Electromagnetic solenoids

[Sunour]

What would be some of the effects of extreme heat (hot enough that you cant leave your hand on it for more than a second or two!) on 24 vdc electromagnetic solenoids opening and closing hydraulics valves on a machine that runs 24/7 for 3.5 months non stop? Part of the heat comes from the hydraulic fluid and part from the solenoid being cycled off/on very frequently.

The problem is with an unusual amount of relay blocks on a PLC output card having to be replaced and they are all outputs to the solenoids.

Any help would be appreciated!

[sPuDd]

I would think its just the usual arcing caused by the

collapsing field as the coil is turned off. Nothing

unusual for DC coils driven by relays. Make sure the

coils have a snubber diode across them, to kill the

back emf.

We use electromagnetic locks in the security industry,

and they give quite a nice back emf. I had one throw me

off a ladder once. A decent voltage and current rated

fast diode across the solenoid coil should solve most

of your problems.

sPuDd..

[powerbuoy]

I agree with sPuDd, please review the following http://power.mcmaster.ca/szabados/courses/4pk4/notes/dcsrc.pdf

page 2 describes the free-wheeling diode (snubber diode)

pb

[Opera House]

The problem with many DC PLC outputs is that they are darlington outputs that do not fully saturate and they produce a lot of heat from the almost two volts across them.  I had to design a solenoid driver for a high speed glue valve that couldn't increase in temperature more than 8 degrees.  Solenoid valve need a lot of current to pull in and then that current can be reduced to only 20% for hold in.  There are solenoid driver (fuel injector) chips that can do this by PWM or an alternate low tech method is to pur a capaciror of 500-1000uF in parallel with a current limiting resistor.  That will be in series with the driver.

[Nando]

This is a common problem which is caused by borderline designs.

The Relays are switching ON OFF an inductance ( Valve solenoids) which may have some current through them and when the relay opens its contact, the current needs to continuously flow causing a very high increase in the voltage across the contacts which generates an electric arc which erodes the relays contact.

Solutions depend on the use of the valve solenoid and its acting speed to open and close the valve.

A MOV ( varistor) or a power Zener is placed across the contacts to reduce the arc to a very low voltage step reducing the erosion by a factor of several decades.

A possible circuit is the one that turns on the voltage of the solenoid, then after the solenoid closes the voltage is reduced enough to keep the valve operated and when the relay opens the energy of the arc may be low that with the additional arc protection the relay has its maximum rated life.

The BEST possible solution would be to add a voltage clamp with a capacitor, a diode and a resistor for the capacitor to eliminate the arc with the diode, then the resistor to discharge the capacitor prior the next valve energizing cycle and the values depend on the inductance of the solenoid and the supply voltage, the current in the energized solenoid, the speed of operation of the valve and the minimum time between the energizing pulses.

If you have the data, then send it to me and I calculate the values which You could install along the solenoid wiring or on the board itself.

AGAIN:

The protection depends on the velocity of the valve cycling

For fast acting valves, a good possible solution is a bipolar supply with is circuit to detect when the choke of the valve solenoid starts to arrive to a negative current de-energizing to stop the opposite supply and open the circuit with an elemental contact protection -- unhappily this design is a bit complicated and most probably your equipment is not that way.

Nando

[Opera House]

Believe me when I say a diode won't solve your problems.  I used to manufacture several types of snubbers.  I towed the company line, but in most cases they do about as much as putting STP in your car engine.  The PLC manufacturers have already included this in the PLC design.  You need to reduce the average current the solenoid draws.  Your best option is to try the resistor combination.  Try using a resistor that is the same resistance as the solenoid.  This will cut the current in half.  The capacitor has to be large enough to supply the current to pull in.  It also must be small enough so that it discharges through the resistor in the off period.  If the solenoid isn't off for at least two seconds a more complex electronic method should be used.  When you have a lot of output cards packed together the heat builds up.  I can almost guarantee that you are operating these cards way beyond the manufacturers detailed spec.  

[coldspot]

"energized solenoid"

Thats something I've wondered-

How long can you keep one activated?

I've collected a few over the years,

a couple new in box for outboard motor

choke systems. A tiny one a few months ago from

I forget, but this is the smallest I've seen.

 less in size than a pencil around and about

a half inch long.

I've always wanted to make something out of the boat ones and thought that it would need to be a short activated time. maybe a locking set-up that would require the solenoid to move something to be able to unlock. I figured that the solenoid couldn't be held in the on stage for long times with out smoking the coil.