Author Topic: Mini Fridge Power  (Read 1903 times)
wattMaster
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Mini Fridge Power « on: June 19, 2016, 06:37:11 PM » Author: wattMaster
In the assisted living, there are mini fridges with labels on them that say: 115/120 Volt (I don't remember), 1.2 Amp, 80 Watt, 1.4 Oz of R134a.
The problem is, what specification do I trust? Watts? Amps? The power label is inconsistent. Is this a power factor issue?
And I can't remember if the 1.2 and 1.4 numbers are the opposite of what I happen to remember.
« Last Edit: June 19, 2016, 06:54:57 PM by wattMaster » Logged

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Ash
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Re: Mini Fridge Power « Reply #1 on: June 20, 2016, 03:42:03 AM » Author: Ash
The fridges may be (and likely are) low power factor, so :

Apparent power = 120V * 1.5A = 180VA
Active power = 80W

Then, the stated current may be higher than what it draws on average, so incorporating a safety factor (for example, for cases like when you want to figure out what else in addition with the fridge, or hw many such fridges, can be connected on an X Amp circuit)
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Re: Mini Fridge Power « Reply #2 on: June 20, 2016, 07:43:48 AM » Author: wattMaster
The fridges may be (and likely are) low power factor, so :

Apparent power = 120V * 1.5A = 180VA
Active power = 80W

Then, the stated current may be higher than what it draws on average, so incorporating a safety factor (for example, for cases like when you want to figure out what else in addition with the fridge, or hw many such fridges, can be connected on an X Amp circuit)
My situation is me comparing a DC compressor to an AC compressor, and the DC one would take 24 Watts, so I would assume this takes 80? Then I would say that the DC compressor would take 3.33 times longer to do the same amount of cooling. Is this correct? So the amps rating is a fluke for situations like this?
Conclusion: Use Amps for how many mini-fridges on a circuit or extension cord for safety, and Watts when comparing DC to AC?
And are AC motors naturally low power factor?
« Last Edit: June 20, 2016, 08:02:09 AM by wattMaster » Logged

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Re: Mini Fridge Power « Reply #3 on: June 20, 2016, 11:13:35 AM » Author: Medved
The DC motors tend to be way more efficient than AC induction types. The AC has about three times the copper losses compare to the DC:
With the DC you just feed the rotor and it has just the losses related to actual power transfer. On top of that are just losses caused by a design "imperfection", so commutator friction (could be reduced by good materials) and commutator arcing (could be eliminated for known load by a proper phase angle shift). As a consequence, in the shaft power levels of around 30W the common efficiency figures are peaking at around 80..90% and that is, where a constant load such as a refrigerator will be designed to operate.

The AC has the same component in the stator (the real part), but on top of that are the losses related to rotor magnetic field generation, so the rotor cage currents and the reactive currents in the stator winding. Normally with an optimized design, all three components are about the same at the rated load; and usually each of them is the same as the DC motor winding losses. Unlike the DC motor, the mains frequency is fixed, so there is one variable less for design optimization, so the copper (and aluminum for the rotor cage) losses will be even higher than corresponding losses in the DC motor. Plus the design of the compressor itself is restricted by the given rpm as well, again reducing the optimization possibilities. Common efficiency figures for the similar 30W of shaft power are around 30..60%, so input power of around 40..100W.

So an 80W induction AC motor compressor would have an equivalent of no more than 40..50W DC motor, so the 25W is not that much less powerful refrigerator than the 80W AC (no more than 2x).

That was all about the real power the appliance draws.
Now because the induction motor itself needs to magnetize the rotor, it yields rather low power factor (around 50..60% is the most common for general purpose motors), so a 80W motor could well mean an input apparent power in the 150..200VA range. It is in "VA" and not "W", because it is not the real delivered power. It just means the wiring is exposed to voltages (120V, that is simple) and currents (1.5A), that may deliver up to 180W if the phase (and shape; but here we have a linear system, so all is a sinewave) would match.
The "VA" is used in place of "W" just to notify, that figure is not the real power, but just an equivalent of the loading of the wires (you have to use that figure, when sizing the circuit wiring and protection, or determining the required number of separate circuits, mainly when speaking not about one, but many such devices in a common installation).
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Re: Mini Fridge Power « Reply #4 on: June 20, 2016, 11:19:57 AM » Author: wattMaster
The DC motors tend to be way more efficient than AC induction types. The AC has about three times the copper losses compare to the DC:
With the DC you just feed the rotor and it has just the losses related to actual power transfer. On top of that are just losses caused by a design "imperfection", so commutator friction (could be reduced by good materials) and commutator arcing (could be eliminated for known load by a proper phase angle shift). As a consequence, in the shaft power levels of around 30W the common efficiency figures are peaking at around 80..90% and that is, where a constant load such as a refrigerator will be designed to operate.

