Voltage drop?

[Dean Hoffman]

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 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. Ideas?

[Mike Jones]

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 solved..

[Ralph Mowery]

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.

[Dean Hoffman]

A bunch cut due to aioe limits.

Thanks, Mike and Ralph. A "couple" more questions. I found a lot of solid state relays that activate from the 3-30 or so volts, dc, and others that activate at 90- 100 and something ac. 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?

[Ralph Mowery]

They make some that will switch on a 3 phase motor from a DC control voltage.

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.