Three Phase Motor Voltage Measurment.

[broc028]

I have a three phase motor which is running a conveyor. I want to measure the phase voltage. When I measure between phase and ground the reading varies between 75V and 160V. Is this normal??

 

[krbernier]

did you read the application note i attached from fluke?

i was simply trying to state that their application notes says the meter will do this. in fact there is a picture of the meter hooked up to a drives output and measuring voltage on the note!

but hey if im wrong im wrong. Thankfully my fluke 289 has served me well. to be honest i never really see the need to use it on the drives output. Seems like they either are putting out voltage or the arent. problems i usually run into are dc bus fuses, output rectifier bridges and bad cooling fans!

i found this forum and it seemed like a great new place for me to gain some knowlege. I never intended it to be a place to argue about things i dont fully understand.

I am just an ordinary industrial electrician. I dont really understand corner frequency,roll off rate or break point. So when fluke says Hey this tool with this builtin filter CAN accurately measure pwm vfd output voltage, i assumed it was correct.

i wish i had just kept my mouth shut.

 

[Skogsgurra]

I am sure that the 289 serves you well. Especially if you, as you say, "never really see the need to use it on the drive's output".

Since this thread is just about that; measuring the output of the drive, it may be an indication that you should pay attention to what Scotty says. He is absolutely right. And, with newer drives (the Sinamics is a recent example) running with switching frequencies down close to 1 kHz, a filter with 1 kHz corner frequency just isn't good enough if you need to measure drive voltage to find out problems with motor overheat, slip, loss of torque and other such problems.

The filter I use is built into a little plastic box that I can keep in my pocket, if needed. It is a single pole filter with 1 Hz corner frequency (Hz, not kHz) and gives me an accurate V/Hz figure over the whole speed range down to around 3 or 4 Hz. Have used it for about twenty years and prefer that over other meters. BTW, the old Metrix MX15 also has a filter built in and does a decent job, too.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[krbernier]

well im learning one thing from this forum. how easily one small statement can get turned all around.

here is a quote from the fluke document i was referring to

"Output side measurements
Measurements on the output
side of a pulse width modulated
(pwm) motor drive have been
difficult or impossible to make,
especially accurate measurements,
that would agree with
the drive controller displayed
values. The traditional true rms
multimeter will not provide accurate
answers. This is because
the voltage applied to the motor
terminals by the VFD is a pulse
width modulated non-sinusoidal
voltage.
A true-rms DMM will give
an accurate reading of the heating
effect of the non-sinusoidal
voltage applied to the motor, but
will not agree with the motor
controller’s output voltage reading
which is displaying the
rms value of the fundamental
component only (typically from
30 Hz to 60 Hz). The issues are
bandwidth and shielding. Many
of today’s true rms digital multimeters
have a wide bandwidth,
sometimes out as far as 20 kHz
or more, which allows the meter
to respond not only to the fundamental
component that the
motor really responds to, but also
to all of the high frequency components
generated by the pwm
drive.
In addition, if the dmm is
not shielded for high frequency
noise, this can also lead to poor
measurement performance due to
the high noise levels generated
by the drive controller. Because
of their higher bandwidth and
shielding issues, many true rms
meters will display readings as
much as 20 to 30 % higher than
the drive controller is indicating.
Fluke’s new 289 multimeter,
with it’s ¼ VGA dot matrix display,
has the ability to display
multiple parameters at the same
time and utilize the selectable
low pass filter, giving the troubleshooter
or engineer the ability
make accurate voltage, current
and frequency measurements
on the output side of the drive
either at the drive itself or at the
motor terminals and also display
the filtered output voltage and
frequency (motor speed) simultaneously.
With the filter selected,
the 289 readings for both voltage
and frequency (motor speed)
should now agree with the
associated drive control display
indications if they are available.
When measuring output current
with a clamp accessory, the low
pass filter allows for accurate
current measurements and frequency
measurements simultaneously
when using the Hall-effect
type clamps. All of these measurements
are especially helpful
when making measurements
at the motor location when the
drives displays are not in view"


all i said was that fluke stated this meter would do this. if it wont then fine flukes information is obviously misleading.

i would like to thank you for making my first experiance in this forum so enjoyable! i will seriously consider any input i may have in the future.\

a simple reply of "in some cases the built in filter will take good readings but in many instances the filter is not sufficient." would have been nice.

but if you must ingnore the explanation of why i gave the statement i made and instead point out things in my response saying well if you said this you must be a moron. By all means have at it...

 

[sibeen]

krbernier, yes, the fluke may measure recent drives perfectly well. This is because it has been designed with these drives in mind and takes into account the switching frequency that they now operate at.

What some of the learnered colleagues here are trying to explain is that this may not work correctly on the older style drives. This is because the switching frequency of the inverter of the older drives was lower than the cut-off frequency of the filter within the fluke multimeter.

If you place your meter on the output of one of these older drives you will get a reading that doesn't match what the drive output is showing on its screen.

 

[ScottyUK]

krbernier,

You are most welcome here, and sorry if my reply was a bit sharp. If you read your own replies you will see why they could be intepreted as being a little argumentative too. Do ya fancy a beer to cool off?

I expect that the bits of the Fluke spec which the engineers wrote are probably very accurate. The stuff the marketing guys write... well, treat it as what it is: marketing! It's not actually outright lies, but Fluke's statement certainly doesn't apply to all situations, and it is risky to assume that it does. To be fair to Fluke, that comment applies to pretty much all measurements regardless of the instrument or the manufacturer: it helps to understand what the capabilities and limitations of the instrument are before trusting it, otherwise you can end relying on readings which have little resemblance to what is going on. I'm sure most of us have been in that position at some time or another.


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If we learn from our mistakes I'm getting a great education!

 

[krbernier]

i think a guiness would do it!

i apologise for seeming argumentative. i didnt realise that the meter wouldnt be accurate on older drives.fortunately for me all of the vfd's i work on are new!(er)

honestly most of the stuff here is over my head. i am just an electrician who got lucky and was the foreman for a large plant upgrade. i impressed the bigwigs at the plant and they offered me the job of head electrician for the plant and surrounding water systems.

my work prior to this was strictly installation at new plants and upgrade at existing plants.Needless to say i had alot to learn about troubleshooting.Thankfully i learn quickly and was able to get lots of usefull info from forums like this one. the people on here really seem to know what they are talking about and hopefully i can learn alot here.

i do have lots of experience with control wiring and issues in the water industry so hopefully i might be able to help someone else aswell!

 

[ScottyUK]

I'll join you in that beer.

Don't worry about what you don't know: the more time I spend learning about engineering the more I realise how much I don't know!


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If we learn from our mistakes I'm getting a great education!