STAL S73 electric motor torque

[Josip]

Hi all,

kindly ask for help...

recently, Stal S73 compressor is imported from Germany,
but used to run with motor of 6kV
now we have to choose new motor low voltage 3phase 400V


kindly request any available data for electric motor torque needed to start up
this compresssor. If available kindly request diagrams with compressor and electric motor torques ...

I have to mention ... long time ago ... somewhere around year 1984. we
faced some problems to start screws S57 and S73 with domestic electrical motors,
but due to too low electric motor torque it was not possible until we discharge pressure by bypass lines with another working compressor (every start-up was a new game with lowering pressure - sometimes funny but not always ). After start up it was not any problem to run that motor for months ...


By policy electric motors were made by domestic companies ("Končar", "Sever" ... quite good quality and some are running still today), but without knowledge about torque needed to start screw compressors design&construction was not good ... it was ...

Unfortunately, but it was also a big school for all of us ....

Thanks for your time and effort for help...

Best regards, Josip

[RANGER1]

Josip,
Good to hear from you.
Obviously Stal information ideal, but would equivalent size machine details in other brands help.
S73 equals Howden 255/165 or Mycom 250L?

[Josip]

Hi Ranger1,

Josip,
Good to hear from you.
Obviously Stal information ideal, but would equivalent size machine details in other brands help.
S73 equals Howden 255/165 or Mycom 250L?

thanks for prompt reply

... to be hones I do not know (logicaly, should be the same torque for similar compressor sizes disregarding brands) and I truly hope someone will pop up with exact data ...


No, it was not due to Stal information ... it was due to mishearing and lack of asking proper questions of my older colleagues designers in chief to Stal ... we dont know what to ask and then kind of shame to ask "basic" questions at all ... old story ... then we find out all answers on our, sometimes, very expensive way ...

Then to be honest it was begining of screw compressor era in this area ... ex Yu
... almost science fiction field ...

A long story, to be told during long evening rest on porch ...

Anyhow, thanks for your answer ...


Best regards, Josip

[josef]

Josip,
I'm sorry, no experience and information about this compressor.

Josef.

[Josip]

Hi, Josef

Josip,
I'm sorry, no experience and information about this compressor.

Josef.

thanks anyway ...

no problem ... still I do hope someone will come with good info ...

Best regards, Josip

[josef]

Josip,
perhaps ask a question at https://www.kompressorteknik.se, maybe help.
They are specialists in all STAL compressors here in Europe.
I also have a STAL S 73 with a large engine.

Josef.

[Josip]

Hi, dear Josef,

Josip,
perhaps ask a question at https://www.kompressorteknik.se, maybe help.
They are specialists in all STAL compressors here in Europe.
I also have a STAL S 73 with a large engine.

Josef.

Thanks for hint ... I'll send a message to them asking for help cost me nothing.

Your compressor is working as a buster or high stage?
Which motor is installed? 160kW or bigger?

Best regards, Josip

[RANGER1]

Josip,
Sent a bit of information on your email.

[Josip]

Hi, Ranger1

Josip,
Sent a bit of information on your email.

received, thanks a lot ...

Best regards, Josip

[Josip]

Hi all,

this should be a first post, but lack of complete data ...

To enlight complete matter, here we are:

Originaly all were:

Type: STAL S 73 B 26
Year around 1985 (not sure)

Original unit with motor SCHORCH
typ KN 5 315 x AH018-Z IEC 315 x IM B3
6000 V, 16,7 A, 5 Hz, 145 kW, IP 23, 2975 1/min, 1987


Those are data for new installation with new motor in question:

Q= 480 kW at -41^oC/-10^oC
N_ef = 120 kW
n= 2950 1/min

In my opinion 145kW low voltage is not enough .... should be 160kw or bigger ... price is not a question, but bigger motor higher the shaft and not compatible with original base frame

Still waiting for proper data ....


Best regards, Josip

[Tycho]

kW is kW.

if it worked with 145 kW on the 6kv motor, it should work with 145 kW on the new motor.

While the 6kv motor draws 15-15 amps, the new motor on 145 kW @ 400v will draw 220-240 amps.

