time delay relay for Air Conditioner Loads

[NAZ55]

Good morning All,
I have an application requiring three of my air conditioner loads to start in step in order to avoid having to buy a bigger size emergency generator.
my three airconditioner loads are as follows:
-240V 60A single Phase AC
-240V 60A single Phase AC
-240V 30A single Phase AC
It's an existing facility with Square D NQOD panel boards.
Do you know of any time delay relay that I could put into the AC circuit to possibly start these units at different time intervals, preferrably more than 10 sec?
Thanks

 

[dpc]

I don't think you're going to find a timer rated for switching 60A at 240V.

If these have a thermostat or some type of control circuit, the best approach would be to put the timer in the control circuit.

If you want or need the brute force approach, you'll need to add an appropriately rated ac contactor and switch it from your timer.

David Castor

http://www.cvoes.com

 

[PHovnanian]

There are a few options possible. The simplest would be to put relays in some low voltage control* input to each AC unit. If such inputs exist. This will save you from having to find 60 Amp time delay relays, or put together a T/D relay, contactor combo.

I don't think you'll want to put any relays and/or contactors in your existing panelboards. An enclosure (or three, one for each a/c unit) will probably be needed. But if you implement a low voltage control scheme, the enclosure requirements will be trivial.

Depending on what sort of conditions you'll want to have initiate this stepped start up, you might want to consider using a cheap PLC. There will be a bit of a learning curve to create the ladder logic. But if you have a voltage sensing input (or three), thermostat inputs and possibly the transfer switch status (utility vs generator), discrete relays could get a bit complicated.

*Or more accurately, low power. 120 or 240 VAC relay contacts are not much more expensive than 24 V stuff. Its the 60 Amps that will cost you.

 

[jraef]

There is no such thing as a "60A time delay relay".

First you will need to determine how your ACs are turning themselves on and off. Most likely there is a contactor, or multiple contactors, that control that kind of power. If so, then then simply trace back the control voltage source for the contactor(s) and put a time delay relay in that control voltage source. You can buy simple inexpensive "On-Delay" timers and just set each one for a progressivly larger amount of delay time.

If the AC units are sealed, or for warranty purposes you don't want to mess with their internal control scheme, you will have to buy 60A rated contactors to feed power to the ACs and add a time delay relay to each of their control circuits.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[Compositepro]

Many air conditioners already have a timer to delay compressor start after power recovery. This prevents the compressor from trying to start against full pressure after a momentary power loss. The time delay between different units is not likely to be identical. In any case, there is still the low possibility that three units can start at the same time at anytime, not just immediately after switching to generator power.

 

[waross]

What compositepro says. See what you have on hand already. I have put in manual restarts for standby generator use. The occupant decides which of his A/Cs to run when there is not enough capacity to run all A/Cs.
This just takes a cheap relay in the control circuit and a push button. Some of these where old A/Cs which did not have delayed re-start and as well, not all could be run at once on the generator.

Bill
--------------------
"Why not the best?"
Jimmy Carter