Newbie question

Hi all, would anyone be willing to help?

I've been thrown a simple enough project, to be done in more or less zero time. It's probably 15 years since I last used a PLC

I just need to slowly sequence through a set of ten 100 Amp mains relays, to build up the power taken from a generator. Say 5 seconds or so between each additional relay step. Although it is a simple requirement, it is imperative that the system be reliable.

I've chosen the Micrologix 1000 PLC almost at random, as it has 12 relay outputs.

I need to know - very quickly - if this PLC has timer functions, and some way of reliably starting a sequence at power up.

Perhaps there are better PLCs available, but at least this one is available from UK dealers. Would anyone care to suggest a substitute?

Thanks

Jeff Adams

I have not used this plc but am sure it has timers. Most brands have good UK distributors - or maybe go for something stocked by RS? Check out how much the software is to buy as well - some are dearer than others.
It sounds simple enough to do. I would feed the contactor auxilliarys into the plc to make sure they are all "Off" before starting the generator and be wary of what may happen if a contactor was out when it should be in (Stuck) or in when it should be out (Welded contacts).

Do you need a fault lamp?
A signal to say the generator is at full speed?
Do you need an input from the generator should it start to overload, which could switch off the contactors?
Think about what might go wrong!

It should be simple enough to cascade the timers at the 5 second intervals you want.



Hope this is a start?

Markie

Thanks for that Markie.

I was thinking of using solid state relays given the high current and the cost. (Need a DC drive of course...) These were 'in fashion' when i was playing with these things last.

Are there any downsides to these things?

Jeff

I personally don't like ssr's but that is just me. I use "Weidmuller" interface relays, then mighty contactors!
I have not worked on loading generators though.

ML1000 has two major defficiencies: it is non-expandable and missing online editing.
On the other hand, any brand/model PLC exists in relay outputs version.
Any PLC has timer instruction of some kind and first pass flag.

From sounds of things
1. you are starting a generator to reenergize a dead bus.
2. Doing sequential loading to prevent stall and to let governor catch up with increased loading.
3. Sequenced loading is total of 1,000 amp

What other load do you have?
What size generator?
FIRST I would have PLC check to ensure the dead bus is really dead. What if you are sensing only one phase and you lose the sense signal?
What controls do you have on the generator as of now?

Think I would check generator current at each step as I add load to prevent overload.

Maybe even check frequency (engine speed?) to ensure governor stays up.

May also want to check voltage to ensure regulator is staying up.

Would take analog control - will the 1000 support that? Not positive.
Dan Bentler

Thanks for the input so far.

The generator is a backup for a bunch of animal heating pens. So the load isn't critical as such, but the animals wil not be happy if the power fails.

At the moment, my working assumption is that the generator has its own control system. This block is just to manage the huge switch on transient.

I will be glad to help you write the ladder logic when you get started.

iice said:

Thanks for the input so far.

The generator is a backup for a bunch of animal heating pens. So the load isn't critical as such, but the animals wil not be happy if the power fails.

At the moment, my working assumption is that the generator has its own control system. This block is just to manage the huge switch on transient.

Click to expand...

The "perfect" way to handle this would be to change the animal heating system to hot water and ideally radiant heat. Would easily allow for zoning and would give a stable heat level ie no spikes. The heat load could be met with the waste heat from the diesel with possible addition of water cooled exhaust manifold and water cooled exhaust stack. This would free up generator capacity to meet electrical only loads.

Dan Bentler

At the moment, my working assumption is that the generator has its own control system.

Click to expand...

Jeff,

You really do need some interlocking between the generator controls and the PLC. If the generator does not have an output that says "generator ready", then you need to add one to tell the PLC that it should now start the loading sequence. If this is mostly a manual operation, it can be as simple as adding a pushbutton to an input on the Micrologix.

The Micrologix does have the S:1/15 First Pass bit, which comes on only on the first scan of the PLC after power-up. However, that bit may be on a long time before the generator is ready to accept loads. If you use only it to trigger your sequence, you will have to shut down the PLC after every generator start. That is not generally a good idea.

Here is a Micrologix program that I wrote for you, to show you the scope of the project. Because you are a self-proclaimed Newbie, I did not use a Sequencer (SQO instruction),so as to keep the logic as transparant as possible. With an SQO, the number of rungs could be reduced significantly, but it would not be so obvious (as the counter that I used) as to how it works.

Best Regards,
Lancie