Voltage Drop Problem.

Ohms law

• Is there a dirty connection somewhere?
• Is there somewhere where a terminal screw has been tightened onto the insulation instead of the conductor?
• What is the end-to-end resistance of the cable?
• Is there a partial short circuit somewhere in the wiring?
• What is the resistance of the contactors?
• Is the reed switch contact rated for the expected current?
• Is the power supply correctly rated to supply the intended load?
• Etc., etc.

The resistance (R) of a wire is related to the resistivity (Rho), the length (L) and the cross-sectional area (A) by the equation R = (Rho) • L / A resistivity Cu 17.2e-9 Ω-m or 17.2e-6 ohm-mm

AC solenoid coils can have a high inrush current which drops down to a fraction of its value when the magnet faces touch. So, the voltage drops due to the high inrush current and then recovers once the relay is closed. Is this happening in your case? Maybe it is not so severe since they are reed relays, nevertheless the phenomenon exists and is worth checking.

Why not use DC. Change the power supply and contactors to 24 VDC and things should be OK. It is amazing how far you can run DC with very little voltage loss.

vtbgiraud,

I disagree. Although the DC resistance of a wire can be less than the AC resistance of a wire, this is significant only as wire sizes get larger (the "skin effect"). For the size in question, the two are virtually the same. The limiting factor is in Ohm/s law, as stated in Anonymous #1's post. Regardless of whether it is AC or DC, with very little current flow, the voltage drop will be very low, and with significant current flow, it will be a problem. The real difficulty in the OP's design is the voltage drop under load is a significant percent of the applied circuit voltage. This then causes the component he is controlling to either work intermittently or not at all. Then when he replaced it with a component of much smaller load (smaller current flow), and sized for a lower voltage, suddenly he found the voltage drop was smaller and now the voltage on the component was too high! No surprise. If he had started with a much higher circuit DC voltage, then the drop would have been a much smaller percentage of the applied voltage and the working voltage would have been within the design range for the components.

If your post is true, then why are the wires from a car battery to the starter so heavy?

--JMM

Just apply simple logic and test procedures, using the law mentioned in #1.
Get a test resistor of kown value, put it across the line and measure the current through it and the voltage across it. You can then work out the equivalent resistence of thesource+ line+connections+faults.
E.G If you have 24v AC at source but only get 12v AC across the resistor then the source+line+faults etc must be the same as the test resistor.

Do the same back at the source and compare the two results.
Then stop and think (whilst drinking tea/coffee etc)
Del

could use triacs at the contactors/relays triggered by signal from reed switches. no problem then with voltage drop through long cable.

In a word, yes, its because of the voltage drop (of the wire).

The voltage drop over the wires is going to be dependent on the load at the end of the wires. The 24AC contactors seems to be a bigger load (lower resistance), increasing current draw, and thus increasing voltage drop over the wires (and associated connection points). You could increase the size of the wires to reduce voltage drop -1 and 1.5 sq. mm seems a little small for the run length you are using (17 and 15 gauge), I would use at least 14 gauge wire.

You need to match your wire gauge to the load as well as calculating the wire resistance for long runs. A smaller load will allow the use of smaller wires.

Best solution would be to increase voltage to 110 or 220VAC, then voltage drop will be divided by 5 or 10 in absolute value and 25 to 100 in relative value.

Surely the best solution is it identify the problem rather than risk a fire by simply increasing the voltage!
Solving problems is easy, the hard thing is identifying the problem.
Del

For similar application I have used the following.

Source supply 24 V.DC

Cable used 2.5 sq.mm Copper.

24 volts DC relay (low burden) to operate from field reed switch

Contactor any voltage to suit the circuit (in my case 230V AC) to be operated thru relay contact.

Distance was 500 met.

System is working fine for the last 6 years

This is my experience