Author Topic: Reducing fridge motor start current  (Read 8658 times)

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Opera House

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Reducing fridge motor start current
« on: May 23, 2008, 09:50:41 AM »
It is time to head off to the camp and I have modified a small chest freezer with a temperature control to use as a refrigerator.  Because of the top load and better insulation, these consume less than 100 watt hours a day compared to 1KWH for a normal fridge. Thought this would be great for my 150W solar panels. The compressor is a capacitor run motor with that capacitor shorted by a PTTC for starting.  Normal wattage is 100W running and 150W starting for about the first 45 seconds.  Thought this would run great on my small 200W inverter.  


Inverter immediately goes into overload.  I put the VAW meter into peak amp hold and it reads about 17A on house current.   I tried a 300uF cap in the starting circuit instead of the short and that didn't help much.  I have a small quiet ONAN 450 watt gas generator and even that immediately dies when the freezer is plugged in.  Anyone have any tricks to reduce the starting current?  Hate to have to buy a 1200W inverter just for this.  

« Last Edit: May 23, 2008, 09:50:41 AM by (unknown) »

imsmooth

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Re: Reducing fridge motor start current
« Reply #1 on: May 23, 2008, 04:24:51 AM »
Use a Hard-start capacitor.  I used this to reduce the starting current for my AC when my backup generator is running.  The extra capacitance shift the peaks of the start and run winding currents, reducing the net current.  A relay device in the unit cuts the capacitor out when a sufficient voltage is achieved.  I dropped a starting current of 80A to 30A when I did this. If you are interested I can look up the source where I got mine.  You need the correct size based on the size of your compressor.  You don't want a cheap one.  I did this first and the relay was junk; it never kicked out and the capacitor went up in smoke.  The next one I got has worked great for the last 3 years.
« Last Edit: May 23, 2008, 04:24:51 AM by (unknown) »

wdyasq

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start current
« Reply #2 on: May 23, 2008, 05:02:15 AM »
I'll be interested in seeing the parts and results. My small older freezer runs on ~120W but has a starting load of ~1200W.


IF a small freezer's starting current can be reduced it will allow for a smaller dedicated inverter to run the thing. This will be a real boom for those who really do live minimally off-grid.


Ron

« Last Edit: May 23, 2008, 05:02:15 AM by (unknown) »
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Bruce S

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Re: Reducing fridge motor start current
« Reply #3 on: May 23, 2008, 07:09:58 AM »
I too would be VERY interested in seeing the results.

I just converted a small 5cu.in. using a new digital programmable control and would like to know about the hard start relays.

Currently takes a 2k inverter to start it. and It's only a 1/10hp.


Do tell us more about matching these and the better ones to use?

Bruce S

« Last Edit: May 23, 2008, 07:09:58 AM by (unknown) »
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DanG

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Re: Reducing fridge motor start current
« Reply #4 on: May 23, 2008, 07:37:10 AM »
I wonder if someone could borrow a car audio amplifier super-capacitor (12V booster cap) and place it in circuit on the inputs of the small inverter that is stumbling on compressor start up. Often times the inverter can be artificially limited by wire size, run length, etc. and other factors...
« Last Edit: May 23, 2008, 07:37:10 AM by (unknown) »

vtpeaknik

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Re: Reducing fridge motor start current
« Reply #5 on: May 23, 2008, 10:07:03 AM »
Could you give us a hint on the needed circuit and how it relates to the circuit as it comes from the manufacturer?  Perhaps a link to a web page that explains it?  Thanks!


I have the same problem: small chest freezer, <100W when running, won't start on an 800W generator.  Kill-a-watt meter says it uses about 8 amps in the first second.  It does start fine off my Prosine 2.0 inverter (rated >4000W surge, with 4/0 cables to the battery).  I wonder if adding the capacitor etc would help the longevity of the inverter, by reducing the current surge through it on each startup of the motor?

« Last Edit: May 23, 2008, 10:07:03 AM by (unknown) »

Opera House

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Re: Reducing fridge motor start current
« Reply #6 on: May 23, 2008, 11:21:00 AM »
I was supposed to go on vacation last Tuesday, now if lucky I will get out next Wednesday. The home projects have been on the back burner.  I did just try the generator again with a 3 ohm resistor in series with the AC line.  When I was searching months ago on the subject I remembered seeing something about that.  That dropped the peak amps to 8 when plugged into the grid.  The generator struggled for about 30 seconds and then recovered. That at least gives me a backup.


