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From: DaveC on 10 Nov 2009 20:49
> The noise issues you mention aren't particularly clear. Many VFDs will
> radiate some HF/RF noise, but a well-designed instrument should be
> relatively immune to radiated noise.
[...]
> The peak phase-to-phase potentials should be addressed by Fluke to insure
> that the DMM can safely withstand these tests.

I don't think it's a fear that the Fluke won't survive a measurement, but
whether the readings will be accurate -- maybe the noise will confuse the DMM
if not lo-pass filtered.

> I would expect that having a handheld, battery powered (floating)
> scope-meter instrument would be more appropriate for analysing VFD outputs,
> than a DMM would be. After all, it can be determined that a motor drive has
an output with common
> light bulbs (LERs - light emitting resistors).

:-) LER -- have to remember that one...

You (and others) make a good point that maybe a DMM -- alone -- isn't the
best tool for troubleshooting a VFD. I have a nice handheld Tek scope (222PS)
that might fit the bill nicely (fully isolated; floatable to 850v/ch; rated
to 400v/ch, 800v in differential mode).

I think I'll go for the Fluke 117. It's got low-Z and cap (to 10K uF) mode
that I need. And I'll bring the Tek along when I go to look at a troubled
VFD.

Thanks for your comments. They're were helpful. You just saved me a few
hundred $$.

Dave

From: Greg O on 10 Nov 2009 22:30

"DaveC" <invalid(a)invalid.net> wrote in message
news:0001HW.C71F09370001469EB08A39AF(a)news.eternal-september.org...
> I'm considering the purchase of a new Fluke DMM. I like the low-impedance
> feature of the 117, but the low-pass filter feature of the 87 is also
> attractive.
>
> I had a conversation with a support engineer at Fluke today to try to help
> me
> decide between these two meters.
>
> It came down to the question of whether the variable-frequency drives I
> come
> across will have noisy outputs which the low-pass filter will be helpful
> in
> dealing with.
>
> Among other services, I install some (3-phase, mostly) VFDs for 230v
> motors
> on old printing equipment to give them a 2nd life.
>
> I have no idea how many (ie, percentage) of VFDs are "noisy" that will
> make
> measurement difficult with a DMM w/o low-pass filtering. Is this a common
> problem?
>
> If you've got some experience in this area I'd like to hear your comments.
>
> Thanks,
> Dave
>

I have been working with VFD's for ~10 years and have never measured the
voltage supplied to the motor. Most VFD's have the capability to show the
output voltage, amps, frequency, so for the most part why would you need to?
Greg

From: James Sweet on 10 Nov 2009 23:55
Jon Slaughter wrote:
> DaveC wrote:
>> I'm considering the purchase of a new Fluke DMM. I like the
>> low-impedance feature of the 117, but the low-pass filter feature of
>> the 87 is also attractive.
>>
>> I had a conversation with a support engineer at Fluke today to try to
>> help me decide between these two meters.
>>
>> It came down to the question of whether the variable-frequency drives
>> I come across will have noisy outputs which the low-pass filter will
>> be helpful in dealing with.
>>
>> Among other services, I install some (3-phase, mostly) VFDs for 230v
>> motors on old printing equipment to give them a 2nd life.
>>
>> I have no idea how many (ie, percentage) of VFDs are "noisy" that
>> will make measurement difficult with a DMM w/o low-pass filtering. Is
>> this a common problem?
>>
>> If you've got some experience in this area I'd like to hear your
>> comments.
>>
>> Thanks,
>> Dave
>
> I would go with the 87 as it seems to be better in just about every way
> looking at the specs. It has better accuracy and resolution in almost
> all categories.



I love my 87, although I do miss the low ohms feature and audible diode
test that my 79 had.
From: Gunner Asch on 11 Nov 2009 00:55
On Tue, 10 Nov 2009 21:30:49 -0600, "Greg O" <goo1959(a)hotmail.com>
wrote:

>
>"DaveC" <invalid(a)invalid.net> wrote in message
>news:0001HW.C71F09370001469EB08A39AF(a)news.eternal-september.org...
>> I'm considering the purchase of a new Fluke DMM. I like the low-impedance
>> feature of the 117, but the low-pass filter feature of the 87 is also
>> attractive.
>>
>> I had a conversation with a support engineer at Fluke today to try to help
>> me
>> decide between these two meters.
>>
>> It came down to the question of whether the variable-frequency drives I
>> come
>> across will have noisy outputs which the low-pass filter will be helpful
>> in
>> dealing with.
>>
>> Among other services, I install some (3-phase, mostly) VFDs for 230v
>> motors
>> on old printing equipment to give them a 2nd life.
>>
>> I have no idea how many (ie, percentage) of VFDs are "noisy" that will
>> make
>> measurement difficult with a DMM w/o low-pass filtering. Is this a common
>> problem?
>>
>> If you've got some experience in this area I'd like to hear your comments.
>>
>> Thanks,
>> Dave
>>
>
>I have been working with VFD's for ~10 years and have never measured the
>voltage supplied to the motor. Most VFD's have the capability to show the
>output voltage, amps, frequency, so for the most part why would you need to?
>Greg


Frankly..I do the same..for longer and the old Fluke 77s do everything I
need done on both ends of a VFD. And frankly..unless one is a
Electronic (component level) tech...one simply doesnt need to diagnose
too deep into them. And diagnosing them down deep takes a significant
amount of (billable) time..so I simply check em out..find out what is
not working properly...fix it if I can easily, or simply order a new
one.

The customer cant afford a lot of diagnosis down deep at $75 an hour for
a $300 5hp vfd.

Gunner

"Aren't cats Libertarian? They just want to be left alone.
I think our dog is a Democrat, as he is always looking for a handout"
Unknown Usnet Poster

Heh, heh, I'm pretty sure my dog is a liberal - he has no balls.
Keyton
From: Phil Hobbs on 11 Nov 2009 08:12
DaveC wrote:
>> I would go with the 87 as it seems to be better in just about every way
>> looking at the specs. It has better accuracy and resolution in almost all
>> categories.
>
> Accuracy and resolution are great, but for me in my work these really come in
> second to features (the low-Z and low-pass features, specifically).
>
> If it were not for the lack of a low-Z feature I'd buy the 87 in a second.
>
> But I need to determine if -- as pertains specifically to VFD voltage
> measurement -- the 117 is deficient (ie, does noise really cripple voltage
> measurement, and how often is this a problem when working in VFDs) in this
> respect.
>
> Thanks.
>

Owning an 87 is the mark of a True Hardware Guy (tm).

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
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