Introduction: NO and NC valves
The solenoid valve we explored in the video was a pilot(diaphragm) operated type with a response time of 30ms. Also note that it is normally in a closed position. Only when the coil is energised the valve is ON.
Normally open or Normally closed valves ?
Let’s test this normally closed valve in an application of a bottle filling machine. Here the valve open time is more than the close time.
The coil can be damaged if it is energised for a long period. A simple trick to achieve a short energising period would be – a normally open solenoid valve (Fig:2a). In this valve the armature is also placed offset to the armature. However this offset is in the other direction.
When the coil is not energised, the valve is open and fluid flows by. When you energise the coil, the armature aligns to the centre of the coil thus closing the valve. Here you can see that now the valve’s open time is more, and the coil is saved! This is a normally open (NO) valve type.
When the NC valve coil is de-energized, the previously compressed spring pushes this armature back down to close the valve. Interesting in actual model two such springs do this task.
AC solenoid valve
So far we have explored the solenoid valves operating on DC supply. You might be wondering what will happen if we give AC supply to this same design? Well it will work but with a problem. You can see the AC sinusoidal waveform drops to zero two times during one complete cycle.
Due to this, the force in the armature will drop to zero twice. This will lead to vibration of the armature, heavy noise and can also damage the valve.
To solve this issue, a thick short circuited copper ring is used. It is attached into the iron cap as seen in this real image.
If this copper ring can hold the armature up while the main coil current goes to zero, our problem is solved. Let’s see how it does that.
In this region of the applied current, the rate of change of current is high. Thus a good amount of current is induced in the copper ring creating its own magnetic field. This field generates enough force to hold the armature here. This generated magnetic field lags 90 degrees to the main magnetic field, which is what we want. Thus there will be a continuous force on the armature and it won’t vibrate.
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