Jump to content

Soft start for 230V ac. freezer


Bro

Featured Posts

My present inverter is too small to fire up a Curry's table top freezer (model, Essentials CTF34W15) and whilst I'm resigned to buying a larger inverter I'm loath to buy one that's over-big for my needs. Would soft start unit such as "United Automation 25 A Soft Starter, 230 V ac" (RS Stock No. 330-5358) be helpful?

Link to comment
Share on other sites

My present inverter is too small to fire up a Curry's table top freezer (model, Essentials CTF34W15) and whilst I'm resigned to buying a larger inverter I'm loath to buy one that's over-big for my needs. Would soft start unit such as "United Automation 25 A Soft Starter, 230 V ac" (RS Stock No. 330-5358) be helpful?

I wouldn't have thought so. The high startup current is due to the motor not spinning. The motor on a freezer etc can be reluctant to spin up due to the pump compressor it's connected to having to overcome residual pressure in the coolant. My guess is that you need X amount of current to start the motor and having soft start won't reduce that peak current demand, but it probably will prolong the agony. As soon as the motor does start to spin, the current will reduce.

Link to comment
Share on other sites

I wouldn't have thought so. The high startup current is due to the motor not spinning. The motor on a freezer etc can be reluctant to spin up due to the pump compressor it's connected to having to overcome residual pressure in the coolant. My guess is that you need X amount of current to start the motor and having soft start won't reduce that peak current demand, but it probably will prolong the agony. As soon as the motor does start to spin, the current will reduce.

Nick's right. When a fridge or freezer starts up the motor is initially stalled, so for a few microseconds appears almost as a dead short to the power supply. The typical inrush current is around 7-8 times the running current.

Link to comment
Share on other sites

And that size would be?

 

Opinion varies, but 5x to 6x the steady-load power requirement of the freezer when its running.

If you know the duty cycle, or accept a guess at 30%, its possible to calculate it from the freezer's annual power consumption, even if you don't know its steady-load power requirement:

 

http://www.canalworld.net/other/annualpower.html

Link to comment
Share on other sites

 

Opinion varies, but 5x to 6x the steady-load power requirement of the freezer when its running.

If you know the duty cycle, or accept a guess at 30%, its possible to calculate it from the freezer's annual power consumption, even if you don't know its steady-load power requirement:

 

http://www.canalworld.net/other/annualpower.html

 

Thanks Paul. I used your link to the calculator which, based on freezer annual power usage of 150kW/hr, suggests a 500W start-up and an inverter having 317W min. output should do it. The inverter I have is rated 800W but doesn't work so something is adrift. Any ideas?

Link to comment
Share on other sites

 

Thanks Paul. I used your link to the calculator which, based on freezer annual power usage of 150kW/hr, suggests a 500W start-up and an inverter having 317W min. output should do it. The inverter I have is rated 800W but doesn't work so something is adrift. Any ideas?

 

The 5x to 6x is based on an educated guess from anecdotal reports of members who have a 230V fridge or freezer on a boat. I'm not sure if PSW (pure sine wave) or MSW (modified sine wave) inverter would make a difference - I suspect not given the simplicity of the components on a freezer.

 

I guess its over to others, who can report their size of inverter that works with their (size of) fridge or freezer.

Link to comment
Share on other sites

Thanks Paul. I used your link to the calculator which, based on freezer annual power usage of 150kW/hr, suggests a 500W start-up and an inverter having 317W min. output should do it. The inverter I have is rated 800W but doesn't work so something is adrift. Any ideas?

150 kilowatts per hour? I suggest connecting to a small power station!

 

Anyway I'd say that 30% duty cycle is wrong, if you don't open the door the compressor will rarely run. 10% duty cycle is more likely. This puts it a little above 1kw at x6. However I'd go for nearer x10 which puts it around 1500w, but that is just the startup transient and many inverters have a x2 transient capability which brings it back to 750w or so.

 

HOWEVER one thing to bear in mind is that inverters only work to spec when they are fed with voltage to spec. Perhaps when the momentary demand for 1500w = 137A at 12v, the batteries and 12v cabling results in excessive voltage drop and thus chokes the inverter. What batteries do you have, what section cable between batteries and inverter, how long is the cable run (each wire), is there an isolator for the inverter?

Link to comment
Share on other sites

What are the details of the inverter?

 

IF it's marginal, might be able to do it with an inrush limiter (sometimes fitted to isolation transformers), cheap but a bit fiddly to work out the best size and fit safely.

