(In the USA)

There are generally two starting mechanisms for single phase refrigeration compressors. They are as follows:

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Method #1): Resistive Start (Split Phase)
An auxiliary winding made of thinner wire and less turns than the main winding is made. Since this winding is less inductive and more resistive than the main winding, it's current will be slightly out of phase from the main winding, producing a rotating magnetic field large enough for starting the compressor motor. This auxiliary winding is connected only temporarily (during starting), by either a current-operated relay or PTC thermistor (not a centrifugal switch).

Method #2): Permanent Split Capacitor (PSC)
An auxiliary winding is made and wired in series with a permanently connected (non-switched) run capacitor. The capacitor shifts the winding's current out of phase from the main winding almost 90 degrees, which is more than plenty for starting. Because the winding stays connected during running, the motor benefits from its constant field even when running at full speed.

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Usually, #1 is used for refrigerators and other appliances with small compressors while #2 is used for air conditioners and appliances with larger compressors.

Compressors usually have 3 protruding electrical connection pins; one common pin, one main winding pin, and one auxiliary winding pin. Many times they are marked to indicate which pin is for which winding, but with no indication as to which method (#1 or #2) is used to run it. My question is as follows:

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What would happen if you used method #1 for a compressor meant for use with method #2 (or the other way around)? They almost certainly aren't meant for this, and it would probably change their operating specifications for the worse, but has anyone tried this / know enough about the insides of these compressors?

Two endpoints are motor won't start or starting winding will burn off, in either case. If you force a PSC (capacitor run) motor into RS mode, capacitor winding likely won't burn off in a few seconds of energizing, but motor certainly will have much lower (or even zero!) starting torque than with a capacitor and may fail to start. Also removing a capacitor from PSC motor will significantly decrease its efficiency and working torque, by tuning rotating stator field into pulsing, so it might overheat or drop out on nominal load.

Depends on the exact motor model, windings resistance etc.

There is also a combined RSCR method where a capacitor is always connected in series with a starting winding, but is shorted with PTC or relay contact on start.

It all depends on how the motor was conceived by its design engineers.





 

PS: Obviously, often a compressor will have a nameplate with a type number intact. Again, obviously, having a type number on hand, a datasheet or a table can be googled, indicating a starting method!

PPS:

Clever people in refrigeration say because small refrigeration systems use a capillary tube in place of expansion valve, pressure difference between high and low pressure sides of the compressor equalizes in some time after the machine is switched off. This makes the compressor to start at relatively low load and allows the use of cheap, but ineffective resistive start method. If the machine is again turned on after insufficient time not allowing the pressure to be equalized, RS starting will fail and overload relay will trip. This is of course quite hard on the motor windings.

 

@RRK
Thanks so much! I have never heard of the RSCR method, that is very interesting!

Obviously running a compressor in a way it isn't supposed to be run will cause issues with reliability.

I found out the hard way that small refrigeration systems need time equalize, I thought I had messed up a compressor because it was stalling / not starting, but it just needed a break. I recently made a small air compressor with a r134a refrigeration compressor (long story), but once it is up to pressure it won't restart, so I will eventually need to add an unloader solenoid for it to work.

I do wonder why they use a capillary tube instead of some sort of machined brass part. It seems like it would be very simple to just have a small orifice that can be machined and brazed instead of using long lengths of expensive copper tubing, but whatever.

My wild guess is because pressure drop over a capillary is gradual it will be more or less silent. Replacing it with a steep orifice will cause a hiss which is undesirable with home appliance.

That is a good theory, that does make sense

The resistive vs "start only capacitor" vs "run capacitor" (these are often really 3 phase machines, the capacitor with the motor impedance at rated load creates the required 3rd phase) compressors each require different winding specification, so when mismatched with the starter, it usually lead to overloads and often no-start.

Sometimes an additional "starting" capacitor with ots relay is added to the "run capacitor" compressor (in a form of hard start kit, consisting of the start capacitor and a relay connecting it for start, either controlled by the voltage from the starting pin, or by the input current) to ensure the proper phasing when the motor is not running, so has way lower impedances. This helps starting mainly when the supply voltage tends to or may be sagging during the inrush current (mainly higher power systems on longer cable feed or when it needs to operate on emergency generators or so).

Sometimes it serves as a bandaid on dying compressor which tends to need more torque to start than originally designed, adding the hard start kit to make it start uses to be more cost effective on old, so soon to be replaced, systems than replacing the compressor for the short time until the system will be scrapped anyway.

The start-only capacitor systems do not rely on winding resistance, byr rhe starting winding is more resistive because designed for just brief use.

