Solenoid Fuse Keeps Blowing

I

Thread Starter

instrumaInt

We recently faced problem with 110 volts AC operated solenoid used to actuate hydraulic control valve. The fuse to the solenoid was repeatedly blowing. Upon opening the solenoid termination compartment, we found a small PCB having 6 diodes. One of the diodes had shorted causing the fuse to blow.

The solenoid nameplate shows it is an AC solenoid. However diodes seem to form rectifier circuit. Is it DC solenoid used with AC voltage?
Can I replace the solenoid with AC solenoid ( i.e. without diodes ) since voltage available is 110 V AC.

My concern is diodes form an additional component which can fail. We cannot afford failures in this application.
 
If the diodes are in a bridge setup, you can substitute a 50 amp 1000 volt bridge rectifier and mount it to the board or cabinet and run the leads to the board. This should eliminate failures like this.

As for replacement, you might have trouble finding something that is the right size with the right pull strength and length. Engineers don't usually add components when they don't have to, so unless you know the engineers used a bit of this surplus and a bit of that retread, I'd be chary of changing it.

One of those six should be inversely polarized across the coil to suppress back EMF spikes which can be much higher in voltage than applied voltage, as you know. You could put an MOV or transient suppressor on the line to the solenoid and make it a monthly maintenance routine to replace it. They go for about 50 cents or less. You could even use faston connectors to the bridge and replace it yearly for $5 or so.
 
Solenoid coils can be AC or DC. When using AC, the current, and thus the magnetic flux, is crossing zero at the frequency rate. Therefore the coil will hum and or chatter if not designed correctly. To avoid that, the coil ends up drawing more power, which becomes more of a burden on the electrical supply going to it and that becomes worse with distance as voltage drops with wire length. A DC coil is preferred because there is no inrush current and the burden current is lower since the magnetic flux remains constant, but DC suffers from voltage drop issues even more than AC. So for devices that are far away from a source, a DC coil that can be fed by AC is the preferred method of making the most reliable system with the lowest installed cost.

If you are a valve maker, making the same DC coil for everything also makes economic sense, but users have a mixture of AC and DC sources in the field. So having that PCB on it that rectifies an AC supply to DC means it can be powered by either (because if supplied by DC, it just goes straight through the rectifier). Most likely that coil is also a lower DC voltage, such as 24VDC, and that PCB is actually a small SMPS (Switch Mode Power Supply) with a very wide AC input voltage range, such as 40-240VAC, which it rectifies and chops down to 24VDC, making it more impervious to line voltage fluctuations.

In your case, replacing the coil with an AC coil may result in other unwanted consequences, such as a larger coil and therefore more severe voltage drop by the time your AC gets to it and attempts to pull it in, or necessitating the addition of a new power source closer to the valve. It might be possible to do a simple change out based on the particulars of your installation, but it must be thoroughly investigated.
 
R
> If the diodes are in a bridge setup, you can substitute a
> 50 amp 1000 volt bridge rectifier and mount it to the board
> or cabinet and run the leads to the board. This should
> eliminate failures like this.

This is a very good idea because you can bolt it to the enclosure to act as a heat sink and it doesn't need to be insulated.

Large DC coils are a bit of a problem because the current required to pull it in doesn't reduce once it's closed the magnetic circuit unlike an AC coil that goes up in impedance once the magnetic circuit is closed.

Sometimes a DC coil has an economy resistor that switches in series to drop the holding current.

Another common example is the solenoid on an automotive starter, it has two coils pulling about 50 Amps dropping to 10 once the main contacts close.
 
W
I am not sure that anyone actually answered one of your original questions, i.e. "Can I replace the solenoid with AC solenoid ( i.e. without diodes ) since voltage available is 110 V AC."

The answer is a "Yes." Buy a 110 VAC solenoid from an reputable industrial supplier of solenoids such as ASCO (http://www.ascovalve.com/) or Versa (www.versa-valves.com). Both of these brand solenoids are commonly used in safety systems and are very reliable. You might look at the ASCO 327 or Red Hat solenoids or the Versa V316 series (please see the links below).

http://www.ascovalve.com/Applications/Products/SolenoidValves.aspx

https://www.asconumatics.eu/images/site/upload/_en/pdf1/80020gb.pdf

http://www.versa-valves.com/sites/default/files/catalogs/LIT_V_Stainless_Steel_USA.pdf#page=

Make sure you select the correct temperature range both high and low. Some people forget the low end but many solenoids are rated only to 32 degrees F (0 degree C) and you can get in trouble if your solenoid is exposed to too cold of temperatures. You should specify a good quality insulation for the coil even if your temperature range is not that high (e.g. epoxy encapsulated, Class H).

William (Bill) L. Mostia, Jr. PE
ISA Fellow, SIS-TECH Fellow,
FS Eng. (TUV Rheinland)
SIS-TECH Solutions, LP

"No trees were killed to send this message, but a large number of electrons were terribly inconvenienced." Neil deGrasse Tyson

Any information is provided on a Caveat Emptor basis.
 
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