JKE_MN

I have a 22kW Generac Guardian Standby Generator model 70422, serial 3001330170 that has been operating for two years now (from Ziller). It worked fine with our old Trane 3.5 ton A/C outdoor unit. (I do have load shed connected to the transfer switch although it never sheds.) We recently had our A/C outdoor unit replaced with a Trane XR 4TTR6042J1000A with a 3.5 ton Climatuff Scroll Compressor. RLA is 16.7. In the interest of being proactive do you think I should install a Sure Start or will I be okay without it? (I could do an experiment by disconnecting service and letting the generator take over but I don’t want to risk damaging my new A/C unit.) If you think a Sure Start is needed, with a RLA of 16.7, should I use the 8-16 RLA or the 16-32 RLA Sure Start? In my mind 16.7 is borderline between the two Sure Start sizes so I want to choose the correct size since the documentation states that incorrect sizing will result in damage to the Sure Start and/or Air Conditioning unit. Thanks for your input.

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murphy
What is the LRA (Locked Rotor Amps) of the compressor?  That is the load that hits the generator when the compressor starts.  The purpose of a sure start is to reduce that value so the generator sees a lower start value.

16.7 is between 16 and 32.  There is no doubt as to which one you should choose.
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JKE_MN
Thanks for the input Murphy. This stuff is confusing to me. ðŸ˜• LRA is 109. Do you think I need a Sure Start unit? Thanks!
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78buckshot
My thoughts are that the A/C will do OK without the soft start BUT the whole shootin' match will be BETTER with it. New A/C , newer generator - protect and prolong them with the soft start.
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murphy
That is more than max current if on NG and very close to max current but still over if on LP.  Presumably that generator can handle a small overload for less than a second.  When the compressor starts turning the back EMF quickly drops the current toward the RLA value.  It depends on what else is on the generator.  If there are other loads that add up to 50% that would be a severe overload for less than a second.  I don't see any documentation for overload performance in Generac's literature.  The generator will start to slow down until the governor can add fuel to bring it back to nominal speed.  I have a 3 ton A/C on a 20 kW generator on LP.  I have a soft start on on it but it was installed when I had a 7 kW generator several years ago.  It did allow the 7 kW to start the compressor.

RLA is the current the compressor draws once it is up to speed and providing maximum back EMF (Electro Motive Force).
LRA is the current the compressor draws before the compressor starts turning when back EMF=0.
A soft start puts a clamp on the LRA current for up to 500 milliseconds.  By that time the compressor should be turning and ramping its current down toward RLA.

Try it and see what the engine sounds like.  If it struggles to get back to speed the Sure Start will help.  
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JKE_MN
Thanks for the input 78buckshot and murphy. I should have mentioned I'm running NG so I believe the max output is 19kW.
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Geoff Z
LRA (locked rotor amps) is typically 6 times RLA (rated load amps). Soft start will typically provide about 60% reduction in LRA motor start current. Good for the generator and a/c. 109 LRA is not pushing up against the rating of a 22kw. Yes 19.5kW on natural gas. However when other loads are used simultaneously during a/c cycling that is when issues can happen.
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grsthegreat
some of the higher end trane units have electronic systems already inplace. I've seen some heat pump systems that look like they have computer boards with launch capability installed in them. There are so much electronics that ive been afraid to add a soft start.
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Geoff Z
grsthegreat wrote:
some of the higher end trane units have electronic systems already inplace. I've seen some heat pump systems that look like they have computer boards with launch capability installed in them. There are so much electronics that ive been afraid to add a soft start.


Agreed. Always best to check with manufacturer to confirm soft start compatibility.
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BillM
JKE_MN wrote:

I have a 22kW Generac Guardian Standby Generator model 70422, serial 3001330170 that has been operating for two years now (from Ziller). It worked fine with our old Trane 3.5 ton A/C outdoor unit. (I do have load shed connected to the transfer switch although it never sheds.) We recently had our A/C outdoor unit replaced with a Trane XR 4TTR6042J1000A with a 3.5 ton Climatuff Scroll Compressor. RLA is 16.7. In the interest of being proactive do you think I should install a Sure Start or will I be okay without it? (I could do an experiment by disconnecting service and letting the generator take over but I don’t want to risk damaging my new A/C unit.) If you think a Sure Start is needed, with a RLA of 16.7, should I use the 8-16 RLA or the 16-32 RLA Sure Start? In my mind 16.7 is borderline between the two Sure Start sizes so I want to choose the correct size since the documentation states that incorrect sizing will result in damage to the Sure Start and/or Air Conditioning unit. Thanks for your input.


I have a 22kW unit, model 6551, I just had a Lennox 5 Ton M/N 13ACXN060-230-21 installed.  It has an LRA of 125, an RLA of 26.4, Min CKT Ampacity of 34.7, likes long walks in the park and is running fine and starting fine while on generator.  I tested the compressor with the gen for over an hour.  Dims the lights on startup but works fine.  No Sure Start.  I want one, but it's too rich for my blood.  

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Brian Baughman
If the new Trane AC unit is variable speed, a soft starter is required for the AC to operate on generator power
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JKE_MN
Thanks all. I did confirm the compressor is not variable speed. Next step will be to test with the generator and no Sure Start. If that doesn't work I will add the Sure Start. 
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Geoff Z
Make sure when you test you do it as you would operate in an outage. It is good to start the AC with no other load and also with other load operating simultaneously. You don't want to find out during a power outage in 100 degrees that you have to sit in the dark to remain cool.
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78buckshot
I installed an owner provided Shure Start today, our company had replace his A/C system late last year, he wants to run the A/C on his Honda portable 7kw. The unit works like a charm, simple new 2 1/2 ton.
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