Variable speed compressors and evaporator fans

[ctscasemod]

On a project to see how much efficiency can be gained by controlling the fridge motor, I have retroffited the fridge at home with an inverter unit, operating at half the nameplate values (25Hz @ 110V).

The inverter supplies a clean sine wave output and a soft start other than that being similar to a VFD. The original PTC starting mechanism has been retained.

Visually everything seems to work well. The temperature is more uniform towards the fridge door. Compressor noise went down with refrigerant flow now being the dominating noise from the unit. The temperatures of condenser and compressor are lower and compressor on time is only 60% (was expecting closer to 80's)

While the compressor power cosumption was cut by half (Excluding about 12% inverter losses), the fridge and freezer evaporator fans now run at their nominal rating for a longer time.

Has anyone that used a variable speed, perhaps on an industrial plant, complemented the ideal speed for the correct operation of the evaporator fans? Or will the increased COP neglect their increased losses?

[Rob White]

..

It is quite common in large commercial applications.
Another energy saving is running the condenser fans faster / longer and then
pressurising the liquid with a liquid pump. It's called liquid amplification
and means you can reduce the compressor speed even more but you
still keep the liquid pressure difference high enough for the TEV's to work.

Rob

.

[Peter_1]

Many small commercials already have an inbuild VFD at their connections

[ctscasemod]

..

It is quite common in large commercial applications.
Another energy saving is running the condenser fans faster / longer and then
pressurising the liquid with a liquid pump. It's called liquid amplification
and means you can reduce the compressor speed even more but you
still keep the liquid pressure difference high enough for the TEV's to work.

Rob

.

That's interesting. Wouldn't an electronic TEV be able to dose the refrigerant even with low return pressure? Or is this just to make sure it actually gets there fully liquid?

36 hours into the test and the compressor started to stall with increasing running current shortly after start. Nothing changed on the inverter side, so i suspect increased friction due to less than ideal lubrification.
Bumped things to nominal and it seemed to stabilize fine, so I raised the inverter frequency to 33Hz and I'm keeping my fingers crossed.

Anyone happens to have any experience about the speed ranges and typical lubrification on these? Other than the refrigerant carrying the oil, what mechanism does a small reciprocating fridge compressor use to lubrify internal parts? Does it splash oil into the internals as small 4 stroke engines do?

Thanks!

[frank]

Small hermetic pots use splash feed lubrication.
I would suspect that the increase in amp draw was due to the windings overheating, as with a lower than design speed, refrigerant flow and working pressure, you are going to get less compressor cooling.

The pot relies on the cold suction gas for cooling and oil return may be compromised by the lower velocities.

[Sandro Baptista]

Yes, that's right Frank. I'm agreed with you.

Ctscasemode have in account that rotating at a lower rpm value the torque is the same, despite a little lower discharge pressure (but an little higher suction pressure) due to lower rpm. So as Frank says the lack of cooling of the motor windings can be a problem. The windings insulation with the heat can get compromised as the motor is not prepared for VSD at so lower rpms and the motor doesn't have thermal reserve for that. Caution. However with PTC sensors active they should automatically cut out the motor operation due to thermal safety reasons.

[ctscasemod]

Yes, that's right Frank. I'm agreed with you.

Ctscasemode have in account that rotating at a lower rpm value the torque is the same, despite a little lower discharge pressure (but an little higher suction pressure) due to lower rpm. So as Frank says the lack of cooling of the motor windings can be a problem. The windings insulation with the heat can get compromised as the motor is not prepared for VSD at so lower rpms and the motor doesn't have thermal reserve for that. Caution. However with PTC sensors active they should automatically cut out the motor operation due to thermal safety reasons.

I don't think it was overheating. The compressor didn't seem to be running very hot and the issue only manifested itself about 30hours later, with the running currents gradually increasing a few hours before the PTC started to cut out. At this point, probably the slip would have increased and the rest we already know...

So I suspect lack of lubrification.

Hum... I also wonder if the PTC would have started to leak some current given the change in voltage... Maybe I should find an old magnetic starter to try.

Running the motor at 33Hz did not change to the symptoms. I could have tried 40, or to periodically increase the compressor speed to allow for oil flow, but for that I will need to re-design the inverter PCB.

I have returned it to direct mains connection for a few days and the symptoms din't return.

[Peter_1]

.... Other than the refrigerant carrying the oil, what mechanism does a small reciprocating fridge compressor use to lubrify internal parts? Does it splash oil into the internals as small 4 stroke engines do?

Thanks!

Inside the central shaft, there's a spiral wound plastic screw which pumps/feeds the oil upside...
https://www.semanticscholar.org/pape...a3e2104ccfa579

and https://www.sciencedirect.com/scienc...4070071730364X
https://www.semanticscholar.org/pape...268b2861fcd6c8