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Discuss Sequential timers, need advice on a good one. in the Industrial Electrician Talk area at ElectriciansForums.net


EF Member
Hi, need some advice on sequential timers.
I need to set up 6 containers with 2 ton (24,000BTU) Inverter Split Air Conditioner units supplied through individual contactors, I am looking for a sequential timer that can allow me to space a starting sequence of 15 seconds between each unit. The ST shall only be operating the coils of the contactors the AC load shall not be through them

The units shall be run off generators and I need the loading of the set to be sequential to avoid the it shutting down.
I need something that is practically idiot proof as when I am gone from this place the quality of the guys maintaining them requires something simple and easy to repalce. So any advice on a good unit out there I can get that will cover this scenario.
Thank you.


EF Member
May I suggest a rephrasing of the requirement? At the moment you are seeking a sequencer which will on the push of a button start up plant 1, then plant 2...and finally plant 6. Is this always what you want to happen?

Close, I need the delays on each AC starting up to allow the diesel generator time to return to its rated RPM after every load is applied, it normally only takes seconds to do this but sometimes if you have a dodgy injection pump or fuel lift pump or governor on the engine it can be problematic. So 15 secs between starting should allow the set to run and settle into its rated RPM (1500)
Applying loads over 30% of the FL to a genset on start up causes issues and shutdowns, and loads should be sequenced, not applied all at once.

Running current on each AC is approx 9.4Amps
(starting current is approx 23amps)
Gen set is 25Kva 400V 50hz and also provides lighting and power in the containers for staff.
Need to think more on this and find the right soloution.
We are storing Medications in these containers for a refugee camp so it has to work properly from day 1 and work all the time.


Respected Member
I had to interrupt writing the post at #3 so deleted it.

I wrote: May I suggest a rephrasing of the requirement? At the moment you are seeking a sequencer which will on the push of a button start up plant 1, then plant 2...and finally plant 6. Is this always what you want to happen?

I was going to go on and ask if you would be satisfied with a controller which allowed any (but only one at a time) of the start buttons to actually start an item of plant once 15 seconds had elapsed since the last item of plant was started.

This scheme could be an automatic start up sequence or a manual start up sequence but prompted by some indicator lamps eg: 'Supply ready to start Plant 2' then the operator presses Start Plant 2 button and the remaining stopped plant indicator lamps extinguish and their start buttons lose control power both for 15 seconds.

For the automatic sequence do you want to be able to pre-select which plant starts?

And what about emergency stop?
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Respected Member
I gather you only have two ACs to start not 6 as I first thought.

What about re-jigging a star-delta motor starter - you could use each of the contactors to DOL start the ACs and swap the SD timer for a 15 second delay on relay such as:

(check the functionality, contacts and voltage, etcetera 'cos I have only quickly looked up before heading out.)



Respected Member
I just had a quick look at the Schneider range of automatic star-delta controllers.

This starter's integral on delay timer LADS2 has a range of 1 to 30 seconds see:

The LADS2 is a pneumatic one:

So re-jigging an SD starter looks a goer for your application.

I suppose you are prepared to spend more on a better make for reliability in the field and reduced risk of loss.
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Respected Member
I thought I'd add that the SD starter idea only sequences the AC starting transients when they are first powered up. Thereafter, the idea relies on/assumes the ACs start (stop) at different times based on the cooling demands on each of them being unsynchronised. Is this a reasonable assumption? Or do the ACs run continuously once started?

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