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Old 04-20-2020, 02:53 PM   #1
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Soft Start vs SUPCO SPP6

I have a 2018 Siesta 24ST with 30 amp service and a Dometic Penguin II 13.5k air conditioner. I've been seeing ads lately for the Soft Start ($299, $$249 with coupon) that reduces the startup surge of the air conditioner, to allow it to start on lower amperage without tripping a breaker, and to allow other appliances to run at the same time.

But I've also seen the SUPCO SPP6 Relay/Capacitor Hard Start Kit ($10.49) on Amazon, a replacement starting capacitor that, to all appearances, seems to do the same thing, and has good user ratings.

Obviously, if I can achieve much the same goal at 4% of the cost, that's the way to go. But has anyone had experience with these, or can explain the difference between the two products?
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Old 04-20-2020, 03:57 PM   #2
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When a motor runs it also acts as a generator, generating voltage opposite to the applied voltage. This is known as Counter ElectroMotive Force (CEMF) because it opposes the ElectroMotive Force (EMF, AKA voltage) that is applied to the motor.

A given motor under a given load will have a running current determined by the difference between the EMF and CEMF. If the load increases and the motor slows down the CEMF will also go down allowing more current to go through the motor.

When the motor is stopped there is no CEMF so the current going to the motor is at maximum for the applied voltage, limited only by the resistance (reactance) of the motor windings until the motor starts turning. To add torque to a single phase AC motor starter windings are used that are off-set from the run windings by some angle. To create a rotating magnetic field the voltage applied to these windings must be out of phase with the voltage applied to the run windings. This is generally accomplished by a starting capacitor which is only in the circuit up to a certain RPM.

The larger the starting capacitor, the more phase difference between the windings which generates more torque and lowering the time and amount of starting current (to a point). That is your simple hard start capacitor.

The "soft start" or easy start kit includes the capacitor as well as a voltage ramping circuit that limits the voltage applied to the motor as it is starting which limits the starting current.

That's your simplified AC motor theory for today. I hope it helps.
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Old 04-20-2020, 04:18 PM   #3
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So are you saying that the Supco capacitor alone won't achieve the amperage smooth-out that keeps the circuit amperage from surging ? I've seen a video on another thread here that shows an a/c start with the Supco, where the amperage slowly increases to 12 and levels off; no surge.
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Old 04-20-2020, 04:18 PM   #4
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I can't speak to the comparison because I've only done one...

I put the soft start kit on our Axis (we have the 13.5k A/C). So far it has been working as advertised:
  • The compressor ramps up in speed when the A/C first starts--no longer hear the "thud" as it kicks on
  • I have an ammeter installed and see the current ramp up slowly to the running current (around 12 amps)
  • I have now successfully ran our A/C for long periods of time on our 15a garage circuit

Oh yeah video, I did take video of the ramp up...let me find it
here it is:
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Old 04-20-2020, 05:53 PM   #5
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JamieGeek: So your video is with the $10 Supco, not the $250 soft start? If so, Supco sounds worth doing.
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Old 04-20-2020, 06:05 PM   #6
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I am a retired master electrician. I have the $15.00 Supco Hard Start. Works just fine and I have very low voltage at my camp.
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Old 05-06-2020, 07:36 PM   #7
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If it can be done for under $10 (assume manufacturers get wholesale prices) why isn’t it standard on all A/Cs?
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Old 05-06-2020, 08:28 PM   #8
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If it can be done for under $10 (assume manufacturers get wholesale prices) why isnít it standard on all A/Cs?
Because it is not required when connected to a sufficient power source, only a marginal power source.
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Old 05-06-2020, 08:36 PM   #9
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But many RVs want to use as small a generator as possible. I should have clarified: for RV A/Cs.
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Old 05-06-2020, 09:33 PM   #10
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My answer was for RV A/Cs.
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Old 05-06-2020, 11:18 PM   #11
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My answer was for RV A/Cs.
Oh. Then it does not make sense to me. Most RVs in my part of the world do not have shore power available.

I see many lugging huge noisy generators or hooking together a couple 2kw hondas or yamahas.
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Old 05-06-2020, 11:43 PM   #12
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my question is 'why' are you interested in this product? You have a 30amp coach with a single air conditioner which runs just fine with 30amp service, even everything at once.
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Old 05-07-2020, 01:19 AM   #13
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my question is 'why' are you interested in this product? You have a 30amp coach with a single air conditioner which runs just fine with 30amp service, even everything at once.
Because when boondocking you can run a little Honda or Yamaha 2k generator all weekend on 2 gallons of fuel and be able to run the AC and the on board that can push the 30 amps burns about half a gallon an hour
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Old 05-07-2020, 01:30 AM   #14
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Oh. Then it does not make sense to me. Most RVs in my part of the world do not have shore power available.

