The AM Forum

This interesting relay has a magnet between the contacts and the contacts are marked + or - for a definite direction of current flow.

If it is for non-inductive loads, why does it have a quenching magnet?

AMF Potter & Brumfield
PRD11DH0
110VDC coil
DPDT 20A 250VDC non inductive contacts

Patrick, At the following link, post #9 talks about your relay. If I read that correctly, without the magnet the contacts are rated at only 50VDC and with the magnet they are rated at 125VDC. So the polarity is important.

http://www.diyelectriccar.com/forums/showthread.php/evdl-tyco-lev200-series-contactor-1443.html

Switching DC is trickier than AC, even if the load is non-inductive. The lack of periodic zero crossing must have something to do with it.

Jim
WD5JKO

DC relays also have a tendancy to "magnetize" and get either sticky or sluggish after long, long periods of being continually energized. The magnet may be to help prevent this by repeling the armature back to the resting (N.C.) position.

I have seen this happen in industrial control circuits.

That is said to be a problem especially when running a.c. relays off DC - something I have done for years.  Usually, a 110v a.c. relay runs perfectly off 24-30 vdc, and much more quietly.  That has never been a problem with mine.  The only relay I ever had to magnetically stick, was designed for DC: a  relay salvaged from an ARC-5.

Of course, I had run that relay in my transmitter over a period of several times the duration of the War.  I'm sure they never designed a life expectancy of 20+ years into WWII anything. Wouldn't the engineers at companies that designed and built BC-610s, BC-348s and ARC-5s, be smiling in their graves if only they knew that some of their handiwork would still be in daily operation 70 years later? I just replaced my relay with a better one, not from an ARC-5, with no problems since.

A simple solution if DC (or DC-operated AC) relays become sticky is to reverse the polarity of the control voltage.  Maybe even install a DPDT switch in the control line and periodically flip the polarity as a measure of routine maintenance (do this only if there are no diodes across any of the relay coils).  If you want to get fancy, design a simple logic circuit that automatically flips the polarity every other activation.  But as I said, except for the ARC-5 relay, some of my relays (both AC and DC) have been running off 24 vdc since the 1970s or earlier, and they have never become sticky.

I use a 24vac contactor relay like that to swing my balanced line to 2 different matching networks. I had some trouble getting it to pull in occasionally, so I built a "special" power supply for it. It whacks the relay in with around 40+ volts of DC to pull it in, then sags down to around 11 volts to hold it in. The much reduced voltage has not caused a magnetizing problem in 10 years of use.

But I have seen it happen to 24vdc relays running continuosly on 24vdc on several occasions. I guess it is just one of those things that just happens on occasion. It never seemed to be equipment brand or relay brand specific.

It's been a very long time since I've seen one of those. The magnet is used the quench the arc created when switching very high power DC. As I dimly recall,the magnet is situated close to the contacts such that when an arc occurs, it stretches the arc and extinguishes it quickly. Otherwise the contacts will wear out pretty fast. In fact, it's never a good idea to switch high power DC. Unfortunately a lot of Engineers didn't know this in the old days. How may DX-60's have I seen with a worn out function switch HV contact?

The 240 vac contactors in my Gates wouldn't pull in until the DC voltage was high enough to burn up the coils if left on for more than 30 seconds or so.  It took over 120 volts DC to reliably pull them in, but once pulled in, it takes less than 48v to reliably hold them in, and the coils run cool at that voltage.  The main relays that use a latching configuration were easy.  I put a 1K wirewound resistor in series with the latching contacts.  They get full wallop from the power supply when  the Filament or Plate switch is pushed, but as soon as the switch is released, the latching circuit that holds them in has the resistor in series, dropping the DC to a safe voltage.

The High/Low power switch does not use a latching circuit.  I fixed that one by using an identical 1K resistor in series with the relay coil, but with a large "computer grade" electrolytic, 1000 mfd or so, across the resistor. When initially switched to Low Power mode, the relay coil immediately gets full wallop and pulls in, since the discharged capacitor acts like a dead short across the resistor.  As soon as the capacitor charges, the current through the resistor and coil drops to the safe value with the resistor in series.