# Energy saving device for freezers



## Dave_G (17 Apr 2006)

Hi,

A while ago I was watching TV prog' and I can recall some type of plug that you fitted to fridge/freezers that reduced the use of electricity - I seem to remeber they were 'smart' in that they allowed the freezer to power up only when required?

Can anyone help me find these?

Thanks,

David


----------



## Alf (17 Apr 2006)

Savaplug?


----------



## Dave_G (17 Apr 2006)

Thanks Alf - the link is very helpful... seems like 'snake oil' :shock:


----------



## Jake (17 Apr 2006)

Not all electronic engineers agree with Tony on this - try a search on uk.diy in google groups, and you'll see that one or two took a look at the literature put out by Savaplug and thought that there was a rational explanation for the claims made. The DTI obviously does as well, for what that is worth.


----------



## RogerS (17 Apr 2006)

Jake":1dogyosc said:


> The DTI obviously does as well, for what that is worth.



Ah well..that answers that then, doesn't it :wink:


----------



## WoodPecker (19 Apr 2006)

From the Savaplug site:



> HOW DOES IT WORK?
> When the thermostat on the appliance switches on the motor to pump the refrigerant around the system full power is required to start the motor. However, once the motor is running full power is no longer needed.
> 
> The SavaPlug senses this and reduces the flow of electricity to match the actual requirements. When the red light glows savings are being made.



What they are claiming is a load of what bulls make :shock: The first section is correct except that they have neglected to mention is that once the motor is running at full "speed" it won't concume the same amount of "power" anyway as the current required to keep the motor running will be much less than at startup. A load only draws as much current as it needs, they are suggesting that the motor continues to draw the same amount of current after startup as during startup and this is complete BS.


----------



## ike (19 Apr 2006)

An auction channel perchance? If so, the price should bid down to nothing cos that's all it will save you :lol: .

cheers,

Ike


----------



## Jake (19 Apr 2006)

I think it is possible that this is too simplistic a view. 

Electronic controls can improve electric motor efficiencies. See for example:

http://www.ecnmag.com/article/CA433417.html

Fridge motors are vastly overrated for their normal running load, because they need to start into a fully compressed load. From what I've read, most (eg workshop) compressors discharge the compression chamber when the motor cuts out. Fridges don't do this as it would add a potential leak into the system. So they have to overcome a good deal of torque on starting up - you can see the lights dim sometimes. That means they have to be vastly overrated for the running load. From there, back to the article above. I guess Mr Godbole is selling stuff, but is he full of bs too?

As I understand it most modern fridges and freezers have this stuff built in already to get their higher energy ratings, so adding an external one will not help and may do harm.


----------



## Anonymous (19 Apr 2006)

Save your money.




> The SavaPlug senses this and reduces the flow of electricity to match the actual requirements.



What utter and complete *rubbish*. The motor itself will draw less current once the initial starup torque has been generated to overcome the striction and frictions in the system. 

"Match the actual requirements"??? Doesn't this sentence say it all? The device will do nothing as the motor will draw the current* it* _requires_ 

The phrase "reduces the flow of electricity" is meaningless too - current flows, not electricity!!

This device will only make you poorer - by £20.


----------



## Jake (19 Apr 2006)

Tony":m9f98ior said:


> device will do nothing as the motor will draw the current it requires


So Mr Godbole is talking out of his rear-end?


----------



## AndyBoyd (19 Apr 2006)

As a senior engineer working for a very large oil company with PhD in physics I feel a lesson comming on. :roll: 

We do this with our large motors/compressors when they run in steady state, we actually reduce the power to them by removing some of the sine wave of the AC (three phase or single phase). The motor pulls more current when we do that to compensate but the overall power consumption is less as we are basically reducing the voltage (hence the motor rotates slower) - but this is averaged down over time so can be hard to get your head round it.

This can realise up to 20 % power consumption savings at steady state (of course motors are rarely run at steady state so the actual total saving is circa 5%). But as we run massive motors this is a consierable cost saving for us.

