Refrigerator battery or mains powered - a different twist

[OldGoat]

In short why do 240V fridges draw so much power on startup?

 

Background

1. Our aged Batts LEC cased fridge freezer is falling about internally with its door shelves held together with waterproof gaffer tape. There is no DC fridge available to match the capacity less that about £1k.
2. There are a few mains units around that would suit but I can't get a definitive size of inverter to power it
3. Looking at the stated and assumed power consumption (based on the types of compressor used) it would appear that about 300W would be adequate. However, discussions on here say that we'd need a 1200W or thereabouts inverter to start the thing.
4. I have a power curve for the Danfoss BD35 DC unit showing a peak current of around 10 amps = 120W (for about 50mS)
5. I can find no similar information for 240V units, only LRA (Locked Rotor Amps) of about 6 amps = 1440 watts

So why such a huge difference?

 

To further confuse the issue Danfoss make a mains unit with a 12/24V DC inverter gives the 240V AC supply to the compressor. However, the inverter is rated at 300W

 

Perhaps the difference is just a matter of cost. The BF150F is around £750, whereas the BD35F about £250, but that may be a matter of production costs for a small volume.

Comparing the 'typical annual consumption' of the largest Shoreline unit with my selection of suitable units - there is no significant difference, indeed the Shoreline one is slightly less efficient than the LEC mains type on which it based..

 

(I know I can get a F/F of my choice for around £550 converted to 12/24V all in)

 

Can anyone shed some more light on the disparity between starting power requirements of the two systems?

 

As an aside and for solar fans - Danfoss do make a variant of the BD35F suitable for direct wiring into solar panels - it has a wider voltage range than the base unit. It's the controller that's different.

Edited August 2, 2015 by OldGoat

[MoominPapa]

So why such a huge difference?

 

To further confuse the issue Danfoss make a mains unit with a 12/24V DC inverter gives the 240V AC supply to the compressor. However, the inverter is rated at 300W

 

Perhaps the difference is just a matter of cost. The BF150F is around £750, whereas the BD35F about £250, but that may be a matter of production costs for a small volume.

Comparing the 'typical annual consumption' of the largest Shoreline unit with my selection of suitable units - there is no significant difference, indeed the Shoreline one is slightly less efficient than the LEC mains type on which it based..

 

It's likely to be cost. Starting the compressor in a fridge takes a lot of oomph. A mains fridge has a lot of energy available from the supply, so the simplest method is to use it. A DC fridge doesn't so has to use a different approach (electrical storage in a capacitor, an unloader valve to get the motor running before it see a pressure load, I honestly don't know.)

 

I do know that our 240V fridge, run from a 1800W inverter, takes a big pulse at power up, and if it's running and you interrupt the power briefly then the compressor can't start against the pressure in the system and the motor stalls and sits there taking five times the normal current until the thermal trip operates, (or it gets switched off for a ten-minute rest, which is mostly what happens.) Starting fridge compressors in non-trivial.

 

MP.

Edited August 2, 2015 by MoominPapa

[Paul C]

1. I have a power curve for the Danfoss BD35 DC unit showing a peak current of around 10 amps = 120W (for about 50mS)
2. I can find no similar information for 240V units, only LRA (Locked Rotor Amps) of about 6 amps = 1440 watts

So why such a huge difference?

 

 

 

Because you're not comparing like for like. It might be that the "locked rotor amps" for the Danfoss compressor is lower, because its specifically designed to be lower, and that's what the money is spent on (well.....the costs are high because compared to mains fridges, the market opportunity for 12V ones is much smaller).

[GUMPY]

Its down to the inverter!

I run our fridge LEC on a 1kw combi big bulky and old tech from caravantechnolgies and its fine. A while ago tried to find an inverter to use as a dedicated fridge inverter, tried 3 from ebay all 1kw none would start the fridge sent them all back for a refund. So my thoughts are that the new inverters just don't have the surge capacity that the older inverters do........

[DHutch]

Interesting thoughts, I don't know enough about the motors/compressors in fridges to know the differences are between a 12-24vdc compressor and a 240vac but while one answer might well be 'because a mains fridge can' there must be something different.

 

Are 240vac fridges two-phase with a phase shifting cap?

 

Daniel

[Neil Smith]

Why not buy a 240v fridge/freezer of your choice and fit your 12v compressor from your old one.

 

Neil

[OldGoat]

Why not buy a 240v fridge/freezer of your choice and fit your 12v compressor from your old one.

 

Neil

Thanks - I had though of that - but the old one is 20 years old. It may be an earlier version of the Danfoss or something else entirely. I was also hoping to do the swap in one hit anyway. I suspect the cost difference would be only around £150+

[Arthur Brown]

Remember that there is no certainty that an old compressor will be compatible with the new(er) refrigerant gasses, and some gasses cannot now be refilled because of legislation on greenhouse gasses.

 

What we need is for someone to adapt the thermal switch from the fridge to switch On the inverter permanently connected to the motor, without breaking the manufacturers warranty!

