volt-amps vs watts

[vavet]

Why are certain things rated as or list requirements for volt-amps instead of watts?
A Watt is, by definition, a Volt-Amp. There has to be some nuance the author is trying to communicate, right?

 

[Bert_]

A watt is only equal to a volt amp in a purely resistive load. A volt amp is simply the current x voltage. A watt is the actual power used.

In anything that is capacitive or inductive like a motor for instance the VA will be higher than the Watts. The relationship between VA and Watts is expressed as power factor.

 

[vavet]

A watt is only equal to a volt amp in a purely resistive load. A volt amp is simply the current x voltage. A watt is the actual power used.

In anything that is capacitive or inductive like a motor for instance the VA will be higher than the Watts. The relationship between VA and Watts is expressed as power factor.

Thank you for the clear and concise answer. Are you new here?

 

[theoldwizard1]

A watt is only equal to a volt amp in a purely resistive load. A volt amp is simply the current x voltage. A watt is the actual power used.

For DC circuits, volts x amps = watts.

 

[Fasthotrod]

Thank you for the clear and concise answer. Are you new here?

Sometimes we get a little too in-depth and lose people... it's not intentional, really. We just geek out on the details when someone asks about something we know/like.

Here's an example of me doing just that regarding Power Factor:

https://www.garagejournal.com/forum/showpost.php?p=7337308&postcount=16

Some of the pictures I referenced in my post have broken links so it's not going to visually express what I was trying to say in my post.

My apologies in advance if you decide to read it.

 

[wyliesdiesels]

Thank you for the clear and concise answer. Are you new here?

Why do you ask that?

He's been here almost 3yrs

 

[vavet]

Why do you ask that?

He's been here almost 3yrs

My demented and dry sense of humor didn’t come through.
The first part of my response was sincere.
The second part was more of a jab at all of us for getting carried away on our answers.

 

[MBfreak]

An attempt to give an answer to the OPs question without delving into physics and math. ( which I am infamous for, but will resist here)

DC and AC deliver real ( active ) power the same way but due to the nature of AC a correction factor ( often known as cos fi) is needed.

DC Watts = DC Amps* DC Volts-

AC Watts = AC Amps*AC Volts*cos fi.


For a heating element on AC, cos fi is VERY close to 1,00
For a typical 3phase squirrel cage motor at rated load cos fi is around 0,85.

I am sure there are very well made graphic tutorials on UT.

Best regards

Ola

 

[Innovate1]

An attempt to give an answer to the OPs question without delving into physics and math. ( which I am infamous for, but will resist here)

DC and AC deliver real ( active ) power the same way but due to the nature of AC a correction factor ( often known as cos fi) is needed.

DC Watts = DC Amps* DC Volts-

AC Watts = AC Amps*AC Volts*cos fi.


For a heating element on AC, cos fi is VERY close to 1,00
For a typical 3phase squirrel cage motor at rated load cos fi is around 0,85.

I am sure there are very well made graphic tutorials on UT.

Best regards

Ola

And for non-linear loads there is "distortion factor" because the current has a phase shift (slight) AND isn't sinusoidal. Typical example is variable speed motor drives which could have a power factor 0f 0.6 to 0.7. The current and voltage line up almost exactly - it is all distortion rather than displacement. The term "power factor" covers both distortion and displacement (phase shift).

 

[Innovate1]

Thank you for the clear and concise answer. Are you new here?

Too funny! I got it right away but still had to add my complex answer...

 

[joel63]

So what does 40 va on a 240 AC control transformer mean?

 

[Stuart in MN]

So what does 40 va on a 240 AC control transformer mean?

It means the transformer is capable of providing 40VA of power at 240VAC, but I'm not sure if that was your question.

 

[joel63]

It means the transformer is capable of providing 40VA of power at 240VAC, but I'm not sure if that was your question.

Does that number have anything to do with the secondary side?

 

[mm08822]

Does that number have anything to do with the secondary side?

Yes, both sides actually.

If the transformer was loaded to rated capacity (40 va), the primary side (240v) would have a current of 1/6 amp. (40va / 240v).

If you know the secondary side voltage, you can also calculate the rated load current it too can support.

e.g. - if 24v, then current would be 10/6 amp. (40va / 24v).

 

[ard]

So what does 40 va on a 240 AC control transformer mean?

a current draw of 0.1667 amps?


edit: 240V x 0.1667A = 40 VA

 

[MikeF2316]

Nobody has mentioned that the number for volt-amps will always be equal or higher than watts. So your 600 VA device may be less powerful than an equivalent 550 watt device.

MBfreak implied that the volt-amp number will be higher by including cosine in his formula, which we all remember from grade 11 has a maximum of 1 at an angle of 0°.

 

[checkthisout]

Out of curiosity, if an a/c 3 phase motor can only achieve 85% efficiency, why not rectify the power?

Is rectification so expensive that it would offset the savings, even in large applications?

