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adding a start capacitor to a circuit

1sharpguy

Jun 9, 2014
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Attached is a wiring diagram for a Kenmore compact refrigerator. For reasons I will explain below, I was wondering if I can install a start capacitor between the thermostat and the overload protector in order to assist in starting the refrigerator compressor's motor?

I am powering the refrigerator with a 12V DC/120VAC inverter connected to three deep cycle 12V 109AH batteries. The inverter has a low voltage protection switch. The cutoff is 10.5V. When there is a power discharge and the voltage as "seen" by the inverter drops below 10.5V the inverter shuts down. This invariably occurs when the refrigerator cycles on. This is happening because the refrigerator does not have a start capacitor, but rather a PTC switch which drops the refrigerator compressor motor start windings out of the circuit once the refrigerator is up and running. The initial surge in power (current) required to start the compressor is causing a low voltage fault in the inverter shutting down the system. Since at this time, I would prefer not to mess with disconnecting the low voltage switch on the inverter, my solution to the problem is to add a start capacitor to the refrigerator circuit.

Bringing us back to my question above, would it make sense (be possible) to connect a capacitor between the thermostat and overprotection switch to start the compressor so that a voltage spike would not be "seen" by my inverter. My thinking is that the capacitor will give the needed boost in power to start the motor running and then be dropped out by the PTC switch. I don't know enough about capacitors or circuitry or the other components to say whether this will work or not and solve my problem. Was wondering if someone on this forum might be able to assist. If this is doable, what microfarad cap would you recommend? The compressor motor is LRA 6.6A and RLA 1.3A. The voltage is 115VAC. Additionally, I have another concern with this set up. Will placing the capacitor in front of the Voltage Overload Protection in the circuit enable that feature and shut down the compressor motor as well?

Another solution that has been recommended is to replace the factory PTC and overload protection switch on the compressor with a Supco 3 in 1 relay, start capacitor and overload. The only issue I have with that is the relay is a potential relay. At a preset voltage surge, the relay opens taking the start capacitor out of the circuit. Thus, not sure how lower voltage from discharging batteries would affect its operation. Unfortunately, this is where I fall down understanding how the AC voltage coming from an inverter is affected by the dicharging of the battery bank with time. I am presuming there will be a voltage drop on the AC side of the inverter. If the potential relay does not "see" a spike in voltage then it will not remove the start capacitor from the windings and thus run the risk of damaging the motor windings over time.2014-06-05 13.50.09.jpg
 
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duke37

Jan 9, 2011
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An induction moter takes 6 or 8 times its full load current to start. There is not much you can do about this particularly if the load has peaks in the torque requirement. I have used a start resistor to limit current on an organ blower motor but here the torque was negligible at low speeds. It enabled the DC offset to decay in the first few cycles before full voltage was applied.

Your best solution would be to change to a 12V DC motor, eliminating the inverter.
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
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Duke, would it be feasible to add a heavy duty NTC thermistor in series with it to limit the start current? Just a crazy thought...
 

duke37

Jan 9, 2011
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Kris
The problem is that you need the torque to start. My brother has an air compressor that was fed by a long cable. It would turn until the piston started to compress the air and then stall, it was moved to the other side of the road so that the cable was much shorter.

The thermistor would reduce the effect of a DC component in the initial surge but would need to pass a large current to get the motor going. This would need experimentation and testing.

We do not know what the invertor turns out, square wave, modified square or sine. If it has a high harmonic content then capacitors could have strange effects.
 

KrisBlueNZ

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So that's a "probably not" then?

How about another crazy idea - a variac driven by a geared motor? The motor would have to wind the variac down to zero when the thermostat switched OFF, then when it switched ON, it would wind the variac slowly up from zero. Sounds too silly? Probably...
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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The problem with refrigerator compressor motors is that they start under maximum load at a point where the motor has low torque. They're a classic case for needing huge starting currents.

If you limit the current you can easily overheat the motor and destroy it.

The correct solution is either bigger batteries and (possibly) bigger inverter, or possibly a variable frequency drive -- but even that's dodgy.

You may need 10x the rated power to start the compressor.
 

britt

Aug 18, 2020
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I know I'm bumping a six year old thread but I'm also wondering about adding a start capacitor to slightly lower surge amperage on a small freezer. My freezer appears to be pulling up to 10 to 12x its rated wattage on startup. Wouldn't a start capacitor help correct the power factor, which could increase startup torque and lower amperage? It seems like power factor would be really bad without one. My freezer also has a PTC relay and I'm not sure if wiring the capacitor into the current relay setup would be alright or if a 3-in-1 would work.

I know these motors need a certain amount of current but it seems like the circuit on this freezer is designed with cost in mind more than efficiency. I'm mostly looking to do this so I can buy a slightly smaller inverter for this 100w freezer.

Thanks!
 

Bluejets

Oct 5, 2014
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A capacitor will do nothing to reduce start current.
The purpose in compressors and the like with the split phase motor is to pull (electrically) the start winding closer to the (again electrical) 90 degrees out of phase to add startup torque.

It might be possible for you to apply a current inrush limiter similar to that shown in the link below.

I have used them on several occasions on electric sliding gate motors where the initial inrush current was sheering off drive gears in the motor gearbox units.
Connect in series with the main incomer active cable.
Not difficult to imagine the sliding gate and all it's mass one minute stationary and the next moment with a boot up the kyber where everything is supposed to just magically move without hesitation. Eventually something has to give...:eek:

link.....some calculations in there somewhere too to find the correct size if i remember correctly.
https://www.ametherm.com/blog/inrush-current/ac-motor-inrush/

here I think...... https://www.ametherm.com/inrush-current/inrush-current-limiters-full-line
 
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debe

Oct 15, 2011
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It will depend on the Compressor, I got lucky with this Sanyo fridge compressor. It would not start with a 300W inverter, but by adding a start capacitor it would start easily. Its really a try it & see.COMPRESSOR SANYO.JPG
 

britt

Aug 18, 2020
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Thanks for the replies! I think I'm going to try it. I do also have a soft-start device for limiting in-rush with ntc thermistors and a relay but I'd be a little concerned that it would make it harder for the compressor to start against the back pressure which could eventually damage it, like Steve said above. I'll be using a full sine inverter for the freezer which might make the cap more effective? I also wondered about running the inverter at 100v 50hz which should also help. I'd probably modify the inverter to do so (if possible), I doubt I'd want to pay much extra for one that had a switchable 100v feature, though I hear they do exist.
 

Bluejets

Oct 5, 2014
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but I'd be a little concerned that it would make it harder for the compressor to start against the back pressure

Mostly the compressors never start against high back pressure unless of course the unit is switched off while running and then switched back on again before the pressure equalises.
Even then, the motor overload/overtemp cut out kicks in due to the locked rotor current.

What you REALLY need to do whichever way you decide to go, is monitor the run current for an appreciable amount of time and under both normal and high back pressure conditions to make sure that the start relay doesn't hang in there and burn the start winding out.
I have seen this happen on line faults that cause low mains supply voltage and nothing will save the compressor in that example.
O/load might eventually kick in but it will be too late as the damage will already have been done.

Safest alternative as always is to use an appropriately sized power unit
BTW....drawing above from Debe does not show the relay unit nor it's contacts.
 

debe

Oct 15, 2011
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It does not use a relay it uses a PTC for start as does all newer fridges as they use R600A (LPG) gas, so relays are not alowed in there build.
 
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