Controlling from far


Thread Starter


I'm interested in starting a pump from a two point level switch located 3 miles from the pump controller, wired not wireless. How can this be done?

Bob Peterson

you really have three chocies.

1. run long wires.

2. use some kind of wireless communications.

3. use wired communications.

If you have a problem with wireless, that leaves you with the other two choices.
I recommend two wired circuits. Use one circuit for control and the other circuit for power to the pump. At a distance of 3 miles, you want AC (alternating current) in both the control and the power circuits. The simplest way is to use a relay with a coil rated at 120 VAC, or whatever voltage your local power company supplies. Direct current, DC, has too much power loss in the wires (heating the wires) at this distance. If your power source is DC (solar panels or batteries) then use a DC-to-AC converter to push the energy over the wires. The relay should be mounted near the pump, and the relay contacts should be rated for the voltage, current, and life cycle of your planned application.

Ken Stewart

Phil Corso PE

Responding to Gabriel's 07-Apr-07 (12:40am) query:

I strongly oppose the use of AC power for the circuit length you mentioned. Such circuits fail because of the phenomenon called "long-wire capacitance!" While possible to energize (close) such a circuit, wire-to-wire or wire-to-ground capacitance introduces de-energization or drop-out problems!

Of course, the capacitance for virtually any configuration can be determined. For example, there are formulas for wire-to-wire, wire-to-ground (earth), wire-to metalic conduit wall, and even wire-to-nonmetallic condit wall, but, such calculations will not solve the problem because you can't effectively negate it.

You might be successful with pole-mounted, open-line construcion, provided you adequately separate the two leads. However, that then, introduces inductive-impedance problems.

If you want additional details, let me know!
Regards, Phil Corso, PE. ([email protected])

Michael Griffin

In reply to Phil Corso PE: Emergency stop circuits in most safety relays use DC for this precise reason.
Mr. Phil Corso,

Thank you for your concern.

Would you give me more detailed about this subject? Some suggest to connect a capacitor accross the stop switch, and believe on reducing the effect of long wire capacitance. Is there any formula to calculate the effects of Long wire transmission? Besides some says one kilometer length can not cause "long wire capacitance effect".

I do appreciate your concern and advice.

Best regards,

Phil Corso, PE

Responding to Zare's 28-Aug (00:12) questions:

Because the penomenon is capacitor related, placing a cap across the stop-switch only exacerbates the problem!

a) There are in fact a number of circuit arrangements associated with the typical 3-wire stop/start station wiring. The "critical" capacitance that prevents the starter-contactor coil from "dropping-out", is calculable. But, frankly, it's easier to "measure" the capacitance using a simple R-L-C measuring instrument!

b) But if you want to pursue it, you can find capacitor calculation formulas in the "Standard Handbook for Electrical Engineers!"

True for cable or wire in a dry environment! But, not true for water-filled conduit, or direct-buried cable, or cable having "lossy" insulation. In fact it can be shown that immersing

a) Adding a resistor across the contactor coil works.. sometimes! However, in my experience the problem grows as the system ages! Also, the additional power consumption presents other problems, like an increased voltage-drop that causes "chattering" in start phase!
b) Convert the entire control circuit to dc. Michael Griffin's suggestion is certainly noteworthy, and should be considered!
c) Use an interposing relay circuit. My company, TRIP-A-LARM (now defunct because of an ISA "edict" against solid-state devices) introduced a patented-product, Smart-A-Larm, in the early '80s! It not only eliminated the problem but could diagnose and indicate an open, short, or ground condition on any of the 3-wires!

Zare, if you want more information, contact me off-list.

Regards, Phil Corso ([email protected])
I would recommend that you install what is known as a mechanically latched contactor. Such a contactor will only follow the 'Rise' State of the commands, i.e. when the power to the closing coil is switched ON the contactor closes. Contactor then latches on mechanically, and the 'close' command can be switched off. This may take a 'long' time as the contactor will still remain on. Normally in such contactors, a normally closed auxiliary contact on the contactor will switch off the supply to the closing coil. The same goes for the trip command, whereby power is sent to the trip coil which de-latches the contactor. A normally open auxiliary contact on the contactor will switch off the supply to the trip coil.

This way you will easily overcome any capacitance effects in the control cables, as the supplies on the control coils are interrupted by the contactor itself. In such a case I would recommend you use AC supply for both closing and tripping commands to avoid un-necessary arcing in the auxiliary contacts.
Mr. Corso,

Thank you for your reply. I wanted to send you email but it cannot be delivered to you. May I have any other email address from you? I do appreciate your concern.

Best regards,
Vahid zare

Elusive Solutions Inc.

Zare, we are now working with Phil to bring back his designs and concepts for the very problem you are trying to solve. Contact me at or [email protected] if you have any questions.
I assume there is no power available at the switch location. Since the cost of your cabling will be considerable perhaps you could consider changing the two point level switch to a 4-20 mA transmitter then you could install inexpensive telephone cable underground or string it along your power line poles. Then you can use an alarm relay at the motor for Start/Stop. This will also allow you to read the level from your pump location.

If you decide to use the switches use DC 24 or 48V with an interposing relay at the pump to minimize the effect of voltage drop
If it were me I would use one of the wireless devices.