120VAC vs 24VDC control
120VAC vs 24VDC control
(OP)
Nowadays with all the excitement about arc flash hazards, people want to get away from 120VAC control. I'm looking for any info on disadvantages of 24VDC control. I know the obvious like voltage drop over long distances, but I was interested more in any experiences with spurious trips and state changes due to routing 24VDC through very RF frequency noisy areas.





RE: 120VAC vs 24VDC control
Besides the voltage drop, a reliability issue comes in with a DC source. If not backed up by station batteries, DC source obtained just through a rectifier power supply could be less reliable as the power supply itself is more prone to failures, as compared to say an ac control transformer. You can add redundant power supplies to address that I suppose. Just my opinion.
RE: 120VAC vs 24VDC control
RE: 120VAC vs 24VDC control
Also, if the capacitive effects of 120V ac conrol circuits are taken into account, you might find that 24 vdc is not at as much of a disadvantage regarding control circuit length as you might expect.
RE: 120VAC vs 24VDC control
GE did some tests a few generations ago and found that about 100 volts was needed on pilot devices to penetrate contact film and ensure reliable operation in some industrial environments.
The old GE Solid State Logic Modules operated at a 12 volt logic level.
GE recommended using over 100 volts for field wiring to ensure reliable operation. We used input interface devices which operated on 120 volts AC in and logic levels out. I seem to remember a 125 volt DC version also.
respectfully
RE: 120VAC vs 24VDC control
That's easy to say, but not always easy to achieve: cable sizes quickly become unreasonable on an industrial plant. Economics and common sense would suggest that running very heavy cables sized solely on volt-drop over long distances is not really a good design.
Another consideration of 24V v's 110V is that 24V contactors are not widely available above a fairly small rating, about 37kW / 50HP being the cutoff in many manufacturer's ranges. Above that level the 24V signal would drive a pilot relay for the main contactor, adding an unneccesary additional component capable of failure and still requiring a higher control voltage.
It really is a question of picking the best solution for your application, considering the physical installation, the size of loads to be controlled, the skill set of those maintaining the equipment, etc.
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Sometimes I only open my mouth to swap feet...
RE: 120VAC vs 24VDC control
Another point is if you are driving a solenoid valve. 24V dc doesn't have any significant change in current as the inductance changes, but 120Vac has a very significant change.
If there is a sticky solenoid, then with ac, the supply ought to have individual fused protection, since otherwise the higher current will burn out the coil.
With dc, the coil is not affected, but you don't have indication of the fault either.
For voltage loss, quite often I have used more than 1 core in parallel for common supply and return.
There seems to be a big drive for 24V dc systems and, although I prefer 110/120Vac for reliability, I have accepted that this is just the way the world is going (but aren't autos moving towards 48Vdc?).
RE: 120VAC vs 24VDC control
This can make for an interesting time trying to locate the fault depending on the amount of things being supplied.
RE: 120VAC vs 24VDC control
42V
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: 120VAC vs 24VDC control
RE: 120VAC vs 24VDC control
RE: 120VAC vs 24VDC control
RE: 120VAC vs 24VDC control
Obviously, cable segregation is important for the 24Vdc signals to avoid intereference but I believe it would not be good practice anyway to have 120Vac control cabling mixed with motor cables, for example - especially if controlled by VSDs.
By employing inteligent PLC programming and even two digital signals from a device (S and S bar, when available) and spurrious events can be avoided.
The control panels in the electric rooms will have a 120Vac or higher voltage (according to customer / country standards) for contactor coils, power-supply mains and so on.
Drivesrock