RV AC 521 compressor saver

My two AC's have run for 12 years without one.
Don't really see the need for one.
Maybe if you had a very small generator you might need one.

What does it do that a SUPCO hard start kit at half the price doesn't do?

I've been researching this circuit for a couple weeks now and, the following is what I've learned. The 521 3-wire circuit uses the usual boost run cap but has the addition of a relay that reads the back emf of the compressor motor. When the motor reaches speed and the back emf is sufficient to open the relay, the boost cap is disconnected from the motor - stated as the safest method. Other circuits such as 521 2-wire or hard start booster PTC/cap don't work consistently or pose extra risk of failure. The 521 3-wire mod reduces start current to half or better which allows much smaller inverter-generators or more additional loads for the big guys. This mod has been used for decades in HVAC and from what I've read, was included by the AC manufacturers many years ago. There is a question about which type of AC it can be used in safely. Since I haven't seen a 521 mod listed for RV's, it may not be suitable. I believe the Supco hard start kit is a PTC/capaitor. From the tests I've seen, this mod doesn't lower the start current. It works by getting the motor up to speed faster giving a shorter duration current pulse. It also appears to be temperature sensitive and may not not work under certain conditions. If anyone finds any error in this, please let me know - I'm still climbing the learning curve.

5-2-1 and SUPCO are both hard start kits and consist of a potential relay and capacitor. Ideally the potential relay operating parameters should match the electrical properties of the compressor. Thus the best and safest hard start accessories to use are those available from the manufacturer of the A/C unit involved.

high5 wrote:
Has anyone installed a 521 compressor saver on an AC unit? I know hard start 'boosters' are popular but, I haven't seen anything about the 5-2-1 compressor saver. They are usually installed on residential units but they do make a kit for 1 to 3 tons (12,000 - 36,000 btu). It looks like a good low cost alternative to the $300 Micro-air EasyStart unit. They both substantially reduce compressor start current.

The 5-2-1 Compressor Saver is clearly a hard start kit as is a Supco SPP6 but the Micro-Air Easy Start Soft Starter Kit is not. Testimonials from those here on rv.net who have installed the Easy Start on their roof top A/C unit indicate this does solve the problem of successfully and repeatedly being able to start & run a 13,500 BTU A/C using a 2K genset such as the Honda EU2000i. If that's your intention the Easy Start is the solution I'd be looking at.

prichardson wrote:
5-2-1 and SUPCO are both hard start kits and consist of a potential relay and capacitor. Ideally the potential relay operating parameters should match the electrical properties of the compressor. Thus the best and safest hard start accessories to use are those available from the manufacturer of the A/C unit involved.

According to the tear-down/installation videos of the Supco, there was no relay - only PTC(s). So, it's operation is dependent upon how fast or slow they heat (and cool) with surge current. A 521 does use a potential relay which is directly dependent on motor back emf. I noticed on an installation video in a 13,500 RV ac, that both the PTC and 521 were shown on the units wiring diagram. So, if the manufacturers wiring diagram includes 521, then I think my question has been answered. That diagram also shows there is no relay in a PTC 2-wire circuit.

SoundGuy wrote:

high5 wrote:
Has anyone installed a 521 compressor saver on an AC unit? I know hard start 'boosters' are popular but, I haven't seen anything about the 5-2-1 compressor saver. They are usually installed on residential units but they do make a kit for 1 to 3 tons (12,000 - 36,000 btu). It looks like a good low cost alternative to the $300 Micro-air EasyStart unit. They both substantially reduce compressor start current.

The 5-2-1 Compressor Saver is clearly a hard start kit as is a Supco SPP6 but the Micro-Air Easy Start Soft Starter Kit is not. Testimonials from those here on rv.net who have installed the Easy Start on their roof top A/C unit indicate this does solve the problem of successfully and repeatedly being able to start & run a 13,500 BTU A/C using a 2K genset such as the Honda EU2000i. If that's your intention the Easy Start is the solution I'd be looking at.

That is my intention and I've seen the EasyStart testing. It clearly is the superior unit but with a cost/benefit issue. In the tests I saw, using equipment capable of capturing very quick peak inrush current measurements, the 521 3-wire reduced peak current by almost 50%. That reduction plus the addition of a delay relay for the fan, brings the starting current well within the capability of a 2000W inv-gen. The total for a 521 3-wire/fan delay mod is ~$50. The EasyStart reduced inrush by 70%+ but the $320+ cost may not be required to achieve the desired result. If the EasyStart was more affordable, I'd buy it in a heart beat - it's a remarkable piece of engineering.

I hav emailed the 521 folks and they do not have a model for 120 volt operation- I haven't measured the back emf voltage on a running unit though. The Supco SPP6E is a relay based unit, and while Supco used to spec it for 90-270 vac, now they spec it at higher- and don't recommend it for 120 volt units. I'm not sure what changed.
Dometic does make a potential relay based start kit- I'll look up the number. It is specifically meant to run a Penguin low profile unit on an Onan 2800 microlight.

Here's the Dometic instruction sheet for the relay based kit-
http://bryantrv.com/docs2/docs/quickstart.pdf

high5 wrote:
Has anyone installed a 521 compressor saver on an AC unit?

It looks like a good low cost alternative to the $300 Micro-air EasyStart unit.

SoundGuy wrote:
Testimonials from those here on rv.net who have installed the Easy Start on their roof top A/C unit indicate this does solve the problem of successfully and repeatedly being able to start & run a 13,500 BTU A/C using a 2K genset such as the Honda EU2000i. If that's your intention the Easy Start is the solution I'd be looking at.

high5 wrote:
That is my intention and I've seen the EasyStart testing. It clearly is the superior unit but with a cost/benefit issue.

The EasyStart reduced inrush by 70%+ but the $320+ cost may not be required to achieve the desired result. If the EasyStart was more affordable, I'd buy it in a heart beat - it's a remarkable piece of engineering.

Your argument 'though certainly understandable is really no different than the argument as to whether a Honda or Yamaha genset is worth a premium price compared to the competition which in some cases cost considerably less. For some the answer is obvious - Honda or Yamaha without question - for others it's a real struggle justifying anything but the cheapest solution they can find. I've owned 2 EU2000i gensets and am convinced the cost / benefit ratio is easily justified ... I just wish I had known about the Easy Start at the time, otherwise I'd have not sold my last EU2000i. :R

Chris Bryant wrote:
I hav emailed the 521 folks and they do not have a model for 120 volt operation- I haven't measured the back emf voltage on a running unit though. The Supco SPP6E is a relay based unit, and while Supco used to spec it for 90-270 vac, now they spec it at higher- and don't recommend it for 120 volt units. I'm not sure what changed.
Dometic does make a potential relay based start kit- I'll look up the number. It is specifically meant to run a Penguin low profile unit on an Onan 2800 microlight.

Here's the Dometic instruction sheet for the relay based kit-
http://bryantrv.com/docs2/docs/quickstart.pdf

The 120 VAC problem may be why I haven't seen a 521 on an RV AC. And, I did not see at what voltage the 521 1 - 3 ton kit was rated. I'm interested in seeing what Dometic has to offer. If someone can tell me how to post an image from my computer, I'll attach a photo of a 13,500 btu(1.125 ton) Airxcel (Coleman Mach?) wiring diagram showing the 521. The Dometic wiring looks somewhat unconventional to me.