Resistive Load Safety

[leachdor]

Another design theory question here...

My application is going to rectify 480vac 3phase to 648VDC and send it into a dummy load. The load is 13Ohms, with 50Adc load, 32.5kW dissipation.

I'm designing the control now, that the user will interface with. I suppose my question is theory, is it safe/what are the effects of just using a heavy duty contactor for the 3 phase AC input...and as soon as we are ready to test...just close the contacts with full load attached.

Will slamming a large load with that much voltage/current cause problems? There is nothing reactive...no caps that need to slowly charge up etc. I don't see any problem with my green eyes...but this is a learning project I don't want to create any hazards.

 

[ScottyUK]

Without wishing to seem rude, is a 480V high energy circuit the best place to learn the basics? Mistakes are likely to be expensive in equipment terms and hazardous to both people and the immediate surroundings. Do you have anyone more experienced working on this project with you? In your position maybe you should consider telling your supervisor that you are working at the limits of your knowledge and that while you are keen to learn you are unwilling to risk the safety of yourself and others while learning.

On the subject of your original question, is this a controlled rectifier or just a 3-phase plain bridge? A controlled bridge would allow you gently ramp up the voltage. If you just switch on a big load like that consider what effect it may have on other users fed from the same bus.


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I don't have an attitude problem. You have a perception problem...

 

[leachdor]

I appreciate your concern.
As it wasn't meant to be rude, no need to be defensive on my end.

(Personally I would have reserved that portion for a private message as a professional courtesy though.)

To alleviate some concern, I will be passing the design by another engineer before implementing of course. Even if I was an expert in this field, I would do the same as its always possible to overlook something. With such a potential safety risk on this particular project, I would certainly hope any engineer would do the same.

As I said its a learning experience, and I do appreciate any and all comments, off-topic, or just helpful one's that may be useful down the road for any post of mine. As a general rule of thumb in my field, I love when questions are asked...to me its a complement via the confidence they have in asking me.

 

[itsmoked]

Sorry no side notes around here... I have never found the side note area. But then I've only visited here 4,694 times.

Could you please answer Scotty's questions?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

If you have not worked with power (I mean power) circuits before, there are quite a lot of things to think about. A few of them are

1 What does this kind of load do to the mains?
2 What cable area do you need?
3 Fusing, temperature cut out.
4 An emergency stop button with tested function and that everyone knows where it is and why.
5 Getting rid of the heat. This is like having at least 30 radiators turned on.
6 How long will this be activated? What will the end temperature be?
7 A lot of code issues have to be addressed. Are you really aware of what the national code says?

If that other engineer is a practising electrical engineer with power background (all EE:s are not power EE:s) I think that things might work out. But, personally, I would turn to a company that is doing these things for their living. They know about these things, they have the right materials available, tooling and know-how. It is not something I would build myself. And I have a power electronics background with 40+ years of experience.

It may seem simple ("It's nothing more than Ohm's law") and that may be so. But power engineering is a little bit more than Ohm's law.

Finally. In many countries, a PE is supposed to supervise and approve power circuits. Where are you situated?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...