load side of a VFD is VERY closely balanced. The balance on the input side is much less of an issue for a VFD as for a motor. 

Obviously, the worst person to listen to would be the VFD salesman. They can come up with all kinds of fancy numbers, very few of which represent the real world. 

Rob


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## miller_elex (Jan 25, 2008)

I tried to upload it.... but the forum will not accept an XLS spreadsheet.

Do a google on VFD Quick Savings Estimator .XLS

It should come up on the Honeywell site.


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## John Valdes (May 17, 2007)

Toronto Sparky said:


> Have you proved this? Makes no sense.. If the flow is restricted the motor would be under more stress.. unless the valve is on the intake of the pump.


No so. The motor will work less if it moves less water. This is the same for intake and output.



Bob Badger said:


> In your friends application I would expect to see a savings with the installation of a VFD.
> Not the same, had you had the motor set for full speed and tried to slow it down with a brake you would see the power used skyrocket.
> Essentially that is what is going on with the pump, it is running full speed but the valve is working much like a brake.


No. The valve is reducing flow, thus reducing current, and pumps are not "full torque loads" they are "variable torque loads" It has nothing to do with braking, only loading.



professor poptart said:


> If it is a chilled water system, you are talking about a closed loop. So with the valve, basically what you are doing is reducing the load on the pump. You can accomplish the same thing with a VFD, but there sre some things to consider. Heating for one. When you slow a motor with a VFD, you are also slowing the motors cooling fan. (unless it has a seperatly powered fan).
> Harmonic distortion and power factor can also be issues but with a single motor installation this probably isn't big problem.
> 
> A better installation could be to install a flow control valve and a pressure transmitter to keep the system at the pressure of you choosing via the valve.
> ...


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## Bob Badger (Apr 19, 2009)

sparky970 said:


> Try for yourself. As the flow rate increases, the motor is working more, thus the increased current. Deadheading a pump is not the same as locked rotor current.





micromind said:


> The HP needed to drive a centrifugal pump is directly related to the flow through it. Nothing else.
> 
> If the flow is reduced by increasing the pressure across it, it'll take less HP.





John Valdes said:


> No so. The motor will work less if it moves less water. This is the same for intake and output.



Yeah I had a brain fart :icon_redface::icon_redface: I agree with all of you.

I have argued the same thing with fans. (Les flow = Less current)




> The opposite is true of any positive displacement pump, like a hydraulic pump. This type of pump will maintain a flow regardless of pressure, and HP is related to pressure.


Agreed.


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## garfield (Jul 30, 2009)

*motor sizing question*

So......I may very well be putting my ignorance on display here. Can you put an amprobe on a motor and determine that the motor is oversized if it's using less than it's rated current.....replace it with a smaller motor based on this?


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## miller_elex (Jan 25, 2008)

I think you can calculate something based on the Slip (from synchronous speed.) Break out your tachometer, and I hope she's cal'd.


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## micromind (Aug 11, 2007)

The HP a motor is producing is somewhat related to its amp draw, but it's not a linear curve. Power factor, among other things, will throw it off.

The best way to determine the actual HP being produced is watts X efficiency. Since efficiency changes with load, the figure used must match the present load.

Rob

P.S. Slip would be accurate, but you'd need to factor in voltage and phase balance.


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## garfield (Jul 30, 2009)

Is this formula applicable to a motor within a certain % range of load. I am puzzled because a 100 hp motor with no load would consume an amount of power that wouldn't have to do with the horsepower being produced.


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## micromind (Aug 11, 2007)

Power consumption always goes somewhere. Energy cannot be created nor destroyed, only changed from one form to another.

In the case of an electric motor, electrical energy is changed into rotating motion and heat. 

Every motor has parasite loads, bearing friction, winding heat, and cooling fans are the two biggest ones. These require energy, but this energy doesn't drive the load. 

Then there's efficiency. This varies greatly depending on a number of factors, but it's almost always lower with lower loading. This is why a 100HP motor driving a 50HP load will consume more energy than a 50HP one will. 

Rob


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