600VAC vs 120VAC design concerns

[babyyoda]

My question is the following.

I have a 600VAC/3/60Hz supply. Originally the motors and instrumentation where specified as 120V/1/60Hz. However the electrical designer is stating that the design will only have 600VAC and 24VDC.

How is the 24VDC supply generated from 600VAC?
Why would you choose to run simple motors (1HP fan motor and 2HP water pump motor) with 600VAC?



Thanks

 

[Lewish]

I assume the designer really means 575VAC/3/60 as that is the most common motor voltage around here. We have some 460V stuff also, but for some reason someone decided 575V was better. It does reduce wire size.
I do know that Baldor makes 575V motors down to 1/3HP.

Just $0.02 worth.

 

[busbar]

Direct your posted questions to the "electrical designer." Sometimes what is a self-aggrandizing, seemingly pat simplification is indeed short-sighted with no concern for the complications of others.

1-2hp 575V 3ø motors should not be impossible to locate, but that may not be available in the most desirable frame [or price or availability.] You could get quotations for 24VDC motors and watch him soil his pants when he sees the 24VDC power supply it will take to run ‘em.

 

[dydt]

hi..
using 600V or 575V will cut down your wire size, conduit diameters, current rating of your protection and even the size of your electrical panel..

on the designers point of view, i think he is referring to 24VDC for your controls like switches, proximity switches, pilot bulbs, relay coils and other sensors instead of using 120V....

you will still need to drop your 600V down to 120V ..and feed this voltage to a 24VDC power supply (120Vin/24VDC out)...for example..i've been using a lot of OMRON power supply lately and one thing i like with it is that if there is a dead short..the 24VDC voltage just drops to zero until you find the fault..

dydt

 

[babyyoda]

Thank you for all the useful information.

Yes, I can locate 575V/3/60Hz for both motors. The 24VDC instrumentation are process switches, solenoid valves etc. Which I can also locate.

All the motors, instrumentation have been ordered and received at 120V/60Hz. Also the electrical engineer has not begun his design. So I had hoped that he would include 120VAC into his design to run the motors.

I have asked the electrical engineer to provide me with his reasons for only having 575VAC and 24VDC, and not 120VAC. Smaller conduit and cables sizes do not justify the $1300 cost adder to the project to now change all the motors.

It is interesting to learn that 120VAC has to be generated to create the 24VDC supply. Why would one have 120VAC and choose not to utilize it?

Thanks Again

 

[Ares]

Even when you standardize on a 600V I can’t imagine not having 120V in the building (How about an outlet for vacuum cleaner?) FHP motors are hard to get for other than 120 V. So even if the MCC is 600V it’s easier to step down to 120V.
Have the design verified – there is a lot of nonsense out there.

 

[dydt]

..hi
how convenient it is if you have a 120V ac in the panel for a small soldering iron, a service light or even a test instrument..

dydt

 

[buzzp]

I believe his reasoning for 24VDC is safety. This is low voltage so the safety hazards are decreased significantly. There seems to be a big push to using low voltage controls for safety, etc. On the other hand, he must not have much experience in the real world to know that 120VAC should also be present for the reasons mentioned above. A simple outlet within the panel would be desirable. 24VDC can be generated from a control power transformer (added cost) that steps it down to 120 (common voltage for off the shelf power supplies) to generate the 24VDC.

 

[babyyoda]

Hi All,

Again thanks for the useful information, and for confirming my suspicions.

Let me clarify that equipment is a remote stand alone unit (a mini industrial gas plant) which will receive 600VAC from an outside power supply.

I agree with Buzzp that 24VDC is wise choice for safety reasons, but would that not negate the fact the now I need a certified electrician to work on the 600V motors? While I have no problems troubleshooting a motor of 120V, a 600V motor greatly reduces the number of candidates (in my company) willing to work on the 600V motors.

So can the 24VDC be generated in two ways? Directly from the 600VAC to 24VDC as well as stepping it down to 120VAC. By not supplying the 120VAC to the motors, would this reduce the size or cost or availability of the step down transformer?

Thanks again.

 

[buzzp]

If the motors are not 120 then the transformer will be much smaller just for control power and much cheaper. Also, if this is at a business, you are supposed to have a licensed electrician, even for 120VAC. Do not be fooled by the reduction in voltage, both are equally as dangerous.

 

[TheBlacksmith]

Dead is dead. More Navy people are killed by 120 than 450/4160 because 120 is "household" voltage and assumed to be safe. I agree a commercial entity should have certified electricians. Improperly connected/maintained 120 volts also fails or starts fires just as well as 600 volts. Even 24 volts canbe hazardous - it won't kill you, but the fall off a ladder or slamming your elbow into a rigid piece in response to the "tingle" hurts.

Blacksmith

 

[busbar]

Apologies if this has been covered, but a 600VAC-24VDC conversion will almost certainly involve an intermediate AC voltage for serving an AC-to-DC power supply—linear or switchmode. {The DC-motors suggestion was in jest—but to point out the occasionally flawed thought-to-text translation of some.} You may be hard pressed to find a supply with other than a 120- or 240-volt AC input, mandating a drytype of some sort, aside from the dreams of the "electrical designer." 575V 3ø 1- and 2-Hp motors may be perfectly attainable. These sizes are less than desirable in 115V 1ø versions.

A false perception of safety may be this: note that in MCC/individual magnetic-starter compartments, in most practical instances, “24VDC” wiring cannot be of the doorbell type. 600V-insulated conductors are very common and there’s really little or no justification for using otherwise in most any sort of industrial-control setting. High-quality 600V FREP/CPE multiconductor external cables are readily available in a number of constructions and typically very reliable for “signal” circuits—and if not already called out, add trivial incremental cost to the project.

I can surely offend someone by saying that in albeit rare circumstance, office-bound PLC-expert/programmer-savant/ScribbleCAD hotshots, regardless or their sometimes elevated self image, make clueless and consequently awful electricians.

 

[busbar]

To add to Blacks’ response, “24V” circuits can breed complacence in troubleshooting and related activities, and be disastrous in compartments with additional, higher voltages, designed as supposedly 'touchsafe' or not. Besides, 600V [and even 24V] coffee pots have long leadtimes.

 

[buzzp]

Quick note: Technically voltage does not kill you. Current as small as 80mA can cause the heart to stop. Generally, the lower voltages can not source enough current across the heart to cause this problem (resistance of bodies varies quit a bit depending on several things). This is the only saving grace in using lower voltages.