Inverter for Whynter mini freezer

MEXICOWANDERER wrote:
GdeTrailer,

Boy! I'll say! Trace boasted their inverters "Would easily operate at X3 rated nameplate" for a half hour, at 25c. I had (and sold) a 12-volt Trace 812 that tolerated a 3,000 watt motor start up. My dumb - the poor inverter survived innumerable clashes trying to fight 120 vac shore power (and auto-protected shutdown). Trace inverters were heavy. Even though they were pseudo sine wave.

Not any more. Those cheap featherweights on the market can barely meet their design wattage spec. I blew my true sine wave Samlex immediately off-line with a 2X load attempt.

Care must be taken matching an inverter to an inductive load start up (inrush). Pseudo sine wave inverters require more percentage safety startup leeway than do true sine wave inverters.

There's no benefit in spending any money to "try and save money" only to guarantee the purchase either fails to do the job or catches fire.

1. Find out the appliance run watt rating

2. Find out what type of motor is used

3. GOOGLE motor-type start up current inrush. It will be a percentage

4. This is valid only for dedicated inverter / motor connections

Trace was a well known quality inverter, thats for sure.

Tripplite does make some heavier inverters built the same way as the PV1250. The PV1250 uses and old school heavy duty 60 hz transformer in the output stage.. This 60hz transformer HELPS greatly to "match" to inductive loads. Also acts as a "filter" to help filter out some of the MSW harmonics in the waveform.

The flip side to modern inverters which makes them extremely lightweight AND CHEAP is the method of how they get the 120V.. The modern day versions use a high frequency "chopper" which also for a very small tiny transformer.. This creates a high voltage at a high frequency on the secondary of the transformer.

This high frequency AC voltage is no good to 120V 60hz appliances so it is rectified and filtered to high voltage DC..

Pretty much the same way a switching power supply works..

Then the high voltage DC is now fed to the output transistors and modulated to 60HZ 120V

Output stage is very similar to say an audio amp except for the fact that it is designed for a single frequency output.

Because of the high frequency chopper/power supply design there may not be enough "reserve" power available for the output stage to sustain large 120V surge currents..

GdeTrailer,

Boy! I'll say! Trace boasted their inverters "Would easily operate at X3 rated nameplate" for a half hour, at 25c. I had (and sold) a 12-volt Trace 812 that tolerated a 3,000 watt motor start up. My dumb - the poor inverter survived innumerable clashes trying to fight 120 vac shore power (and auto-protected shutdown). Trace inverters were heavy. Even though they were pseudo sine wave.

Not any more. Those cheap featherweights on the market can barely meet their design wattage spec. I blew my true sine wave Samlex immediately off-line with a 2X load attempt.

Care must be taken matching an inverter to an inductive load start up (inrush). Pseudo sine wave inverters require more percentage safety startup leeway than do true sine wave inverters.

There's no benefit in spending any money to "try and save money" only to guarantee the purchase either fails to do the job or catches fire.

1. Find out the appliance run watt rating

2. Find out what type of motor is used

3. GOOGLE motor-type start up current inrush. It will be a percentage

4. This is valid only for dedicated inverter / motor connections

Naio wrote:

sidney wrote:
Thanks for all the replies. I did not consider how high the start up surge could be.

For now I'm going to stick with running the freezer on shore power.

Cheers

When my mini freezer is stuffed full, it keeps things frozen for 3.5 days with no power, if highs are in the upper 60s / low 70s. At higher temps it's more like 3 days.

It's very important that it be full, though. Not-full, it might be less than 12 hours. Fill yours with frozen water bottles if needed.

My freezer uses 65 watts when running. My 1500 watt inverter will SOMETIMES handle the startup surge, but not reliably.

Not ALL inverters are created "equally" and have long enough surge current capacity to reliably start an induction motor under a load.

If the inverter does not include the amount of time it can handle the surge rating in the specifications you most likely WILL have a hit or miss type situation.

Most cheap inverters fit this category..

Better inverters WILL include the length of time that it can sustain in the surge current area..

Example is the PV1250 that I have..

Here is the specs..