I see many lugging huge noisy generators or hooking together a couple 2kw hondas or yamahas.
That's why motorhomes come with onboard generators.
Travel Trailers have to use external generators and if a properly sized one is used the A/C runs fine. It's only when you have a limited power generator that you would need/want a soft start kit. RV A/C manufacturers build their units assuming the RV will be connected to a power supply of sufficient capacity. They do not engineer/build the A/Cs to operate with marginal power that will cause an early demise to their product.
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Old 05-07-2020, 02:05 AM   #15
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BUT....

from a practical 'financial' comparison, and using the assumption that you DO have an onboard generator, that you have ALREADY PURCHASED as part of the RV, you are then purchasing ANOTHER generator, let's say at $500, and ALSO then purchasing the 'special a/c starting capacitor' for another $200, you are now $700 in the 'hole', versus just using the BRAND NEW onboard generator, which DOES have the capacity to 'correctly' power your a/c unit WITHOUT any additional cost or device to purchase.

With that in mind, $700 is worth maybe 350 gallons of gas, if a $2 per gallon is used.
The 350 gallons of gas, if used at a rate of 1/4 gallon per hour(not 1/2 gal per hour because that is at the MAX output rate of the generator, which is not required to simply run an air conditioner, since as you stated, that's ALL you would be powering), you could then extrapolate that out to 1,400 HOURS of onboard generator run.

1,400 HOURS of your generator 'running', and 'just for a/c usage' for this comparison, since that's all you'll be using any additional separate 'generator' for, is a LONG, LONG amount of 'camping' time to be anywhere CLOSE to comparing to the dollar purchase of the aforementioned items.
1,400 hours equates to 175 DAYS OF CAMPING if you used this method for 8 HOURS straight per day!

175 Days of 'camping', for most anyone using their camper for weekends, or vacation during the summer, etc., makes for probably several YEARS of general usage, as every day is NOT a day where the air conditioner will be needed, especially during the temperate months during Spring and Fall, and certainly not during the winter, or during any storage.

I get that 'fuel' usage might be a first thought for some, but since the onboard generator is already there, ready to be used, is already very capable of powering everything onboard, and at the same time, and is easy to start, is easy to store, is easy to carry, is easy to fuel, carrying a smaller generator, then.... well, you get the idea.

Yes, if money is not object, then go for it.
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Old 05-07-2020, 02:18 AM   #16
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BUT....

from a practical 'financial' comparison, and using the assumption that you DO have an onboard generator, that you have ALREADY PURCHASED as part of the RV, you are then purchasing ANOTHER generator, let's say at $500, and ALSO then purchasing the 'special a/c starting capacitor' for another $200, you are now $700 in the 'hole', versus just using the BRAND NEW onboard generator, which DOES have the capacity to 'correctly' power your a/c unit WITHOUT any additional cost or device to purchase.

With that in mind, $700 is worth maybe 350 gallons of gas, if a $2 per gallon is used.
The 350 gallons of gas, if used at a rate of 1/4 gallon per hour(not 1/2 gal per hour because that is at the MAX output rate of the generator, which is not required to simply run an air conditioner, since as you stated, that's ALL you would be powering), you could then extrapolate that out to 1,400 HOURS of onboard generator run.

1,400 HOURS of your generator 'running', and 'just for a/c usage' for this comparison, since that's all you'll be using any additional separate 'generator' for, is a LONG, LONG amount of 'camping' time to be anywhere CLOSE to comparing to the dollar purchase of the aforementioned items.
1,400 hours equates to 175 DAYS OF CAMPING if you used this method for 8 HOURS straight per day!

175 Days of 'camping', for most anyone using their camper for weekends, or vacation during the summer, etc., makes for probably several YEARS of general usage, as every day is NOT a day where the air conditioner will be needed, especially during the temperate months during Spring and Fall, and certainly not during the winter, or during any storage.

I get that 'fuel' usage might be a first thought for some, but since the onboard generator is already there, ready to be used, is already very capable of powering everything onboard, and at the same time, and is easy to start, is easy to store, is easy to carry, is easy to fuel, carrying a smaller generator, then.... well, you get the idea.

Yes, if money is not object, then go for it.

WOW!!!

Quarantine induced calculations.

Somebody needs to go camping!
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Old 05-07-2020, 02:52 AM   #17
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I am not familiar with "soft start". So I have a question.
My husband is on oxygen and when the A/C comes on it causes the oxygen concentrator to shut down. Would this "soft start" keep that from happening?
Thanks for information.
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Old 05-07-2020, 02:54 AM   #18
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Sorry meant to include while we are running the generator, we have an Onan 4000
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Old 05-07-2020, 03:55 AM   #19
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Originally Posted by 16ACE27 View Post
That's why motorhomes come with onboard generators.
Travel Trailers have to use external generators and if a properly sized one is used the A/C runs fine. It's only when you have a limited power generator that you would need/want a soft start kit. RV A/C manufacturers build their units assuming the RV will be connected to a power supply of sufficient capacity. They do not engineer/build the A/Cs to operate with marginal power that will cause an early demise to their product.
If they do it is a stupid assumption for the western US at least. Most camping is in National Parks, National Forests, or on BLM land, very few of which have AC power. Most use trailers or pickup campers that do not have generators.

At $10 it should be standard.
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Old 05-07-2020, 03:57 AM   #20
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Originally Posted by mccallsd View Post
I am not familiar with "soft start". So I have a question.
My husband is on oxygen and when the A/C comes on it causes the oxygen concentrator to shut down. Would this "soft start" keep that from happening?
Thanks for information.
Shore power or generator?

If shore power, unlikely. If generator, possibly.
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