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Line Voltage monitoring - do I need to...

So I'm installing a line voltage monitor [if not a then similar] to protect a single-phase 240V residential heatpump from self-destructing during windstorm-instigated power flaps. It will trigger a 5-10 minute shutdown period.

Since I've failed at finding such a monitor surplus [and NO, I'm not going to build a one-off when I can buy..] I have the choice of ordering a 240, 120 or 24V model.

Now, normally I'd say "Of course, look at the 240; that's what powers the heatpump..." But that means a separate box, or have 240v lurking in the otherwise LV controls cabinet [no...].

If I looked at 120, yea one side might droop, but in this case... The more I think, the more I like looking at the 24V from the heatpump's own control transformer.

See, this house is fed by its own 7600V in/240/120 out "yard pig" transformer. [It can't be a pole pig with no pole, right?] It's the end of the line on that leg -- fed by a 2000' buried cable from the public road, and there, there's another 3500' until it meets up with other users.

And the heat pump is the only 240v load of note. So the way I see it, the 240/120 & 24v are all going to flap in lockstep, set by the windings ratios; when the 240 drops 15%, so will the 24v, etc.

Is my thinking wrong? Do I really need to be looking directly at the 240 for some reason I'm overlooking?

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A host is a host from coast to coast.................wb8foz@nrk.com
& no one will talk to a host that\'s close........[v].(301) 56-LINUX
Unless the host (that isn\'t close).........................pob 1433
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Reply to
David Lesher
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What you're after is called "short cycle protection." Most modern units have these built in, but if not then you can buy an "aftermarket" so-called as in:

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That adjustable potential relay from global spec is going to cost, their description of operation is screwy, and it is *not* appropriate for the job. As for brownout conditions, the compressor is already protected, the

240V nominal rated compressor will happily work at 208VAC. If you don't know anything about your unit or what your doing, leave it alone.
Reply to
Fred Bloggs

The system has short-cycle protection, but it's designed to delay restarts, not drop the unit off-line during these kind of flaps. {Or if it was, it's mis-designed...}

--
A host is a host from coast to coast.................wb8foz@nrk.com
& no one will talk to a host that\'s close........[v].(301) 56-LINUX
Unless the host (that isn\'t close).........................pob 1433
is busy, hung or dead....................................20915-1433
Reply to
David Lesher

from

trigger

units

If the heat pump uses a permanent split capacitor motor what you really need to ensure ultimate protection is something like this

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Whichever method you use the heat pump should be wired via a dedicated circuit breaker in the mains distribution box.

Reply to
Ross Herbert

Trust me, it is. It's wired into the #2 panel, being the one dedicated to 240V non-PF loads {i.e. not covered by the generator transfer switch}. [#1 - main disconnect; #3, normal 120V loads; #4 generator loads. #2&3 could be the same panel, but there are too many breakers to fit.] Yes, this place is overbuilt; I won't even go into the rest of the HVAC system.

I don't know if it's PSC or not. But that is an interesting gadget. We have no such regulation here. I also recall from EE412 class that reduced voltage/current limiting starters were harder on the motors than across-the-line, but not exactly why...just that it was a day's lecture & discussion.

--
A host is a host from coast to coast.................wb8foz@nrk.com
& no one will talk to a host that\'s close........[v].(301) 56-LINUX
Unless the host (that isn\'t close).........................pob 1433
is busy, hung or dead....................................20915-1433
Reply to
David Lesher

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