The LRA (locked Rotor amperage/starting current) will vary depending on starting gear.
direct online: 5-9 times FLA (full load amperage (220 A)
star/delta: 2-3 times FLA

with a frequency drive or a softstarter LRA will not be an issue.


if you increace the motor kW, you will have power to spare during pulldown, and it will draw less current during full load.


I can't find any info on the compressor itself, but on the pictures I found it looks close to a Howden wrv 204 in rotor diameter, and 145 kW doesn't seem to be to far away from the truth.

[Josip]

Hi all,

thanks for help, but... always that but ... sorry "mea culpa"

I asked a "little" bit wrong question
.... due to described problems within my first post
... the question should be .. what is compressor starting torque
... to compare with motor torque before ordering new motor ...

to enlight the problem ....
back in old days with starting problems due to a little bit lower motor torque
(motor power was ok ... by calculation we needed about 210-220kW) next one
standard type was motor of 250kW which we use ... due to bigger power we got
bigger torque too, bringing us very close to required torque to start compressor ...

unfortunately, still it was not enough to start compressor without discharging
pressure from oil separator i.e. almost unloaded start via by-pass lines, later on
some compressors were equipped with by-pass valve between suction and discharge
- can be dangerous with possible malfunction of by-pass valve ...

As I described in my first post screws were something new and noone was familiar
with special construction of electric motor to deliver enough torque to start
screw compressor what we learned later but with price.

So we are again at start ... with question about screw compressor starting torque
what I believe will be hard to find out but hope dies last ...



Best regards, Josip

[Tycho]

Just remembered i had this in my "fridge porn" folder

https://drive.google.com/file/d/1xso...w?usp=drivesdk

[Josip]

Hi, Tycho

kW is kW.

if it worked with 145 kW on the 6kv motor, it should work with 145 kW on the new motor.

While the 6kv motor draws 15-15 amps, the new motor on 145 kW @ 400v will draw 220-240 amps.

The LRA (locked Rotor amperage/starting current) will vary depending on starting gear.
direct online: 5-9 times FLA (full load amperage (220 A)
star/delta: 2-3 times FLA

with a frequency drive or a softstarter LRA will not be an issue.


if you increace the motor kW, you will have power to spare during pulldown, and it will draw less current during full load.


I can't find any info on the compressor itself, but on the pictures I found it looks close to a Howden wrv 204 in rotor diameter, and 145 kW doesn't seem to be to far away from the truth.

agree with you kW is kW, but we learned those motors are a kind of special
construction (something like a double cage deep grooves and maybe something
more but I do not know those requirements regarding construction) to deliver
enough torque to start screw compressor ...

starting gear is for sure a big factor to reduce starting current,
but compresor's torque remain unchanged and motor should deliver
higher torque

according to STAL diagram 4846 -E- 30aE page 4/4
"Refrigeration capacity and power consumption" motor of 145kW
is above required input power of 120kW at 50Hz for this compressor
working at -40/-10.

So if we get compressor torque or not ... we are going to use motor
of 160kW which should deliver enough torque to start this compressor.

Anyhow, thanks for your input and thanks to all for this discussion,
time and effort to help.

Best regards, Josip

[Josip]

Hi, Tycho

Just remembered i had this in my "fridge porn" folder

https://drive.google.com/file/d/1xso...w?usp=drivesdk

Thanks a lot, this manual is perfect for field service engineers
unfortunately, nothing about torques and capacity or electric motror.

I believe those data are in STAL files for design engineers.


Best regards, Josip

[RANGER1]

Josip,
Other brands on motors have different shaft heights for bigger Kw, maybe check out.
May also depend if you want TEFC, or open drop proof.
In Australia we use a lot of Teco, Toshiba & WEG.
European motors seem to have different frame sizes to us for same Kw.
Recently on a S80 Stal supplied ex Denmark without motor had to have special motor made at 2.5 times the price for standard motor in our part of the world.
Normally 150 kw motor ok for this application, 185 Kw no problem.
Teco make a 315 shaft height up to 265 kw
High torque could be called deep wound rotor.

