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Fridge start current peak & soft start
#11
Ok so I tried using PICAXE pre-programmed PIC chips (can use easy flowchart to code & easier programming) but they proved to be way too slow & not able to run commands needed in millisecond timings here.
I changed to standard PIC chips & settled on using an easy to get 16F84A that I have a few of.
I've managed to get a socket wired in just above the board & the pins connecting to where they need to.
Once I got the programmer hardware working (windows seems to have tightened some security stopping older chip programmers running), then I was able to test & debug some live code.
Now I've got the code running it pretty close to what I think I'll need.

It starts the triac at approx 40% & ramps it up quickly to 100% in under 1 sec, then switches on one of the relays & leaves it on.
So the circuit will end up feeding the run winding via the NTC first while the triac ramps up energy into the start winding, then cut off, & last the relay shorts across the NTC so full power t to the run winding + run cap connected as normal.

Bit more tweaking of the code in mind, then have to convince the wife it's time for some live testing on a fridge full of food - unless I can find a similar compressor to test on ;-)
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#12
Well i got around to trying the soft starter circuit live on the fridge.

So the original circuit has a base line like below (taken with a Mastech clamp/voltmeter).
I had to wait for the PTC device to "reset" (cool down again) & gas back pressure to drop between starts (did this most times...!).
I typically measured a peak current of ~7.4A at start, varied a bit, sometimes down to only ~5A.
When running normally, the current was ~0.7A per the nameplate.
- mains in to neutral was about 239VAC, 50Hz here (also = run to neutral)
- between run & start was ~267VAC
- between start & neutral was ~128VAC
(ie the capacitor was providing a phase shift & reduced voltage to the start winding)

During tests below when the motor attempted a start but stayed stalled, current was about 5.4A

The motor & capacitor arrangement in mine is called a "permanent split capacitor" (PSC)-style compressor.
Mine does have a "run" capacitor fitted (4uF, rated 470VAC).
In the original fridge circuit, the PTC device (when "cold") is low resistance & connects the start winding to the run winding.
The capacitor isn't involved yet as it's across the PTC (which is a short at first).
The PTC quickly heats (in approx 0.5 sec) & goes open circuit leaving the capacitor there.

Here's a link to a similar PTC device: http://www.carbonbrush.com/8ea.pdf

Trouble is, this is quite different to how the ramped start would be working (or even the soft starter board's original working as posted above).

So I connected up the soft starter "Mk1".
The soft starter circuit put an NTC (resistance starts higher, goes low) in series with the run winding then shorting that with a relay after ~0.4secs.
At the same time it ramped up the start winding phase from about 40% to full wave, then going open to leave the capacitor there just like the original circuit.
I tried multiple starts & made the ramp time quite fast (~0.4 seconds) - it started only properly once out of at least about 10 attempts - not good!
Because the compressor is rubber mounted, I could see it "kick" by varying amounts as it tried to start.
It also did a weird one where it grumbled to a rough shaking but was not "running" - turned that off pronto!....
Had to give it a few minutes to cool between attempts & did click its thermal overload a few times.
Unfortunately it seems this type of PSC motor does not like a ramped up start - at least not how I've tried so far!.
Bit disappointing, but hey, learning ;-)

So now I'm considering should I leave the start winding connected at 100% for a short while (eg 0.5 sec) before turning off the triac?

And also thinking about how sometimes when it starts (original circuit) it doesn't seem to dip the lights & other times it does.
It's operating just by simple thermostat contact closure normally so "luck of the draw" as to phase of the mains when juice is applied.
I think I'll try modifying the connections & CPUs program so it can control the phase point on the AC waveform that it attempts to start via the original PTC circuit & see if starting at eg zero crossing is best or maybe some other position on the cycle gets a reliable start with least "current pulse" impact.

I might also try ramped up phase into the whole original circuit too.
Running off solar, DIY & electronics fan :-)
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#13
(10-14-2018, 11:29 AM)Redpacket Wrote: Well i got around to trying the soft starter circuit live on the fridge.

So the original circuit has a base line like below (taken with a Mastech clamp/voltmeter).
I had to wait for the PTC device to "reset" (cool down again) & gas back pressure to drop between starts (did this most times...!).
I typically measured a peak current of ~7.4A at start, varied a bit, sometimes down to only ~5A.
When running normally, the current was ~0.7A per the nameplate.
 - mains in to neutral was about 239VAC, 50Hz here (also = run to neutral)
 - between run & start was ~267VAC
 - between start & neutral was ~128VAC
 (ie the capacitor was providing a phase shift & reduced voltage to the start winding)

During tests below when the motor attempted a start but stayed stalled, current was about 5.4A

The motor & capacitor arrangement in mine is called a "permanent split capacitor" (PSC)-style compressor.  
Mine does have a "run" capacitor fitted (4uF, rated 470VAC).
In the original fridge circuit, the PTC device (when "cold") is low resistance & connects the start winding to the run winding.
The capacitor isn't involved yet as it's across the PTC (which is a short at first).
The PTC quickly heats (in approx 0.5 sec) & goes open circuit leaving the capacitor there.

Here's a link to a similar PTC device: http://www.carbonbrush.com/8ea.pdf

Trouble is, this is quite different to how the ramped start would be working (or even the soft starter board's original working as posted above).

So I connected up the soft starter "Mk1".
The soft starter circuit put an NTC (resistance starts higher, goes low) in series with the run winding then shorting that with a relay after ~0.4secs.
At the same time it ramped up the start winding phase from about 40% to full wave, then going open to leave the capacitor there just like the original circuit.
I tried multiple starts & made the ramp time quite fast (~0.4 seconds) - it started only properly once out of at least about 10 attempts - not good!
Because the compressor is rubber mounted, I could see it "kick" by varying amounts as it tried to start.
It also did a weird one where it grumbled to a rough shaking but was not "running" - turned that off pronto!....
Had to give it a few minutes to cool between attempts & did click its thermal overload a few times.
Unfortunately it seems this type of PSC motor does not like a ramped up start - at least not how I've tried so far!.
Bit disappointing, but hey, learning ;-)

So now I'm considering should I leave the start winding connected at 100% for a short while (eg 0.5 sec) before turning off the triac?

And also thinking about how sometimes when it starts (original circuit) it doesn't seem to dip the lights & other times it does.
It's operating just by simple thermostat contact closure normally so "luck of the draw" as to phase of the mains when juice is applied.
I think I'll try modifying the connections & CPUs program so it can control the phase point on the AC waveform that it attempts to start via the original PTC circuit & see if starting at eg zero crossing is best or maybe some other position on the cycle gets a reliable start with least "current pulse" impact.

I might also try ramped up phase into the whole original circuit too.

maybe the motor is cogging due to phase shift?
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#14
(10-14-2018, 03:21 PM)rtgunner Wrote:
(10-14-2018, 11:29 AM)Redpacket Wrote: Well i got around to trying the soft starter circuit live on the fridge
........
I might also try ramped up phase into the whole original circuit too.

maybe the motor is cogging due to phase shift?

Did you have any more info you could share on that? Eg explain what you're thinking a bit more? - I'm still figuring out how it works, especially at start up!
When running normally, I like found above, the start winding does have quite a bit of phase shift on it.
The pdf I linked (& others I'd read) also talk about phase shift during start but then this motor has the start & the run windings directly in parallel at least initially (ie no phase shift in the applied power).
So maybe the start winding is physically offset vs the run winding?
Or maybe the magnetic field moves differently/lags differently at startup?

Any thoughts would be welcome....!
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