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Vfd Versus Alternatives


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#1 puzzler

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Posted 17 November 2007 - 05:22 PM

What is the experience of those of you who used VFD versus Intelligent Motor Controllers like POWERBOSS/MOTORBOSS

comparison with motor controllers

#2 marke

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Posted 20 November 2007 - 08:55 AM

Hello puzzler

Welcome to the forum.

There is no comparison between VFDs and Powerboss because they are totally different products and have totally different uses.
The Powerboss is a device that is designed to dynamically reduce the voltage applied to an induction motor when it is operating at reduced efficiency (very light load). The motor operates at fixed speed.
The VFD is designed to alter the speed of an induction motor.
Powerboss - Fixed speed
VFD - Variable speed, - totally different products.

Please feel free to ask specific questions if there are answers you are looking for.

Best regards,

#3 puzzler

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Posted 22 November 2007 - 05:42 PM

QUOTE (marke @ Nov 20 2007, 02:55 AM) <{POST_SNAPBACK}>
Hello puzzler

Welcome to the forum.

There is no comparison between VFDs and Powerboss because they are totally different products and have totally different uses.
The Powerboss is a device that is designed to dynamically reduce the voltage applied to an induction motor when it is operating at reduced efficiency (very light load). The motor operates at fixed speed.
The VFD is designed to alter the speed of an induction motor.
Powerboss - Fixed speed
VFD - Variable speed, - totally different products.

Please feel free to ask specific questions if there are answers you are looking for.

Best regards,



#4 puzzler

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Posted 22 November 2007 - 05:49 PM

OK I agree these are different devices, but what I was getting at is the following. Many people claim that you can save energy with the VFD's. I read a report that the VFD's are generating lots of heat in the motor which consumes more energy than it saves and as such are inferior to the Powerboss, which controls the voltage and the amps leaving enough torque for the various stages of the load. Furthermore I measured a motor with and without the VFD and found 50% more consumption of energy and lots of harmonics. So basically I wanted to refer to their task as energy savers.

#5 marke

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Posted 22 November 2007 - 06:02 PM

Hello puzzler

OK, you wish to compare the energy saving ability of both devices for reducing the energy used by the motor.

If I can be blunt, I would say forget about saving energy consumed by the motor except under extreme circumstances.
The induction motor is pretty efficient and you are better to concentrate on other areas of losses than the induction motor.

If you compare the energy saved on a lightly loaded motor operating at a fixed speed, then the VFD will use more energy, but as above, I do not consider that to be a worthwhile endeavor.
If you look at a number of applications, there is a machine driven by the motor that is operating inefficiently. If you slow the machine down, the losses will be reduced, and provided that at the lower speed the machine still does what is required, you will save energy.
For example:
A centrifugal pump operating at a fixed speed with a varying flow. The pump is sized for the highest flow that may be required. When a reduce flow is required, the pump continues to operate at full speed and cavitates. In doing so, it increases the output pressure, and spins the water within the housing. If we slow the pump down so that it operates at the speed required to produce the flow, we elliminate the cavitation losses and save energy.
Typically, the speed is controlled using a PID controller with a pressure transducer as a feedback source. - Considerable energy can be saved, but not in the motor, rather in the pump.

Best regards,

#6 kens

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Posted 25 November 2007 - 09:22 PM

Hi Puzzler, if you remember that energy in = energy out (load) + losses, and then understand that a device such as powerboss etc is designed to reduce the losses while a VFD is designed to reduce the load. Most modern electric motors have a very high efficiency allready (>95%) so there is little savings to be made there.
The real savings are to be made by reducing the load whenever possible. As marke has mentioned any centrifugal pump or fan load should be investigated as there is often potential for savings. These devices operate under the centrifugal laws whereby input power is proportinonal to pump or fan speed3. So the theoretical example would be that is we reduce the speed of a pump by 20 % then the power input is reduced by 50%. In reality the result will be a lttle different but not too far away.
The powerboss type controllers can work on certain types of load but remember that these are only saving a percentage of the losses, not a percentage of the total load. They also are only working when the motor is lightly loaded so this is then a percentage of the losses at the lower load. If we imagine a theoretical 55kW motor that has a half load efficiency of 90 percent and we install a powerboss to save some of this energy. At half load the motor is consuming 27.5kW (output) / 90% (efficiency) = 30.5 kW input. Therefore there is a little over 3kW of losses. This is all that the powerboss can work on. It will be able to save a percentage of 3kW at the times that the motor is unloaded.

Out of interest could you tell me a little more about the experiment that you carried out with the VFD that showed a 50% increase in energy used?


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