I have a 24 volt relay that starts a 220 volt pump motor for a sprinkler system. The relay and pump are about 800 yards from the 24 volt signal generator.

I have had four relays go bad and fail.

I am thinking that a voltage spike gets into the line - but I do not know from where it occurs.

I would like to fuse the whole system so I can find where the voltage comes from.

Do you think 250 volt AC fuses are sufficient for the pump lines and automotive fuses are good for the signal? Should I use fast or slow blow? Will a Zener diode offer any additional help.

I am a mechanical engineer and know just enough to be dangerous. I would really love to have someone design or even build this for me. $$

  • \$\begingroup\$ There are basically 3 ways the relay can fail - a) the contacts weld shut, b) the contacts are so damaged that they no longer make contact or they don't allow current flow when they do, or c) the coil opens up and the contacts don't move. Which of these are you experiencing? Generally, it's b, but I thought I'd ask. Oh yes, and what gauge wire are you using to send the 24 volts to the relay? My first guess is that you have so much voltage drop in the coil drive that the contacts chatter and burn out. \$\endgroup\$ – WhatRoughBeast Apr 21 '14 at 22:30
  • \$\begingroup\$ I do not know exactly how the relays fail. The relay is inside a box. What I do know is they fail open - meaning that when I apply the 24 volts the pump motor does not run. These were wired by an electrician and they use heavy duty wire. The 24 volt wire is about 1/2 mile long but I am getting 25 to 26 volts at the relay. \$\endgroup\$ – user40681 Apr 29 '14 at 14:30

I believe you're right, and it's a voltage spike that's destroying your relays. A motor is an inductive load. One of the characteristics of inductance is that it will resist a change in current. In other words, if current is flowing through the motor when the relay opens, the motor windings will try to keep current flowing through the (now-open) circuit. This causes arcing and fouling in the relay contacts, which is likely why your relays are failing.

A fusing system won't help you in diagnosing the problem, because they only pop with excessive current (not voltage).

There are two ways for you to go:

1) (easy) Find a relay made for inductive (motor) loads. Look at their spec sheets. Generally, if the relay is rated for inductive loads, it will either say "Inductive" in the contact ratings, or will have an HP (horsepower) rating. An HP rating implies a motor load, which implies inductance.

2) ("fun") Add an MOV (metal-oxide varister) across the relay contacts. It acts kind of like a pressure relief valve. To make it even better, you could add an MOV and an RC snubber, which would make the power very clean.

As an example of relay ratings, look at this one from Schneider Electric. (You'll want to choose your own, tailored to your motor power ratings, etc...)

In the specifications, they show different ratings:


In this case, the one that you're interested in is the Horsepower rating. This relay can safely switch current to a 1-HP motor at 250VAC (or 0.5-HP at 120VAC).

I expect choice #1 will work just fine. If you choose to add an MOV / RC snubber, let us know, and we'll show you how to spec the parts. It's 3 parts at the most, so is not difficult to assemble...

Good luck!

  • \$\begingroup\$ Do you know how these inductive load relays work? I'm just curious. \$\endgroup\$ – Vladimir Cravero Apr 21 '14 at 23:21
  • \$\begingroup\$ @VladimirCravero In some cases, they have an integral MOV :) I'm not really sure what other magic the designers use for these, sorry... \$\endgroup\$ – bitsmack Apr 21 '14 at 23:37
  • \$\begingroup\$ @user40681 I added an example, in case it helps :) \$\endgroup\$ – bitsmack Apr 22 '14 at 0:25
  • \$\begingroup\$ I really appreciate your assistance. This is a great web site and service. It is what the internet should be all about. I have decided to build the device instead of specking a new relay. The motor is 2.0 HP, 3450 RPM, 230 VOLTS, 13.3 AMPS SF, with a service factor of 1.7 Your help with the components needed is greatly appreciated. \$\endgroup\$ – user40681 Apr 25 '14 at 12:30
  • \$\begingroup\$ @user40681 I agree, this is a fun and helpful forum :) I'll type up the info on the MOV and RC Snubber later today. I might suggest that you ask a new question, specifically about protecting relays from voltage spikes. Not only might you get an answer before I am able to get to it, but you may also get a selection of good answers to choose from! If you do post another question, please leave a comment here and let me know. \$\endgroup\$ – bitsmack Apr 25 '14 at 14:09

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