Installing a hard start capacitor kit on a compact refrigerator
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  1. #1
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    Installing a hard start capacitor kit on a compact refrigerator

    I purchased a Kenmore compact refrigerator(http://www.sears.com/kenmore-2.4-cu-...p-04692489000P) to run off an inverter installed inside a van. The inverter has a low voltage protection alarm which shuts off the inverter when the voltage reaches 10.5V DC (system is powered by 12V DC deep cycle batteries). Over time as the batteries discharge and lose power, the cycling (on current surge) of the refrigerator will cause the inverter to "think" the batteries have discharged below the 10.5V cutoff when in fact they are actually charged to 11.7 volts. The refrigerator has a PTC (positive temperature coefficient) thermistor installed to regulate the cycling of the refrigerator. I would like to remove PTC and replace it with a relay and start capacitor (such as a hard start capacitor kit). Has anyone ever done this for a compact refrigerator or know if it can be done? If so, can you recommend a hard start kit or set up that can be used with this type of refrigerator? The compressor is a Jiangsu Baixue Electric, AC 110-120V, 60Hz, LRA 6.6A 1PH, Model #: QDH2511G. The thermistor is attached to two prongs (one I presume is the start and the other is the run connections). Can I simply remove the thermistor and attach a two pole run capacitor to the compressor? Or should I leave the PTC in place and add a relay and a start capacitor in series with the PTC?


    I have attached a photo of the refrigerator's wiring diagram. There is a blue wire from the thermostat that connects to the overload protector and a white wire connected to the thermistor.
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  2. #2
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    You should be able to get a "3 in 1" that replaces the existing starter assembly.

    Also, if the inverter supports outputting 100V/50Hz (the better ones do, sometimes called "eco mode" or similar), try that.

  3. #3
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    What is a "3 in 1" and where can I find one?

  4. #4
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  5. #5
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    The inverter's low battery cutoff is working as designed. You really don't want to draw a true deep cycle lead-acid battery (read medical AGM or gel or golf car flooded) down below 50% SOC, and for a faux deep cycle like a marine battery even 50% is too much.







    The best bang for your buck in terms of kilowatt hours delivered per battery dollar spent is at the knee of the last chart. Too shallow of discharge and the battery dies from old age before you get your money's worth out of it, too deep and you kill it.
    In my neck of the woods to most people "Maintenance" equals cursing at and/or kicking something when it breaks down.

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  6. #6
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    Quote Originally Posted by danielthechskid View Post
    The inverter's low battery cutoff is working as designed. You really don't want to draw a true deep cycle lead-acid battery (read medical AGM or gel or golf car flooded) down below 50% SOC, and for a faux deep cycle like a marine battery even 50% is too much.







    The best bang for your buck in terms of kilowatt hours delivered per battery dollar spent is at the knee of the last chart. Too shallow of discharge and the battery dies from old age before you get your money's worth out of it, too deep and you kill it.

    Although helpful and enlightening, this discussion about battery life expectancy isn't really relevant to my post. The point of the post was not to discuss or seek answers to battery life expectancy or operating practices, but rather the fact that my inverter's low battery protection alarm is being engaged as a result of a surge current required to start my compact refrigerator. The draining of the battery and inadvertant shutting down of the inverter is a result of using a mini refrigerator not designed to be used with an inverter (except when the batteries are in a fully charged state which isn't realistic. For the purpose of this application, I am shooting for 36 to 48 hours between charges). Rather than employing a relay and capacitor, this refrigerator uses a PTC to drop out the start windings once the refrigerator begins running. Thus, the solution offered by NiHaoMike appears to be a good one. My only concern has been being able to find a 3 in 1 kit with a capacitor that won't fry the windings in my compressor motor. It appears the Supco URCO810 may be compatible. Once I have some experience and history with this installation, I hope to be reporting back here with my findings. Thanks again for the review of battery SOC and life and the charts. Although, I am familiar with the data I did not know the exact cutoffs. Appreciate the heads up. Have added the information to my "toolbox".

  7. #7
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    Ideally, you'll have a proper 2 phase VFD (like the Shannon Liu Quadrature Drive), but those are really hard to find.

    A 10uF or so run cap might help but only with a sine wave inverter. Another trick to cut out the idling losses is to have the thermostat control the inverter. Setting the inverter to 100V/50Hz (if possible) should help improve efficiency by reducing the condensing temperature and reduce resistance losses in the wiring. It would also reduce inrush without reducing the starting torque (actually probably improves it) because the V/Hz ratio is the same.

    Also, many (if not most) inverters do have a low voltage cutoff that reacts too quickly. Ideally, they should have a few seconds of delay to avoid false trigger from inrush.

  8. #8
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    Quote Originally Posted by NiHaoMike View Post
    Ideally, you'll have a proper 2 phase VFD (like the Shannon Liu Quadrature Drive), but those are really hard to find.

    A 10uF or so run cap might help but only with a sine wave inverter. Another trick to cut out the idling losses is to have the thermostat control the inverter. Setting the inverter to 100V/50Hz (if possible) should help improve efficiency by reducing the condensing temperature and reduce resistance losses in the wiring. It would also reduce inrush without reducing the starting torque (actually probably improves it) because the V/Hz ratio is the same.

    Also, many (if not most) inverters do have a low voltage cutoff that reacts too quickly. Ideally, they should have a few seconds of delay to avoid false trigger from inrush.


    Hello Mike...

