Leakage using SSR old topic but still need help

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jgrade
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Leakage using SSR old topic but still need help

Post by jgrade » Thu Nov 13, 2008 12:10 pm

I have a need to switch a relay using 240VAC 3 phase to drive an appliance that is 120VAC 1 phase. The question is can I use a standard relay rated for 120VAC on the coil and connect it to L1 and L2 of the 240VAC 3 phase?

I am lost when it comes to phases and if the relay will chatter with the sin; or does taking only one leg of the 240 line give me 120V with a 1 phase sin.

The application is simple. I need to take the three lines coming off a contactor running a 3 phase 240V motor and switch a relay that will drive a 120VAC 1 phase appliance. The reason i need the relay is because there are 3 contactors and 3 motors that need to drive the same 120V appliance, so i thought I would connect the contacts of the relay in parallel and when each motor is switched on the corresponding relay would energize and the 120 appliance would turn on.

Or am I way off base here?

Bigglez
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Re: Connecting realy to 240VAC line for 120VAC use

Post by Bigglez » Thu Nov 13, 2008 12:41 pm

jgrade wrote:I have a need to switch a relay using 240VAC 3 phase to drive an appliance that is 120VAC 1 phase. The question is can I use a standard relay rated for 120VAC on the coil and connect it to L1 and L2 of the 240VAC 3 phase?
Are you in the USA or Canada? Three-phase utility supplies
are not normally provided for domestic customers.

However, domestic customers do get "split-phase" 240V
on two hot conductors called L1 and L2 and a neutral
called N, and a ground.

If this is an industrial or commercial installation you
may indeed have a three-phase supply, and there will
be three line conductors, a neutral, and a ground.

For light industrial loads the three-phase power may
be either 208Y/120 volt or 480Y/277 volt both are wye
systems with a neutral conductor (i.e. four wires).

Its important to understand what you have before
proceeding further.

jgrade
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Post by jgrade » Thu Nov 13, 2008 12:58 pm

Image

Here is a crude example of what I want to do.

jgrade
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Post by jgrade » Thu Nov 13, 2008 1:00 pm

In the US. The application is turning on a chemical feed pump (120V about .6A) whenever any one of three 3 phase 240V pumps activate to add makeup water to a steam boiler. This is an industrial application not domestic. All HVAC stuff.

Bigglez
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Post by Bigglez » Thu Nov 13, 2008 1:13 pm

jgrade wrote:In the US. The application is turning on a chemical feed pump (120V about .6A) whenever any one of three 3 phase 240V pumps activate to add makeup water to a steam boiler. This is an industrial application not domestic. All HVAC stuff.
Your diagram will work. Are the phase voltages
240V? (120/208, 277/480 is common).

Alternatively, you can operate the relay form just
one phase of the three and the wye or neutral.

If you have a DMM handy, measure the phase to phase
and phase to neutral voltages.

Dean Huster
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Post by Dean Huster » Thu Nov 13, 2008 3:45 pm

If the requirement is that any one of THREE pumps will activate the water-adding pump, you'll need an auxiliary contact on each of the three motor starters for the pumps, these auxiliary contacts in parallel and that parallel combination in series with the water-adder so that it will run whenever any one of the three 3-phase pumps is operating.

Dean
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

Robert Reed
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Re: Connecting realy to 240VAC line for 120VAC use

Post by Robert Reed » Thu Nov 13, 2008 7:36 pm

jgrade wrote:I have a need to switch a relay using 240VAC 3 phase to drive an appliance that is 120VAC 1 phase. The question is can I use a standard relay rated for 120VAC on the coil and connect it to L1 and L2 of the 240VAC 3 phase?
If your line is really 240 , why not use standard 240VAC relay and wire phase to phase. It is just as cheap as a 120 VAC relay. Most relays are spec'd with "Must pull-in and Must Drop out" voltages. The Pull-ins are usually 80% of rated coil voltage and would easily work down to a 208VAC line.. One thing I did not see mentioned is - from what source do you intend to get your 120VAC appliance voltage from?

