Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: jaiii on 04/11/2013 12:49:07
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Hello.
1 We have a magnetic field generated 3-phase electric current flow.
What will be the angular velocity of the magnetic intensity?
2 will rotate the magnetic field at the center of the solenoid powered DC current?
Thank.
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I interpret the questions as follows:
1 We have a magnetic field generated [by a] 3-phase electric current flow.
I assume this means that we have a series of coils (such as from a 3-phase electric motor), connected to the AC mains.
If you remove the rotor, these coils will create a rotating magnetic field (http://en.wikipedia.org/wiki/Induction_motor#Principle_of_operation) in the center of the motor (where the rotor was originally).
What will be the angular velocity of the magnetic intensity?
If the motor has just 3 coils, the magnetic field will rotate 360° =2π radians for every cycle of the applied AC voltage.
The mains AC voltage operates at 60Hz in North America, and 50Hz in some other parts of the world.
So in a 50Hz country, a synchronous motor (http://en.wikipedia.org/wiki/Synchronous_motor) will rotate at 50 revolutions per second, or 50*60=3000 revolutions per minute (rpm).
The angular velocity is measured in radians per second; in a 50Hz country, the magnetic field will rotate at 50*2π=314.159 radians per second.
Some motors are wound with (say) 15 coils, which means that the motor will rotate once for every 5 AC cycles, or only 10 revolutions per second.
Another kind of motor is called an induction motor (http://en.wikipedia.org/wiki/Induction_motor). These do not quite reach the precise speed of a synchronous motor, but reach a lower speed that depends on the load applied to the motor shaft.
2 will the magnetic field rotate at the center of the solenoid [if it is] powered [by] DC current?
If the coils are powered by a DC current, there will be a steady magnetic field in the center of the coil; it will not rotate.
There is another kind of electric motor called a stepper motor. When driven from a DC voltage by electronic circuits and a computer, it is possible to accurately move a fixed number of rotations and stop at a specified angle. Stepper motors are widely used in printers, disk drives and motor cars.
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Hi.
Thank very nice.
But I have one question:
Will the magnetic field rotate at the center of the solenoid [if it is] powered [by] 1 phase AC current (sinus)?
Thank
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With single-phase AC, the magnetic field "oscillates backwards and forwards", rather than "rotates around and around".
If you had a synchronous motor which you could bring up to 3000rpm, it would stay locked with 50Hz single-phase AC. But this is inconvenient, since you need a motor to start your motor!
In practice, single-phase AC motors have a mechanism that provides a slight phase shift in the oscillating magnetic field, to give the rotor an initial "kick" to get it going, by using an inductance or capacitance.
See http://en.wikipedia.org/wiki/Induction_motor#Starting
Once the motor is running, a phase shift between the current in the stator and the current in the rotor causes the rotor to always try to "catch up" with the 50Hz field - but it never quite makes it.
See: http://en.wikipedia.org/wiki/AC_motor#Single-phase_induction_motor