 

Far better and easier to source a suitable size inverter. Would help to know the cold resistance of the compressor, to do this unplug a while (say 10 to 15 mins), turn thermostat to max, then measure the resistance across the L and N terminals of the plug.

 

If you have shoreline access and a plug in energy monitor there's a more accurate and useful way to measure the starting current.

Edited by smileypete
Link to comment
Share on other sites

Thanks for all the good stuff. I'm away from the boat at present so can't give details about inverter and haven't tried using it with engine running. I can say though that batteries are 110Ahr x 2, fully charged and cabling is of substantial thickness. Circuit from batteries (+) to inverter is through isolator, 150A shunt, 120a relay then back to batteries (-). Apart from the 120A relay this circuit is in accordance with instructions for MCS Boat Products "15Amp DC Panel".

Edited by Bro
Link to comment
Share on other sites

IF you really do want a module to drop the starting current, the one mentioned in this topic should help:

 

http://www.canalworld.net/forums/index.php?showtopic=46931&p=871398

http://www.sollatek.com/products/freosave/

 

But bear in mind it will might be priced more like commercial type product, and the inverter still might not be big enough!

 

ETA: Also it'll need a PSW inverter, but might save money long term for off grid liveaboards.

Edited by smileypete
Link to comment
Share on other sites

Has anyone tried inverters from Sunshine Solar? Their 'Sunshine Power Inverter 1200W 12V Pure Sine Wave' claims "2400W SURGE POWER; 1800W for 10sec, 2400w for 2sec". For £149 a pop it sounds a snip (if it's for real and is reliable).

Link to comment
Share on other sites

Has anyone tried inverters from Sunshine Solar? Their 'Sunshine Power Inverter 1200W 12V Pure Sine Wave' claims "2400W SURGE POWER; 1800W for 10sec, 2400w for 2sec". For £149 a pop it sounds a snip (if it's for real and is reliable).

www.canalworld.net/forums/index.php?showtopic=51376

Link to comment
Share on other sites

IF you really do want a module to drop the starting current, the one mentioned in this topic should help:

 

http://www.canalworld.net/forums/index.php?showtopic=46931&p=871398

http://www.sollatek.com/products/freosave/

 

But bear in mind it will might be priced more like commercial type product, and the inverter still might not be big enough!

 

ETA: Also it'll need a PSW inverter, but might save money long term for off grid liveaboards.

I spoke with Sollatek who say the Freosave is intended for OEM supply to be inbuilt to fridges. They would not be happy to supply over the counter for after fitment. Shame!

Link to comment
Share on other sites

Has anyone tried inverters from Sunshine Solar? Their 'Sunshine Power Inverter 1200W 12V Pure Sine Wave' claims "2400W SURGE POWER; 1800W for 10sec, 2400w for 2sec". For £149 a pop it sounds a snip (if it's for real and is reliable).

 

Another topic here:

 

http://www.canalworld.net/forums/index.php?showtopic=63249

 

Looks like a good price as they don't seem to be the el cheapo 'barrel scrapings', should cover a typical domestic fridge easy peasy.

Link to comment
Share on other sites

 

Another topic here:

 

http://www.canalworld.net/forums/index.php?showtopic=63249

 

Looks like a good price as they don't seem to be the el cheapo 'barrel scrapings', should cover a typical domestic fridge easy peasy.

 

Thanks for the link Smiley. Seems there are no adverse reports on the Sunshine Solar inverters so I reckon I'll go for the 1200W jobbie at £159 which will hopefully deal with the fridge start-up load.

Link to comment
Share on other sites

150 kilowatts per hour? I suggest connecting to a small power station!

 

Anyway I'd say that 30% duty cycle is wrong, if you don't open the door the compressor will rarely run. 10% duty cycle is more likely. This puts it a little above 1kw at x6. However I'd go for nearer x10 which puts it around 1500w, but that is just the startup transient and many inverters have a x2 transient capability which brings it back to 750w or so.

 

HOWEVER one thing to bear in mind is that inverters only work to spec when they are fed with voltage to spec. Perhaps when the momentary demand for 1500w = 137A at 12v, the batteries and 12v cabling results in excessive voltage drop and thus chokes the inverter. What batteries do you have, what section cable between batteries and inverter, how long is the cable run (each wire), is there an isolator for the inverter?

 

I got it.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.