power consumption of freezer

[pcmdude]

Hello,

I have connected a household freezer up to a power meter (plugged this power meter into the mains and then the freezer into it). I have noticed that when I turn the freezer on (initial internal temperature is warm ambient temperature) the power meter reads about 120-150 W. Once it reaches steady "on/off" cycling (when the freezer is cold), the power meter reads about 100 W (during the cycle period when the compressor is on).
I am pretty sure it is a fixed speed compressor because it operates with an "on/off" cycle, so I cannot understand why the power consumption varies.
Does anyone have any ideas as to why?

Thanks

[mad fridgie]

Hello,

I have connected a household freezer up to a power meter (plugged this power meter into the mains and then the freezer into it). I have noticed that when I turn the freezer on (initial internal temperature is warm ambient temperature) the power meter reads about 120-150 W. Once it reaches steady "on/off" cycling (when the freezer is cold), the power meter reads about 100 W (during the cycle period when the compressor is on).
I am pretty sure it is a fixed speed compressor because it operates with an "on/off" cycle, so I cannot understand why the power consumption varies.
Does anyone have any ideas as to why?

Thanks
Basic principles of refrigeration!

[Yuri B.]

If You still do not need to know the "basic principles of refrigeration" for Your every-day life then compare the process with, lets say: while speeding up a car and riding it at a reached steady speed - the two fuel consumption rates wil be different.

[pcmdude]

If You still do not need to know the "basic principles of refrigeration" for Your every-day life then compare the process with, lets say: while speeding up a car and riding it at a reached steady speed - the two fuel consumption rates wil be different.

Regarding the analogy of a car, there is higher fuel consumption during accelaration due to greater fuel injection isn't it? Once a steady speed is reached, then the accelarator pedal is released a little and thus there is lower fuel injection, which gives the difference in fuel consumption rates.

Now, regarding the freezer, if the compressor is rated at say, 240 V and 0.5A, that would mean it draws 120 W of power (can only draw 0.5 A - steady fuel injection using the car analogy). Now, the inefficiencies of the refrigeration cycle would govern how much of that power is turned into cooling power. However, since it is a fixed speed compressor, doesn't that mean it can only operate at 120 W electrical power? During cooling from ambient, the compressor runs for a prolonged amount of time, thus the energy consumption would be high (measured over a given time interval), but shouldn't the measured electrical power drawn still be 120 W at any given time? Then, when steady state is reached, the compressor switches on and off, thus the energy consumption measured over a particular time interval will be lower compared to pull down. So I understand the difference from an energy consumption point of view. But it is the difference in electrical power drawn that does not make sense to me. From my understanding (or am I wrong?) a fixed speed compressor can only run at a given electrical power (which is different from energy consumption).

[mad fridgie]

Regarding the analogy of a car, there is higher fuel consumption during accelaration due to greater fuel injection isn't it? Once a steady speed is reached, then the accelarator pedal is released a little and thus there is lower fuel injection, which gives the difference in fuel consumption rates.

Now, regarding the freezer, if the compressor is rated at say, 240 V and 0.5A, that would mean it draws 120 W of power (can only draw 0.5 A - steady fuel injection using the car analogy). Now, the inefficiencies of the refrigeration cycle would govern how much of that power is turned into cooling power. However, since it is a fixed speed compressor, doesn't that mean it can only operate at 120 W electrical power? During cooling from ambient, the compressor runs for a prolonged amount of time, thus the energy consumption would be high (measured over a given time interval), but shouldn't the measured electrical power drawn still be 120 W at any given time? Then, when steady state is reached, the compressor switches on and off, thus the energy consumption measured over a particular time interval will be lower compared to pull down. So I understand the difference from an energy consumption point of view. But it is the difference in electrical power drawn that does not make sense to me. From my understanding (or am I wrong?) a fixed speed compressor can only run at a given electrical power (which is different from energy consumption).
The internal moter of the compressor spins a fixed speed. It drives a fixed volume piston. What changes is the mass it is moving and where it si moving to.
On pull down it is moving a greater mass to a higher pressure elevation. Hence more work done, Work done = Power.
Your electrical internals are not like an electric element.
A compressor DOES NOT run at a given electrical power

[pcmdude]

The internal moter of the compressor spins a fixed speed. It drives a fixed volume piston. What changes is the mass it is moving and where it si moving to.
On pull down it is moving a greater mass to a higher pressure elevation. Hence more work done, Work done = Power.
Your electrical internals are not like an electric element.
A compressor DOES NOT run at a given electrical power

Ok, thanks for that. I think I understand now.

[Yuri B.]

And mass flow depends on the absolute pressures difference: the less the difference, the more mass flow and the more power consumption. So the flow is the biggest at the beginning of freezing. After a time temperature is dropping in the freezer - alongside and according to the pressure (or vice versa the pressure does alongside the temperature), the difference has increased, mass flow decreased - power consumption decreased too.

[pcmdude]

And mass flow depends on the absolute pressures difference: the less the difference, the more mass flow and the more power consumption. So the flow is the biggest at the beginning of freezing. After a time temperature is dropping in the freezer - alongside and according to the pressure (or vice versa the pressure does alongside the temperature), the difference has increased, mass flow decreased - power consumption decreased too.

Thank you Yuri B and mad fridgie for your fast and helpful responses. I really appreciate it.

[cool runings]

.

I know you have the answer.

But using the analogy of the car..

Think of a car on a flat road, it uses a fixed amount of fuel to maintane a fixed speed. Now the same car drives up a hill at the same speed, it needs more power to push up the hill therefore uses more fuel..

Fridges are the same. It is doing more work so costs more to do the work, when it achieves the corect temp the workload eases.
All the best

coolrunnings

.

[pcmdude]

.

I know you have the answer.

But using the analogy of the car..

Think of a car on a flat road, it uses a fixed amount of fuel to maintane a fixed speed. Now the same car drives up a hill at the same speed, it needs more power to push up the hill therefore uses more fuel..

Fridges are the same. It is doing more work so costs more to do the work, when it achieves the corect temp the workload eases.
All the best

coolrunnings

.

yup, thanks for that. I came to that same conclusion the other day when I was driving.

[JohnP]

If the freezer is an auto-defrost model, on startup it will go through a defrost cycle which involves a heater, before it starts freezing. The defrost lasts about 15-30 minutes, depending on model. You are probably seeing the heater power.

[Sumit]

Hi friends ,

Good explanation of the refrigeration basics.Hats off to all my seniors, this website is really very beneficial for newbies like me.