So I think they do that, the trick is we have very smart system to detect when we can do this and when we need to turn it off, I'd be interested to see how savaplug senses that. (Our sensing systems are very expensive)

I've just checked this with our electric engineering gurus (literally round the corner from me) and they confirm the above

Old fashioned drill speed controllers work in the same principle


----------



## ike (19 Apr 2006)

Tony wrote:


> This device will only make you poorer - by £20



Totally agree, but just think of the massive savings!... erm that'll be bout £2.50 a year then?.. :roll: 

A case of "if something sounds to good to be true, it usually is" I suspect. :lol: 

Ike


----------



## Jake (19 Apr 2006)

That's £2.50 more a year than it was a few hours ago. At this rate a savaplug will be earning cheques from the lecky company by tomorrow evening!


----------



## ike (19 Apr 2006)

That was based on your 5% saving!


----------



## Anonymous (19 Apr 2006)

Andy

Zero crossing phase delay triggering (which you describe) can be done quite cheaply if one is not looking for a sophisiticated system capable of controlling and monitoring a large motor. 
If we assume the supply is steady 50Hz, we can simply wait for a small period (a few hundred microseconds) after the zero crossing point in the sinewave and then turn the switching device on.

I have designed many motor speed controllers using a similar principle in my 20+ years as a design engineer, mostly using cheap microcontrollers.

However, I still see no saving on a small motor which runs on low load.


----------



## omega1 (19 Apr 2006)

Hi
Just been readding your post about the control of large AC induction motors, I always believed that the speed of rotation was governed by the number of poles and the frequency of the supply, or are you using other types of AC motors? 

I thought that fridge compressors were run by induction motors.

Regards karl


----------



## Jake (19 Apr 2006)

ike":1nwyki8n said:


> That was based on your 5% saving!



Not mine...


----------



## Jake (19 Apr 2006)

Tony":1nnp5yb3 said:


> I have designed many motor speed controllers using a similar principle in my 20+ years as a design engineer, mostly using cheap microcontrollers.
> 
> However, I still see no saving on a small motor which runs on low load.



Mr Godbole or whatever his name seems to have at least equal experience but appears to disagree though? Is he just talking about more sophisticated techniques or something?


----------



## AndyBoyd (20 Apr 2006)

Tony - you must be right, we apply this technique and a few other very fancy ones :wink: on motors larger than your house.


----------



## CHJ (20 Apr 2006)

omega1":1tz43vpq said:


> Hi
> Just been readding your post about the control of large AC induction motors, I always believed that the speed of rotation was governed by the number of poles and the frequency of the supply, or are you using other types of AC motors?



In an ideal world it is, (a motor running with no load). However in the real world there is a certain amount of 'slippage' as the rotor tries to align with the next pole in its path (think of the magnetic connection as an elastic band that gets distorted as load is applied)

I *think* that what is being described is managing this 'slippage' by reducing the supplied energy without the rotor getting too far behind the poles and stalling. (done at the phase crossover point to reduce heat in control components because you are switching off power at it's least effective point) * (Tony?)*



omega1":1tz43vpq said:


> I thought that fridge compressors were run by induction motors.


They are.


----------



## omega1 (21 Apr 2006)

What he is saying though actually appears to defy ohms law. Surely if a a motor is drawing more current then the power consumption is going to go up which inturn means the motor will heat up more and more energy will be lost as heat. If they are doing this with large motors then ir will not be beneficial to the life of the motor.

I still do not understand what he saying about controlling the speed of the motor if you reduce the voltage the motor will not run slower if it is an induction motor otherwise speed control would be easily done by varying the voltage but this makes no difference, so thyristor soft start units and inverters have been designed to do it. At the power station i work at when the reactor trips the supplies to the gas circulators are lost, the supply is then switched back onto them and the only way that can be used to control their speed is to use inverter drive units to vary the frequency.
these motors are a lot larger than what is found on compressor though!

cheers karl


----------



## CHJ (21 Apr 2006)

omega1":26wcunhd said:


> ...snip...if you reduce the voltage the motor will not run slower if it is an induction motor otherwise speed control would be easily done by varying the voltage ...snip...