[Tycho]

Hi, Tycho



agree with you kW is kW, but we learned those motors are a kind of special
construction (something like a double cage deep grooves and maybe something
more but I do not know those requirements regarding construction) to deliver
enough torque to start screw compressor ...

starting gear is for sure a big factor to reduce starting current,
but compresor's torque remain unchanged and motor should deliver
higher torque

according to STAL diagram 4846 -E- 30aE page 4/4
"Refrigeration capacity and power consumption" motor of 145kW
is above required input power of 120kW at 50Hz for this compressor
working at -40/-10.

So if we get compressor torque or not ... we are going to use motor
of 160kW which should deliver enough torque to start this compressor.

Anyhow, thanks for your input and thanks to all for this discussion,
time and effort to help.

Best regards, Josip

Hi josip, i reached out to my Stal guy, he said i should call him in the morning and he'll see what he can find

[Josip]

Hi, Tycho

Hi josip, i reached out to my Stal guy, he said i should call him in the morning and he'll see what he can find

Good news
Thanks, I'll wait, no problem.


Best regarads, Josip

[Josip]

Hi, Ranger1,

Josip,
Other brands on motors have different shaft heights for bigger Kw, maybe check out.
May also depend if you want TEFC, or open drop proof.
In Australia we use a lot of Teco, Toshiba & WEG.
European motors seem to have different frame sizes to us for same Kw.
Recently on a S80 Stal supplied ex Denmark without motor had to have special motor made at 2.5 times the price for standard motor in our part of the world.
Normally 150 kw motor ok for this application, 185 Kw no problem.
Teco make a 315 shaft height up to 265 kw
High torque could be called deep wound rotor.

Very good info ... not sure what will be a final decision
regarding new motor ...

reply to your email sent ...


Best regards, Josip

[cricri]

Hi Josip

check your Email Josip,
starting torque curve for a little bigger Z compressor from Grasso and pictures of a S73B (-42/-12°C) motor and compressor plates. 132 kw 380 V. old plant still operating!!

[Josip]

Hi, cricri

Hi Josip

check your Email Josip,
starting torque curve for a little bigger Z compressor from Grasso and pictures of a S73B (-42/-12°C) motor and compressor plates. 132 kw 380 V. old plant still operating!!

Thanks for info ... email info forwarded to coleague ...


Best regards, Josip

[Tycho]

Hi Josip,

My Stal guy came up empty, but is reaching out to his contacts.

Spoke to my electrical guy, and he confirmed what I said, as long as the kw is the same there shouldn't be any issue.

The difference is that a 2 pole motor (3600 rpm) is "weaker" than a 4 pole motor (1800 rpm) (I'm just spitballing the rpms)

But the 2 pole motor is more efficient and smaller in design, while the 4 pole is slightly stronger and larger in design.

https://www.difference.wiki/2-pole-m...4-pole-motors/

But to go one step up on motor size is playing it safe, also you will have more power to draw of if you need.

Worst thing I know is package suppliers that skimp on motor size so you are always running on the edge of tripping the whole thing.


Best of luck

[Josip]

Hi all!

Little update to this thread ... definitely for new discussion ...


Owner of this unit is going to rewinding original motor of 6kV
to make a low voltage (400V) motor ....



They found a second hand motor,
but not possible to install it due to
misalignment of holes on motor foot and frame holes ...

Not sure if rewinding is applicable in this case,
but hope that someone will put some light on this matter...


Best regards, Josip

[RANGER1]

Josip,
Only motor rewinder could say, but sounds messy & risky.
From then on motor is special, so if something happens to it no direct change over.
To me modifying base usually not such a big deal, or in this case probably same price when all done (if motor can be modified for lower voltage).

[Tycho]

I'm with Ranger1 here...

Rewinding a motor sounds a whole lot like spending a lot of money to save very little money.

and ending up with a 1 of a kind motor.

Redoing the base is not such a big of a deal, like ranger said. You need someone who is well versed with a cutting torch, and a good welder I know a guy that can do both

The whole rewinding of the motor sounds like something that came up in a project meeting where all the economic guys were gathered...
-Boss:so how do we solve this as quick as possible?
-Engineer one: Well, we can rebuild the frame and fit a factory standard motor?
-Boss:anyone else?
-Engineer two: well, we could ask a company to rewind the existing motor, and then we wouldn't have to rebuild the fra...
-Boss: Excellent! let's rewind the motor, Engineer two, you are in charge of this! Make it happen!
-Engineer two: instantly wishes he kept his mouth shut

[RANGER1]

PS get a new motor , not secondhand.
Think of electrical efficiency with latest & greatest $$$ savings over time.

