Prove vfd energy saving at 50Hz frequency

It has nothing to do with the 50 Hz. If you are pumping a fluid and there is a control valve downstream, then a constant speed pump will waste energy (as heat) pumping against the restriction. With a VFD/VSD you will only pump as hard as needed to meet the process conditions.

If your supply voltage is 50Hz and you are running the VFD at 50Hz, you will be wasting energy. If you are seeing a lower current, that is likely a measurement error. Where are you measuring that current and how are you measuring it, as in what kind of meter? The harmonics in VFDs can cause a lot of metering errors.

Beware that you don't fall into the same trap as free-energy generator inventors. The VFD converts the 50Hz ac into DC and then uses electronic switches to chop the output up into a PULSE-WIDTH MODULATED waveform, meaning that it chops the 50Hz waveform up and outputs it as a series of square waves to create a sine wave. Trying to measure the current without a true RMS measuring ammeter will give you erroneous results.

Think of the VFD as a switch mode power supply with an efficiency of say 95%. What goes in will always be less than what goes out. The beauty of the VFD is that it allows you to run the motor in a more efficient way in certain applications (such as at low speeds, soft starting by ramping output up, etc). This is where you will achieve energy efficiency gains over other methods (such as a big variable resistor bank, motor soft starter, etc).

You will not achieve energy gains if you are just running the motor at full speed using a VFD (unless perhaps you are starting and stopping it regularly).

Hope this helps clear up the misunderstanding. Further information can be obtained from the VFD manufacturers website (in the form of application notes, etc) and your VFD handbook.

The posts above are correct in pointing out that you need to be mindful of the validity of your measuring instruments/techniques. If you are comparing the amps as reported by the VFD front panel meter vs a clamp on ammeter, then you are comparing "apples to oranges".

Jack of all trades was also correct in pointing out that the efficiency at full speed will be less than an "across the line" contactor.

But that brings me to another thought: Since where you are at is 50 Hz, have you checked to see what frequency the drive is setup for at full speed? American standards are 60 Hz=full speed, and the drive might be setup for that as a default. If that is the case the (50/60)*30=25 Amps, which is in the ballpark of what you are seeing (that is a down and dirty estimate). The reduction from 60 Hz to 50 Hz is your measure of improvement from only pumping what you needed, and the 50 Hz itself is only coincidental.

Just a thought

Interesting point, I see where you are going with that.

• He was running at 50Hz without the VFD
• His VFD was (maybe) defaulted to 60Hz as a maximum speed, so his V/Hz ratio was pre-set for hitting peak voltage at 60Hz.
• He set the speed to 50Hz
• The voltage output at 50Hz would then be reduced to 83% of maximum
• If the motor was significantly unloaded, it would be the same net effect of a "Nola Energy Saver" in that the iron losses in the motor would be reduced, hence a slightly lower current. I don't know if 17% lower current would be actualized, but it is remotely possible.

The problem issue then would be that essentially, he is starving the motor of torque. Specifically, he would be reducing the motor output torque by over 32% (.8333 x .8333). The reason it is reducing the current is because he didn't really need as much torque as his motor was producing anyway, so it continues to work. if however the load at any time starts to demand more torque, he is in deep trouble right away. The motor will not have it to give and will overload.

I still think it is a measurement error though. Most VFDs are not made in America so the default settings are usually based on 50Hz, not 60Hz.

Dear jraef.....our vfd's standard frequency is 50 hz....so voltage is not reduced it is standard... is there any role of power factor? also tell me wat is 'Nola enery saver' ? I havt heard it earlier ...

If your VFD is truly set up to deliver full voltage at 50Hz, then any reduction in current that you are reading is almost certainly a measurement error. There is now way that a properly tuned VFD running at full speed is going to use LESS energy than the same motor running across-the-line (DOL).

As to the other issue. If you do a search on "Nola Energy Saver" you should find plenty of articles describing it. Many many people are tryingf to sell these things no (again). Frank Nola was a NASA scientist who invented a device in 1969 as a means to reduce power consumption in unloaded AC motors that would potentially be used in a Space Station. Because he worked for the government, his patents were openly available to the public. People used them throughout the 1970s as "Energy Savers" or "Power Factor Controllers".

What they do is monitor the motor's power factor (it does NOT control it) and uses that measurement to determine if the motor is loaded or unloaded. When an AC motor goes less than 50% loaded, the power factor becomes very very poor. Using that knowledge then, the Nola device uses SCRs to phase back the firing angle, effectively reducing the RMS voltage to the motor. Since a small portion of a motor's current draw is used to just energize the windings and iron core, the "iron losses" in that process are reduced because they are a factor of the applied voltage. There is also a corresponding loss of motor torque, but again, you already determined you didn't need it all because the power factor was low, indicating that there was little load on the shaft. The device also had the ability to quickly return to full voltage should the shaft load return.

These things worked in theory, but in actual applications it turned out to not be worth much. Most motors are NOT run in an unloaded state for very long, they are just turned off (there is no better "energy saver" than that). Hundreds of small companies started up in the 1970s making these things, almost all of them went bankrupt when they ran out of suckers customers who had not already heard the news that the energy savings didn't really materialize. A few went on to become the Soft Starter manufacturers we know and love today, but the majority of them abandoned this idea in favor of the other more tangible benefits of Soft Starters. With the advent of the Internet though, new scams are being perpetrated at an alarming rate with these things.

One side issue though is that VFDs can inherently accomplish the same basic thing, as I described earlier. They do many more things that truly can save measurable amounts of energy as well. I just brought it up as a possible explanation of your phenomenon.

Think 'power factor'.

With the presumption that the driven equipment is a pump I submit this .Most pumps will be selected based on max requirements of flow and head.In normal operation or under reduced requirements the flow at max conditions may not be required.It used to be reduced by throttling the valve which is most inefficient method.With the advent of V/f drives pumps could be operated at reduced speeds.Pumps Follow a square law torque with ref to speed.The flow varies proportional to speed and Power as a cube of speed.If 80% say fow can meet proces requirement then the power required by the motor will be 51% of the full rated power.The savings comes actually from this .Speed is reduced by keeping the ratio of V/f constant.You have to examine pump speed Vs Flow and power curves to ascertain what min.flow can meet process requiremts.If a drive is used at full rated conditions then the system is most inefficient.selection and application of Inverters must be done carefully.One reason for current to go down could bethe PF. pf.Pf improves after V/f due to capacitors provided on the DC Bus.The point is saving comes mainly from reduced power requrement of motor than from other sources like lower losses etc.Also it must be noted when V/f is held constant the magnetic conditions remain the same as for rated Volts and Freq.However actual case working is little more complicated as one has to take the sytem curve of the pumping system into account,This is the reason many tmes people don't get the savings projected in actual use.

Yes, all well and good (well, sort of) but his speed was the same both times.

That should have ben the first question as to why he needs a VFD when he is going to run at same speed.The saving is I feel mainly due to PF improvement which is always there due to the presence of capacitors.That certainly will reduce the curent for the same power requirement.Any way all iswell that ends well

Best wishes and regards