120Vac vs. 24Vdc control power in harsh environments 2

[Lawrence220]

thread237-164753
I am looking into the pros and cons of using 120Vac or 24Vdc with respect to industrial environments. I have had some problems with 24Vdc failing in places where the adjacent 120Vac has continued to work and would like to find some papers or studies that look into this. I have found a thread in which it is mentioned that GE had done test finding that about 100volts was needed to penetrant contact film, does anyone know where I can see these tests or any other ones like it? or does anyone just have any pros or cons they know of the dome piece?

 

[waross]

One major is battery power versus grid power.
For a substation the ability to maintain protection and switching during a fault condition or outage is very important.
Battery power provides that ability. Batteries require dependable care and maintenance.
On the other hand, in an industrial plant where a power outage brings all motors to a halt, loss of control power is a non issue.
A companion issue is choice of voltage.
Davidbeach is much more qualifies than I to comment of voltage selection for a substation.
For an industrial plant, voltage selection is a compromise between a number of things.
Cost, safety, immunity from interference, contact film issues, etc.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[cranky108]

Even in substations 120VAC is used in limited control fashion, such as transformer controls for cooling and LTC applications. If there is no power, then there is no need for cooling.
Except that the station power often comes from a different source than the transformer high voltage.
Many times a short loss of power may not be much of an issue.

The choice of controls power is a thoughtful process.

However sloppy maintenance should not be a factor.

 

[RedSnake]

In EU almost all control circuits today is built 24VDC for industri for the reasons I explained before, low power consumption, less space, less material, less environmental impacts, low freight cost, less manufacturing cost, etc. All other voltages products that can be bought is more or less for maintaining old systems or grid AC for home use where transformers aren't so common.

For Ex classed environments, and now I am making an assumption, it's easier to comply with the regulations with 24 VDC since the energi needed is less, easier to "contain".

We have used UPS for shorter start up times for PLC:s or PC:s but my experience from the past is that they stand for more of the stop time then the eventual time you save at a power drop.
Now this is for old UPS systems, maybe the new ones are better.

But as it is sade they need maintaining and if that does not work it's often better to be without.

/A



“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[stevenal]

Does the industrial process rely on simple plug in style relays? Sometimes the 120 V coils will drop out during a voltage sag the motors can ride through. Battery backed up DC coils will not be affected by supply sags.

 

[TugboatEng]

On the subject of coils, AC coils have a high inrush current compared to holding and not all can tolerate rapid cycling without overheating. DC coils can cycle continuously.

 

[unclebob]

125 Vdc hurts, but it works!

 

[FreddyNurk]

Why consider anything other than 125VDC?

One major is battery power versus grid power.
For a substation the ability to maintain protection and switching during a fault condition or outage is very important.
Battery power provides that ability. Batteries require dependable care and maintenance.
On the other hand, in an industrial plant where a power outage brings all motors to a halt, loss of control power is a non issue.
A companion issue is choice of voltage.
Davidbeach is much more qualifies than I to comment of voltage selection for a substation.
For an industrial plant, voltage selection is a compromise between a number of things.
Cost, safety, immunity from interference, contact film issues, etc.

Indeed, choice of voltage can also be governed by factors such as power required for the equipment to operate and availability.
All of my work in Power Generation has been done with 24V control voltages, but I do LV projects where all the operating coils for breakers are small enough to be able to use 24V, and all of the other control equipment is standardised on 24V DC, so its a sensible choice.
In substations, the power required for breaker operating coils (as far as I'm aware, anyway) is often higher and justifies the use of a higher voltage, but still with the backup requirement. In that context, I'd agree with davidbeach.

To me though, use of a UPS as compared to a battery supply to keep the control power up is an unnecessary complication, but that can also be governed by the control equipment and whether it has a DC supply option.

Positive grounded 48V DC is another one, although that appears to be limited to a specific industry.

EDMS Australia

 

[waross]

On the subject of coils, AC coils have a high inrush current compared to holding and not all can tolerate rapid cycling without overheating. DC coils can cycle continuously.

A correct but misleading statement.
AC inrush depends on the air gap or armature travel. As the relay operates and the air gap closes, the inductive reactance increases, dropping the current.
DC on the other hand needs a similar level of current to magnetize the air gap, but when the air gap closes the current doesn't dropas it does with an AC coil.
The inrush of larger DC contactors is so great that economizer circuits must be used to prevent rapid burnout of the DC coil.
For a given relay, the minimum RMS AC current required to pull in won't be that much different than the minimum DC current required to pull in.
The minimum AC current may even be less due to the peak currents, but that depends on a lot of mechanical factors such as the inertia of the relay armature and the distance it moves at the peak current.


Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

Or, said another way:
AC coils draw a large current to pull in. As the armature closes the air gap, there is a reduction of current.
DC coils draw a large current to pull in. As the armature closes the air gap,there is no reduction of current.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[TFinstad]

Great question! Great answers!

Fact is we are stuck with 24V AC/DC control systems. It has to do with circuit boards and board level circuitry components. Everything is getting smaller and cost savings continue to drive all trends. Ideally 120 Volt control circuitry works best across most control panels and systems because of its hold in capability, ability to work in dusty environments, and its aversion to droop from fluxuating control power, transformers or otherwise. I've seen control circuits on MCC in paper mills spec out for 480 volt control just because of the hold in requirement. (Mike 480 volt Glen - you know who you are)

Best regards,

TF