In some very rare cases -- less then 1% of applications-- certain system criteria may have an impact on the shifting of dual coil, 3-position, 4-way and 5-way valves. Most applications using these dual coils valves have no issues at all. However, should your valve appear to be shifting slowly, sluggishly, or in extreme cases, not at all, the bright side is; there is a very simple solution! No, the solution is not beating the valve with a wrench (although we have heard this) but a simple switch of the (+) positive and (–) negative terminals, on ONE of the coils, will most likely perk up your system.
I once went on a troubleshooting call where the top coil of a dual coil valve would not shift the valve, but instead would actually energize the bottom coil position of the valve. Huh, say what? Upon investigating, it turned out that there were four issues going on: 1) the machine's electrical system had a residual trickle voltage in the system that never dropped current to zero when de-energizing the bottom coil; 2) the coils had diodes; 3) fine particle contamination; and 4) both coils had the same polarity. Polarity, in this case, means that both coils used the same terminal for the plus (+) side/battery and the other terminal for the minus (-) side/ground.
To start, let’s understand what happens when the bottom coil is energized. When you apply voltage to the bottom coil, a magneto-motive force is created to move the plunger, thus shifting the valve (see Fig. 1 below). This magnetic force essentially polarizes the plunger with a north and south pole, creating a magnet. When you de-energize this bottom coil, the magnetic field of flux collapses and the internal spring sends the valve back to the centered or neutral position (Fig. 2). The top coil’s magnetic field is created when it is energized and the same magnetic flux induces the same magneto-motive forces on the plunger (Fig. 3).
Click here for a full-page PDF of Figs. 1, 2 & 3 below.
There are several factors that must be present simultaneously in your hydraulic scheme that can add to this slow, sluggish or non-shifting condition of the dual coil valves. Let’s look at each of these separately:
In some slow-response cases – if the top coil is energized too quickly the bottom coil may not have time to drop out completely. However, speed alone will not cause a non-shifting or slow-actuating system. There are several factors that need to be present in a system, simultaneously to see this condition. One factor alone will not cause an issue.
Residual or Trickle Voltage: There are several reasons why an electrical system might require this minimal voltage, such as keeping a safety device powered with a low-voltage signal. Some controllers resist going to zero current in order to maintain data logging capabilities, etc.
Relay Set-ups: In some cases, the controllers can act as a resistor/capacitor network and this capacitance issue can delay the collapsing magnetic flux from the coils.
Drop-Out Current: We also have a minimum drop-out current requirement for all the valves, and some controllers have a diagnostic current that is close to this value. When other factors come into play this diagnostic current may be too great to overcome. By using meters in the electrical system, you can determine if any of these conditions exist.
Coil Voltage: We know that the electromotive forces created from a 12-volt coil and a 24-volt coil are the same, giving us the same valve performance. However, due to the fact that the 24- volt coil has two-times the windings of a 12-volt coil, the inductance in the 24-volt coil is increased by four times. Dropping out (or collapsing the magnetic field) of a 12-volt coil happens in roughly 4 milliseconds while dropping out the 24-volt coil takes about 16 milliseconds (see the Time Constant graphs below). This inductance might add to the potential delay in shifting from one coil to the next. Again, voltage alone is not enough to slow down your system. Several of these factors must be present.
Diodes: Having a diode in the coil could potentially delay the collapsing magnetic field. If a diode is required to protect the system from the back EMF created from this collapsing magnetic field, use coils with Zener diodes. The Zener diode removes the polarity requirement from the coil terminals. HydraForce has a wide variety of coils released with Zener diodes. Diodes alone will not delay the shifting of the valves.
Temperature and Resistance: If the coil temperature is below room temperature, the resistance of that coil is lower and thus the current draw is slightly higher. Higher than normal current draw could add to the time required to collapse the magnetic field.
Polarity: As mentioned above, the polarity of the top and bottom coil can play a key role in contributing to or solving an issue of slow or sluggish response of a dual-coil valve. When the same terminal on each of the coils is used as the (+) battery side and (–) ground side, polarity is considered the same, thus inducing the same magnetic north and south poles to the plunger (see Fig. 4 below). By simply switching the connections +/- terminals to only one of the coils, opposite polarity is created (Fig. 5).
Click here for a full-page PDF of Figs. 4 & 5 below.
In the case of opposing polarity (Fig. 5), we have created an opposing magnetic field around the top coil, in essence reversing the north and south poles of the induced plunger magnet. Since we know from our school days that opposites attract and like poles repel, this opposing magnetic flux actually helps to collapse the first or bottom coil's magnetic field. It is just that simple. Well, just that simple, if your system has a combination of these variables in play and is acting slow or sluggish.
In summary, if your dual coil, 3 position, 4-way and 5-way valves are acting sluggish, and you have ruled out contamination, over-tightening of the valve's coil nut, over-torquing into a valve housing, have the right voltage to the coils, and you suspect you have more than 3 of the other criteria in your system, try switching the (+) plus and (–) minus wires on only ONE of the coils. No need to abuse the valve with a wrench . . .
For more information on solenoid valve operation see our solenoid school.
About the Author:
Lisa DeBenedetto is a Regional Manager at HydraForce with more than 20 years of
hydraulic experience. She has been with HydraForce for over 16 years. Contact Lisa