Electrical Power Measurement Of VFD

We use power meters between the VFD and the motor.

You cannot do that because the motor waveform out of a VFD is not a sine wave, but a collection of chopped square waves. You need to measure the power before the VFD. This also takes into account the drive inefficiency.

Dear Sir,

I think You have confused of my question.I have asked for measurement of Input Power onlly.

Let me know the correct measureing instrument to measure the Power at the Input of VFD.

As we are getting diffrent readings with diffrent meters.

Regards

Aruna Kumar

Dear sir,

I am interested in measuring In put power to the Equipment.

As we have to calculate Power saving or Energy saving Without VFD and With VFD.

So please give me the details of Measuring Instrument and method of Power measurement.

Regards

Aruna kumar

We are interested in measuring the power at the input of VFD so that we can calculate the exact energy saving at supply point and to reduce energy cost,

Regards

Aruna Kumar

Jack's right. You can't measure the output of the VFD. Well, you can try but your readings will not be accurate. I don't know what meters you're using to measure at the input side but most VFD have a display. The ones we use show voltage, current and power. If your VFD has a display, then use that.

As your aim is at saving energy after use VFD. you can simply use a electroenergy meter for your measuring. put donw its number in Hour or day or week, so that you can work out the engergy save. because before vfd is only simple ordinary sine wave. or you can use a composition electric quality gauge for your measurng, from it you can get all paramters include instant power and unit time energy consuming so that you can compare the value before and after use vfd. The meter can also record all parameter auto for your reference afterward.can display in led or screen mubmer.

Hope it can be useful. [the ordinary instrument cannt be satisfaction with your requirement. because the valuse in differnet recording section is different.its variable during working time.]

You need to start with wide bandwidth sensors to measure current and voltage. Normal VTs and CTs are often not suitable. Rogowski current sensors and carefully designed wide band width voltage sensors are required as a start. You need sensors to measure 1Hz or less, and up to 10kHz or more

Then comes the question, why do you need to measure this power, how accurate, and what update rate? better control of you plant? control of the VSD itself? metering for billing? energy monitoring? harmonic problems?...???...??? etc..

The answer to this question will lead you to what topology of power measuring instrument will be appropriate; such as analog, fast sampling digital, DSP.

We design & manufacture some of the above, so I could probably help you further. I'm out of office for a few days though.

NeilJ

Try this: http://www.professionalequipment.com/xq/ASP/ProductID.4125/id.5/subID.482/qx/default.htm

What methods or devices are you using now? Normally the drive manufacturer gives a HMI interface showing current and power measurements alongwith the drive when installed in a switchgear.

Dear Sir,

Normally the manufactuerer will give output current and power display in the panels.

But we are interested in knowing how to measure the input current and power respectively.

Please let us know the procedure and the accurate measuring instrument.

Aruna Kumar

You may put a rectifier with good filter between AC line and VFD. Measurement of DC current and DC voltage may be performed with very high accuracy.

Depending on the size of you drive, your switchgear should have readings as well as your drive. Large drives (ours are 500+HP) provide input and output information. If it is low voltage, I have had good luck with both a Dranetz 658 and Fluke 434. Both are good manufacturers in my opinion.

Just a thought... When it is mentioned different meters give you different readings are you certain that the meters are incorrect? Consider the application. Likely the choice of VFD was based on a changing load and efficiently driving it. So unless you had the different meters connected at the same time how can you be sure one or both is not operating correctly. The design of a analog power meter is relatively simple. The utility company is pretty accurate if you are trying to calculate savings. How accurate do you need to be on a VFD by VFD basis? Depending on your budget a good multifunction protective relay can give you just about any statistic you need.

Jgunders makes a good point. Unless you connected all the meters at the same time, there would be differences in the readings due to changing loads and vfd speed.

We have a power monitor going to our MCC and we take readings regularly. When we installed the VFD, we noted a change in the power readings toward the economic side. The VFD was definitely helping but we couldn't determine the power consumption on the VFD line itself since the power monitor was for the whole building. If we had connected a power monitor on the motor before we installed the VFD, we would have acquired the kwh consumed per day or month by the motor and then be able to get the kwh consumed after the VFD was installed. The difference would then be the savings in kwh from the motor itself.

That's right. As others have already said, use a simple Watt-Hour meter, preferable the exact same type that your utility uses. If you are interested in energy savings, that is the only number that will matter. Hard precise numbers from descete instuments need to be corerected for harmonics etc., but those values will be meaningless if they have no effect on the revenue metering system anyway.

Measure with microrpocessor based energy standard meter