    The VFD is an interesting solution to my dilemma. Unfortunately, I'm afraid it is beyond my level of technical competency...at least at this time. As well, there are probably no electrical engineers or techs walking around this area of the world that are willing to install one at my price point. Thus, sticking with the 3 in 1 is probably a more practical solution for the time being. However, I am glad you directed me to the VFD solution. I just spent the last two hours studying VFD on youTube and certainly have a better understanding of and appreciation for the device and it's utility than I did two hours ago. However, that doesn't make me the least bit confident I could go out and source one, install it and have it do the job it was designed to do by a long shot. However, you have certainly peaked my curiosity and as a result I have even learned more about electrical equipment than I knew two hours ago. I now understand what a VFD is, what a rectifier is and how a VFD is a combination rectifier and inverter. As well, I have learned that the speed of motors can be controlled by VFDs and that sensors can provide feedback loops to VFDs to alter the frequency and thus speed. This is beginning to sound like a programmable logic exercise which again is beyond my competency in this area and the scope of my project. Of course, if we were talking about building a multi-million dollar factory designed around automation that would be one thing and certainly the incorporation of these high end automation gadgets for monitoring and controlling processes would be critical and not simply an intellectual curiosity. However, in my case, we are talking about a few batteries and a cheap inverter running a few appliances in a van. I can definitely see how a VFD (especially the Shannon Liu) would reduce the voltage surges on start up as well as in combination with a thermostatic control reduce the demand on my batteries and thus extend the life and lengthen the discharge cycles. Cool stuff.

    Instead of going with a 3 in 1 kit, the idea of inserting a start cap in the compressor motor circuitry had occurred to me. I was thinking I could insert it between the thermostat and Overload Protector, the blue wire in the wiring diagram (see earlier thread). However, I am not sure how to go about sizing the cap for the motor windings. The motor data plate indicates the LRA is 6.6A while the RLA is 1.3A. Pretty small motor. Another concern was whether the voltage surge from the start cap would trip the overload protection and shut down the motor anyway. So as you can see, I am not really sure what I am doing in this regard and therefore believe that sticking with replacing the PTC with the potential relay and start cap is the way to go.

    Finally, I whole heartedly agree with your last statement that most inverters have low voltage cutoffs that engage way too quickly. I would hardly be having the problems I am facing today if that was not the case and had incorporated in them a momentary delay as you suggest. I can't tell you how many online reviews of this particular inverter I have read where people are having the same issue and they are just lost. Most do not have the time to try and understand what is going on and since the inverter is relatively cheap they prefer to just pitch it or turn it into a boat anchor. Speaking of boats, it is now time for me to move on to my next project which is repairing the wood rot in the entrance threshold of my motor yacht. Cheers and thanks again for your input and advise!!

  9. #9
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    Quote Originally Posted by 1sharpguy View Post

    Instead of going with a 3 in 1 kit, the idea of inserting a start cap in the compressor motor circuitry had occurred to me. I was thinking I could insert it between the thermostat and Overload Protector, the blue wire in the wiring diagram (see earlier thread). However, I am not sure how to go about sizing the cap for the motor windings.
    What you are describing sounds more like a run cap, only the way you are describing to wire it sounds wrong. A start capacitor needs a start relay to go with it. A 3 in 1 has a start capacitor, start relay, and an overload built into it. The size of the start capacitor isn't super critical since it only remains energized for a fraction of a second. It is a cheap, simple device that would probably work good in your situation. You can get them from the internet, an industrial supply store such as Graingers, or any hvac/r wholesaler.

  10. #10
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    100uF or so is about what you should look for in a start cap. (In fact, those start caps have a very wide tolerance and you'll probably find one marked 88-145uF or something like that. It's not at all critical.) It should go in series with the PTC. (If the existing PTC doesn't allow you to, just use a 3 in 1.) Also add a 10uF or so run cap between the start and run terminals if you have a sine wave inverter.

    If you're willing to open up the inverter, many have internal adjustments for output voltage and frequency. Or you can get a Japanese market inverter that outputs 100V/50Hz out of the box or a good inverter that has an "eco mode" switch on the front.

  11. #11
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    My inverter is a modified sine wave. In that case, would you still recommend a 10 uF run cap?

    Your suggestion to open the inverter and make voltage and frequency adjustments is probably beyond my understanding. However, I may try contacting the manufacturer to see if it is possible to make such adjustments as you recommend to this inverter.

    Can you explain how switching from a 120V/60Hz inverter to a 100V/50Hz would benefit me? What would be the purpose? How would it help matters?

  12. #12
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    My suggestion is to just get an Engel fridge. The hermetic compressor used in household fridges can not be used while driving. The little bumps on the road messes it up. It thunks every time you hit a bump even when it isn't powered. Over time, I think it will destruct itself just from going over bumps all the time.

    Quote Originally Posted by NiHaoMike View Post
    Shannon Liu Quadrature Drive
    What are u on? I have no clue what it is and there is no such thing other than in your posts, according to google.
    Supercooling and Subheating

  13. #13
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    In that case, it would probably be better to leave it out. It's next to impossible to know for sure without checking with a scope. With modified sine (or unfiltered sine like most VFDs), you'll also ideally want the compressor (motor) within a few feet of the inverter to avoid standing wave issues.

    100V/50Hz has the same V/Hz ratio as 120V/60Hz so motors operate properly, just at reduced speed. In your application, it means that the compressor has to run longer per cycle, but the condensing temperature would be lower so the efficiency is higher. Basically a poor man's VFD. Ideally, you'll want to be able to vary the voltage and frequency down to as low as 60V/30Hz with a PID thermostat controlling it, but that's getting a little complex.


    BTW, if you're planning to run other stuff from the inverter, 100V/50Hz will run just about everything just fine (just ask the Japanese!), the most notable exception being old audio equipment with synchronous motors.

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