VFD to Use on Piston Type Refrigeration Compressor

Good answer JRaef. I think many people think that using a VFD automatically saves energy.

I think that OP is trying to emulate what happens inside a modern "invertor" type of refrigeration system (aircon?)

In these, instead of the compressor turning on and off in concert with the thermostat, the speed is modulated to effectively change the heat pumping capacity.

Energy savings come from eliminating cyclical high starting currents.

The energy savings are real and have been independently verified. Google it to find out for yourself, I too was a doubter once!

The comment on lubrication is a valid one....I'm not sure whether invertor systems have special compressors or not.

Give it a try OP and let us know what you discover. The thermostat may need some mods too.

Since the PRIME focus is to reduce costs, and- since you are looking at reducing capacity, which indicates a refrigeration system that is larger than required, I suggest that you install a cold storage system and run the compressor at night when you can get less expensive power.

Depending on your Actual load vs. Installed capacity and how much storage you can install, you might still have to augment the storage with daytime cooling, but you can "schedule" that using run-time timers. Check your rate structure to see the times that are defined as "peak" hours and the "window" used to determine electrical demand- it is usually about usage during a 30 minute rolling window. Assuming that is the case, set the ON timer for, say 10 minutes ON (or no more than 33% of the "demand window"), followed by ("Demand Window" minus ON time) minutes OFF. That will limit your "demand" (cost per kW usage during "peak" operating hours) to no more than 33% of what you are now paying for THAT load IF you actually need to augment.

For example- Assuming that this load is "process" and operates 24/7, and your electrical costs are, say, $15.00 per on-peak kW "demand" with NO off-peak demand charges and $0.085 per on-peak kWh / $0.040 per off-peak kWh "usage" with an "on-peak" window of 9 hours a day, 5 days a week and your refrigeration system (compressor and condenser fans) energy load is 1.25 kW per ton with a total of 300 tons installed and a 200 ton average load, your current monthly cost for power is-

(300 x 1.25) x $15.00 + ((200 x 1.25) x ((5 x 9) x $0.085 + (720 - (5 x 9)) x $0.040)) or $13,331.25.

The 200 ton average load means a monthly load of about (720 x 200) or 144,000 ton-hours with (5 x 9) x 200 = 9000 On-Peak ton-hours. Now, assuming that you can install (9 x 5) x 200 x 1.1 reserve = 9900 ton-hours of storage, your NEW monthly bill would be

First, IF you ran the system fully loaded during the entire "Off-Peak" period, it would produce (300 x (720- (5 x 9)) or 202,500 ton-hours of cooling. Since your actual load is only 144,000 ton-hours, the Actual operation during "Off-Peak" storage would only be (144,000 / 202,500) or 71.2% of total capacity, so the actual new bill would be

(300 x 1.25) x 71.2% x (720 - (5 x 9)) x 0.04, or $7,209 which is a MONTHLY savings of $6,122.25 or $73,467 annually. Obviously, YOU need to fill in the blanks using your actual electrical rates and operating schedules.

That means, based on an assumed 2-year payback window, you could spend up to about $147,000 on the storage system. It will very likely be much less expensive.

Also- your Electrical peak usage window is Only during the five "summer" months, but it is not worth the effort to worry about seasonal operating time shifts.

I am sure that the savings indicated are well above whatever MIGHT be possible if you used the VFD as originally defined, with absolutely NO risks to any machinery. In fact, the machinery will work "better" because it will be running at essentially constant load for several hours, then shutting OFF for a few hours.

What type of refrigeration system is this? Application?

Opps.. who told you that a VSD would save you money?

In fact it will cost you even more. The replies you have so far have covered most of the problems that you will get, so I'll not dwell, what you don't seem to realise is that a VFD will cost you more in power quality due to increased harmonics, and that will affect ALL your systems, your power quality will drop and your pF will be out of wack, if not already.

If you are serious in your efforts to save energy and money, first get your power quality analysed, then invest in voltage optimisation, pf correction & harmonic mitigration (or any or all of them), and that will save you money in so many ways.