Sorry that no one could read my mind in answering this question last time. The answers were not to the point. quot;To maintain same power at lesser voltage you need more currentquot;. Well does the motor know what power it has to deliver? Isnt the power delivered determined by voltage that is applied? It is an open loop circuit right? So what is the factor that induces the motor to draw more current at lesser voltage? Please.....
The design of an AC motor causes the motor to operate at a speed that is determined by the frequency of the power. The common speeds are 1000, 1500 and 3000 RPM for 50 Hz and 1200, 1800 and 3600 RPM for 60 Hz. The motor will produce (or burn itself up attempting to produce) whatever turning force, torque, is necessary to turn the load at the design speed. The speed can only drop a little as the load increases without the current increasing out of proportion to the load. The electromagnetic mechanism inside the motor is essentially a closed loop circuit that regulates speed and causes the torque supplied to match whatever is needed to turn the load at the motor’s design speed. If the voltage drops, more current is required to allow the motor to produce the torque required to maintain the design speed. Maintaining a specific torque at a specific speed requires a specific power proportional to torque times speed. Maintaining a specific mechanical power delivered by the shaft of the motor requires a specific electrical power input proportional to voltage times current. The above is essentially the same as Bramble’s answer, but put a different way.
I didn't see your first posting but anyway let me have a go! Let's assume it's about an ac motor. As long as such a motor is working reasonably close to its design parameters its speed will be pretty well constant (if it's a synchronous motor then exactly constant). Its load will apply a torque to the motor shaft and this torque multiplied by the speed defines the load (mechanical power). If the speed is constant most loads (e.g. pumps) will demand a fixed torque so the motor will see a fixed mechanical power demand even as voltage varies. The motor does not draw voltage. The voltage is supplied by the user to the motor. The product of voltage and current is the electrical power to the motor ( to a first approximation - there is an adjustment for something called power factor but for explanation purposes we may set that aside for the moment). So the motor draws current to make its electrical power intake equal its mechanical power output. If we drop the voltage V supplied, the motor will automatically compensate by drawing more current I to keep V.I constant. How the motor knows to draw more current requires a fair depth of explanation. Basically an induction motor will slow down very slightly which increases the current demand and in a synchronous motor the load angle will increase slightly. Somewhat more basically, we have to do with two equations you may know : F=Bli and V = Blv which give the force F and voltage V in a conductor of length l carrying current i and moving at speed v in a magnetic field B. So if we reduce V then (at constant l and v) B must reduce. Reduced B (see first eqation) demands increased i in order to maintain F (F being the source of the output torque). Edit your question if you need more explanation.
it somewhat is thoroughly linear, extra modern is had to do the same paintings with much less voltage, and vice-versa. With AC the situation is extra laid low with the fee sensor circuits, which will immediately call for extra modern to maintain shaft velocity. If the present headroom isn't there the motor can lag right into a at the back of schedule phasing state and right this moment overheat and cook dinner the windings.
