Please explain transformer ratings

Transformer ratings are independent of the load characteristics. VA (the product of volts x amps) is a unit of apparent power in alternating current circuits. For a transformer, it represents the output that can be delivered for the time specified at rated secondary voltage and frequency without exceeding the specified temperature limitations.

Watts (KW, MW, etc.) is a measure of true power being consumed by an alternating current load.

The difference between apparent and true power is the load power factor.

AC power has two components - a real and an imaginary part. The two parts are related by the phase angle of the current with respect to the voltage.

The real part is measured in watts, and is (almost) always less than the total power. Total power is measured in volt-amperes, VA.

So, if for example you wanted 10 KW out of your transformer, you might need 12KVA to pass through it to get your 10KW, so you want to rate the transformer at 12KVA, not 10KW.

Its really not very complicated even tho it might seem so. Basic Ohm's law is the actual, tho hidden, key. The windings of the transformer generate heat from the current going thru the wire's inherent resistance in both the primary and secondary windings. The copper really doesn't care what the power factor is. The losses in the iron core don't care what the power factor is. The physical transformer can only dissipate so much heat without damage to the winding insulation. So my current rating is roughly determined by how much heat can be dissipated in the physical arrangement, and the operating voltage rating is, in general, determined by how much generated magnetic field is made before I saturate the iron core of the unit and then seriously lower its impedance. You can easily demonstrate this: I can connect a large AC capacitor (with a clamp-on ammeter around one of the leads) to the secondary with a wattmeter measuring the primary input power and the wattmeter will just show the energy being dissipated in the copper losses, iron losses and insulation ac losses while the ammeter will show up to rated secondary current. I can be at the rated kVA of the transformer but no power is being transfered thru the transformer except for the losses in the capacitor.

The transformer rating in kVA accounts for the real power or useful power or resistive power(kW) and the reactive or imaginary power (kVAR) of the transfomer plus all other losses (hyteresis,core loss,etc). Hence the useful energy derived is less than what the commercial rating is. Nevertheless, i believe that they are rated as such to show that these other phenomena exists within the transformer.

The rating of a transformer is the the actual maximum rated load which can be connected to the Transformer safely with out damaging the same (over heating of winding) as per the design parameters and specification. The connected load ( KW ) depend on the user like weather it is inductive load ,capacitive load or resistive load .So the manufacturer cannot predetermine the load conditions and the KW requirement depends on these parameters. So The designer of the Transformer designs the capacity as VA (KVA ) rating which is independent of load conditions.The user has to select the rating depends on his actual load and operating power factor .

Simply the term of transformer is considered as source of power supply stated in VA, while others downstream as loads stated in Watt or VAR. But power generation station also stated its capacity in MW or GW, means that those power generation able to supply loads in such MW as real power, assumed that terms of apparent power, real power, and reactive power, have been well-known.

KVA and MVA are measures of the capacity to transform power between voltages. KW and MW are measures of heat dissipation. The continuous heat dissipation of the transformer in KW or MW will be considerably less than the KVA or MVA rating, otherwise the transformer is faulty and possibly not long for this world.

By using KVA or MVA as a measure of capacity, the ability of the transformer to cope with loads with a less-than-unity power factor can be taken into account in the system design (power factor is a measure of the difference in phase between the applied voltage and the current taken by an AC load that is not purely resistive).

Hi

Sorry to change the conversation slightly but I came across this forum question while googling and I wonder if anyone can help me.

I have just purchased a 110 volt circular saw which connects to a special mobile dust extractor now on the details of the extractor it said maximum appliance connected load 2400w the power consumption of the vac is 400 - 1200w variable and the consumption of the saw is 400-1200w also. Where it all goes funny is when you look in the instruction book and it says max connected load 500watt (for 110v).

Also it has a sticker on the socket saying this also.

Now I am running this from a 3.3kva 110v transformer and I have spoken to someone on the phone at the manufacturer and they said ignore the 500watt warning and just use it.

Can anyone work out the amperage and so on for this little set up and and tell me if they are right that I can just ignore the warning.

I have divided watts by volts etc and it comes out at high amps but I am only a carpenter so electrical engineering is not my thing maybe someone can help thanks and sorry for diverting the conversation slightly.

Anyone ????????