Rating Of Power Machines

I never looked at it that way because 1KW=1KVA, since W=V*A. All motors have an inductive component and a real work (resistive?) component. Together they have the effect of reducing the power factor (PF less than 1.0). The apparent power is the voltage times the current without regard to phase shift effects. That is what the generator has to produce. The motor can not take more than the generator is capable of producing.

So, if a lot of current flows due to the inductance, then it requires large conductors but it produces no work. Only the portion of current that is in phase with the voltage is capable of producing a force that can be used for some purpose.

The terms are used interchangeably. That is to say that either KW or KVA describes capacity for useful power. You must also include power factor to describe the whole arrangement, except for DC devices for which, generally speaking, PF=1 at all times.

Motors deliver mechanical power as their output. So they are specified by this, which is kW or HP. The power they draw is not at unity PF, so the input kVA is higher than the output kW by factors of PF and efficiency.

When one makes and sells a transformer, one does not know what load will be put on it. Could be motors(PF=0.8) , resistance furnaces (PF=1), capacitors(PF=0)...etc. The load will determine the PF. So,the tfr is rated in kVA. If you use it to supply power to a resistance furnace or incandescent lights, PF=1. so kVA=kW. But not with any other load.

Generator and motor are the extreme end power system components i.e. one at the most upstream component and the second is the most downstream component.

In case of motor, mechanical loads are driven by it. And mechanical loads are rated in KW. Therefore motor is rated at KW.

To visualise the demand and supply equality it is also necessary to speek the generator capacity in KW and that the reson generator is rated at KW.

Transformer is the is the intermediate device and we do not know what type of load we will feed through it and therefore it is rated as KVA.

This matter was discussed in detail a year or so back. There will be a lot of information in that thread. Take the effort to check the previous posts. in case you are not aware there is, on the right hand side, one box for "search this forum"

(BTW that applies to a lot of questions being asked again and again eg the multiples of 11KV too)

For motors, on their output side one is interested in the mechanical power (hp or kw) produced. Hence motors are rated in hp/kw.

On the electrical side, both power out (hp/kw) and electrical input (volts x amps = va or kva) are important. kw = kva x cos φ. [i.e, kw and kva are different by the factor of cos φ, which is called "power factor."]

A typical generator will have both a kw and a kva rating, commonly differing by a power factor of 0.8. For example, 80 kw and 100 kva. Both ratings need to be satisfied.

WHY ELECTRIC MOTORS ARE RATED IN HORSE POWER?

Introduction: As electrical professionals, many of us would have been handling electric motors in hundreds of numbers over the years in our careers. And, whatever the SI System of units might suggest, many of us would still prefer to refer to electric motors' power rating in horse power only rather than in watts or kilowatts as is prescribed in the SI System of units. But, how many of us know why this is so? Why electric motors are rated in horsepower? Why not in Elephant Power, as somebody asked this author?

The story: One would be surprised to note that the rating of electric motors in horsepower has nothing to do with any electrical professional in the first place. It all started with one James Watt. Recall him? He is a Scottish engineer, associated with the Steam Engine. In most of the children's general knowledge books, James Watt is wrongly credited with the invention of the Steam Engine. That credit goes to one Thomas Newcomen of England, who, in fact, had invented the Steam engine in 1705. James Watt only made improvements to the Newcomen Engine, to improve its efficiency and to make it commercially viable. This was in the year 1769.

Having made this improvement, Watt started manufacturing these engines, in a partnership with a businessman called Matthew Boulton and started looking for markets for his engine. Remember! It was the 18^th century. And the industrial revolution was in its primitive stages in England. The main profession of world's population was only agriculture. The only customers, to whom Watt could sell his engine, were farmers. Watt started talking to farmers, with a view to market his engine to them. They wanted to know what his engine could do for them. Watt knew that an engine is a device that could do some work. What work that a farmer is more interested in? That of drawing water from a deep well and of irrigating his fields.

Farmers in India were (and are) using bulls and oxen for the purpose. But, in Europe and in England, farmers were using horses for the job. Hence, Watt pointed out to the farmers that his engine could do the work of the horses - that of drawing water from the deep well. But the farmers wanted to know how many horses that his engine would replace. Watt did not have an answer. For, he only knew what his engine could do and not what a horse could do. But, without answering this, the farmer wouldn't buy his engine.

So, he bought a healthy horse and wanted to find out what is the power of one horse. What is power? Power is the rate of doing work. Isn't it? What is work? Work is Mass x Displacement. Watt tied a weight to the horse that he bought and chased it to move. He found out that the horse was capable of doing about 550 foot-pounds (lb-ft) of work in one second. That is, it could move 550 pounds of weight over a distance of one foot in one second or 55 pounds of weight over a distance of 10 feet in one second.

But, for his engine, one second is too early to calculate. So, he converted the power of the horse into work done per minute. That is 550 lb-ft/sec. x 60 seconds (i.e.) 33000 foot-pounds per minute. That, in fact, is the power of one horse or one HP. When he tied the same weight to his engine and measured the work done in one minute, he could compare it with the power of the horse now. If his engine moved 66000 foot-pounds in one minute it was having the power of two horses or 2HP and so on. Now, he had an answer to the farmer.

Later, when steam engines were replaced by electric motors as prime movers, the custom of rating them in horsepower continued.

Subsequently, as a mark of respect to the great inventor, the British Association gave his name to the unit of electrical power, which, later was adopted by the SI System of units.

And, the rest, as they say, is History.

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When you talk about any machine (or equipment) You have three things

a) Input

b) Output

c) failure modes/ Constraints/ Limitations (and mostly these are external to the equipment/machine the internal constraints are usually taken care of in the design stage)

Let us see transformer

Input - Electrical Power - paremeters, V, I, CosΦ

output- Electrical Power - paremeters, V, I, CosΦ

Major constraints - Voltage (Insulation failure), Current (Conductor failure), heat Loss Dissipation (again depends on the external factor I). Note- negligible mechanical constraints here.

It may be seen that the powerfactor- which is one of the external factors do not affect the limitations in any way. So we rate it in terms of Applied voltages (Input, Output), Total Current ie V and I or V and VA (after all the I is directly VA deducible with V known)

Now I would like the OP to do the same with generators and Motors and tell us back about his/her analysis.