As I said before, I am using a capacitor in the start winding instead of tying the start and run windings together.  Are you referring to using large capacitor in series with the AC power feed.  That might help with the generator as the frequency drops.  Still have to try a 400W inverter with some modifications.  


I bought a new 5 CF GE freezer for $100 on special at SAM'S.  Figured I might as well start with something current.  I as using a Norcold 12V 60W fridge at the camp, but it just ran too often.  That compressor was a two wire 24V AC 60 HZ motor with a rather dinky inverter.  Knowing this now, I might have been better off buying an old freezer and moving this compressor over to it.  I will solve this, just not this trip.

« Last Edit: May 23, 2008, 11:21:00 AM by (unknown) »

PeterAVT

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Re: Reducing fridge motor start current
« Reply #7 on: May 23, 2008, 11:37:50 AM »
I am very interested in this topic although I do not feel qualified to comment. I have no RE so I am interested in saving energy instead. I live in an urban trailer park in the north east USA and the landlord, neighbors, and town are very strict.


That said, it seems the whole matter is one of power factor correction. Super caps across the input to the inverter are a great idea, but what capacity? You can order about a farad at 10 - 16 volts from digi-key or mouser.com for about 60 USD and do some wiring in series/parallel like batteries. And they would be good for about 5 time constants till they reach 0.63 charge, but is that enough for 40 seconds? My brain is fried too much for the math at the moment, but I'm very interested.


The other way to do it is inductance. Both are a form of energy storage, and an inductance will oppose a change in current, which seems to be the desired outcome. Hence get a solid bar of steel and wrap it with enough wire and place it into one of the "hot" legs... put it on the inverter output instead. But how much energy to store at (assuming) the bog-standard 60 cycles? And for what length of time constants?


Now I gotta get pencil and paper and think.


You are trying to PF a fridge which was a brilliant idea, by the way to convert a chest freezer. And that is a lot easier to do with sine waves and a known inductance for the motor. It's a bunch of maths, and the power company will hate you.


That said, let me go and chew on this a bit. Good luck! Also remember: caps and coils will eith lead or lag the voltage or current draw, depending. At a given frequency. Very likely you'll need to measure and do the maths for each individual appliance to make it work, but its definitely a good possibility. Old technology but nobody else bothers anymore. I would love to "power factor" (PF) my entire house but that is a project for later.

« Last Edit: May 23, 2008, 11:37:50 AM by (unknown) »

dnix71

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Re: Reducing fridge motor start current
« Reply #8 on: May 23, 2008, 12:19:33 PM »
I have a small upright freezer that won't start off my inverters because of it's high start load. I would be interested in trying things, too. I was going to give it away if I couldn't make it work, so I don't mind the gamble on breaking it.


It would be a nice backup for hurricane season, since once you get it cold, it will stay that way as long as you keep the lid down.

« Last Edit: May 23, 2008, 12:19:33 PM by (unknown) »

dnix71

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Re: Reducing fridge motor start current
« Reply #9 on: May 23, 2008, 03:44:54 PM »
I did some googling after taking the covers off my upright freezer. I got it at Brandsmart 'cheap'


After opening it up, I see there is no start cap, and the manufacturer has a specs page on the internet that says it is sold with or without a start cap.


Guangzhou model AS43U6 is the compressor in the Avanti freezer. For $17 from ebay I ordered a Supco SPP6 hard-start solid state cap/relay. It should be here the middle of next week. I'll let you all know how it goes.

« Last Edit: May 23, 2008, 03:44:54 PM by (unknown) »

imsmooth

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Re: Reducing fridge motor start current
« Reply #10 on: May 23, 2008, 08:57:01 PM »
Ok.  I got mine from http://bestbuyheatingandairconditioning.com


Here is a link after searching there for Hard Start

http://bestbuyheatingandairconditioning.com/Merchant2/merchant.mvc?


