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Old February 6th 04, 06:22 PM
gradstdnt
 
Posts: n/a
Default Potential Relay for phase converter

I am working on building a 7.5 hp rotary converter. I plan on using a
start circuit with start cap that drops out after a potential relay
senses enough voltage in the generated leg. Grainger has a listing of
potential relays but I really don't like dealing with them as I am not
a business and their past stock levels leave much to be desired. I
preferr McMaster Carr for service and ease of ordering. Strangely
enough, McMaster has zero listings for "potential relay" but has
plenty of double throw relays.

It appears to me, a double throw relay, one that can be wired either
normally off or normall on, should be able to work just fine as long
as the voltages for the relay cutout are appropriate. Is a potential
relay simply a "normally on" relay with a certain or adjustable cutoff
voltage?

I've got most all the hardware to start putting together this
converter except for the start cap and relay. I know what I need for
a start cap. Any help on specs for a relay that will do the job is
greatly appreciated.

Here is a shot of my 3 hp converter I built over three years ago. I
outgrew it and thus a 7.5 hp unit. Will be up for sale as soon as I
get my 7.5 hp unit together.

http://www.metalworking.com/dropbox/...otary_conv.jpg

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Old February 6th 04, 06:58 PM
Toolbert
 
Posts: n/a
Default Potential Relay for phase converter

"gradstdnt" wrote in message
om...
I am working on building a 7.5 hp rotary converter. I plan on using a
start circuit with start cap that drops out after a potential relay
senses enough voltage in the generated leg. Grainger has a listing of
potential relays but I really don't like dealing with them as I am not
a business and their past stock levels leave much to be desired. I
preferr McMaster Carr for service and ease of ordering. Strangely
enough, McMaster has zero listings for "potential relay" but has
plenty of double throw relays.

It appears to me, a double throw relay, one that can be wired either
normally off or normall on, should be able to work just fine as long
as the voltages for the relay cutout are appropriate. Is a potential
relay simply a "normally on" relay with a certain or adjustable cutoff
voltage?

I've got most all the hardware to start putting together this
converter except for the start cap and relay. I know what I need for
a start cap. Any help on specs for a relay that will do the job is
greatly appreciated.

....

After some searching, Grainger is the only place I found that sells
potential relays. They aren't normal relays. Your best bet for an
alternative source is a commercial air conditioning / refrigeration shop as
that is where these are normally used. (Starting single-phase hermetic
motors)

A potential relay differs from a normal relay in that it has a precise spec
and some hysteresis over the opening and closing voltages. A normal relay
will not predictably pull in and firmly engage the contacts as the coil
voltage slowly increases. The potential relay stays open until a high
threshold is reached and then stays closed until a low threshold is reached.
Your mileage may vary... maybe you can find a 240V coil DPDT relay that
will work in your application.

If you don't use a potential relay I suggest using a different start
circuit - either a one-shot timing relay that is adjustable down to the
sub-second range, or the "Jim Hanrahan" pushbutton start circuit which I
don't like for safety reasons but that is OK if you're the only one using
the shop.

If you or a friend can get into some experimenting, you can build hysteresis
into a DPDT relay by using one set of contacts and some power resistors
wired to add a "boost" to the coil voltage once the relay pulls in.
Resistance and power rating would depend on the particular coil and current.
Seems like something that ought to work but that is only ni theory.

Bob


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Old February 6th 04, 07:03 PM
Peter H.
 
Posts: n/a
Default Potential Relay for phase converter



I am working on building a 7.5 hp rotary converter. I plan on using a start
circuit with start cap that drops out after a potential relay senses enough
voltage in the generated leg. Grainger has a listing of potential relays but I
really don't like dealing with them as I am not a business and their past stock
levels leave much to be desired. I preferr McMaster Carr for service and ease
of ordering.


Grainger is just as easy, if not easier, to do business with, as McM-C. YMMV,
of course.

Grainger stocks General Electric and Steveco potential relays.

The Steveco 90-66 is the one most folks have had the greatest success with.

G.E. potential relays are rated 3 HP.

Steveco potential relays are rated 5 HP.

