DC Power vs AC Power

[bendesa]

Hi all,

Is there a difference between DC Power and AC Power?

For example, a DC appliance of 1200 Watt is that the same power as an AC appliance of 1200 Watt?

Why am I asking?

I've seen an DC Freezer 12/24 volt but 1200Watt. They claim it can be used in Off Grid situations.
But wowwww 1200 Watt for a freezer isn't that huge ?

I see AC Freezer from 300 to 700Watt

Why should you use an 1200 Watt 12V Freezer?

Thx

Regards

Ben

 

[ggunn]

Hi all,

Is there a difference between DC Power and AC Power?

For example, a DC appliance of 1200 Watt is that the same power as an AC appliance of 1200 Watt?

Why am I asking?

I've seen an DC Freezer 12/24 volt but 1200Watt. They claim it can be used in Off Grid situations.
But wowwww 1200 Watt for a freezer isn't that huge ?

I see AC Freezer from 300 to 700Watt

Why should you use an 1200 Watt 12V Freezer?

Thx

Regards

Ben

There is no difference in AC vs DC power calculation; V = IR and P = IV. 1200W at 12V is 100A, and yes, that would be huge. Are you sure those numbers are correct?

 

[bendesa]

There is no difference in AC vs DC power calculation; V = IR and P = IV. 1200W at 12V is 100A, and yes, that would be huge. Are you sure those numbers are correct?

Hi Ggunn,

Thanks for your reply.

I think it's a no go

Regards


Ben

 

[bendesa]

Hi Ggunn,

Thanks for your reply.

I think it's a no go

Regards


Ben

Yes the numbers are correct according to the supplier. I checked with them because I also thought that can't be correct.

 

[jaggedben]

There is no difference in AC vs DC power calculation; V = IR and P = IV. 1200W at 12V is 100A, and yes, that would be huge. Are you sure those numbers are correct?

Well, there's power factor with AC.

 

[ggunn]

Well, there's power factor with AC.

Of course, but I am pretty sure that PF is beyond the scope of his question.

 

[curt swartz]

Are you sure the 1,200 is not Watt Hours per Day (24 Hrs)? Many DC applainces give the watt hours to calculate battery storage needed.

 

[Strathead]

Are you sure the 1,200 is not Watt Hours per Day (24 Hrs)? Many DC applainces give the watt hours to calculate battery storage needed.

good question!

 

[ggunn]

Are you sure the 1,200 is not Watt Hours per Day (24 Hrs)? Many DC applainces give the watt hours to calculate battery storage needed.

That would make more sense than a 12VDC refrigerator that needs 100A to run. At a 50% duty cycle that would be, what, 14.4 kWh/day?

 

[kwired]

Are you sure the 1,200 is not Watt Hours per Day (24 Hrs)? Many DC applainces give the watt hours to calculate battery storage needed.

1200 watts is kind of high for typical household refrigerator as well

 

[kwired]

Why should you use an 1200 Watt 12V Freezer?

because AC isn't readily available like when off grid or in a RV or similar circumstances.

And yes 1200 watts is high for household types fridge/freezer but is kind of the low end on commercial and industrial units with any larger storage capacity.

 

[Besoeker3]

1200 watts is kind of high for typical household refrigerator as well

And intermittent.

 

[Carultch]

Hi all,

Is there a difference between DC Power and AC Power?

For example, a DC appliance of 1200 Watt is that the same power as an AC appliance of 1200 Watt?

Why am I asking?

When you see the nominal ratings for AC Power, it is either an average, or a special kind of averaging called RMS (I'll get back to that one), that enables us to use the same calculations as we do for DC. As far as user experience is concerned, since you don't experience individual grid frequency cycles, average power is ultimately what you care about. The power will come in pulses, that are as low as 0W and as high as 2400W, and on average, it will draw its nominal amount of power. We're talking 120 Hz cycles of AC power for our 60 Hz AC grid in the USA. The power is constant for a 3-phase load, but for a single phase load, it will pulsate.

The nominal ratings for power is the average power, while the nominal ratings for voltage and current are RMS values (root mean square). For our nominal 120V household power, it is really a waveform that is as high as 170V, and as low as -170V. The ratio of 170V to 120V in theory, is the square root of 2. I can share this Geogebra file that demonstrates this concept. Adjust A to change the voltage amplitude, and R to set the resistance of the load.

RMS Voltage Concept

RMS Voltage Concept

www.geogebra.org

The reason we do this, is so that P=I*V can still work for single phase AC as it does for DC. The peak values of I and V are both scaled down by sqrt(2) to get RMS values, so that they multiply to average power.

 

[kwired]

When you see the nominal ratings for AC Power, it is either an average, or a special kind of averaging called RMS (I'll get back to that one), that enables us to use the same calculations as we do for DC. As far as user experience is concerned, since you don't experience individual grid frequency cycles, average power is ultimately what you care about. The power will come in pulses, that are as low as 0W and as high as 2400W, and on average, it will draw its nominal amount of power. We're talking 120 Hz cycles of AC power for our 60 Hz AC grid in the USA. The power is constant for a 3-phase load, but for a single phase load, it will pulsate.

The nominal ratings for power is the average power, while the nominal ratings for voltage and current are RMS values (root mean square). For our nominal 120V household power, it is really a waveform that is as high as 170V, and as low as -170V. The ratio of 170V to 120V in theory, is the square root of 2. I can share this Geogebra file that demonstrates this concept. Adjust A to change the voltage amplitude, and R to set the resistance of the load.

RMS Voltage Concept

RMS Voltage Concept

www.geogebra.org

The reason we do this, is so that P=I*V can still work for single phase AC as it does for DC. The peak values of I and V are both scaled down by sqrt(2) to get RMS values, so that they multiply to average power.