The AC has the same component in the stator (the real part), but on top of that are the losses related to rotor magnetic field generation, so the rotor cage currents and the reactive currents in the stator winding. Normally with an optimized design, all three components are about the same at the rated load; and usually each of them is the same as the DC motor winding losses. Unlike the DC motor, the mains frequency is fixed, so there is one variable less for design optimization, so the copper (and aluminum for the rotor cage) losses will be even higher than corresponding losses in the DC motor. Plus the design of the compressor itself is restricted by the given rpm as well, again reducing the optimization possibilities. Common efficiency figures for the similar 30W of shaft power are around 30..60%, so input power of around 40..100W.

So an 80W induction AC motor compressor would have an equivalent of no more than 40..50W DC motor, so the 25W is not that much less powerful refrigerator than the 80W AC (no more than 2x).

That was all about the real power the appliance draws.
Now because the induction motor itself needs to magnetize the rotor, it yields rather low power factor (around 50..60% is the most common for general purpose motors), so a 80W motor could well mean an input apparent power in the 150..200VA range. It is in "VA" and not "W", because it is not the real delivered power. It just means the wiring is exposed to voltages (120V, that is simple) and currents (1.5A), that may deliver up to 180W if the phase (and shape; but here we have a linear system, so all is a sinewave) would match.
The "VA" is used in place of "W" just to notify, that figure is not the real power, but just an equivalent of the loading of the wires (you have to use that figure, when sizing the circuit wiring and protection, or determining the required number of separate circuits, mainly when speaking not about one, but many such devices in a common installation).

Now I understand. So it's really 80 Watts.
And I assume the same applies to inrush currents? It says: "7A LRA".
And our portable air conditioner says: "1160W 10.8A 115V". So I assume it was optimized for better power factor?
« Last Edit: June 20, 2016, 11:24:27 AM by wattMaster » Logged

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Re: Mini Fridge Power « Reply #5 on: June 20, 2016, 11:55:55 AM » Author: Medved
Now I understand. So it's really 80 Watts.
And I assume the same applies to inrush currents? It says: "7A LRA".
The inrush current is really treated as the current - so a momentary loading of the wiring. The real transferred power is not important with the inrush, as it's time is so low, the related energy transfer is nearly none at all. But the peak current may become problematic, when it exceeds the fast trigger (designed to act on real short circuits) of the circuit breakers (not from a single fridges, but with multiple of them it may become real pain, mainly when the power gets restored after a longer time, so all start at once). I do not know the US code that well, so I do not know what exactly the "LRA" stands for, but it looks to me like the necessary equivalent rating of the fuse/breaker to not false trip on inrush. But for that you should check with someone more "fluent" in the US code...


And our portable air conditioner says: "1160W 10.8A 115V". So I assume it was optimized for better power factor?

The thing may contain a capacitor for the power factor correction, then the power factor may easily be above 0.9. With induction motors the capacitor uses to be used as well as a phase shift component, making two, shifted phase feed for the motor (so close to a three phase system). It yields better utilization of the winding space, as there is no separate winding used just for the motor start (as with the fridges), but all the windings are used to really handle the power and/or the reactive rotor excitation currents at the same time.

With modern units you may find another way: The mains get rectified (even using an active PFC circuit) and then the main motor is designed as an electronically comutated permanent magnet DC motor. This motor is practically inverted version of a DC motor (permanent magnet on the rotor, windings on the stator, the commutation is not done by brush contacts, but using a solid state switches, usually in the form of a 3-phase bridge). Someone may look at it as a synchronous AC motor fed from an inverter, but because the operating frequency is really designed to follow the rotor movement, the "DC motor" classification is according to me more appropriate. But how it is called does not matter.
What does matter is, the losses are less than 1/3 compare to the induction: It is a DC motor, so the winding handles only the real delivered power, the rotor magnetization ensures a permanent magnet without any power needs. And again, the system designer is free to choose the operating rpm to maximize the overall efficiency and/or size and weight (include the compressor itself; the second is important for the mobile units). Plus compare to a DC motor, the solid state commutator ensures there is no friction, nor arcing and related losses, hence improved efficiency even compare to a real old fashioned DC motor. Plus it is not that much difficult to integrate an efficient power regulation speed control into the inverter (it just just few lines of code more for the control firmware to make the target speed not just a constant).
All these make the refrigeration machinery to consume easily about 20..30% less energy (with the power reduction by changing the rpm instead of cyclic operation the difference is even greater) than with the classical induction motor. Of course, that makes sense for larger power levels, so you will find it mostly in the AC and not as much in residential fridges.
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Re: Mini Fridge Power « Reply #6 on: June 20, 2016, 12:02:02 PM » Author: wattMaster
Now I understand. So it's really 80 Watts.
And I assume the same applies to inrush currents? It says: "7A LRA".
The inrush current is really treated as the current - so a momentary loading of the wiring. The real transferred power is not important with the inrush, as it's time is so low, the related energy transfer is nearly none at all. But the peak current may become problematic, when it exceeds the fast trigger (designed to act on real short circuits) of the circuit breakers (not from a single fridges, but with multiple of them it may become real pain, mainly when the power gets restored after a longer time, so all start at once). I do not know the US code that well, so I do not know what exactly the "LRA" stands for, but it looks to me like the necessary equivalent rating of the fuse/breaker to not false trip on inrush. But for that you should check with someone more "fluent" in the US code...