Or,

Would the rectification process itself also consume almost as much power?

Or?

 

[SGKent]

a 120V AC single phase wave starts at -0- volts rises to 120V then falls back to -0-. A 240 V AC wave starts at -0- volts rises to 120V say above neutral then falls to -0-, then goes to 120V below neutral then rises back to -0-. Since the actual voltage is not constant, there is a correction factor to compensate for the rise and fall of the voltage. The industry using the devices attached to the AC may use different correction factors or methods of calculating it. Likewise the actual amperage being sent thru the wire is rising and falling too. It is governed differently by say resistance than capacitance or magnetism. Hence a resistance load will respond differently than a transformer (magnetism involved) or motor - inductive or magnetic.

For most lay quick calculations in AC or DC, the volts x amps will give you the approximate wattage.

 

[joel63]

Yes, both sides actually.

If the transformer was loaded to rated capacity (40 va), the primary side (240v) would have a current of 1/6 amp. (40va / 240v).

If you know the secondary side voltage, you can also calculate the rated load current it too can support.

e.g. - if 24v, then current would be 10/6 amp. (40va / 24v).

Thank you MM, that's what I was looking for.

 

[MattT]

Out of curiosity, if an a/c 3 phase motor can only achieve 85% efficiency, why not rectify the power?

Is rectification so expensive that it would offset the savings, even in large applications?

Or,

Would the rectification process itself also consume almost as much power?

Or?

First off I don't know what a dc motors efficiency is but I guarantee you it isn't 100% efficient. Might actually be worse than AC 3 phase judging by how hot the motors get. And yes there are rectification losses too.

But the main reason DC has only been used for variable speed applications is reliability. Back in the day DC was the cheap and easy way to get variable speed but the motors are nowhere near as reliable. Now VFDs are cheap DC motors are pretty much obsolete in industrial applications.

Even battery powered applications like tools are moving away from DC motors. DC brushless is actually a crude 3 phase motor.

 

[Innovate1]

First off I don't know what a dc motors efficiency is but I guarantee you it isn't 100% efficient. Might actually be worse than AC 3 phase judging by how hot the motors get. And yes there are rectification losses too.

But the main reason DC has only been used for variable speed applications is reliability. Back in the day DC was the cheap and easy way to get variable speed but the motors are nowhere near as reliable. Now VFDs are cheap DC motors are pretty much obsolete in industrial applications.

Even battery powered applications like tools are moving away from DC motors. DC brushless is actually a crude 3 phase motor.

Nothing crude about them - although they could be cheaply made. The are often premium 3 phase motors. But they are synchronous and you can't just run them on 60 Hz power.

 

[dogdog]

dam feels like I am back in college again

 

[Citation]

a 120V AC single phase wave starts at -0- volts rises to 120V then falls back to -0-. A 240 V AC wave starts at -0- volts rises to 120V say above neutral then falls to -0-, then goes to 120V below neutral then rises back to -0-. Since the actual voltage is not constant, there is a correction factor to compensate for the rise and fall of the voltage. The industry using the devices attached to the AC may use different correction factors or methods of calculating it. Likewise the actual amperage being sent thru the wire is rising and falling too. It is governed differently by say resistance than capacitance or magnetism. Hence a resistance load will respond differently than a transformer (magnetism involved) or motor - inductive or magnetic.

For most lay quick calculations in AC or DC, the volts x amps will give you the approximate wattage.

120V is the RMS average. The voltage actually rises to something like 170V peak.

 

[MBfreak]

Cost of power in 60/50 Hz residential or small business networks is measured by the supplier/seller in WattHours ( usually kWh )


The VAh rating in an AC network is V*A*h and the Wh rating is V*A*cosfi*h.
Multiply both with sqrt3 for 3 phase .
It is the Wh that is metered and what you pay for.
The VAh-Wh differential results in some load losses in the supply system and can be reduced by installing phase compensation capacitors, which very few utilities require from residential and/or small business customers.

For those of you that want to dig deeper , start by reading up on the interesting concept of the square root of a negative number ( j in electrogeekish) . Once mastered , AC power is easily understood.
But 99,999999 % of us will do very well without that.

ola

 

[checkthisout]

First off I don't know what a dc motors efficiency is but I guarantee you it isn't 100% efficient. Might actually be worse than AC 3 phase judging by how hot the motors get. And yes there are rectification losses too.

But the main reason DC has only been used for variable speed applications is reliability. Back in the day DC was the cheap and easy way to get variable speed but the motors are nowhere near as reliable. Now VFDs are cheap DC motors are pretty much obsolete in industrial applications.

Even battery powered applications like tools are moving away from DC motors. DC brushless is actually a crude 3 phase motor.

Yes, I know they aren't 100%. They dont have a power factor rating though.

A/C motors waste power because the counter emf is out of phase with the incoming power. The motor makes more waste heat per unit of work done than a dc motor.