If my understanding is correct they are not talking about reducing the voltage, that just generates heat from waste power.

I believe they are referring to switching off the power for a fraction of a second at the phase crossover point (0volts) which wastes little power in the switching devices but has a small effect on the overall power delivered to the device, and therefore allows a small amount of rotational slippage to occur in the motor under load reducing the effective rpm..

As far as Induction motors are concerned in non critical speed situations, like a wood lathe, it is possible to reduce its rotational speed by loading it, if taken too far to the point where it is going so slow that the load overcomes the electrical torque it stalls. You will also see this on an induction motor table saw if cutting too vigorously. Taking things this far will obviously be detrimental to the motor eventually because of increased non effective current draw heating it up. 

But I can see that under controlled conditions on something like a large compressor that is not speed critical that you can manage power supplied, against rotational speed loss due to load, to effect a small percentage saving.

I am no expert on High Energy power control but the above is what I understand of the subject from my time in an Aircraft electrical design office where I had to interface 60kw alternators with various aircraft systems.


----------



## Anonymous (21 Apr 2006)

Yep

What is happening is that the peak voltage is the same, however, the time period where voltage is applied (1/2 cycle of mains voltage) is reduced by a small period of time and so the power delivered to the motor is effectively reduced by a small amount - as is torque


----------



## omega1 (22 Apr 2006)

cheers for the reply Tony at least you know what you are saying, the other guy was saying that if you adjust the voltage then you would adjust the speed which is not true. Adjusting the time period though will vary the speed because you are adjusting the frequency of the supply which is what i was trying to say.

If he believes that adjust the voltage make a difference to the speed then I suggest he goes back to school/college as this is basic electrical principles that I learned on my onc in the 1st year! :shock: 

cheers karl


----------



## CHJ (22 Apr 2006)

omega1":2syltkjb said:


> ...snip.... Adjusting the time period though will vary the speed because you are adjusting the frequency of the supply which is what i was trying to say.



They are not adjusting the frequency, Just the period of time the power is switched on. They are interrupting the supply for a very short period in every cycle.








The power during the dotted line period is switched off. An induction motor under no load will still run at its design speed.


----------



## omega1 (22 Apr 2006)

Hi Chas 
What you have described is correct for constant speed induction motors that are used continuously but the load is either on or off and during the off period the motor power can be limited thus saving energy.
Andy has said that the motor will draw more current but the power will be lower as you are basically reducing the voltage and hence the motor would run slower. This statement is not true because P= IsqR so if the resistance windings is constant and the current increase then so will the power as the equation shows. Also varying the voltage will not adjust the motors speed.
Regarding the fridge motor then that would assume that the motor is either vastly overrated or runs continuously 24Hrs a day.

Regards karl


----------



## CHJ (22 Apr 2006)

omega1":1y01rxhy said:


> ...snip...This statement is not true because P= IsqR so if the resistance windings is constant and the current increase then so will the power as the equation shows. Also varying the voltage will not adjust the motors speed.



Looks as though we are talking at cross purposes, and wandering away from the original topic.

I do know that an induction motor windings as the name implies are as much inductive as resistive so current draw can be quite a way out of step with the voltage peak causing considerable (paid for via a watt meter) energy losses and makes any calculations theoretical and in the real world only controlled monitoring will show the effect of all factors.
The Power factors in aircraft supplies are always checked in real time for new installations and not just signed off on the calcs.

The efforts that were put into balancing the inductive-capacitive factors involved in the 3+ megawatts that my last shop consumed resulted in 'charged power' savings equivalent to running several lathes and milling machines.


----------



## trevtheturner (23 Apr 2006)

"......... and wandering away from the original topic" ............

which was Dave_G asking where he could find a Savaplug!! :lol: :lol: 

Cheers,

Trev.


----------



## CHJ (23 Apr 2006)

trevtheturner":tvd23mgf said:


> "......... and wandering away from the original topic" ............
> 
> which was Dave_G asking where he could find a Savaplug!! :lol: :lol:
> 
> ...



I don't know either *Trevor*, :lol:


----------

