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"54Vac Three Phase" means what?

S

Scrim

Jan 1, 1970
0
I want to drive a motor described as requiring "54Vac Three Phase". It uses
three connection wires, one for each phase.

Unfortunately I can get no further information, but need to know the pk-pk
voltage for the sine wave on each phase.

Am I correct in assuming that the 54Vac is probably a 3 phase RMS value?

If so then am I correct that the Pk-Pk voltage on each phase is (54* 2 *
sqareroot2)/ (sqareroot3) = 88Vpk-pk

Scrim
 
Z

Zak

Jan 1, 1970
0
Scrim said:
I want to drive a motor described as requiring "54Vac Three Phase". It uses
three connection wires, one for each phase.

Unfortunately I can get no further information, but need to know the pk-pk
voltage for the sine wave on each phase.

The first question is: is the voltage between pases or between pase and
null?

Over here it usually is between phases. While ground-to-phase is 230
volts, the voltage between phases is 400.

So a 54 volt-between-phases motor would need 31 volts from zero on each
phase.

For each of those phases the normal sqrt(2) applies, meaning the peak
voltage is 44 and peak to peak is 88.

Which seems to match your original post nicely - but it all depends on
the original definition of the running voltage.

If the motor can be wired for star or triangle...


Thomas
 
S

Scrim

Jan 1, 1970
0
Zak said:
The first question is: is the voltage between pases or between pase and
null?

Over here it usually is between phases. While ground-to-phase is 230
volts, the voltage between phases is 400.

So a 54 volt-between-phases motor would need 31 volts from zero on each
phase.

For each of those phases the normal sqrt(2) applies, meaning the peak
voltage is 44 and peak to peak is 88.

Which seems to match your original post nicely - but it all depends on
the original definition of the running voltage.

If the motor can be wired for star or triangle...


Thomas

It's an induction motor with just three connections to the windings,
intended to run at 1250Hz, 75,000rpm, 54Vac three phase, (Yes, 75,000rpm! -
it's a turbomolecular pump). I can't judge how the windings are arranged
internally.

Scrim
 
Z

Zak

Jan 1, 1970
0
Scrim said:
It's an induction motor with just three connections to the windings,
intended to run at 1250Hz, 75,000rpm, 54Vac three phase, (Yes, 75,000rpm! -
it's a turbomolecular pump). I can't judge how the windings are arranged
internally.

My guess is that the 54Vac is effective between phases - if it is
specified like that. AC is alway spec'd as effective AFAIK.

If it doesn't get up to speed it might be that it needs to be 54 volts
to ground or there may besome other problem. And it won't get up to any
speed unless started in a decent vacuum, I guess.

Interesting beast. How are they protected against objects falling in?



Thomas
 
S

Scrim

Jan 1, 1970
0
Zak said:
My guess is that the 54Vac is effective between phases - if it is
specified like that. AC is alway spec'd as effective AFAIK.

If it doesn't get up to speed it might be that it needs to be 54 volts
to ground or there may besome other problem. And it won't get up to any
speed unless started in a decent vacuum, I guess.

Interesting beast. How are they protected against objects falling in?



Thomas

By 54V between phases do you mean 54v rms, so single phase rms =
54/(squareroot3) = 31v , so single phase pk-pk = 31*2 *(squareroot2) = 88V ?

They often fit a domed metal wire gauze over the large diameter input, at
the expense of 10% lost pumping speed - mine has nothing at the moment. I
picked it up cheap and a bit messed up, but I think it should be ok if I can
get the psu sorted. I'll be hiding some distance away though, the first few
time I rev it up!

Scrim
 
Z

Zak

Jan 1, 1970
0
Scrim said:
By 54V between phases do you mean 54v rms, so single phase rms =
54/(squareroot3) = 31v , so single phase pk-pk = 31*2 *(squareroot2) = 88V ?

That seems the most likely to me. But I never used these things.
They often fit a domed metal wire gauze over the large diameter input, at
the expense of 10% lost pumping speed - mine has nothing at the moment. I
picked it up cheap and a bit messed up, but I think it should be ok if I can
get the psu sorted. I'll be hiding some distance away though, the first few
time I rev it up!

Good luck...


Thomas
 
H

Hal Murray

Jan 1, 1970
0
It's an induction motor with just three connections to the windings,
intended to run at 1250Hz, 75,000rpm, 54Vac three phase, (Yes, 75,000rpm! -
it's a turbomolecular pump). I can't judge how the windings are arranged
internally.

The magic word is "squirrel cage". Neat idea. No brushes.

It's the standard way to make industrial size motors when you have
3 phase available.

Roughly, the 3 phases are connected to 3 coils on the stator
that are oriented 60 degrees from eachother. That forms a
rotating magnetic field. (Swap a pair to go in the other
direction.)