When I installed this I measured the currents in both windings and the net current.  One needs to remember that AC current is sinusoidal.  They are only additive if the phases line up perfectly; otherwise, they add up as vectors and can even be subtracted.  The added capacitance shifts the phase of the start winding so the sums of the two currents are significantly smaller.  The problem is that the start winding sees a higher current (remember, it sees a higher current, but the phase is shifted so the sum of the run wave and start wave don't add up like before).  THis current will damage the windings over time; this is why the relay needs to take the added capacitance out as soon as the motor is running.

« Last Edit: May 23, 2008, 08:57:01 PM by (unknown) »

dnix71

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Re: Reducing fridge motor start current
« Reply #11 on: May 24, 2008, 04:04:50 AM »
Some Supco's has two safeties in it according to their page. The cap is only in circuit for .6 to .8 seconds, not matter what else happens. Using a capacitor alone is risky.


http://www.filterace.com/detail.aspx?ID=1112


I'm going to hang around for a while watching things and deliberately short-cycle the freezer a few times just to see what happens. The freezer already has a klixon to protect it from short-cycle rotor lock.

« Last Edit: May 24, 2008, 04:04:50 AM by (unknown) »

wdyasq

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Re: Reducing fridge motor start current
« Reply #12 on: May 24, 2008, 07:19:43 AM »
I looked around a bit and found these capacitors could be found for $80 and less. My best 'find' was this:


http://www.azpartsmaster.com/Products/Compressor-Hard-Start-Kit---HS5_SPP5.aspx


Less than $7 before shipping...


Ron

« Last Edit: May 24, 2008, 07:19:43 AM by (unknown) »
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Norm

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Re: Reducing fridge motor start current
« Reply #13 on: May 24, 2008, 03:30:05 PM »
Hence get a solid bar of steel and wrap it with enough wire and place it into one of the "hot" legs.


.....isn't that a choke coil?

« Last Edit: May 24, 2008, 03:30:05 PM by (unknown) »

PeterAVT

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Re: Reducing fridge motor start current
« Reply #14 on: May 24, 2008, 05:41:37 PM »
Yes that is exactly the idea. But you can tune it to store energy at a specific frequency like a cap, and it might be cheaper.
« Last Edit: May 24, 2008, 05:41:37 PM by (unknown) »

FuddyDuddy

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Re: Reducing fridge motor start current
« Reply #15 on: May 24, 2008, 09:33:27 PM »
In 1971, NASA published an article on Power Factor Correction.

It's still available on the web. Google "NASA Power Factor Correction Circuit" and you'll get a bunch of links for this type of circuitry.

FuddyDuddy

« Last Edit: May 24, 2008, 09:33:27 PM by (unknown) »

Jon Miller

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Re: Reducing fridge motor start current
« Reply #16 on: May 25, 2008, 02:29:05 AM »
Has any one made a DC motor powered compressor?


Using a DC motor it would be easy to add a capacitor to absorb peak power.

« Last Edit: May 25, 2008, 02:29:05 AM by (unknown) »


Opera House

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Re: Reducing fridge motor start current
« Reply #17 on: May 26, 2008, 02:04:29 AM »
I was looking for a little soft start theory.  Something to reduce what the H bridge fets would see.  I tried a capacitor bank of 12,000uF to dump some charge on the 140V DC power supply section.  This would take the strain off the boost up inverter for a short time.  Alone will light a 100W lamp for several seconds. A little small, but enough for a test.  This capacitor bank is fed by 150 ohms and discharges through a diode. It didn't make any difference and was nearly at full voltage when inverter went into alarm.  FETs have a current sense resistor in the ground leg and appears to have an instant and average current limit.  I will try changing that with adding a capacitor time delay to take advantage of the high voltage capacitor dump.  I think the fets can handle it for a short period.  The inverter is 500W.
« Last Edit: May 26, 2008, 02:04:29 AM by (unknown) »

dnix71

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Re: Reducing fridge motor start current
« Reply #18 on: May 27, 2008, 05:02:53 PM »
My Suppco hard start cap came in today. After putting it in the start wattage dropped from 698 to 430, but still neither of my inverters will start it. The power factor starting is .19, running aboout .6


It does start and run quieter now, so I don't mind the money spent.


The hard part was the install. The non-cap compressor motors have two relays, one on the common (a klixon) and a bimetallic disc bridging the run and start prongs (inside a little plastic box) on the compressor. The start side had an external 1/4" quick connect terminal which hooked right up to one side of the Suppco, but the start side had no external connector. I ran a wire through a little hole I made in the side of the bridge connector and into the start terminal. On the other end of the wire I put a quick disconnect to attach the other wire of the Suppco.

