Transformer and Motors

This is an off-asked question and there are threads somewhere that would help.

But the short answer is that the function of a motor is to convert electrical energy to mechanical work, so horsepower is the right measure. The function of a transformer is to maintain electrical power at (almost) a constant value while changing the ratio of voltage and current, so VA is the right measure.

Another way to think about it is, that for a given motor type such as single phase induction, you can actually tell the differences in horsepower by weight. For a given transformer type, say 60 Hz power, you can tell the differences in VA by weight. {BTW, this is an idea you might actually have to physically try before you accept it}

<.....for a given motor type such as single phase induction.....tell the differences in horsepower by weight. For a given transformer type...tell the differences in VA by weight....>

Wow! That would be a handy thing to have tucked away! Is there a chart or charts showing these relationships, please?

Just for a laugh (as if I didn't have enough to do!) I looked up some control panel toroidal transformers (Nuvotem, from RS). There's the plot:

[CR4 is so dam'd addictive!]

I used to have one for the different lamination types (EE, EI, etc) for 60 Hz single phase transformers, but I've long since lost it. I've never seen one for motors, but I've observed that it generally holds.

Transformers supply, motors work.

Transformers supply volts and amps, so VA.

Motors use volts and amps, but they also affect the supply by making them lag each other in time, which is what we call "Power Factor". So the power motors use is measured as a function of all 3 issues, volts x amps x power factor and that new value can no longer be expressed simply as "VA and something else", so it is called... watts!

The motor's rating is kW/W/HP because usually you are interested in the mechanical power of a particular motor, so the output of a motor is mechanical whereas a transformer has electrical power output. Characteristic to the electrical power is the aparent power, measured in VA/kVA/MVA..this includes the active power measured in W/kW and another component - the reactive power measured in VAr/kVAr. The reactive power is used by transformers and motors to magnetise the iron core for the induction process to take place.

Generators are also rated in KVA for reasons mentioned in some of the replies.

Maybe you refer to small generators because large generators(eg: turbogenerators ) are rated in kW/MW. This is because it is powered by a turbine whose power is given in kW/MW, so is about mechanical power,

you are right.Yes large generators are rated in MW or kW depending upon the size .even in DG sets 10MW sets are known though it is not uncommon to see %000kva , 2000kva geneartors