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11 comments
Participant

Join Date: Apr 2009
Location: Chicago, IL, USA
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Motor power measurement after a VFD

04/26/2009 1:43 PM

Hello everyone.

I need to know power consumption fluctuations of a mixer motor as an indicator of the strength of the material being mixed in the mixer. Currently I use a Hioki 3169 clamp on power meter to do the power measurement. As a portable R&D tool, I use the power meter together with a data acquisition system we built around it for many different mixers, from single phase lab scale Hobart type mixers to 3 phase plant scale mixers. Often the time, I encounter VFDs, so my first question is, where should I attach my power meter to if a VFD exists, between power supply and VFD or between VFD and motor? and why?

I sort of have a hunch that after VFD is the correct choice. I know my power meter should only work with its rated 50/60 Hz frequency (although Hioki told me 45~65 Hz might be OK). My second question is: If I use the power meter after a VFD and the frequency is quite out of the range, let's say either 100 Hz or 10 Hz, will the power meter be damaged? If not, is the power data it measures accurate? If not, is there any way to correct the data?

Up to now, all the mixers with VFDs I have dealt with had a frequency output range of about 40 ~ 70 Hz and the power meter was still recording data. I am concerned about the data's accuracy though. Hioki just told me to buy their other wide bandwidth power meters which are much more expensive. I would like to see if someone here can help me clarify the questions I asked above before I make any purchase decision.

Thank you.

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Power-User

Join Date: Mar 2008
Posts: 344
#1

Re: Motor power measurement after a VFD

04/26/2009 2:03 PM

Gosssshhhhssh... 'Lock and Tag' first for safety, then, I'll be installing a 'Juice Recorder' on such Puppy in order to have a good clue of what's going on, I believe. We know it going to varied up and down as load decrease-increase definetetly. With a recorder online it will help ya' out determine how much 'WATTS' went during processing and at what time did happened. So you may have something good to show there. Allset you have the technology go for it, it will come to ya' Buddy.

Crank that Puppy-Up,

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#2

Re: Motor power measurement after a VFD

04/26/2009 3:38 PM

If I use the power meter after a VFD and the frequency is quite out of the range, let's say either 100 Hz or 10 Hz, will the power meter be damaged?

Probably not.

If not, is the power data it measures accurate?

Probably not, the waveform is chopped DC, not standard 50/60Hz AC. From the Hioki data sheet the measurement frequency range limits are 40-70Hz. It doesn't indicate anywhere that it is capable of accurately measuring any waveform other than a sine wave.

If not, is there any way to correct the data?

If an accurate indication is all you are after, measure BEFORE the VFD. If you know the efficiency of the VFD (which varies a little depending on output load) you can compensate for this and get a dead accurate result for the motor load after the VFD.

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Participant

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#3
In reply to #2

Re: Motor power measurement after a VFD

04/26/2009 4:54 PM

Thank you Jack.

I was advised that to get accurate power fluctuation data I need to hook up after VFD. I guess, since a VFD is remanufacturing voltage, current and frequency, the load fluctuation of the motor may be masked or distorted if measured before the VFD. I don't really know the dynamics of power in a motor and VFD system.

HIOKI does offer wide frequency range power meters, some of which claim frequency range from DC to 2MHz. Other selling point of the expensive power meters is its ability to measure higher orders of harmonics. Do I need to worry about harmonics?

Other than higher prices, I am reluctant to switch to those wide bandwidth power meters because I already built my system around the HIOKI 3169 and I need to find a way to validate/correct the power data I collected with 3169 anyway. If there is no way to do that, then I will have to give up 3169 and the data I collected after VFD and start over with those higher priced power meters.

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#4
In reply to #3

Re: Motor power measurement after a VFD

04/26/2009 6:35 PM

I was advised that to get accurate power fluctuation data I need to hook up after VFD.

For a standard mixing load there shouldn't be any power fluctuations (the speed and likes of which the VFD's input capacitors would filter out), the motor load should be fairly smooth and ramp up or down as the mixture consistency changes. In your application you are not trying to measure harmonics, only get a (material consistency indication?) based on measured current. The Hioki 3169 should be fine on the VFD input side.

Can you give us more information on your application, what are you mixing? how accurate do you have to measure? what is the size of the motor and mixing vat? the overall start and finish consistency of the material? your estimated measured currents and mixing times, etc.