You will have to use an auxiliary contactor to start your converter, because
your idler rating exceeds the potential relay rating. Select a contactor rated
at least 7.5 HP at 240 volts, assuming a 240 volt converter, of course.

Figure on about 900 microfarads for your start capacitor, also assuming a 240
volt converter.

Figure on about 90 microfarads for your A-B capacitor (across which would be
connected the 900 microfarad start capacitor, switched by the auxiliary
contactor).

Figure on about 60 microfarads for your C-B capacitor, thereby giving you the
nearly ideal 60/40 percent balance (actually, an intentional imbalance).

Figure on about 30 microfarads for your A-C power factor correcting capacitor.

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Old February 6th 04, 07:12 PM
Artemia Salina
 
Posts: n/a
Default Potential Relay for phase converter

On Fri, 06 Feb 2004 19:03:52 +0000, Peter H. wrote:


Grainger is just as easy, if not easier, to do business with, as McM-C. YMMV,
of course.


Unless things have changed in the past two years Grainger will not sell to
anyone unless they have a tax number (i.e. must be buying for a registered
business). I know this from personal experience.

McMaster-Carr, Enco, MSC, etc. on the other hand make no such distinctions.

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Old February 6th 04, 07:25 PM
Spehro Pefhany
 
Posts: n/a
Default Potential Relay for phase converter

On Fri, 06 Feb 2004 18:58:18 GMT, the renowned "Toolbert"
wrote:


If you or a friend can get into some experimenting, you can build hysteresis
into a DPDT relay by using one set of contacts and some power resistors
wired to add a "boost" to the coil voltage once the relay pulls in.
Resistance and power rating would depend on the particular coil and current.
Seems like something that ought to work but that is only ni theory.
Bob


I have to make a 10HP rotary converter.. what do you suggest for the
capacitors and what are the potential relay on/off voltages? I might
whip up something electronic to switch a regular contactor if someone
can tell me what it should do.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com


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Old February 6th 04, 08:25 PM
Toolbert
 
Posts: n/a
Default Potential Relay for phase converter

"Spehro Pefhany " Spehro Pefhany wrote
in message ...
On Fri, 06 Feb 2004 18:58:18 GMT, the renowned "Toolbert"
wrote:


If you or a friend can get into some experimenting, you can build

hysteresis
into a DPDT relay by using one set of contacts and some power resistors
wired to add a "boost" to the coil voltage once the relay pulls in.
Resistance and power rating would depend on the particular coil and

current.
Seems like something that ought to work but that is only ni theory.
Bob


I have to make a 10HP rotary converter.. what do you suggest for the
capacitors and what are the potential relay on/off voltages? I might
whip up something electronic to switch a regular contactor if someone
can tell me what it should do.


The contact opens when the coil voltage passes ~ 170 volts and closes when
it drops below 90 volts. In a rotary phase converter that is connected
across the 3rd leg to neutral not phase-to-phase. Keep in mind the 3 phase
neutral is offset from the single phase neutral. 3rd leg to single phase
neutral is typically 200 volts.

I don't know if the 170 / 90 is optimal. With an undersized idler the 3rd
leg will drop low enough to kick in the relay when starting a large motor.
That isn't good for the start capacitors but it does give a useful boost to
the load - engaging the start capacitors to get the load motor up to speed
faster. If you build something electronic, probably better to build it as a
one-shot that can't kick in again until the entire unit is power cycled.

Capacitors just use the r.c.m. guidelines ... 15 uF per hp per phase run, 70
uF per hp start I think. I've built or collaborated on four 7.5 hp
converters with good results using 92 uF run capacitors (one each side) and
500 uF start capacitors (two 250 uF in parallel).

Bob


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Old February 7th 04, 12:11 AM
keith bowers
 
Posts: n/a
Default Potential Relay for phase converter

Peter H. wrote:



I am working on building a 7.5 hp rotary converter. I plan on using a
start circuit with start cap that drops out after a potential relay senses
enough
voltage in the generated leg. Grainger has a listing of potential relays
but I really don't like dealing with them as I am not a business and their
past stock
levels leave much to be desired. I preferr McMaster Carr for service and
ease of ordering.


Grainger is just as easy, if not easier, to do business with, as McM-C.
YMMV, of course.

