Technician1002 wrote:The inductance of the motor is matched to the capacitance of the starting cap to provide a phase shift. This is used to provide a rotational field to start the motor turning. Too big of a cap is bad and will result in too much current and an incorrect phase shift to properly start the motor. It may result in burning out the motor.
Always match the size of the starting cap to the size of the motor. Like a cannon Chamber Barrel ratio, there is an optimum range. Efficiency and power drops when run far outside the range.
Sorry about my first response. But I really was thinking in terms of electrical current as being able to be shared. So at least now I know that I need four capacitors. One for each compressor. It was your analogy that through me off here.
This really has to be hilarious. A computer cooling tech asking for help from potato gun shooters. How many of you actually use compressors of this type for your application? I'm just now getting ready to calculate the total CFM of these four compressors.
When I worked in a classified data center, the techs learned refrigeration so the operation didn't need to be shut down very often for outside contractors. I learned on several 100 ton chilled water systems.
When you calculate the CFM, remember for use in air the inlet pressure is much lower than the compressor inlet design pressure. If you are using an R22 compressor that is for A/C use, it is referred to as a high temp compressor. It has a lower displacement than a freezer compressor of the same power. The freezer compressor using a lower boiling temperature has a lower vapor pressure so the compressor is designed with a larger displacement for the lower volume of inlet gas due to the lower pressure. In general AC compressors are higher power due to the higher BTU load unless you are talking about icemakers which use a low temp compressor at high BTU's.
If you are going for high efficiency at high pressure, taking the 4 compressors you speak of and stacking them so 3 feed one, you can get high volume at high pressure. The first 3 would be in parallel as a first stage to take one atm (0bar) and boost it to 3 atm (2 bar). The volume of air then feeding the single second stage at 2 bar will have 3 times the CFM than it being fed an inlet of atmospheric pressure. The second stage will be in normal operating range with an inlet of about 45 PSI and an outlet of about 200 PSI.
To prevent disaster in case the second stage fails, use a 150 PSI pop off between the stages. Never feed more than 200 PSI into the inlet of a fridge compressor. The shell is not designed for it.
As a computer cooling tech, you probably knew most of the compressor limitations and normal operation pressure range for an R22 or R134a system. I posted this for the general spudder population.
Be safe. Condensation will damage the compressors over time.
To clear up the function of the starting capacitor, if you look at a motor like a steam engine with a flywheel and piston and connecting rod, it is easier to visualize mechanically what is going on electrically.
If a steam engine is parked with the piston fully extended or retracted, applying power won't make it start rotating. A single phase electrical motor is electrically the same. A magnetic field is alternating back and forth. The rotor in the middle can't go back and forth.
Once started a flywheel on a steam engine takes care of getting the rod off center for the next stroke. An AC electrical motor is the same. Once rotating the back and forth magnetic field applies power to the rotating part.
To get either started, an off center force needs applied. In a steam engine a second cylinder or more is used. Steam trains have the two cylinders on each side 90 degrees out from each other so a train can start. If one is at top or bottom stoke, the one on the other side is mid stroke and can apply motion.
A 3 phase electric motor is much like the steam train and has 3 coils, 120 degrees apart. A 3 phase electric motor does not need a starting capacitor.
Homes don't have 3 phase electric power, so this in a refrigerator is like a single cylinder steam engine always parked at top or bottom dead center. It needs an off center cylinder to push it off center.
The capacitor provides an electrical phase shift. This in conjunction with the main run winding provides the off center rotation needed to start the motor. After the motor is started, this coil is shut off for efficiency. To get the proper phase shift and proper start current, the capacitor is matched to the motor size. This is why only one cap can't be shared between the motors. I hope this helps.