amps @ 12 volt question

[The Cobbler]

I'm a bit rusty on electric formulas.
I have a friend that has a motor home, he says the fridge is drawing 35 amps at 12 volts thru the inverter . he thinks that is high.
I tell him that's like 3.5 amps at 120 volts, I don't think that's high for a fridge .
he says I'm wrong.

 

[Bert_]

I'm a bit rusty on electric formulas.
I have a friend that has a motor home, he says the fridge is drawing 35 amps at 12 volts thru the inverter . he thinks that is high.
I tell him that's like 3.5 amps at 120 volts, I don't think that's high for a fridge .
he says I'm wrong.

Your math is right on. Watts is Watts. 35 x 12 = 420w. 3.5 x 120 = 420w

Add in a little inefficiency from the inverter and the fridge is under 400w

 

[Kevin Essiambre]

12 volts is a wonderful number to compare to 120 volts.

Easy way that I do it is to move the decimal point.

10 amps at 120 volts = 100 amps at 12 volts

Of course, the proper way is to convert to watts.

Sent from my SM-G920W8 using Tapatalk

 

[Lightning rod]

You are correct sir.

What does the nameplate rating say the rated current is supposed to be?

 

[The Cobbler]

You are correct sir.

What does the nameplate rating say the rated current is supposed to be?

I asked him but he said he can't find a nameplate. he has a service guy coming tomorrow to test his fridge. 'I have done what I can do
thanks all

 

[SGKent]

the question on an inverter are the amps he is quoting going to the inverter or leaving the inverter. There is an efficiency loss. 3.5 amps going into the inverter at 12V might be 3 amps at 120V coming out. But the general math is correct as others have stated. A hair dryer is like 1500 watts or 12.5 amps so 2.9 to 3.5 doesn't seem all that high unless it is something like ammonia based and uses a candlepower or so to drive it. Come to think of it I have to wonder if the real issue is that he is running his batteries down supporting it when the RV is off shore power.

 

[checkthisout]

Inverter means the fridge is running on 120 off an inverter drawing off the battery.

Those heating elements are about 350 watts so 35 amps is about right.

Why is he running it that way? Is he without hookups and doesn't want to use propane?

 

[theoldwizard1]

I'm a bit rusty on electric formulas.
I have a friend that has a motor home, he says the fridge is drawing 35 amps at 12 volts thru the inverter . he thinks that is high.

There are some losses in the inverter, probably less than 10%. I hope he is using a pure sine wave inverter. The modified sine wave inverters are even LESS efficient when driving motors.

Segue - True DC compressor refrigerators and air conditioner have been around for several years now. It took me awhile to figure out "how do they do that" !

DC motors are not very efficient. Nature if the beast, plus the arcing of the commutator is not a great idea. Single phase AC are better because there are only slip rings, no arcing. But as the voltages moves away from the peak the power goes down. as it crosses from positive to negative it is only the momentum of the armature that keeps it spinning. This is why 3 phase is so much more efficient as the overlapping of the phases fills in this power "gap".

So those DC refrigeration compressors actually have a small dc-to-3-phase inverter ! They may of may or may not have variable frequency control and I will bet the motors are wound for less than 120V per phase, but the concepts are all the same !

 

[MikeF2316]

There are some losses in the inverter, probably less than 10%. I hope he is using a pure sine wave inverter. The modified sine wave inverters are even LESS efficient when driving motors.

Segue - True DC compressor refrigerators and air conditioner have been around for several years now. It took me awhile to figure out "how do they do that" !

DC motors are not very efficient. Nature if the beast, plus the arcing of the commutator is not a great idea. Single phase AC are better because there are only slip rings, no arcing. But as the voltages moves away from the peak the power goes down. as it crosses from positive to negative it is only the momentum of the armature that keeps it spinning. This is why 3 phase is so much more efficient as the overlapping of the phases fills in this power "gap".

So those DC refrigeration compressors actually have a small dc-to-3-phase inverter ! They may of may or may not have variable frequency control and I will bet the motors are wound for less than 120V per phase, but the concepts are all the same !

For those that don't know, this is exactly how an electric car works too! Except we know that electric cars do have variable frequency control, and are wound for higher voltage.

 

[Dagny]

What ac motors have slip rings?

 

[Lightning rod]

Most AC motors are squirrel cage (permanently shorted rotor windings- low start up torque) but there is a type Of AC motor that has a wound rotor. You can vary the start up torque by inserting resistance into the slip ring circuit and get better start up but the rings are shorted at full speed usually depending on application.

 

[pancho400cid]

What ac motors have slip rings?

I don't know much about small single phase "appliance type" AC motors. In the three-phase industrial motor world, by far, most AC motors are induction motors and don't have slip rings

Lighting Rod is correct - wound-rotor induction motors (WRIM ) have slip rings that let external resistance get added to the rotor circuit to modify starting torque.

Some synchronous motors also have slip rings so DC power from an external source can be applied to the rotating field circuit of the rotor.