[Josip]

Hi guys,

thanks for reply ...

but the owner of the unit in question is a kinda electrical engineer
and he is going to try his way ... rewinding motor for low voltage use ...
regarding this motor ... he is deaf for any suggestion or advice ....

anyhow... let's wait for outcome ... I'll get back with info if any ....


furthermore .... regarding compressor starting torque ...
finally we found that info ... but no practical value now ...

Anyhow thanks for all inputs and help ...


Best regards, Josip

[Josip]

Hi, Ranger1

PS get a new motor , not secondhand.
Think of electrical efficiency with latest & greatest $$$ savings over time.

good advice ...

... but this guy (owner of above unit and motor)
... is buying, using and selling mostly secondhand things ...

he has his own moto how to run refrigeration business ...

seems it is not so bad,

but that is definitely another movie ... we do not fit within


Best regards, Josip

[RANGER1]

Hi, Ranger1



good advice ...

... but this guy (owner of above unit and motor)
... is buying, using and selling mostly secondhand things ...

he has his own moto how to run refrigeration business ...

seems it is not so bad,

but that is definitely another movie ... we do not fit within


Best regards, Josip

Josip,
Thanks, all kinds of people out there.
Probably will work, who knows.
Have a friend who runs things past me every now & then.
I’m probably conservative & advise against, he always does it anyway.
Now I just say yes & agree, he does it, no problem.

[Josip]

Hi all,

another enigma with this and other compressors ...

two S57 and one S75B are equipped with something (see photos, please)
like a float ...

it is connected with two pipes at top and bottom and there is some
electrical cable ...

to be honest we do not know what is that and how it works ...

never saw similar installation ... but never too late to learn something

I do hope someone should put some light on that piece of equipment...


20191017_135534 res.jpg 20191017_135552 res.jpg
20191017_135633 res.jpg 20191017_135646 res.jpg

It is installed on S57 on discharge cover and on S75B on suction cover...
we assume it is kind of protection ... can be only for oil ...

It was actually 2 stage plant S57 at high side and S73B and S75B at low stage.
After reinstallation it will be again two stage ammonia refrigeration plant

Thanks for your time and effort to help about this.

Best regards, Josip

[RANGER1]

Josip,
Also never seen, but maybe see what they are trying to achieve.
A few years ago had a few Stals get seriously damaged, as packages started full of liquid due to leaking discharge check valve.
Pressures up, so no ice.
Also potential for oil to flood rotor chamber if to much oil was added to retro-fitted oil separator.
The original Stal discharge pipe entered half way up oil separator, new was closer to bottom, basically horizontal after it left compressor.

So my guess is to prevent starting machine if any level of oil/refrigerant to avoid damage.

In a lot of cases as well rotor driven oil pump could have been replaced with remote oil pump, so also possibility of flooding compressor with oil on a few failed starts.

[Josip]

Hi RANGER1,

Thanks for your time and effort ....

I believe you described a lot of possible causes to prevent compressor start due to compressor flood with oil or liquid refrigerant ...

In some Stal document is written that difference between compressor bottom and inlet of discharge pipe into oil separator must not exceed 600-700mm ...

For leaking check valve you are right, been there (compressor not damaged, but the noise was not Beethoven music ...

On one petrochemical plant with remote oil pumps (and of course a few failed starts) compressor was full of oil, but fortunately motor not started at all and we noticed low oil level within oil separator and then use another oil pump to pump oil form discharge pipe into oil separator

... we were lucky ... it was plant commissioning phase ...

another problem here was too high concrete base of oil separator (oil separator inlet was quite high, but compressor must run almost all time at 100% and there was not too much oil in discharge pipe when compressor stops ... it was allowed to have one failed start, the second one must be good or we need to drain oil from discharge pipe ...

so we wrote new starting procedure .... only that was possible in that moment ... later on that plant was running without problem for long time ...


Probably design of plant with Stal compressors was not the best and they faced one or more possible problems and to protect compressor the best was to install some protection device like this one ...

Best regards,
Josip