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/_2001_retired_files/rotary_conv.jpg
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...
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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On Fri, 06 Feb 2004 18:58:18 GMT, the renowned "Toolbert"

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
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in message wrote:

hysteresis
current.
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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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
in message Info for manufacturers: http://www.trexon.com

http://www.speff.com
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On Fri, 6 Feb 2004 18:24:06 -0600, the renowned "Bob Swinney"

Okay, that sounds like the way to go, thanks, Bob.
Best regards, Spehro Pefhany
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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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On Fri, 06 Feb 2004 19:03:52 +0000, Peter H. wrote:

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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On Fri, 06 Feb 2004 14:12:30 -0500, Artemia Salina

Maybe they just don't like you. Grainger in southern MS sells over the counter to anyone.
Ron Thompson Was On the Beautiful Mississippi Gulf Coast, Now On the Beautiful Florida Space Coast, right beside the Kennedy Space Center, USA
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'If you're standing in a puddle, don't touch anything that hums' From the Red Green show
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Peter H. wrote:

You might try USAMfg href="http://ww2.usamfg.net/cgi-bin/onramp.exe?custnum=&password=&pgm=itemnum.bbx&itemno 617&button=Submit" Shipping/handling will be more than the relay, but you will have a relay. Real nice people and prompt service.
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Keith Bowers - Thomasville, NC

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replying to Peter H., Jim wrote:

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.

connected the 900 microfarad start capacitor, switched by the auxiliary contactor).

Hi Peter,
Would there be a wiring diagram to accompany the details above?
Have a good day,
Jim
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replying to Peter H., Jim wrote:

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.

volt converter.

connected the 900 microfarad start capacitor, switched by the auxiliary contactor).

nearly ideal 60/40 percent balance (actually, an intentional imbalance).

Hi Peter,
Would there be a wiring diagram to accompany the details above?
Have a good day,
Jim
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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 snipped-for-privacy@yahoo.com (gradstdnt) wrote:

Visit my website: http://www.frugalmachinist.com Opinions expressed are those of my wifes, I had no input whatsoever. Remove "nospam" from email addy.
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---------------------------- I used a double pole NO start push button. One pole to close the motor contactor and the other to close a relay that connected the start caps. After the convertor starts release the start button and the relay drops out the start caps. No expensive potential relay needed. Don Warner ---------------------------

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