jgrade
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Post by jgrade » Fri Nov 14, 2008 5:39 am

Thanks for all the replies.
If your line is really 240 , why not use standard 240VAC relay and wire phase to phase. It is just as cheap as a 120 VAC relay. Most relays are spec'd with "Must pull-in and Must Drop out" voltages. The Pull-ins are usually 80% of rated coil voltage and would easily work down to a 208VAC line.. One thing I did not see mentioned is - from what source do you intend to get your 120VAC appliance voltage from?
I plan to use a 240V relay, but was unsure about the phase issue. The 120V line is already in the conduit so taping that voltage is simple.
If the requirement is that any one of THREE pumps will activate the water-adding pump, you'll need an auxiliary contact on each of the three motor starters for the pumps, these auxiliary contacts in parallel and that parallel combination in series with the water-adder so that it will run whenever any one of the three 3-phase pumps is operating.
Correct. To make the drawing simple, I just added the one relay. I plan to use three relays and wire the feed pump in parallel.

jgrade
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Post by jgrade » Fri Nov 14, 2008 5:54 am

Updated drawing to reflect actual installation.

Anyone have an opinion about using solid state relays instead of mechanical? I shouldn't have to worry about heat since the cycle will be about 15 seconds max.

dyarker
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Post by dyarker » Fri Nov 14, 2008 6:00 am

Correct. To make the drawing simple, I just added the one relay. I plan to use three relays and wire the feed pump in parallel
Good, you and Robert already thought of that. I didn't till ride home from work.

Steam boiler huh? How about alarms or auto-shutdown if none of the water pumps or the chemical pump don't have flow when they're supposed to?

Cheers,
Dale Y

Robert Reed
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Post by Robert Reed » Fri Nov 14, 2008 7:17 am

With all the plusses of triac switching, I see no advantage to using them in your sitution. The relays would no doubt be a bit cheaper and are self contained and redy to mount easily. Also the triac sheme will still involve some sort of trigger signal - fairly simple but still just adds to the complexity. All said and done, many times its hard to beat the simple and reliable old mechanical relay.

jgrade
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Post by jgrade » Fri Nov 14, 2008 8:03 am

dyarker wrote:
Correct. To make the drawing simple, I just added the one relay. I plan to use three relays and wire the feed pump in parallel
Good, you and Robert already thought of that. I didn't till ride home from work.

Steam boiler huh? How about alarms or auto-shutdown if none of the water pumps or the chemical pump don't have flow when they're supposed to?

Cheers,
That part is already taken care of. I am just adding the sulfite, amine and polymer to keep the boiler working for many years.

Bigglez
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Post by Bigglez » Fri Nov 14, 2008 8:39 am

Robert Reed wrote:With all the plusses of triac switching, I see no advantage to using them in your sitution....All said and done, many times its hard to beat the simple and reliable old mechanical relay.
Generic mechanical relay (240VAC coil, 240V AC 10A) $13ea
Generic SSR (240VAC input, 240V AC 10A) $25ea

Although the initial cost is higher, the SSR has several
advantages (zero-cross switching, no wear) and would
pay for itself after the mechanical relay was replaced
once.

No additional components are required in either
solution - just wiring and mechanical mounting.

many times its hard to beat the simple and reliable

jgrade
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Post by jgrade » Fri Nov 14, 2008 1:37 pm

I don't know where you are shopping for SSR's but I can't find anything less than about $38~39. I was thinking about this SSR. The model I want is the TA1210.

OK now I see. I did not notice the link to Digikey. Thanks.

Robert Reed
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Post by Robert Reed » Fri Nov 14, 2008 2:43 pm

TYCO
240 VAC Coil
30 amp DPDT Contacts
Mouser - aprox. $7
Self contained and with QC conectors
2 screw mounting screws
no heat sink required
no emitted RFI (if important)
depending on frequency usage it may outlive equipment its connected to!
Keep it Simple and keep in mind the savings are times three.

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