Grainger stocks General Electric and Steveco potential relays.

The Steveco 90-66 is the one most folks have had the greatest success
with.

G.E. potential relays are rated 3 HP.

Steveco potential relays are rated 5 HP.

You will have to use an auxiliary contactor to start your converter,
because your idler rating exceeds the potential relay rating. Select a
contactor rated at least 7.5 HP at 240 volts, assuming a 240 volt
converter, of course.

Figure on about 900 microfarads for your start capacitor, also assuming a
240 volt converter.

Figure on about 90 microfarads for your A-B capacitor (across which would
be connected the 900 microfarad start capacitor, switched by the auxiliary
contactor).

Figure on about 60 microfarads for your C-B capacitor, thereby giving you
the nearly ideal 60/40 percent balance (actually, an intentional
imbalance).

Figure on about 30 microfarads for your A-C power factor correcting
capacitor.

You might try USAMfg
href="http://ww2.usamfg.net/cgi-bin/onramp.exe?custnum=&password=&pgm=itemnum.bbx&item no=88617&button=Submit"
Shipping/handling will be more than the relay, but you will have a relay.
Real nice people and prompt service.
--
Keith Bowers - Thomasville, NC
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Old February 7th 04, 12:24 AM
Bob Swinney
 
Posts: n/a
Default Potential Relay for phase converter

You can get a Steveco No. 90-65 potential relay rated 50 amps at a motor or
AC service establishment.
They are around !2 - 15 $.

Bob Swinney
"Spehro Pefhany " Spehro Pefhany wrote
in message ...
On Fri, 06 Feb 2004 18:58:18 GMT, the renowned "Toolbert"
wrote:


If you or a friend can get into some experimenting, you can build

hysteresis
into a DPDT relay by using one set of contacts and some power resistors
wired to add a "boost" to the coil voltage once the relay pulls in.
Resistance and power rating would depend on the particular coil and

current.
Seems like something that ought to work but that is only ni theory.
Bob


I have to make a 10HP rotary converter.. what do you suggest for the
capacitors and what are the potential relay on/off voltages? I might
whip up something electronic to switch a regular contactor if someone
can tell me what it should do.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers:

http://www.trexon.com
Embedded software/hardware/analog Info for designers:

http://www.speff.com


  #9   Report Post  
Old February 7th 04, 12:38 AM
Spehro Pefhany
 
Posts: n/a
Default Potential Relay for phase converter

On Fri, 6 Feb 2004 18:24:06 -0600, the renowned "Bob Swinney"
wrote:

You can get a Steveco No. 90-65 potential relay rated 50 amps at a motor or
AC service establishment.
They are around !2 - 15 $.


Okay, that sounds like the way to go, thanks, Bob.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
  #10   Report Post  
Old February 7th 04, 02:48 AM
Roy
 
Posts: n/a
Default Potential Relay for phase converter

here are many laces that sells potential relays other than Grainger.
HVAC, and lots of electrical supply houses sells them.


On 6 Feb 2004 10:22:12 -0800, (gradstdnt)
wrote:

===I am working on building a 7.5 hp rotary converter. I plan on using a
===start circuit with start cap that drops out after a potential relay
===senses enough voltage in the generated leg. Grainger has a listing of
===potential relays but I really don't like dealing with them as I am not
===a business and their past stock levels leave much to be desired. I
===preferr McMaster Carr for service and ease of ordering. Strangely
===enough, McMaster has zero listings for "potential relay" but has
===plenty of double throw relays.
===
===It appears to me, a double throw relay, one that can be wired either
===normally off or normall on, should be able to work just fine as long
===as the voltages for the relay cutout are appropriate. Is a potential
===relay simply a "normally on" relay with a certain or adjustable cutoff
===voltage?
===
===I've got most all the hardware to start putting together this
===converter except for the start cap and relay. I know what I need for
===a start cap. Any help on specs for a relay that will do the job is
===greatly appreciated.
===
===Here is a shot of my 3 hp converter I built over three years ago. I
===outgrew it and thus a 7.5 hp unit. Will be up for sale as soon as I
===get my 7.5 hp unit together.
===
===
http://www.metalworking.com/dropbox/...otary_conv.jpg

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