« Last Edit: May 27, 2008, 05:02:53 PM by (unknown) »

Bruce S

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Re: Reducing fridge motor start current
« Reply #19 on: May 28, 2008, 11:30:09 AM »
jon;

 Ive seen a few of these for sale, even looked into them to have when traveling down home for annual beef fill up.

Then saw the costs for a unit that isn't very big at all WOW it's up around $1000.00 US.

I've also seen a few walkin that are run on large DC motors hooked up with a pulley and external compressor ( Walkin beer coolers in resturaunts).

But that's about it.


Cheers

Bruce S

« Last Edit: May 28, 2008, 11:30:09 AM by (unknown) »
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vtpeaknik

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Re: Reducing fridge motor start current
« Reply #20 on: May 28, 2008, 04:56:44 PM »
So what are these "hard start kits"?  

http://www.azpartsmaster.com/Products/Compressor-Hard-Start-Kit---HS5_SPP5.aspx

Can't tell if it's a capacitor or what?  What's the "solid state" part?  Says there:


"Increases compressor starting torque for faster recycling. For all Permanent Split Capacitor, Single phase air conditioning and heat pump systems. Total solid state design for maximum reliability. Wired with "Piggy-back terminals for easy installation on run capacitors. Designed for 115 Volt thru 277 Volt systems. Mounts easily with either screw or clip.


Compressor Rating:

BTU: 4,000 thru 60,000

Horse Power: 1/2 thru 5

Replaces:

MARS: 32701

Robertshaw: 600-057

SUPCO: SPP5"

« Last Edit: May 28, 2008, 04:56:44 PM by (unknown) »

hiker

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« Reply #21 on: May 28, 2008, 09:59:03 PM »
try a green plug-_it gives a motor a soft start.... Google search green plug  ..
« Last Edit: May 28, 2008, 09:59:03 PM by (unknown) »
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scottsAI

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Re: Reducing fridge motor start current
« Reply #22 on: June 05, 2008, 08:52:29 AM »
Opera House,


I can think of other things you can do if you have not solve it yet.

Since this is a couple weeks old, will not take the time to write it up.

Reply if still interested or email me. You do not have email address.


Have fun,

Scott.

« Last Edit: June 05, 2008, 08:52:29 AM by (unknown) »

cdog

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Re: Reducing fridge motor start current
« Reply #23 on: June 05, 2008, 05:31:42 PM »
I'd love to hear your idea/solution, I am in the same boat with my inverter/fridge.

would a rotary inverter work for this, do they run continuously or turn on when required?

Thanks for any help at all,

Cdog.
« Last Edit: June 05, 2008, 05:31:42 PM by (unknown) »

(unknown)

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Re: Reducing fridge motor start current
« Reply #24 on: June 09, 2008, 04:25:50 PM »
dnix71,


I am about to try this with a sylvania SE80133-1 which has the same Guangzhou model AS43U6 compressor (it might be the same Avanti freezer, just a different no name brand).  I have a few questions:


-I found a few wiring diagrams for the Guangzhou compressors (including the AS43U6):

http://en.wanbao-compressor.com.cn/en/Exhibit/typelist_AS.asp?cid=4#

(some of it is Chinese, but you can get the gist).

Just wondering exactly where you connected the start side of the suppco.  By "start terminal" do you mean the "#6 PTC Starter"

(in diag#1 Without Capicitor(RSIR) http://en.wanbao-compressor.com.cn/images2/pro/AS_azjx.jpg )


-Do you think a larger capacity Hard Start Kit would allow you to run off an inverter?  I see the RCO-410 says it will do 1/4 to 1/3 Horsepower...do you know how many HP the AS43U6 is?


-I am also thinking about bypassing the existing termistat and putting in something like:

http://www.rancoetc.com/ranco-etc111000000-prewired-digital-temperature-controller-p-87.html?osCsid=
2cf386676204ac4aa9b8b6be409cfe4e

Are you just setting the built in thermistat to the lowest setting of "1"?  Is that warm enought?  (ie above freezing?)


Thanks in advance,

BoB

« Last Edit: June 09, 2008, 04:25:50 PM by (unknown) »