Prevent Simultaneous Compressor Start-up ?

[JayH]

Anybody know of a way to prevent two refrigeration compressors from starting at the same time and overloading an inverter? I suppose a similar problem/solution could exist for air conditioners.

I installed a small dedicated inverter for the AC refrigeration in the hopes that it would be more efficient, and allow the bigger unit to sleep or shut down. The small inverter can handle one unit starting with the other one already running, but it can't handle both starting at the same time. The surge only lasts about 1-2 seconds, so it remains to be seen if this ever really happens in practice.

[Blue Stocking]

Set the thermostats 5 degrees difference.

[Sailmonkey]

You can install a start delay timer on one of the units. This wouldn't be a 100% guarantee that through some freak coincidence that both wouldn't start at the same time but would reduce the chance to near zero. All it does is go between the thermostat and the compressor (over simplified) and not allow the compressor to start until a preset amount of time has passed. See link

Relay, Time Delay, Dpdt - Time Delay Relays - Relays - Electrical : Grainger Industrial Supply

[JayH]

Originally Posted by Sailmonkey

You can install a start delay timer on one of the units.

Thanks Sailmonkey. You're right it won't prevent them starting at the same time out of coincidence. A time delay on the thermostat will stop them starting at the same time when the inverter is turned on or DC power is applied, so that's something. Currently I'll have to unplug one of the units in that scenario.

I'm still hoping for some magical device that just does this, preferably on an AC level so I don't have to modify the units.

Seems this could all be easily avoided if the inverter staggered the startup times of the outlets and only shut down the outlet causing the overload for a short time. Instead, it shuts down power to both outlets until the unit is reset.

[Sailmonkey]

Realisticly I think that the start delay timer would do just what you need. I'm no statician but the possibilty of having the unit with no timer turn on it's compressor at the exact same instant as the one with the timer would be next to nill. You may also try adding a hard start capacitor to one or both units.

[JayH]

Originally Posted by Sailmonkey

I'm no statician but the possibilty of having the unit with no timer turn on it's compressor at the exact same instant as the one with the timer would be next to nill.

Since the units are governed by separate thermostats the timer on one of them probably doesn't have any effect on the odds they'll start at the same time. Assuming the start times are already random, random plus a time delay is still random.

Originally Posted by Sailmonkey

You may also try adding a hard start capacitor to one or both units.

That is an option I'm trying to learn more about. Is it likely these units already have capacitors? If they do, can they be increased in size to reduce the surge?

[Nauticatarcher]

Just put a relay in power line to each compressor that when its energised opens circuit to other compressor, when compressor shuts down other compressor can start and vice a versa

[senormechanico]

Originally Posted by Nauticatarcher

Just put a relay in power line to each compressor that when its energised opens circuit to other compressor, when compressor shuts down other compressor can start and vice a versa

My first thought, but it would take even less $$ if you used small relays in the thermostat circuits.

I did a similar circuit in a house where it would have been a major undertaking to add a 220 volt circuit for an irrigation pump.
Instead, I added a relay so when our hot tub clock said it was time to run, the irrigation pump ( if running ) would stop. When the hot tub heater got done, the irrigation pump would go back to its cycle.

Worked like a charm.

[Cowboy Sailer]

How about 110 Vac normally closed relays on one unit's start winding that opens the other unit's thermostat line? It takes two relays but they are not handling significant power so they would be cheap to buy.

[Goofy Foot]

Hi

How about a 2 stage thermostat. If worried about equal run times per compressor install a lead/ lag switch.

[JayH]

Getting into the windings and changing the thermostats sound too invasive. I hope it doesn't need to be that complicated. Other suggestions aren't taking into account that the units need to be able run concurrently, and that they have different duty cycles, each over 50%.

How about 4 normally closed relays, two of them with timers (if such a thing exists)? When Unit 1 comes on, the load actives a timed relay that interrupts the supply of Unit 2 for a few seconds. Unit 2 has a normally closed relay that interrupts the coil of the timed relay if Unit 2 is already running. The other two relays are to do the reciprocal.

If I'm thinking about it correctly, this should prevent anything but a simultaneous startup of both units within the speed of the relays, which I'm willing to assume is impossible in any scenario except inverter startup. I'm already assuming that it is highly unlikely to happen within the 1-2 seconds of surge current, so maybe the complexity is unnecessary.

For the inverter startup case, I'll need a way to delay one unit very briefly. Can't think of a way for a load to delay itself...

Can I solve the whole surge problem with start capacitors?

[Goofy Foot]

Hey Jay

Double check your wiring diagrams, you shouldn't have to get into any windings. There should be a thermostat relay that starts and stops the compressor.

[goboatingnow]

get a standard AC motor protection unit. set the current to the max the invertor is happy with, use the contacts to disable the other compressor while the fault ( ie too high current exisits). All ac wiring.

[Opie91]

If you want a fool proof method go with the large relays and cut off the power to the compressor when the other is running. They have to be cheaper than your inverter. Probably the simplest installation also.

[btrayfors]

If I understand the OP's question correctly, it's not just a matter of cutting power to one compressor while the other starts.

With two A/Cs or two friges, you want BOTH of them running at the same time. If your inverter or other ac power source can handle both loads while they're running, but has problems if -- by chance -- both compressors attempt to start at the same time, then you can't just solve the problem by using relays which will cut power to one compressor while the other is starting. The reason is that the other one -- the one you cut power to -- could well have been RUNNING (not starting), and if you cut power to the compressor you can't just turn the power back on without waiting a "decent interval" or you could REALLY provoke a high amperage draw, as the compressor's head pressure would be high. Some installations have a "wait time" after power is cut, but others don't.

I don't know what the solution is, but I'm pretty sure it's not just a couple of relays. At least that's how it seems to me this late in the evening.

BTW, GREAT FIREWORKS in DC this evening....maybe the best ever! Now the crazies are headed downriver at high speeds, half in the bag. Hope nothing serious happens.

Bill