Can you explain that for us?

Isn't power the work being done? When expressing it for electrical power it is V x A and if balanced three phase it is V x A x square root of 3.

Voltage and current will cycle at whatever the frequency is but a 1000 watt load is a 1000 watt load to the prime mover of the source regardless what volts, amps, frequency or even if DC current is used to transmit that power to the load.

 

[Carultch]

Can you explain that for us?

Isn't power the work being done? When expressing it for electrical power it is V x A and if balanced three phase it is V x A x square root of 3.

Voltage and current will cycle at whatever the frequency is but a 1000 watt load is a 1000 watt load to the prime mover of the source regardless what volts, amps, frequency or even if DC current is used to transmit that power to the load.

We think in terms of the RMS voltage and RMS current all the time, because that's how it matters the most for an application. But, if you look at it in terms of waveform amplitude, you'll see different formulas apply. RMS is a special kind of time averaging, that allows our AC formulas to look just like DC formulas.

On an instantaneous basis, the first principles equation is power = current * voltage, and this is true, regardless of the format of the power. For steady state DC, we don't need to think about the difference between instantaneous and average, because the voltage and current are constant. Calculus turns into simple Algebra.

For AC, both current and voltage are sine wave functions of time. Given voltage V(t) = Vmax*sin(w*t), a pure resistive load's current that follows this voltage will be I(t) = Imax*sin(w*t), where the current amplitude Imax = Vmax/R. The w is a term that directly relates to the frequency (f), via w=2*pi*f, since sine functions use radians. Multiply these together, for instantaneous power, P(t)=Vmax*Imax*sin(w*t)^2. Since the maximum of sin(w*t)^2 is 1, this shows us that Pmax = Vmax*Imax.

Take the average value of the function of sin(w*t)^2, over a full cycle, and get 1/2. So the average power, Pavg = 1/2*Vmax*Imax. In order to define RMS values, we distribute the 1/2 factor equally to both current and voltage, which means dividing both terms by sqrt(2). Such that Vrms = Vmax/sqrt(2), and Irms = Imax/sqrt(2).

 

[Carultch]

and if balanced three phase it is V x A x square root of 3.

For three phase, you have 3 voltage functions of time, that are phase shifted from each other by 2*pi/3 radians. In a clockwise phase sequence, the phase-to-neutral voltages look like this. Vmax = sqrt(2)*Vpn, where Vpn is the nominal phase-to-neutral voltage like 120V or 277V.
Va = Vmax*sin(w*t)
Vb = Vmax*sin(w*t - 2*pi/3)
Vc = Vmax*sin(w*t + 2*pi/3)

A simple load of 3 resistances in a Y-formation, gives us currents to each resistor of:
Ia = Imax*sin(w*t)
Ib = Imax*sin(w*t - 2*pi/3)
Ic = Imax*sin(w*t + 2*pi/3)

Multiply corresponding voltages and currents, and add these up to find the total power. Factor out Vmax*Imax, common to each term:
P = Va*Ia + Vb*Ib + Vc*Ic
P = Vmax*Imax*(sin(w*t)^2 + sin(w*t - 2*pi/3)^2 + sin(w*t +2*pi/3)^2)

Graph that mess of sine terms, and you'll see a steady line equal to 1.5. With trig identities, you can prove it is exactly a constant of 1.5. So this gives us three phase power as:
P = Vmax*Imax*1.5

Recall that Vmax = sqrt(2)*Vpn, and Imax = sqrt(2)*Inom, and after substituting & simplifying, we get the following for three phase power:
P = 3*Vpn*Inom

If we want phase-to-phase voltages, which equal Vpn*sqrt(3), multiply by 1 in a fancy way, to generate Vpn*sqrt(3) to replace with Vpp:
P = 3*Vpn*sqrt(3)/sqrt(3) * Inom
P = 3/sqrt(3) * Vpp*Inom
P = sqrt(3)*Vpp*Inom

 

[bendesa]

Hi all,

Is there a difference between DC Power and AC Power?

For example, a DC appliance of 1200 Watt is that the same power as an AC appliance of 1200 Watt?

Why am I asking?

I've seen an DC Freezer 12/24 volt but 1200Watt. They claim it can be used in Off Grid situations.
But wowwww 1200 Watt for a freezer isn't that huge ?

I see AC Freezer from 300 to 700Watt

Why should you use an 1200 Watt 12V Freezer?

Thx

Regards

Ben

I just don't understand the specs of this product. They claim its 1200 watt but they also claim it will run for 20 hours on a 12v 100ah battery. How is that possible?

 

[Carultch]

I just don't understand the specs of this product. They claim its 1200 watt but they also claim it will run for 20 hours on a 12v 100ah battery. How is that possible?

It's probably not running at its full load continuously. But rather, it is controlled by a thermostat to cycle on and off.

The 1200 Watts refers to the power it consumes during its on-time, but it would have to average at no greater than 60 Watts to run on a 12V / 100A-hr battery for 20 hours. This would mean it is only predicted to cycle-on for 5% of the time. I'm skeptical that this number is realistic.

The environmental temperature will play a roll in its on-time as well, for a multitude of reasons. The greater the temperature across its walls, the more cooling power you will need, and the more warm air you will let in when you open the door. Refrigeration cycles are are also less efficient, the higher the temperature of the outside environment where it has to dump its heat.

 

[Besoeker3]

Just my take.
My field was industrial variable speed drives and many of those were in paper mills. DC drives for the most part. To that extent I was quite conversant with quite large direct current systems. For me it was the flexibility that had the merits of these systems.

 

[wwhitney]

The 1200 Watts refers to the power it consumes during its on-time

Presumably it refers to the maximum power it consumes. Maybe it's large due to motor startup current?

Cheers, Wayne