And our portable air conditioner says: "1160W 10.8A 115V". So I assume it was optimized for better power factor?

The thing may contain a capacitor for the power factor correction, then the power factor may easily be above 0.9. With induction motors the capacitor uses to be used as well as a phase shift component, making two, shifted phase feed for the motor (so close to a three phase system). It yields better utilization of the winding space, as there is no separate winding used just for the motor start (as with the fridges), but all the windings are used to really handle the power and/or the reactive rotor excitation currents at the same time.

With modern units you may find another way: The mains get rectified (even using an active PFC circuit) and then the main motor is designed as an electronically comutated permanent magnet DC motor. This motor is practically inverted version of a DC motor (permanent magnet on the rotor, windings on the stator, the commutation is not done by brush contacts, but using a solid state switches, usually in the form of a 3-phase bridge). Someone may look at it as a synchronous AC motor fed from an inverter, but because the operating frequency is really designed to follow the rotor movement, the "DC motor" classification is according to me more appropriate. But how it is called does not matter.
What does matter is, the losses are less than 1/3 compare to the induction: It is a DC motor, so the winding handles only the real delivered power, the rotor magnetization ensures a permanent magnet without any power needs. And again, the system designer is free to choose the operating rpm to maximize the overall efficiency and/or size and weight (include the compressor itself; the second is important for the mobile units). Plus compare to a DC motor, the solid state commutator ensures there is no friction, nor arcing and related losses, hence improved efficiency even compare to a real old fashioned DC motor. Plus it is not that much difficult to integrate an efficient power regulation speed control into the inverter (it just just few lines of code more for the control firmware to make the target speed not just a constant).
All these make the refrigeration machinery to consume easily about 20..30% less energy (with the power reduction by changing the rpm instead of cyclic operation the difference is even greater) than with the classical induction motor. Of course, that makes sense for larger power levels, so you will find it mostly in the AC and not as much in residential fridges.
LRA means "Locked Rotor Amperage", code for inrush current, or, more scientifically, the amps drawn by the compressor when not running.
And I guess if the compressor uses a capacitor, my Air-Conditioner-On-An-Inverter plan is busted.  >:(
I don't think the portable AC uses a DC motor because the label on the compressor says: "120V 60Hz", and because of the dismal efficiency. (8.6 BTU/W)
Edit: I'm testing the portable air conditioner and the power meter is saying that it is using 850 Watts. Why is this happening? Then it would only be outputting about 7,500 BTUs.
« Last Edit: June 20, 2016, 12:59:02 PM by wattMaster » Logged

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Re: Mini Fridge Power « Reply #7 on: June 20, 2016, 05:43:35 PM » Author: Ash
Are "traditional" (brush) DC motors in the 10's Watt range permanent magnet ? If no then there are additionally the excitation (stator) coil current that all goes to losses
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Re: Mini Fridge Power « Reply #8 on: June 20, 2016, 05:47:50 PM » Author: wattMaster
Are "traditional" (brush) DC motors in the 10's Watt range permanent magnet ? If no then there are additionally the excitation (stator) coil current that all goes to losses
I have absolutely no idea, best to buy a lot and take them apart.
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Re: Mini Fridge Power « Reply #9 on: June 21, 2016, 02:09:37 AM » Author: Medved
Are "traditional" (brush) DC motors in the 10's Watt range permanent magnet ? If no then there are additionally the excitation (stator) coil current that all goes to losses

Up to 100W, at least since WW2 all real duty motors (and dynamos; so not the toys) use only permanent magnets. The exceptions are just the types, where the stator field should be made variable for some reason (starters require high steady torque to overcome the compression, but then rather high rpm to reach required engine rpm for starting; dynamos need the output voltage to be possible to regulate,...).
These days, with the availability of the rare earth magnet materials, there is practically no high power border - TGV trains use permanent magnet traction motors (in the brushless form, but that makes no difference) and there we are talking about MW range. The only limitation is the ability to manage the huge forces of such strong magnets when the motor is being assembled (with the TGV, the motor assembling is one of the most guarded secrets).

Even many AC motors, where the inertia could be made really low (e.g. home appliance pumps,...) use permanent magnet synchronous motors on just AC as well - even with the required reserve for starting (within 1/2 of the mains cycle it has to reach full rpm, otherwise it won't start at all), it means cutting the losses and cost very significantly (a ferrite cylinder is easier to make than a caged rotor, plus it may be submerged in the dirty water without any corrosion consequences; that means eliminating all the shaft seals and all such problems)
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