The starting problems most fridges have after short power off are not because of the system pressure, but because the resistive start winding uses to be controlled by a simple PTC and that PTC takes time to cool down and connect back the starting winding.
Old frifges did not have this problem, they use to use current controlled relay, which detect the high motor stall current to connect the start winding, so in my experience did not have any problem starting any time after power cut.

Don't know the reason, why the stsrting relay got replaced by the PTC in the late 80's/early 90's, whether it was just the cost, or there was some safety concern (the need to not make sparks, as non-freon refrigerant that started to be used at that time use to be flammable, but at the time there were well known flame suppression techniques costing nothing that should be sufficient)
 

"The "capacitor run" are often 3 phase motors (under designed load the voltage and current phasing really becomes symmetrical 3 phase), so the 3 pins are equal." - sorry but this is absolute rubbish theory. I would be grateful to see a practical confirmation that someone ever did this in mass produced equipment. 3-phase motor has 3 pole pairs on the stator, placed within 120 degrees. Or 3*N number of pole pairs at 120/N degrees for reduced RPM. For  motors designed for capacitor run, this is 2 pole pairs at 90 degrees, motor runs on artificially made 2-phase power with 90 degrees phase shift ideally.  And to create an even round magnetic field, capacitor winding often is deliberately made with slightly higher inductance than the other one.

Sure, today this is all completely irrelevant as it has become cheap to implement 3-phase inverter circuit  to run 3-phase motor from single phase mains.

PTC starter has many advantages over a starting relay. It is of course cheaper.  You don't need a relay with massive silver contacts to switch huge ~10A. starting current. It is completely silent. Generally, more reliable. It does not generate awful interference when operating. Starter relays have a nasty surprise of being positional sensitive - so if you place the equipment on the side, relay may fail to release, and you are at mercy of overload switch to save the motor.

Refrigeration compressors of course DO have a problem starting due to pressure difference in addition to the fact that today's PTC starters also need some time to cool down to operate a second time. Typically, overload switch thermal inertia is designed to take care of this all.

@Medved
I am pretty sure the only way to make 3 phase with a capacitor is by making the North American 120/240 Delta system, I am unsure how your proposed method works.

I have heard of those so-called "hard start kits", I never really knew why they were used until now!

I have never heard of a start-only capacitor motor being used inside a refrigeration compressor, could you elaborate?

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@RRK
This is exactly what I was thinking, I don't think running a 3-phase motor on single phase + capacitor would ever work right in countries that don't use the North American 120/240 Delta system.

"I don't think running a 3-phase motor on single phase + capacitor would ever work right in countries that don't use the North American 120/240 Delta system."


Depending of what you think of "right"

You will get about 60% of rated mechanical power. That may be OK for amateur application, like running a "borrowed" 3ph lathe in a garage. You connect line and neutral to phases A and B, and connect phase C to A or B through a capacitor, depending on rotation direction desired. Sometimes an extra starting capacitor is added for a heavy start.

 

@RRK
What meant by "right" was without electrical / mechanical damage, I agree power will be lacking. With semi-cheap converters being available now, this method doesn't really have an application anymore except for running rotary phase converters, but that is an entirely different topic.

In a refrigeration context, I don't think this capacitor approach would be a good idea, especially from a starting torque standpoint. It might never start up, and just sit there and bake until something blows up.

@Medved
I have never heard of a start-only capacitor motor being used inside a refrigeration compressor, could you elaborate?

Me neither, but a lot of things that seem to not have much sense to use but are possible happen to be used from time to time, so I meant it as it is possible so someone may find a reason to use it for certain application. Just wanted "to cover" an arrangement which is possible.

But agree, there seem to be not much advantage over simple resistive start (still need a starting relay, just need thicker winding and extra capacitor, so extra cost...). Maybe higher torque when the supply is weak, but then the higher power factor of the run-capacitor system (combined with the hard start kit) would bring way more benefit for the tolerance of e.g. weak power supply or so...

By the way what I would also expect is the use of synchronous motor with an electronic starter - similar system that uses to be used with water pumps in dish washers and many fans - a triac switch fires when the mains happen to have polarity that pushes the rotor further. This way it is able to start even rather heavy rotor even when the motor itself is synchronous.
After start the triac remains permanently ON and the motor then runs in a standard synchronous mode. The advantage is, synchronous motors do not need to spend any power to magnetize the rotor (induction use slip and electromagnetic induction, so it causes losses in the rotor cage, as well as the reactive power in the stator), plus it may be easily phased with the crank in a way the piston is compressing exactly at the moment the single phase feed delivers torque, so theoretically allow for lower vibration. But maybe the complications like sensing the rotor position do not make it that much simpler over VFD feeding the same synchronous motor, where the VFD can control the speed and it is a similar "electronic box" from the wiring complexity perspective...