The rotor is a bunch of shorted turns. Current induced by the
rotating field makes the rotor want to go in the same direction
as the field is rotating.

Torque is proportional to induced current which is proportional
to the slip speed.

For example:
http://ww1.microchip.com/downloads/en/AppNotes/00843a.pdf
(has a picture of the rotor - looks like a squirrel cage :)

Use a permanent magnet in the rotor and you get a synchronous
motor.
 
R

Rich Grise

Jan 1, 1970
0
By 54V between phases do you mean 54v rms, so single phase rms =
54/(squareroot3) = 31v , so single phase pk-pk = 31*2 *(squareroot2) = 88V
?

They often fit a domed metal wire gauze over the large diameter input, at
the expense of 10% lost pumping speed - mine has nothing at the moment. I
picked it up cheap and a bit messed up, but I think it should be ok if I
can get the psu sorted. I'll be hiding some distance away though, the
first few time I rev it up!

Scrim

Well, for heaven's sakes, don't try to run it up in air, or it _will_
be junk. In fact, before you power it up at all, you should probably
bake out it under as hard a vacuum as you can get, and keep it in
vacuum. (you can do this with a valve. ;-) )

But it sure sounds to me like the 54V is from phase to phase, i.e.
delta, i.e. your multimeter will read 54VAC (if it goes to that freq)
from A to B, from B to C, and from A to C. I'm pretty sure you did
the arithmetic right, i.e. if you add a neutral in the middle, and
simulate it with a Y, then, yes, each leg would be 31VRMS to
neutral.

It sounds like you're trying to design a power supply for the thing.
Would that be an accurate guess?
 
S

Scrim

Jan 1, 1970
0
Rich Grise said:
Well, for heaven's sakes, don't try to run it up in air, or it _will_
be junk. In fact, before you power it up at all, you should probably
bake out it under as hard a vacuum as you can get, and keep it in
vacuum. (you can do this with a valve. ;-) )

But it sure sounds to me like the 54V is from phase to phase, i.e.
delta, i.e. your multimeter will read 54VAC (if it goes to that freq)
from A to B, from B to C, and from A to C. I'm pretty sure you did
the arithmetic right, i.e. if you add a neutral in the middle, and
simulate it with a Y, then, yes, each leg would be 31VRMS to
neutral.

It sounds like you're trying to design a power supply for the thing.
Would that be an accurate guess?
I don't plan to try running the turbo pump beyond short tests until it's
under vacuum. I've carefully cleaned it and baked it at atmospheric for now.
The real bake out will have to wait until it's running.
Yes, I'm trying to build a power supply for it. I really want to try to keep
it as low cost and easy to build as possible, but I don't mind spending time
on the design and I'm patient. My original plan was to use the 3 phase
variable frequency digital sine wave generator I've designed (with help from
this group - see thread: Digital Sine Wave Generation) to drive three audio
ic power amps. Unfortunately I now realise that the required 3 phase 54Vac
means I'll need an 88Voutput swing, which is beyond the IC's I've been able
to track down. There are plenty of high power audio amp chips and power op
amps able to provide far more than the 35W max I'll demand from each, but
none at sensible prices that will work with 90V-100V supplies :-( I'm a
little concerned too about how these devices would behave when driving a 3
phase induction motor anyway.

The alternative idea I've been considering is to use some logic to produce
3 square wave signals which overlap in the required 120 degrees (3 phase)
fashion. I'd connect these to something like the HIP4086 3-Phase Bridge
Driver and 6 FETS.
The HIP4086 can't quite cope with the required 88V either, but I should be
able to find something given the amount of mains powered switch mode stuff
around these days. I'm concerned that this approximation to a sine wave
might be just too poor, especially given that it seems that the limiting
factor with turbo pumps is power dissipation in the motor windings. However,
I don't need the pump to run up to speed in just 60 seconds (like I
understand it would with the recommended supply), and think I might get away
with a fixed frequency 1250Hz 3 phase square wave supply - who knows, but I
don't want to invest too much effort in this, given that the pump may turn
out to be beyond use anyway.

Any pointers to great integrated solutions to this would be much
appreciated.

Scrim
 
Z

Zak

Jan 1, 1970
0
Scrim said:
I don't plan to try running the turbo pump beyond short tests until it's
under vacuum.