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#6
In reply to #4

Re: Motor power measurement after a VFD

04/26/2009 10:49 PM

Jack, the material being mixed can be highly viscoelastic such as rubbers, polymer blends or surfactant solutions. When being mixed, the viscoelasticity of the material causes a wide range of torque fluctuations of the mixing motor which is reflected in the motor power consumption fluctuations. I am trying to monitor the bandwidth of the power consumption fluctuation as well as how fast the fluctuation is (frequency) and use it as an indicator of the consistency change of the material due to the mixing action. My concern with HIOKI 3169 being hooked up on the VFD input side is that can the motor load fluctuations pass through the VFD? From what I know about VFDs, they are rectifier+inverter so they may filter out any power fluctuation frequencies the motor picked up from the mixing process.

what are you mixing?

high polymer blends, rubbers, etc.

how accurate do you have to measure?

as long as the fluctuation motor load can give me an indication of how well the material is mixed. Maybe the sampling rate is more important here since the fluctuation may be quite fast. We chose HIOKI 3169 mainly because it has a fast sampling rate (100ms).

what is the size of the motor and mixing vat?

Size can be from 1,2 kw to 100 kw. Mixing speed usually in the range of 20 ~ 100 RPM

the overall start and finish consistency of the material?

Start consistency is usually 1e4 Pa-S and finish can be as high as 1e6 or 1e7 Pa-S.

your estimated measured currents and mixing times, etc.

Currents from several A to 100 A. Mixing time around 15 mins.

Thank you

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#9
In reply to #6

Re: Motor power measurement after a VFD

04/27/2009 9:02 AM

In my opinion it appears you are attempting to compensate for problems in your batch mixing process. Likely you are experiencing batch-to-batch inconsistencies so want to measure power in an attempt to optimize mix cycle times.

For high viscous polymers we have specified the Readco Continuous Blender with excellent results. Continuous mixing can additionally lower your KW requirements, drastically shorten the process time and minimize cleanup between batches.

Click: http://www.readco.com/conmix.html

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#5

Re: Motor power measurement after a VFD

04/26/2009 9:22 PM

Generally, only the most expensive meters have enough filtering to be capable of accurately reading the PWM output of a VFD. Ones that say "True RMS" are not necessarily good enough (although many claim to be), it takes very specific, and expensive, additional filter circuitry. Without that, your reading will be wildly inaccurate because of the high harmonic content and the fact that the output is actually pulsed DC, not a true AC sine wave.

That said, almost all modern VFDs now come with the ability to read output power statistical data built-in. These are far more accurate than even the best expensive external metering will be. What kind of VFD do you have? I'd be willing to be you can just read the necessary data right off of the keypad.

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#7
In reply to #5

Re: Motor power measurement after a VFD

04/26/2009 10:54 PM

Thank you JRaef.

I don't know much about VFDs and it surprises me to know they can measure output motor load accurately. I am trying to develop a universal portable tool for mixers with or without VFDs. I had no idea VFDs could give me so much trouble though ...

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#8
In reply to #5

Re: Motor power measurement after a VFD

04/27/2009 12:14 AM

In addition to JRaef's good point, many VFD's can provide this data in the form of an analog output that your existing equipment can connect to directly. This would require a small amount of programming to change the VFD's parameters to give an analog output measuring the desired parameter (motor power) and the wiring to bring this output's terminals to an appropriate location for connection of your meter. Doing it this way would allow the keypad display to continue its normal or previous function.

--JMM

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#10

Re: Motor power measurement after a VFD

04/27/2009 10:25 AM

Hi,

Use the three phase multyfunction meter like this:

http://revalco.com/2005/catalogo_eng/50-64%20gb.pdf

Regards!

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Power-User

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#11

Re: Motor power measurement after a VFD

04/28/2009 12:51 PM

Firstly, I don't think there's any chance of damaging your meter as the voltages involved are all well inside its range, pwm or not.

Whether its accurate may be subject to opinion but why not try it and compare to the inverter readout?

In my experience, the inverter output is more than good enough for watt hour counting applications (like dough) although they generally don't offer a counter with reset as you will need for a similar application.

Having dine similar mixing applications from 5.5 to about 110kW, another interesting feature is a small offset from dol to VFD useage. This eventually equated to the watts 'lost' to heat with a dol start whereas the VFD is more efficient. This outweighed the VFD loss!

Generally, the current clamp should be OK but you may add filtering to the voltage sensing to get a better result when measuring at the output. Conversely, the input is hard to measure on current because of the harmonic content that requires better frequency response as mentioned elsewhere.

On balance, measuring at output is closer to what you want and is a little easier to meter, I think, and the better way to go.

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Users who posted comments:

chinagewu (3); Delmar (1); jack of all trades (2); jmueller (1); JRaef (1); KeyMan (1); magwer (1); MalcolmK (1)

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