Control Voltage


Thread Starter


I have a situation where the 480volt level is high resulting in 131volts out of my control transformer. As a result we are losing 120volt
solenoids in a faily short period of time. Is there a way (Easy and cheap) to drop this voltage into a range that is more acceptable to the coil of the solenoid? We are looking at the upstream side to see why the 480v is not 480v.



Bouchard, James [CPCCA]

Some control transformers have primary taps for different voltages. Just change the tap if there is one. If not change the transformer for one with taps. Actually 131 volts is just within the =/- 10% margin most utilities give for their voltage supply so your solenoids should be able
to take it. The =/- 10% range is 108 to 132 volts. You could also look at your supply transformer ( if you have one ) it should also have taps that can be changed to get you closer to the desired voltage level.

James Bouchard

Hakan Ozevin

You can use varistors to protect the selenoid coils. It is difficult to determine the voltage increase, but it can be from mismatched capacitor banks, starting of big motors, welding equipment, etc.

Steve Myres, PE

Power transformers come with multiple taps on the high side, usually used for undervoltage line side, but I think they usually include some for overvoltage as well. You'll probably get way too much transformer, and you may end up setting it
on the ground net to the control panel, but it should work as a temporary measure. If the 480V can't be fixed, order a custom control transformer from "": Osborne Transformer. Even if the 480V does get fixed eventually, you'll still get about 110V, which should be OK.

Or maybe put an SCR on the output of the control transformer. Cutting 10% should snarfle up the waveform too badly, and the solenoids shouldn't be too picky about that anyway. Be careful about noise sensitive components in the panel if you use
this method, though.
Don't try chaning the taps on your utility supply transformer yourself. While its a simple procedure, have a utility company lineman do the change. Typical supply is >13KV, enough to electro-zap you at 6'. Contact your utility supplier if your voltage is running high. There can be a number of causes both on their side or your side of the transformer. A common cause is that you transformer was originally tapped to supply 480 under certain load/power factor contitions,and now those conditions have changed. Your utility supplier can help you determine and correct the cause. They will sometimes help with the diagnostic without charge, since it benefits both you and them.
It sounds like you are discribing A machine tool transformer used in the control pannel of your equiptment.

the line side of your power measured phase to phase ( not phase to ground )will be probably measure in the 510 volt to 520 volt range. if this is true, you need to have the primary power tap on your facility main transformer changed. this should be done with your plant shut down and the help of a journeyman electricion and the
power company. if there has been a large plant in your area shut down or a shift cut-back, this will cause the line voltage to rise. the overvoltage condition may be time dependant. there may be some other signs of the overvoltage on other machines such as intermittent strange hapenings.

if the problem is isolated to this machine, and it has an isolation transformer, it may have taps in this transformer. if no isolation transformer, or the 480volt 3 phase is measured around 460 to 490 volts, change the control transformer. it will
be cheeper than shuting down the plant.

if the aplication is critical, a constant voltage transformer can be wired into the pannel and will give a good voltage regulation, harmonic protection, and surge supresion. these are expensive and can come with ratings from .25kva up to 3kva.
We have used CVR (Constant Voltage Regulator) transformers with good results. Acme, Tripp-Lite, and Sola are possible sources. In particularly high failure rates, we have fed a UPS with the control transformer and powered the PLC and devices with the UPS. Individual grounding of each machine with a ground rod next to it also helps in some cases.

Hope this helps.
It is probably well worth your while in using a constant voltage transformer. They maintain a constant voltage on the secondary even when the primary voltage varies.

Keep in mind these transformers generate a lot of heat because the transformer is running in a saturated condition. I have used sola control transformers in the past which have worked well.

M Griffin

Curt Wuollet

It should be relatively easy, if not cheap, to find a control transformer with taps specifically to cover this situation. Other solutions involve
a bucking transformer or other gadgets that could well cost more than a tapped control transformer. About the cheapest would be a "variac" (a
variable autotransformer on the 110 side). These typically allow adjustment from 0-130 Volts from a 117 source. $50.00 and up depending on what power level you need. I don't like using these long term but a lot of people do.