Don't run it in air. It doesn't run very much at all in that case, and
agood enough starting vacuum will not be that hard to reach.
Yes, I'm trying to build a power supply for it. I really want to try to keep
it as low cost and easy to build as possible, but I don't mind spending time
on the design and I'm patient. My original plan was to use the 3 phase
variable frequency digital sine wave generator I've designed (with help from
this group - see thread: Digital Sine Wave Generation) to drive three audio
ic power amps. Unfortunately I now realise that the required 3 phase 54Vac
means I'll need an 88Voutput swing, which is beyond the IC's I've been able
to track down. There are plenty of high power audio amp chips and power op
amps able to provide far more than the 35W max I'll demand from each, but
none at sensible prices that will work with 90V-100V supplies :-( I'm a
little concerned too about how these devices would behave when driving a 3
phase induction motor anyway.

The alternative idea I've been considering is to use some logic to produce
3 square wave signals which overlap in the required 120 degrees (3 phase)
fashion. I'd connect these to something like the HIP4086 3-Phase Bridge
Driver and 6 FETS.

This would be a whole lot easier. I'd make high side drivers myself and
run the whole thing on square waves. Use the desired RMS value as DC
power voltage. 31 V RMS to neutral means + and - 31 volt power.

To drive you could even use optocouplers. 3 phase can be generated with
a 4017...

But run the thing in vacuum only or the motor will burn out. You may be
able to find data sheets on line of these pumps - I found them at least
for Alcatel pumps.


Thomas
 
R

Rich Grise

Jan 1, 1970
0
....
My original plan was to use
the 3 phase variable frequency digital sine wave generator I've designed
(with help from this group - see thread: Digital Sine Wave Generation) to
drive three audio ic power amps. Unfortunately I now realise that the
required 3 phase 54Vac means I'll need an 88Voutput swing, which is beyond
the IC's I've been able to track down. ....
The alternative idea I've been considering is to use some logic to
produce 3 square wave signals which overlap in the required 120 degrees (3
phase) fashion. I'd connect these to something like the HIP4086 3-Phase
Bridge Driver and 6 FETS.
....

This sounds good to me - Plus, isn't the RMS of a square wave equal to the
peak value?
The HIP4086 can't quite cope with the required 88V either, but I should be
able to find something given the amount of mains powered switch mode stuff
around these days. I'm concerned that this approximation to a sine wave
might be just too poor, especially given that it seems that the limiting
factor with turbo pumps is power dissipation in the motor windings.

I souldn't worry about this at all - You might need antiparallel diodes,
but I'd think a motor's inductance would make a dandy low-pass filter. ;-)
However, I don't need the pump to run up to speed in just 60 seconds (like
I understand it would with the recommended supply), and think I might get
away with a fixed frequency 1250Hz 3 phase square wave supply - who knows,
but I don't want to invest too much effort in this, given that the pump
may turn out to be beyond use anyway.

I don't really get what you're saying here - do you mean that they used to
ramp up the drive freq, or that it takes that long to spin up even with
just the constant power supply? In either case, I'd try to duplicate the
"real" operation as closely as I could.
 
S

Scrim

Jan 1, 1970
0
Rich Grise said:
...

This sounds good to me - Plus, isn't the RMS of a square wave equal to the
peak value?


I souldn't worry about this at all - You might need antiparallel diodes,
but I'd think a motor's inductance would make a dandy low-pass filter. ;-)


I don't really get what you're saying here - do you mean that they used to
ramp up the drive freq, or that it takes that long to spin up even with
just the constant power supply? In either case, I'd try to duplicate the
"real" operation as closely as I could.
The rms/square wave thing is quite confusing, since, as you say, the motor
will filter it. It would certainly simplify things if I can just drive each
phase with a 0 to 54v square wave! I'm starting to feel motivated again :)
Cheers - back to the HIP4086 3-Phase Bridge Driver and 6 FETS.

I'm not certain, but suspect the proper controller ramps frequency to get
maximum acceleration on the rotor all the way up to full speed. (I've read
that during the start-up phase it increases the "Volt/Hz" ratio, but that
doesn't reveal much). I also suspect it takes advantage of the windings
being cold at switch on, so it can dump more heat into them for a short time
without causing damage and hence get the motor from 0 to 75,000rpm in just
60 seconds. (The winding are fitted with a temperature sensing thermistor,
by the way). But I suspect the voltage might need to be reduced to some
extent at lower frequency to avoid excessive currents. This is too
complicated so I just intend to try using a fixed frequency 1250Hz supply
and accept it might take, say, 5 minutes to get to full speed. If it's
slower than that I'll try some frequency ramping.

Scrim
 
H

Hal Murray

Jan 1, 1970
0
I'm not certain, but suspect the proper controller ramps frequency to get
maximum acceleration on the rotor all the way up to full speed. (I've read
that during the start-up phase it increases the "Volt/Hz" ratio, but that
doesn't reveal much). ...

Is the rotor a permanent magnet, or a squirrel cage?

