Ideal Electric Power

Don't worry about the earth-to-neutral voltage, Guvnor, as all it is is an expression of the neutral current, as has been stated in these columns many times before.

Just make sure your installation is fully compliant with the electrical codes applicable in your country. And if it isn't, then get someone to fix it.

Lyn already did the "hard" work, but that is exactly what I was going to suggest. By using localized UPS modules, you will protect yourself from out-of-range voltage, spikes AND localized power outages.

AND- they are relatively inexpensive and easy to purchase.

I'm with Lyn on this one, A UPS to protect your PLCs & controls would be the ideal solution.

Ensure the model you select is a double conversion type (AC->DC - DC->AC).

Unless the process is critical I would avoid running the motors on the UPS(s) simply due to cost and the fact that most motors can handle voltage spike & droops reasonable well.

Having said that, if you decide that the motors need UPS support (This will be expensive) ensure the UPS is also capable of Starting the motor this will mean it need to be oversized (think about the rules for fuses or Circuit breakers on motors).

This is primarily due to the fact that (I'm make assumptions here) if a process is running, motors will be starting & stopping automatically.

This will require a Larger battery bank and larger rated UPS(s).

The Double Conversion gives the cleanest & most reliable power, however efficiency is slightly less due to the double conversion = double losses.

Regards,
Sapper

Go with the UPS it give the constant voltage and maintain harmonics level

Hi Sandoo

Do you have many motors (variable speed) in the plant, under power failure conditions you may get back emf from the motors which can damage solid state equipment.

Hello you all. Just few wold about my experience with ups. It is true that they are getting cheaper every day, it seems like, but all depend of how bad isthe power plant you dealing with. You may have to consider a ups that always "make" the wave form than who are in stand by mode. That is probably what they say by power conditionner. Any way have fun!

A good reply by Lyn.

Some folks recommend UPS - but remember most packaged UPS keep you connected to the mains unless it goes drastically out of normal - all the usual short spikes will pass through unreduced, except insofar as the UPS has filters and surge limiters to protect its own circuits. The "real pro" UPS run the output always from the inverter, with mains as a standby.

I can recommend the straightforward ferro-resonant type of voltage regulating /isolating transformer, so long as you do not have wide frequency swings

Hello friends,

thanks for your reply...

Many of you have focussed on UPS but in my case its not possible. One important thing I need to mention here that my plant is much bigger i.e. with 40 hydraulic injection molding machines, 6 robots & thus many induction motors. Maximum demand is 1300KVA. Daily electricity cosumption approx. 20,000 Kwh.

Requesting to suggest accordingly..

You just install relatively small units at the power supply to each

Dear Sanoo,

Following your description of the plant, I feel the surges and disturbances you get are locally generated by that machinery. In other words, you do not have ideal electric power and never will.

Starting from scratch, single phase power for controls direct from a breaker on the primary distribution (using its own separate trunking, well separated) would minimise shared impedance disturbance by high power loads.

As I suggested, in my previous post #7, the ferro-resonant voltage stabilisers usually have very good rejection of dips, surges and spikes (the resonance stores energy to bridge dips and the actual construction with primary and secondary on opposite ends of the transfo core promotes isolation). An individual stabilising transfo for the controls suffering most failures is a good place to start.

Using a direct local single phase of a 3 phase machine for electronic controls can expose them to the motor starting/stopping disturbances. Even a simple 1:1 isolating transfo can reduce the spikes - a voltage stabilising transfo is better.

You have not described which kind of equipment suffers most failures - power supplies, on-machine sensors, PLC???

You mention "improper earthing" as a cause of failure you have had. What was "improper earthing" in those cases??

Actual Earthing as a solution to interaction problems is a myth. Power generation sites, where it is really necessary to have a good earth often struggle to put enough rods into the ground to get the necessary 0.5 ohm. In comparison, even 25 metres of 1 sq. mm copper wire gives an Earth Bonding resistance of 0.5 ohm back to the common "earth point" [local electricity regulations may specify 2.5 sq.mm as a minimum bonding].

The benefit of a "separate ground" for electronics is that it made the designers really separate that cabling from the rest.

Aeroplanes cannot have an earth. They get struck by lightning regularly. It is the bonding together of everything which enables them to survive, plus the fact that the electronics are insulated, as far as possible, from the structure (to reduce the effect of voltage differences around the structure, unavoidable when many kA pass through it). You can only get a "good" earth on a salt marsh or a ship!

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