This involves controlling irrigation well motors. The well
motors run off 3 phase 480. Two panels with starters, each running
its own well motor. The nearest well supplying water to that system
about 3,900 feet away. The second well pumping water to that system is
probably another 1000 feet away from that system. There's 12-2 w/g
running from the system to the first well, then from the first well to
the second. We want to kill both well motors if the system stops.
We'd like to stay under 30vac for the controls for safety and NEC
reasons. The 24 volt relays we normally use have just under 12 ohms
resistance. My meter showed the amp draw of one at just under .6 amps.
The must activate rating of the relay is about 20 volts.
A voltage drop calculator
<http://www.calculator.net/voltage-drop-calculator.html shows we can't
run both relays. The best idea so far is to control the nearest well.
We'd add a 480x24 transformer there to activate a 24v relay to control
the second well.
You might change the relays to the solid state kind. They are available
from Ebay to handle around 40A for under ten dollars, typically. They
will switch with a DC control signal of 3 to 30 volts at around 10 ma or
so. Check out the specs and if they suit your needs, your problem is
Use some of the solid state 'relays' that only need a few milliamps to
activate. They operate over a wide range of input voltage and the
voltage drop would not be much either. YOu may have to put a seperate
relay operateing off the 480 volts at each pump starter.
Have the solid state pull in the other interposing relay.
On 5/28/17 8:42 AM, Ralph Mowery wrote:
A bunch cut due to aioe limits.
Thanks, Mike and Ralph.
A "couple" more questions. I found a lot of solid state relays that
from the 3-30 or so volts, dc, and others that activate at 90- 100 and
We usually have 24 vac available for control voltage. That's easy
to fix if not.
Is a bridge rectifier the right thing to use to get from the ac to the
dc? How would I size it if so? I've found a lot of them with
voltages listed at well over the 24 we have. Do they have to be sized
with the correct voltage like a relay coil? Would a 1000 volt one work
but just be overkill?
And lastly, for now, how well do solid state relays and bridge
rectifiers handle lightning?
They make some that will switch on a 3 phase motor from a DC control
If you can not find any that work with AC, you can use about any 1 amp
100 volt or more diode, or diode bridge. Usually the small 1 to 5 amp
diodes and bridges don't care about the voltage as long as it is high
enough.. If you only have 24 volts of AC , you can use a diode of 100
volts to 1000 volts without any problem, just cost,but when only buying
10 or less you are only talking about a dollar or less difference.
If you do use the single diode or bridge, put about a 20 UF capacitor
rated from around 100 to 300 volts at the output of the diode to smooth
out the voltage. You may have to play with the value of the capacitor
as to how long you want the motor to keep on running. At work where we
used them, we often put a led with a series resistor across the dc
control voltage so we could see if it was suspose to be off or on.
If you are worried about lightning and do not want to go solid state,
you may want to install just a small relay to pull in that larger relay.
You could even go to say a 12 or 6 volt coil and put a resistor in
series with the coil to compensate for the voltage drop.
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