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  • What is meant by "power" ?

    zaveri

    Member

    Updated: Oct 25, 2024
    Views: 1.4K
    What exactly do they mean, when they say that the power output of a powerplant is some kW ?

    and also what do they mean when they say that a particular ac device requires some watts to operate ?

    now supposing the motor tag of a fan reads " 220 volts and 5 amps", does it mean that it requires 1100 watts (since power = volts * amps) ?
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  • zaveri

    Member • May 11, 2013

    No replies here ?

    #-Link-Snipped-# #-Link-Snipped-# #-Link-Snipped-#

    what do you have to say to this guys ?
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  • lal

    Member • May 12, 2013

    Power is work done per unit time. Or the energy consumed or transferred per unit time-Joules per second.

    The fan described in the post will consume 1100 Watts when operating at its rated speed when the supply voltage is 220V (that is such a huge fan!! 😁 ). This doesn't mean it always consume 5A. When the voltage is not 220V the device will not be consuming 5A or 1.1kW power, neither shall it be running at the rated speed at which it is designed to.

    Take the case of an incandescent lamp. If the voltage is fluctuating, the glow of the bulb can be seen as fluctuating too. If the power consumption was always the same, there should not be any kind of fluctuations. That means, if the supply voltage was lower, it should have consumed more current than the rated current to keep the power consumption constant, which is impossible! The ratings on AC devices define that at the rated voltage that particular device will consume that rated current and so, 'that much' power.

    Devices will work even without consuming rated power at a lower voltage, but might not always give an output which they are supposed/designed to give.
    What exactly do they mean, when they say that the power output of a power plant is some kW ?
    It means the power plant is able to meet a maximum power transfer rate equal to the described power output, beyond which it will fail. In other words, power output of a power plant is the maximum energy that the power plant can generate and transfer in a second.
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  • zaveri

    Member • May 13, 2013

    lal
    The fan described in the post will consume 1100 Watts when operating at its rated speed when the supply voltage is 220V (that is such a huge fan!! 😁 ). .

    this is no big fan ! the fan is question is just a (small) 600 mm ceiling fan.

    But then certain things are still not clear to me. Now take this fan as an example. i want to calculate the power consumed by it at any given instant. now how do i do that ?
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  • lal

    Member • May 13, 2013

    Oops! Seems like I messed up some things describing that. I'll get back to that very shortly.
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  • zaveri

    Member • May 13, 2013

    lal
    Oops! Seems like I messed up some things describing that. I'll get back to that very shortly.
    please do so
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  • Anand Tamariya

    Member • May 13, 2013

    Current and voltage rating just tells about the max. capacity of the appliance. Power consumed, e.g. in case of fan, will depend on the resistance of circuit inside fan.
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  • tre

    Member • May 14, 2013

    the power consumed by the fan at any given time (which is the instantaneous power consumption) it is equal to the the instantaneous current squared multiplied by the resistance of the internal windings. (inst I X I X R).
    Basically the term power is a general term.
    Concerning your fan there two types of power involved
    Reactive Power - this is power which is responcible for providing the magnetism for fan operation
    Active Power - this is power which drives the mechanical load ie the fan on the shaft of motor . This is the one i have described above
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  • lal

    Member • May 14, 2013

    Alright, let me try this again. I mistook 1100 watts as the indicated rated power output on the name plate of the fan. Well, that is not the case here actually. Is there a rated power indicated?

    The voltage and current shown there are the maximum values as pointed by Anand. The current indicated on an induction motor name plate is actually the full load ampere rating. The device will be consuming much less current under normal operating conditions. A house hold ceiling fan generally consumes less than 100 watts.

    To measure power, you may use a watt-meter. Or by knowing the supply voltage and motor power factor, measure the current consumed. Multiplying all the three values will give power consumption.

    (I still think the descriptions are not complete and do not answer your questions fully. Will come back again for sure!)
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  • lal

    Member • May 21, 2013

    Let us try to make this more transparent.

    The name plate reads 220V and 5A. In the case of induction motors, the indicated voltage on the name plate is the rated voltage. The motor is expected to provide its best performance when the voltage is 220V. Now, the confusion is current. The current indicated is actually the full load current. Which means, under full load condition, the motor will draw 5A from the supply if the voltage is 220V.

    So that happens only under full load. An induction motor draws the highest amount of current when it is starting from rest (provided it will be loaded only till full load). Then the amount of current drawn decreases once the rotor starts speeding up. This is because, once the rotor starts rotating, a back emf will be induced which reduces the current drawn. In our case - the ceiling fan, motor will not be loaded fully and hence it won't draw 5A from the supply. Therefore the power consumed by the motor will be much lesser.

    I hope that makes things much more transparent. Can you post all the name plate readings on the fan motor?
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  • n.rajeev sharma

    Member • May 31, 2013

    i think that's a ideal thing but in practical it is the cosine of angle between voltage and current so now p=vi cos(angle)
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  • Jeffrey Arulraj

    Member • May 31, 2013

    n.rajeev sharma
    i think that's a ideal thing but in practical it is the cosine of angle between voltage and current so now p=vi cos(angle)
    Let us put it in other words

    For DC frequency F=0 and this makes

    P=VI*Cos0

    P=VI

    But since in an AC we have variations in F we tend to use the Cos term also in the formula for power
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  • satyajit rules

    Member • Feb 12, 2014

    in case of fan.. it is a single phase induction motor..its power can be calculated by
    P=VI cos(angle) not by P=VI

    what is the difference between the two formulas is: former one is mentioned in its name plate as rated power. rated power means the output power that only converts into work done. it is called used power or active power.

    the later shows the power drawn by the circuit or line.this consists of both active power(VI cos0) & reactive power(VI sin0).reactive is used to magnetise the circuit.so this is unused power or loss. so in motors or fans P=VI has no meaning a sreactive power can vary with the same motor with different circuits or lines.
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  • zaveri

    Member • Feb 13, 2014

    satyajit rules
    in case of fan.. it is a single phase induction motor..its power can be calculated by
    P=VI cos(angle) not by P=VI
    What can be the angle in this case ? 90 degrees ?
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