Modes Of Operation Of Static Eliminators

Passive Eliminators
A charged object will generate an electric field between itself and any surrounding earthed object (or indeed any object of differing voltage).

In the case of passive eliminator the field is between the surface and the tips of the Carbon Fibre or Stainless Steel earthed brush. See Diagram 1. The fine point at the end of the individual bristles causes the electric field to be highly concentrated at this point. When the strength of this electric field reaches a sufficient value, ionisation of the air molecules surrounding the tip occurs.
In the example in Diagram 1 the positive charge on the surface of the material will cause electrons from the tip of the brush to jump to surrounding air molecules which will then have a net negative charge and are thus negative ions.


Diagram 1

From the simple rule that opposite charges attract, the negative ion will be drawn to the positively charged surface of the material. When the ion reaches the surface the extra electron the molecule is carrying is drawn from the molecule and delivered to the surface of the material. When this process has occurred in sufficient quantity the positive charge on the surface will be reduced. In this process there will come a time when the electric field generated between the surface and the tip of the brush is no longer sufficient to cause ionisation of the air and no further neutralisation takes place. In the instance of a negatively charged surface the opposite procedure takes place.
Passive eliminators are thus useful for reducing high levels of static charge, tens of kV’s down to levels of a few kV’s. However by their very nature they are not able to completely neutralise the surface charge.
Radioactive Eliminators
Radioactive eliminators employ polonium 210 or other low level radioactive source. In the process of radioactive decay alpha particles are emitted to the surrounding atmosphere. These high speed particles collide with the air molecules and in doing so cause the air to become ionised. This ionised air then neutralises nearby surfaces in similar fashion to the passive eliminators.
AC Eliminators
AC Eliminators operate at supply frequency. The mains voltage, 110 or 240 etc. is greatly increased using a ferro-resonating transformer to generate voltages of between 4.5 and 7 kV. This high voltage is fed to the ionising pins, whilst the casing of the bar is connected to earth. See Diagram 2.

Diagram 2

If we look at the positive cycle of the input waveform we will see that the electrode pin will be at a positive voltage compared to the casing. This generates a strong electric field between the two which is highly concentrated at the sharp point of the electrode pin. This in similar fashion to the passive bar generates positive ions at the pin point. These molecules are then repelled from the pin due to their like charge. On the negative half of the cycle the opposite occurs and negative ions are created at the pin point. These again are repelled from the electrode pin due to their like voltage. Thus around the ionising pin a cloud of positive and negative ions is produced. In the absence of outside influences the positive and negative ions are attracted to each other or to a nearby earth, such as the casing of the bar or nozzle, and would either neutralise each other or be dissipated to earth. However with the presence of a nearby static charge an ion will be attracted to on opposite charge on the surface of the material. At the surface of the material the electrons will be exchanged and the surface will be neutralised.
As the ionisation at the bar is not dependent upon the surface charge and ions are produced regardless of the proximity of a surface charge, complete neutralisation of a surface can be achieved. This is a significant advantage over the passive eliminators. The speed at which charges can be neutralised is dependent upon the rate of ion production and speed of repulsion of the ions from the emitter pins, which in turn is dependent on the voltage at the pin.
Pulsed DC Eliminators
Pulsed DC Eliminators like their AC counterparts produce ionised air by using high voltage. Whereas the AC units operate at supply frequency the Pulsed DC units operate at lower frequencies, between 0.5-20 Hz. The ionising bar consists of a series of emitters connected alternately to the negative and positive outputs. See diagram 3. The casing of the bar is made of plastic and hence there is no proximity earth. The output from the power supply is effectively a square wave switching from negative to positive at the chosen frequency.

Diagram 3

Looking at the positive half of the wave form the controller switches on the high output voltage connected to the positive emitters. This then sets up an electric field between the emitter and the surrounding earthed objects. At the sharp point of the emitter this field is extremely strong, and in similar fashion to the AC eliminators, positive ions are produced. The similar charge of the ion and the emitter drives the ions away from the bar.
On the negative half of the cycle the power supply delivers a high negative voltage to the alternate set of emitters. Again in similar fashion to the AC eliminators negative ions are produced at the emitter point.
A statically charged object in the vicinity of the ionising bar will attract or repel the ions, dependent upon their relative polarities. When the ions reach the statically charged surface, neutralisation takes place in a similar manner as described in the AC eliminators section.
The low frequency of operation lends Pulsed DC equipment to long range neutralisation. The relatively long duration of each half of the cycle cause large “clouds” of ions of alternating polarity to be emitted from the bar. This distance between the positive and negative ions close to the bar greatly reduces the rate of re-combination, (positive and negative ions coming together and cancelling each other out).
At long distances from the bar, less ions are deliverable to a statically charged surface and hence the speed of neutralisation is reduced. Hence when utilising Pulsed DC equipment on dynamic applications, such as moving webs, thought must be given to distance at which the bar will be mounted from the target surface.
An additional feature of the Pulsed DC system is that the output wave form can be altered and the duration of the negative and positive section of the wave form can be increased or decreased. For instance if the charge to be neutralised is known to be positive the duration of the negative part of the output can be increased and conversely the positive part of the wave form reduced.
This will increase the production of negative ions and decrease the production of positive ions, making the system more efficient at neutralising the positive charge. In similar fashion for a known negative charge the output can be biased towards positive ion production.

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