[DHutch]

Yeah, fitting a new 12vdc compressor to convert a fridge is an option, and something some companies do, but I don't think fitting a 20yo compressor to a new fridge would stack up as part because as said they will have different gases.

 

Daniel

[OldGoat]

Interesting thoughts, I don't know enough about the motors/compressors in fridges to know the differences are between a 12-24vdc compressor and a 240vac but while one answer might well be 'because a mains fridge can' there must be something different.

 

Are 240vac fridges two-phase with a phase shifting cap?

 

Daniel

 

Daniel, thanks - got me thinking...

 

One of the problems that I have is that Danfoss-Secop is the only manufacturer with any data on their kit and my initial searches focussed on the model numbers that I knew. Not only that but the data available has different parameters, For BD** compressors for 240V no graph so LRA has to be a guide (?).

 

I never thought (didn't need to know) that capacitor start implied two phase, just thought it gave an extra kick(now I know why my aged pillar drill goes 'click...) So maybe 'yes' - I found a diagram for a Danfoss mains unit with a starter winding, though it was for a special duty version. However, that all makes sense (even if I'm wrong) - the capacitor has a large inrush of energy when the unit starts, that accounts for the very large peak current that stalls the inverter and not the motor run-time demand.

 

Capacitor start is cheap, the internal wiring is cheap and as we know what the consumer wants 'good value' = cheap products.

 

I'll buy that analysis - unless of course somebody comes back to say it's rubbish.

 

From my cost point of view if I have to buy a 12-1400W inverter, it makes the project more expensive than converting a new fridge to 12V. I don't have any need for a large power unit. An old 'square wave' runs the vacuum cleaner, a small 350W sine wave runs the tv etc etc. No big washing machine, toaster or microwave. We're only leisure users.

 

Overall DC fridges are not more efficient that their AC cousins - so no plus point there.

 

 

I still welcome more input from folks - and many thanks for the contributions thus far.

[GUMPY]

Thanks - I had though of that - but the old one is 20 years old. It may be an earlier version of the Danfoss or something else entirely. I was also hoping to do the swap in one hit anyway. I suspect the cost difference would be only around £150+

If its that old it may well have R4 as the refrigerant if it has its a bin job as modern refrigerants will not work in the old system. that's why mine went in the bin as I was told by a fridge tech that even with a new compressor it wouldn't have worked.

[OldGoat]

If its that old it may well have R4 as the refrigerant if it has its a bin job as modern refrigerants will not work in the old system. that's why mine went in the bin as I was told by a fridge tech that even with a new compressor it wouldn't have worked.

 

I hadn't even got that far, the late lamented (?) M.Batts didn't get as far as any technical details in his blurb (a copy of which I have). Being a 1995 model perhaps not.

Despite this an even our use of for holidays etc, I don't think swapping it would be 'a good idea'.

 

Thanks anyway.

[MoominPapa]

 

I never thought (didn't need to know) that capacitor start implied two phase, just thought it gave an extra kick(now I know why my aged pillar drill goes 'click...) So maybe 'yes' - I found a diagram for a Danfoss mains unit with a starter winding, though it was for a special duty version. However, that all makes sense (even if I'm wrong) - the capacitor has a large inrush of energy when the unit starts, that accounts for the very large peak current that stalls the inverter and not the motor run-time demand.

 

You got me googling, and I found a circuit diagram, and description.

 

http://www.jimdow.com/jimdow/sales/diagrams/StartRelayStartCapacitorOverload/StartRelayStartCapacitorOverload.htm.

 

 

So fridge start-up surge is not just due to lack of motor back-emf, they actually have a circuit which connects an extra start winding across the supply during run-up.

 

MP.

Edited August 2, 2015 by MoominPapa

[smileypete]

In short why do 240V fridges draw so much power on startup?

 

Background

1. Our aged Batts LEC cased fridge freezer is falling about internally with its door shelves held together with waterproof gaffer tape. There is no DC fridge available to match the capacity less that about £1k.
2. There are a few mains units around that would suit but I can't get a definitive size of inverter to power it
3. Looking at the stated and assumed power consumption (based on the types of compressor used) it would appear that about 300W would be adequate. However, discussions on here say that we'd need a 1200W or thereabouts inverter to start the thing.
4. I have a power curve for the Danfoss BD35 DC unit showing a peak current of around 10 amps = 120W (for about 50mS)
5. I can find no similar information for 240V units, only LRA (Locked Rotor Amps) of about 6 amps = 1440 watts

So why such a huge difference?

 

To further confuse the issue Danfoss make a mains unit with a 12/24V DC inverter gives the 240V AC supply to the compressor. However, the inverter is rated at 300W

 

Perhaps the difference is just a matter of cost. The BF150F is around £750, whereas the BD35F about £250, but that may be a matter of production costs for a small volume.

Comparing the 'typical annual consumption' of the largest Shoreline unit with my selection of suitable units - there is no significant difference, indeed the Shoreline one is slightly less efficient than the LEC mains type on which it based..

