120VAC transformer for 480VAC Motor

[iamcaring]

Hi

I was creating motor schematics and suddenly found out that vendor is not providing 120VAC transformer so my question is if we don't have input 480 VAC transformer convert to 120VAC(output)then how we can use motor permissive and PLC input/outputs? as normally output realys requires 120VAC. Do you think 480VAc can solve the purpose for Motor controls and status? I doubted that 480VAc can do all this as well On/Off switch also requires 120VAC so we must required 120VAC. Am I thinking right or is there possibility ro run switch and realys at 480VAC? Please confirm?

Thanks in advance

 

[dpc]

In the US, it used to be fairly common to use 480 V control circuit and pilot devices for motor control. It is nearly unheard of today for any new installation AFAIK. I believe it is still allowed by the NEC. Since you don't tell us the location of the project, it is hard to say what is possible and what is not in terms of code requirements.

For PLC I/O, you'd have to use inputs and outputs rated for 600 V, or (more likely) use interposing relays.

But maybe there is another possibility: The 120 V ac does not have to come from a control power transformer tapped off the 480 V motor power. That is common, but not the only possibility. Sometimes a common 120 V source is used for an entire MCC if the loads are related to the same process.

David Castor

http://www.cvoes.com

 

[jraef]

I seriously doubt anyone makes PLC I/O that is rated over 240VAC.

If you have a chance to change things, I like using 24VDC now.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[purplepete]

480 volt AC controls are a nightmare, especially in fault-finding where each side of a contact or coil will always be at phase-neutral volts above ground. I would go straight to 120 VAC contactor coils and controls with one side grounded. 24 VDC is good in some circumstances.

 

[ScottyUK]

"The vendor isn't providing..."

What did you (or the client) specify?

24V DC is good on smaller MCCs, but you end up with very large buswires just to control volt-drop if it is a big MCC with a lot of medium-sized contactors. Large contactors usually have a 24V pilot for a line voltage coil so they aren't so troublesome. Long field control circuits are troublesome too for the same reason: volt-drop.

24V DC is great for implementing a cheap and robust no-break dual redundant system because you can use series diodes to parallel supplies. The only way to do achieve no-break with an AC scheme is to use a semiconductor ATS.


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If we learn from our mistakes I'm getting a great education!

 

[iamcaring]

You guys amzed me. Thanks all. That was my first post and response time was excleent. Actually, the MCC is in USA and we are designing the work from Canada and after reading "dps"response I am certain now that in US there is possibilty to use 480VAC even for HOA switch.

I am still confused how we can bring PLC input/Outputs from PLC to MCC as the voltage is 480VAC and PLC can provide either 24V (Input for motor status) or 120VAC(utility outputfor motor controls/permissives). Please advise how we can handle this plc input and ouput for 480VAC and motor schematics should indicate all this.

Thanks again.

 

[iamcaring]

I forgot to asnwer "ScottyUK".
We request 120VAC from vendor.Thanks

 

[potteryshard]

Although dpc indicated that 480 V controls were used previously, and are still nominally accepted by the NEC, I don't read the message as an endorsement. 480 V controls no longer seem to represent good practice.

The inclusion of a 480/120 V control power transformer is likely to cost far less than the special PLC I/O, interposing relays and/or other adaptations required to permit use of 480 V. And, will be far less dangerous.

 

[iamcaring]

Thanks I will request and recommend my US client to get 120VAC for controls. I really appreciate all of you sharing your knowledge in this forum with me. I am still new to EPCM industry and may pose question time to time here with a hope of getting answers in timely manner?
Cheers

 

[jraef]

480V controls on MCCs, although not common by any stretch, is actually still the "standard". 120V controls is an "option" in most cases, because it requires an engineering decision to be made in advance: is the 120V control coming from CPTs in every bucket (added expense), or is the 120V control power coming from a separate source (adds interlocking issues)? Remember, one of the key original concepts behind an MCC is the idea that every starter bucket is the same as another of the same size, so they are universally interchangeable to support decreased down time for changeovers or repairs. 480V control allows for common control circuits in every bucket at the lowest possible cost. I'm not endorsing it here, just stating the facts.

Because a LOT of MCCs are still bought without engineering support (therein lies the problem), many times they end up in the field with 480V "standard" controls simply because nobody thought to ask and the purchasing dept put the tender offer out without that "option" requested, awarding the project to the lowest bidder. Sad, but all too common even in this day and age.

When I worked for Siemens, I can't tell you how many times I got a call from an electrician in the field installing an MCC and complaining that there were no CPTs in each bucket. When I would research the original RFQ, PO and submittals, there was no mention of it, so the default was 480V. Distributors that cared and added the extra for the CPTs were under cut on price by those who did not, and nobody doing the review before awarding the PO would notice.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[iamcaring]

Jraef Thanks for the explaination and will keep in mind when designing MCC for other projects.

Cheers