"Model: PV1250FC
Voltage compatibility: 12 VDC
Frequency compatibility: 60 Hz
Output voltage: 120 VAC
Output capacity (continuous): 1250 watts
Output capacity (OverPower): 1875 watts/1 hour
Output capacity (DoubleBoost): 2500 watts/10 seconds
Overload protection: Output circuit breaker
Cooling method: Fan
Input connection: 2 x DC input terminals (user supplies cables)
Outlet quantity/type: 2 x 5-15R
Switches/LEDs/alarms: 3-positon On/Off/Remote switch
6 x diagnostic LEDs
Dimensions (HWD): 7.0 x 8.75 x 9.0 in.
Weight: 23.2 lbs.
Material of construction: Black polycarbonate"

Notice the area I bolded!

The PV1250 CAN handle 1875W for ONE HR!!!

And can handle 2500W (double the continuous rating) for TEN SECONDS!

Cheap inverter manufacturers over state their surge capacity and the time it can handle the surge at the level they stated will be on the order of MICROSECONDS which is not enough to get an induction motor started!

Yes, The PV1250 IS expensive (currently about $250) compared to a $79 1500W import but it WILL run rings around any cheap import inverter..

sidney wrote:
Thanks for all the replies. I did not consider how high the start up surge could be.

For now I'm going to stick with running the freezer on shore power.

Cheers

When my mini freezer is stuffed full, it keeps things frozen for 3.5 days with no power, if highs are in the upper 60s / low 70s. At higher temps it's more like 3 days.

It's very important that it be full, though. Not-full, it might be less than 12 hours. Fill yours with frozen water bottles if needed.

My freezer uses 65 watts when running. My 1500 watt inverter will SOMETIMES handle the startup surge, but not reliably.

my fridge draws approx 100w at 120v while running
but the start surge is over 900w for the compressor start
my 300w PSW go power will not start the fridge
my tripplite 1250 has been running it for 6yrs come next month

with any 120v compressor fridge, going small on the inverter is hit or miss

Thanks for all the replies. I did not consider how high the start up surge could be.

For now I'm going to stick with running the freezer on shore power.

Cheers

ScottG wrote:

2oldman wrote:
Oh, around 150-300w.

This will work fine.

Not even close..

Now if you are talking about a NON INDUCTIVE load like a TV that would not be an issue.

BUT, since the subject IS a highly inductive load that small of an inverter won't have a snow balls chance in a very hot place of attempting to start the compressor.

I MEASURED my home fridge conversion, running was 80W, BUT the startup surge was 10 to 11 TIMES the running wattage.

With that said, some folks have reported success with 700W inverters but I think that is very sketchy at best. Most cheap inverters have very short surge wattage, so short they don't bother listing the time..

With that said, you NEED at least 1000W inverter if you are going dirt cheap, that will at least get an inverter that has a continuous wattage rating ABOVE the startup surge...

DON'T SKIMP on the 12V wire size, go to the max the inverter will accept! AND KEEP THE 12V WIRE AS SHORT AS POSSIBLE!

For my home fridge conversion I went with a Tripplite PV1250.. That is an MSW inverter that IS designed to handle INDUCTIVE LOADS.. Yes, it is not cheap but I have confidence it will handle the fridge compressor without going up in smoke or burning up the compressor.

MEXICOWANDERER wrote:
How can folks guess at this without knowing the type of AC motor used? Got gnus for some - a split phase motor inrush is WILDLY different than a capacitor start motor.

I suggest an inquiry to the distributor would be productive.

Unfortunately the companies website seems to have issues so I could not peruse it for the specs

How can folks guess at this without knowing the type of AC motor used? Got gnus for some - a split phase motor inrush is WILDLY different than a capacitor start motor.

I suggest an inquiry to the distributor would be productive.

I tried a 450 watt inverter on a 2.5 cu mini frig and it didn't work. There are other things to also keep in mind when hooking up an inverter. Wire size, the length of the wire run, placement of the inverter (closer to the battery is best}.

I found putting a frozen water bottle (half gallon or smaller) cooled the frig down enough to travel 12 hours before hooking up to shore power.

This subject was talked about in great detail on this forum about 8 months ago. Try searching for the other posts they had lots of great information posted.