 

(I know I can get a F/F of my choice for around £550 converted to 12/24V all in)

 

Can anyone shed some more light on the disparity between starting power requirements of the two systems?

 

As an aside and for solar fans - Danfoss do make a variant of the BD35F suitable for direct wiring into solar panels - it has a wider voltage range than the base unit. It's the controller that's different.

 

Mains fridges have an induction motor which is totally different type of motor to the Danfoss 12V ones, the latter have a brushless DC motor with quite sophisticated control circuitry.

 

The Danfoss compressors are more efficient as they have a permanent magnet and variable speed, but modern mains fridges now use a better refrigerant than what the Danfoss compressor uses, which closes the gap in efficiency between them. Also their efficiency drops off as the load increases, so don't perform as well in larger units. Lastly they are often fitted to units with rather so-so insulation.

 

So in all an A+++ or larger A++ mains fridge may come pretty close to what the 12V equivalent uses.

 

Its down to the inverter!

I run our fridge LEC on a 1kw combi big bulky and old tech from caravantechnolgies and its fine. A while ago tried to find an inverter to use as a dedicated fridge inverter, tried 3 from ebay all 1kw none would start the fridge sent them all back for a refund. So my thoughts are that the new inverters just don't have the surge capacity that the older inverters do........

 

What was the compressor stall current a.k.a. locked rotor amps/LRA for the LEC?

 

I'd use this to gauge the required continuous rating of the inverter. There will be a very short higher current blip at switch on as the motor magnetises, but would have thought the surge rating of the inverter would cover this fine.

 

My own rather elderly mains fridge draws 3.83A LRA, so would have thought a 1kW inverter would cover it fine.

 

Interesting thoughts, I don't know enough about the motors/compressors in fridges to know the differences are between a 12-24vdc compressor and a 240vac but while one answer might well be 'because a mains fridge can' there must be something different.

 

Are 240vac fridges two-phase with a phase shifting cap?

 

Some use a start capacitor, some use a run capacitor, some use neither.

 

They all use a PTC thermistor or relay to switch power, or switch extra power, to a start winding which is phase shifted to the main winding, the capacitance or AIUI even the difference in resistance of the start winding does the phase shift.

 

What we need is for someone to adapt the thermal switch from the fridge to switch On the inverter permanently connected to the motor, without breaking the manufacturers warranty!

 

One way is to turn the fridge to max and use an external temperature controller.

 

cheers, Pete.

~smpt~

Edited August 2, 2015 by smileypete

[rupertbear]

 

Mains fridges have an induction motor which is totally different type of motor to the Danfoss 12V ones, the latter have a brushless DC motor with quite sophisticated control circuitry.

 

The Danfoss compressors are more efficient as they have a permanent magnet and variable speed, but modern mains fridges now use a better refrigerant than what the Danfoss compressor uses, which closes the gap in efficiency between them. Also their efficiency drops off as the load increases, so don't perform as well in larger units. Lastly they are often fitted to units with rather so-so insulation.

 

So in all an A+++ or larger A++ mains fridge may come pretty close to what the 12V equivalent uses.

 

 

What was the compressor stall current a.k.a. locked rotor amps/LRA for the LEC?

 

I'd use this to gauge the required continuous rating of the inverter. There will be a very short higher current blip at switch on as the motor magnetises, but would have thought the surge rating of the inverter would cover this fine.

 

My own rather elderly mains fridge draws 3.83A LRA, so would have thought a 1kW inverter would cover it fine.

 

 

Some use a start capacitor, some use a run capacitor, some use neither.

 

They all use a PTC thermistor or relay to switch power, or switch extra power, to a start winding which is phase shifted to the main winding, the capacitance or AIUI even the difference in resistance of the start winding does the phase shift.

 

 

One way is to turn the fridge to max and use an external temperature controller.

 

cheers, Pete.

~smpt~

 

I'm not sure that I'm understanding the last comment. Does this mean that the thermostat is controlling the LV supply to the inverter rather than the 230V supply to the compressor, so that the inverter starts up when the temperature rises with the compressor load already connected?

 

If so, what effect would this have on the startup surge and the inverter's ability to cope with it?

[by'eck]

 

I'm not sure that I'm understanding the last comment. Does this mean that the thermostat is controlling the LV supply to the inverter rather than the 230V supply to the compressor, so that the inverter starts up when the temperature rises with the compressor load already connected?

 

If so, what effect would this have on the startup surge and the inverter's ability to cope with it?

 

I think its assumed the inverter has software controlled on/off switch.

[sjc]

Mains fridges use single phase induction motors, which I believe have a very low starting torque. I assume this is why they require the starter circuit to over-drive it to get it to start turning. For this reason I think you can burn out the motor if the startup time is excessive ( since during this time it is being over-driven ). DC motors simply have an inrush current then produce more torque from a standstill. However, single phase induction motors are relatively cheap and reliable, hence their use.