Step Up Transformer Issue

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Besoeker

Besoeker

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Ingenieur said:
obviously
it is to smooth the output...make it more 'sine' like..
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If you have ever looked at the output current of a VFD, it is quite sinusoidal.
The only one I came across that wasn't very good was on a bore hole pump with a switching frequency of under 1kHz.
 
Besoeker

Besoeker

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Ingenieur said:
it is xfmr 875 > 845 motor
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Foe those figures you need to make an assumption about factor. In my experience such a power factor is unlikely on a 4-pole machine.
The 760kW units I mentioned above the pf at full load was 0.81.
 
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Ingenieur

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Besoeker said:
If you have ever looked at the output current of a VFD, it is quite sinusoidal.
The only one I came across that wasn't very good was on a bore hole pump with a switching frequency of under 1kHz.
Click to expand...

but yet they use a 'sine' filter

as you know the output i will have noise, basically harmonics of the switching f
 
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Ingenieur

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Besoeker

Besoeker

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MattSW said:
Hello,

I wanted to post a question on an issue we recently saw. Unfortunately I wasn't at the site so I have to take all the information third hand but here is what I know.

We sold a 875 KVA step up transformer to a customer to put on a SWD running a 1000 hp motor and yaskawa drive. The motor is 4160 volts and 122 amps, thus the step up transformer. They added a sine wave filter to the yaskawa drive. Yaskawa was rated at 1200 amps.

This is where I am losing info though. They said they were only able to get 840 KVA out of the transformer. I'm not sure where they were measuring, if they were reading that off the drive or taking measurements, I'm not sure on that. I found a similar motor online and used the numbers from it to get that it would take 788 kilowatts to attain full load. Now, the transformer is rated somewhere around a K-4 and with 100% being electronic load, it should probably be somewhere in the k-20 range. My question is this, has anyone seen a transformer not able to take additional loading like that? I don't believe they were increasing voltage to cause saturation. I've never seen it before, usually it just runs over temp. I've never seen it before and was wondering if anyone here had seen a similar experience. Either way, I believe they need a new transformer. But I wanted to get some information from anyone that could rationally explain the failure.
Click to expand...

I think Jraef is right. The filter means that the transformer is presented with lower than nominal input voltage resulting in a reduction of the 4160V the motor is expecting.
 
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Ingenieur

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the only apparent measurement we have is 840 kva
we don't know the point at which it was taken (at xfmr sec would be logical since they say that was what was being limited)
looking at it from a basic understanding I did the following
I'm a simple EE, not a physicist lol

these are rough numbers for the 3 possible scenarios
assume 840 at
drive out
filter out
xfmr out/motor in
calc'ed xfmr loss and filter kva on approx std values

worse case (obviously) at the motor
and it is ok imo
I can't delve into complex physics yet because not sure there is an issue and we basically have no data
being key, how did they determine the xfmr flatlined?
where was the 840 measured?
 
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Ingenieur

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Besoeker said:
I think Jraef is right. The filter means that the transformer is presented with lower than nominal input voltage resulting in a reduction of the 4160V the motor is expecting.
Click to expand...

he stated the xfmr has a wide range of taps
perhaps not used, but would seem one of the first steps in commissioning
imo the motor is rated at 4000, regardless
but even if 10% low 0.9 x 4160 = 3750 would this not increase i and kva ~ constant and determined by the load (which I assume is constant)
even if i is 10% over rated (but kva is < rated) how would this flatline output at 840?
just overheat with higher losses as noted by the op?

I think we can agree on one thing
we have very limited info as noted by the op
perhaps he'll chime in again
 
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Russs57

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Miami, Florida, USA
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Maintenance Engineer
The first post I read about these "step up drives" blew my mind. Transformers don't like running below rated hertz and they down right hate DC current in the primary.

Okay, I get we can limit the low end of the hertz. I get that IR compensation can be used.

Still it seems to me, as an end user, I would want a single sourced complete package from a major player. No way I'd want a drive from X, transformer from Y, and motor from Z.....and installation and start-up from "I don't know jack about this brand Inc.".

I guess what I'm really wondering is just how much money are we saving compared to buying a medium voltage drive?
 
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Ingenieur

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Russs57 said:
The first post I read about these "step up drives" blew my mind. Transformers don't like running below rated hertz and they down right hate DC current in the primary.

Okay, I get we can limit the low end of the hertz. I get that IR compensation can be used.

Still it seems to me, as an end user, I would want a single sourced complete package from a major player. No way I'd want a drive from X, transformer from Y, and motor from Z.....and installation and start-up from "I don't know jack about this brand Inc.".

I guess what I'm really wondering is just how much money are we saving compared to buying a medium voltage drive?
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as far as cost
if the bldg only has 480 you need a step up xfmr anyways
or take service at 4160 and step down for other uses, basically a larger xfmr
or 2 services
med v stuff costs alot more: switches, cb's, etc
I would guess the vfd is at least 2x
 
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mike_kilroy

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Location
United States
Besoeker said:
If there is any asymmetry in the PWM output then it could cause saturation. That's why we used transformers with an air gap in the core design. I don't know if the output sine filter would reduce that possibility.
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A good output sine filter totally eliminates pwm spikes and makes voltage pure sine wave. There will be NO Drive pwm effecting the transformer.

Since xfmr ratio usvadjustable, there is no need for motor derate due to lower than expected voltage.

Need more info! Get drive display of output volts and amps when concerned.

Sent from my SM-G900V using Tapatalk
 
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mike_kilroy

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United States
Besoeker said:
If we could be absolutely certain that there was perfect symmetry we wouldn't go with gapped transformers. But we do.

One such was a pumping station in Yorkshire where we installed four 760kW VSDs that needed output step up transformers. So very similar rating to that of the OP which is why I picked thaat one.
This isn't an illustration. It is a real life example.
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Not valid comparison at all. You did not have output sine filter so you REQUIRED gapped core or oversized xfmr. This OP needs neither.

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Ingenieur

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assume at motor
delta connected
840 kva
4000 v
121 a line, phase 70
phase Z 4000/70 = 57.3 Ohm
xfmr 875 kva

no one is going to convince me that if Z is lowered to 55 that i (and S) will not increase due to the xfmr
do not care if v is 3600, i = 133 and Z is 46.8 and is lowered to 44

throw a short on the sec
still 840???
 
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