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Fridge compressor: What happens when PTC relay is still hot during start?

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harvie

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Hello,
i am building the diy silent air compressor from old fridge and and fire extinguisher. It has some sort of starting relay and no visible capacitor (i suppose start capacitor is sealed inside compressor. right?).

What i understood is that when it's connected to power the relay (probably ptc) is closed (=cold ptc). And current flows to the start winding. As times go the ptc warms up and disconnects start winding, leaving only run winding to do all the job.

But i have pressure switch which disconnects compressor at 8 bar and reconnects it when it drops to 6 bar. But when i use the compressed air too quickly it thereforem may happen that ptc will not yet be ready and cold for next start, which means that power will be applied to run winding without energizing the start winding. Compressor does not start in such case.

What should i expect when this happens? I am afraid that it can damage run winding because current flows thru it without motor really rotating which may result in bad motor cooling and burning up the run winding. I've heard that some PTCs have integrated thermal protection of run winding, but i am not sure if it's ok to trigger it too often (during normal everyday use).

I've been thinking about replacing ptc with electronic timer relay or something that would engage start winding for few seconds every time power is applied. But i don't know if it's good idea and i also would rather keep things simple.


Cycling fridge compressor without unloader valve?
Next thing i am afraid of is what will happen if compressor will start with air tank pressurized to 6 or 10bars? I am not going to use unloader valve to simplify the design. First experiments shown that it was starting at around 7 bars few times, but i didn't done any repeated tests or tests at higher pressures. What pressures are in fridge when compressors start? is something around 8 bars ok? i've heart these beasts can go up to over 100 bars, so i guess 10 bars should not be considered "load". but i don't know.
 

PTC compressor start circuit usually works without a capacitor, as discussed in a recent thread. https://www.edaboard.com/threads/358826/?highlight=fridge

I guess we are talking about a few 10 seconds maximum required for PTC cool-down, shouldn't be a problem with pressure switch control?

My main concern would be that hermetic fridge compressors are designed to operate with refrigerant/oil mixture. I fear the motor won't like to run dry for an extended time.
 

FvM said:
I guess we are talking about a few 10 seconds maximum required for PTC cool-down, shouldn't be a problem with pressure switch control?
Click to expand...

Well air tank is relatively small and going from 8 down to 6 bar can be easily achieved in few seconds. Eg. when using dusting gun without pressure regulator.

However my question is hypothetic: What can i expect if this really happens?

FvM said:
My main concern would be that hermetic fridge compressor are designed to operate with refrigerant/oil mixture. The motor won't like to run dry for an extended time.
Click to expand...

I'll replace the oil with compressor oil suitable to work in non-hermetic conditions and top it regulary as it leaves the engine with air.
 

There may be an overriding consideration about cycling times for your compressor. Appliance instructions advise waiting a few minutes after shutting off a refrigerator or air conditioner, before we start it again. The reason is because liquid refrigerant remains inside the compressor when stopped. Since the liquid cannot be compressed, it causes greater power consumption if you immediately restart the compressor. However startup becomes easier as the refrigerant returns to gas.
 

As i will use it only for air (with bit of oil), there's probabably very little probability that liquid air will be created in compressor.
 

So i've tried and it's real problem. When i press duster gun at full pressure it evacuates the pressure vessel so quickly that it starts with PTC relay still hot. which means it does not start. it only hums.

Is there something i can do about this? Is it okay to replace PTC relay with electromagnetic starter relay or something? I guess electromagnetic starter relays engage starter coils for much shorter period than PTCs as PTCs take some time to heat up, while electromagnetics act almost imediately.
 

So. i've done some research on this topic.
I've found interesting slides: https://slideplayer.com/slide/5924445/

Long story short:

It seems that replacing PTC with Electromagnetic starter is completely possible. It works as current relay that switches start winding on when there's high (LRA) current passing thru main winding before motor is started and switches it off once it falls down to lower (FLA or less) current as motor starts. This should allow rapid cycling of compressor without risking stalled rotor.

But you need electromagnetic starter to be matched for wattage of your compressor. Also electromagnetic starters don't have integrated overload protectors (as opposed to some PTCs - i think at least some of them have it). So when you use electromagnetic starters, you need to add overload protector on common line too. These should be also matched to wattage of your compressor.

(Technicaly speaking both of these devices should be matched by amperage, not wattage. But as voltage is constant and sellers identify them by target wattage, i say they should be matched by wattage to keep things simple)

So i guess if you are going to build air compressor from old fridge and cycling it using pressure switch, you should absolutely replace the PTC with properly rated electromagnetic starter and overload protector to ensure long life of compressor. (Besides of using inlet air filter and replacing oil with low-viscosity one, that's also suitable for non-sealed use, therefore not degraded by air humidity. Toping it regulary as it gets spitted out with air.)

Typical magnetic starter is for $4 and overload protector for $0.5 (both on aliexpress), so i guess it's worth it.

Also the reason why there's no capacitor in RSIR (resistive start induction run) single phase compressors is because there's no need to create phase shift by external capacitor, when start winding is mainly resistive and run winding is mainly inductive, which creates phase shift between these two per se. Even without using capacitor. However some compressors may need capacitor or even two to work properly.
 
Last edited:

harvie said:
Hello,
i am building the diy silent air compressor from old fridge and and fire extinguisher. It has some sort of starting relay and no visible capacitor (i suppose start capacitor is sealed inside compressor. right?).

What i understood is that when it's connected to power the relay (probably ptc) is closed (=cold ptc). And current flows to the start winding. As times go the ptc warms up and disconnects start winding, leaving only run winding to do all the job.

But i have pressure switch which disconnects compressor at 8 bar and reconnects it when it drops to 6 bar. But when i use the compressed air too quickly it thereforem may happen that ptc will not yet be ready and cold for next start, which means that power will be applied to run winding without energizing the start winding. Compressor does not start in such case.

What should i expect when this happens? I am afraid that it can damage run winding because current flows thru it without motor really rotating which may result in bad motor cooling and burning up the run winding. I've heard that some PTCs have integrated thermal protection of run winding, but i am not sure if it's ok to trigger it too often (during normal everyday use).

I've been thinking about replacing ptc with electronic timer relay or something that would engage start winding for few seconds every time power is applied. But i don't know if it's good idea and i also would rather keep things simple.


Cycling fridge compressor without unloader valve?
Next thing i am afraid of is what will happen if compressor will start with air tank pressurized to 6 or 10bars? I am not going to use unloader valve to simplify the design. First experiments shown that it was starting at around 7 bars few times, but i didn't done any repeated tests or tests at higher pressures. What pressures are in fridge when compressors start? is something around 8 bars ok? i've heart these beasts can go up to over 100 bars, so i guess 10 bars should not be considered "load". but i don't know.
Click to expand...

if roto of motor is rotation, ptc is hot, but lt's smaller temperature to ptc change statuts.
if motor connected to power and roto is not rotation. lock roto current is verry big. ptc is verry hot and turn off
i realy want to talk about this topic. but my english is bad.
 

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