It is strange observation. Usually an increase in operating voltage to an electromagnetic device, like a solenoid, will result in a lower current, hence a lower temperature-rise above ambient. However, this is only true if the device is in a normal operating condition. What is the
"on/off duty?

Phil Corso, PE
(Boca Raton, FL)
How's that again Phil? Using higher voltage coils will usually require less current, but for a given coil, higher voltage is going to mean more current or physics are denied.


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Michael Griffin

If the voltage is too high, the iron will saturate at the voltage peaks and end up drawing *more* current which ends up as heat. A colleague of mine had this happen with control transformers on some new equipment which was sent to another country. The supply voltage to the plant was poorly regulated (too high), and the transformers were drawing excessive primary current even
though the secondary current was within the rating. This was solved when I sent him new control transformers with a slightly higher primary voltage rating. These were a standard catalogue item from Hammond.

I imagine something similar would apply to solenoid valves, although I am surprised that this is a problem at only 131 volts (+ 9% above 120 VAC). Perhaps these are 110 volt rated? In this case they would be running 19% above the rated voltage. 110 and 120 volt systems are not equivalent when you take voltage tolerance into account.

Michael Griffin
London, Ont. Canada
> How's that again Phil? Using higher voltage
> coils will usually require less
> current, but for a given coil, higher voltage
> is going to mean more current or physics are
> denied.

think in terms of work performed: mechinal load of shifting the coil remains constant. higher voltage, lower current, same horsepower.

well pump motors powered with 120volt use twice as much current as 240volt well pumps.

I agree with phil.

however vickers dosen't make a sol. valve coil rated for 175volts. 120v, 240v, or 480v are the only choices.

I'm going to have to side with Curt. When you connect a motor you use different windings for 240V -vs- 120V. Connect your 120V motor winding to 240V and your motor will start sizzling and crackling (but not for long). It is true that a solenoid wound for 240 will draw less than a 120 solenoid. But a 120V solenoid on 130V will draw more current. If its a 110 solenoid you can start getting into trouble, especailly if its a 110 50/60 hz as opposed to a pure 60hz solenoid. The best solution is to address the overvoltage problem on the 480 feeder. Its indicative of problems elsewhere in the system.
"cure the body, don't amputate the finger"
Curt, I apologize. My response was too quick.

You are correct for the case where a coil is subject to an overvoltage that drives its magnetic structure into saturation. Then, the
temperature-rise-above ambient will be proportional to the square of the current-ratio. Remember the halve rule - every 10% increase insulation temperature will halve life-expectancy! Additionally, saturation effects
will also raise the temperature of the magnetic structure. The resultant increase in iron losses aggravates the situation.

I was too quick to respond to the observed consequence. I should have said that the overvoltage effect on a solenoid is very dependent on the degree of saturation. But my real concern was the very rapid increase (assumed) in failure rate. Unless, it can be determined that the coil failures were from temperature effects would question the duty cycle of the solenoids.

One last point, my statement about current decreasing when voltage increases addresses the constant impednce model of induction motors.

Phil Corso, PE
(Boca Raton, FL)
The impedance of a solenoid valve coil once energized and plunger is pulled in is fixed for a 60 Hz frequency,

If the supply voltage increases, then the current
also will proportionately increase. It will increase more once the core is saturated (because of less back emf).
Did You see any tranformer there are an capacitor to estabilizer the courrent ? And know how to calculate this capacitor ?
An easy relatively cheap method would be to hook up a 12 VAC control transformer in a bucking arrangement. Primary in parallel with the 131 volts (120 volt primaries are much cheaper than high voltage) and the secondary in series with the 131 volts after the connection for your bucking transformer. Since the secondary will run a little high with 131 v in, you should end up with 115-117 volts. Secondary current should be rated the same as your original transformer.
If you get one with a tapped primary you can tweak your voltage in.