If it's a permanant magnet, then you have a giant stepper motor
and you have to drive it in sync with the actual rotation. Torque
is porpotional to the angle between the rotor magnet and the
stator field that it is trying to track.

If it's a normal squirrel cage induction motor (which seems like
a good fit for this use), then torque is proportional to the
difference in speed between the rotor and the rotating magnetic
field. The usual startup problem is that the motor tries too
hard and will rip itself apart if you don't limit the current.

Some sort of current limiting would probably be a good idea.
I'm not sure if you gain anything by slowly ramping up the
frequency, but I could easily be missing something.
 
S

Scrim

Jan 1, 1970
0
Hal Murray said:
Is the rotor a permanent magnet, or a squirrel cage?

If it's a permanant magnet, then you have a giant stepper motor
and you have to drive it in sync with the actual rotation. Torque
is porpotional to the angle between the rotor magnet and the
stator field that it is trying to track.

If it's a normal squirrel cage induction motor (which seems like
a good fit for this use), then torque is proportional to the
difference in speed between the rotor and the rotating magnetic
field. The usual startup problem is that the motor tries too
hard and will rip itself apart if you don't limit the current.

Some sort of current limiting would probably be a good idea.
I'm not sure if you gain anything by slowly ramping up the
frequency, but I could easily be missing something.

--
The suespammers.org mail server is located in California. So are all my
other mailboxes. Please do not send unsolicited bulk e-mail or unsolicited
commercial e-mail to my suespammers.org address or any of my other addresses.
These are my opinions, not necessarily my employer's. I hate spam.
Thanks for that. Definately no magnets in there and clear signs of a
squirrel cage, so I am certain this is an induction motor. From what you say
it sounds like what I should do is use a fixed 1250Hx frequency, but ramp up
the supply voltage or current as the motor heads twards full speed ???

Scrim
 
S

Scrim

Jan 1, 1970
0
Zak said:
Don't run it in air. It doesn't run very much at all in that case, and
agood enough starting vacuum will not be that hard to reach.


This would be a whole lot easier. I'd make high side drivers myself and
run the whole thing on square waves. Use the desired RMS value as DC
power voltage. 31 V RMS to neutral means + and - 31 volt power.

To drive you could even use optocouplers. 3 phase can be generated with
a 4017...

But run the thing in vacuum only or the motor will burn out. You may be
able to find data sheets on line of these pumps - I found them at least
for Alcatel pumps.


Thomas

Thanks Thomas. Unfortunately Varian don't play ball. They won't even give
advice on cleaning your pump! It's a full on case of 'No User Servisable
Parts'. Some of the other sellers of turbomolecular pumps are very helpful,
but Varian systems are strictly suited to users who don't want to touch the
system themselves. You have to work hard to glean information from the
sparse available sources.

Scrim
 
H

Hal Murray

Jan 1, 1970
0
Thanks for that. Definately no magnets in there and clear signs of a
squirrel cage, so I am certain this is an induction motor. From what you say
it sounds like what I should do is use a fixed 1250Hx frequency, but ramp up
the supply voltage or current as the motor heads twards full speed ???

That's what I would try first. I can't think of anything bad
that can happen if you don't give it enough power.

You can also try a lower frequency if it's easier to generate.

Do you have any way of measuring rotor speed?
 
T

Tony Williams

Jan 1, 1970
0
Hal Murray said:
That's what I would try first. I can't think of anything bad
that can happen if you don't give it enough power.
You can also try a lower frequency if it's easier to generate.

The centrifuges I saw many years ago were spun up
slowly, ramping up both the frequency and the AC
voltage.

At 100,000 rpm they were also in armoured 'dustbins'
because of the dangers of a crash.
Do you have any way of measuring rotor speed?

Our instrument took the power off for 10 seconds
and measured the frequencies of the back emf at
that 10 second interval. The first freqency gave
the rotor speed and the difference between the
two frequencies allowed the mechanical losses to
be calculated. If the watts-loss was too high
then a crash was imminent and the centrifuge could
be taken offline.
 
S

Scrim

Jan 1, 1970
0
Hal Murray said:
That's what I would try first. I can't think of anything bad
that can happen if you don't give it enough power.

You can also try a lower frequency if it's easier to generate.

Do you have any way of measuring rotor speed?

--
The suespammers.org mail server is located in California. So are all my
other mailboxes. Please do not send unsolicited bulk e-mail or unsolicited
commercial e-mail to my suespammers.org address or any of my other addresses.
These are my opinions, not necessarily my employer's. I hate spam.
I intend to find a way to measure rotor speed. If I find a suitable glass
vacuum vessel it will be quite simple to combine a laser pointer and
photodiode to generate an ac rpm signal with a reflection from the rotor.

Scrim
 
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