# The Naked Scientists Forum

### Author Topic: Direct wire  (Read 1198 times)

#### jaiii

• Sr. Member
• Posts: 114
##### Direct wire
« on: 04/11/2013 12:49:07 »
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.
« Last Edit: 04/11/2013 17:37:54 by jaiii »

#### evan_au

• Neilep Level Member
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##### Re: Direct wire
« Reply #1 on: 05/11/2013 09:48:41 »
I interpret the questions as follows:
Quote
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 in the center of the motor (where the rotor was originally).

Quote
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 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. 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.

Quote
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.
« Last Edit: 05/11/2013 09:51:10 by evan_au »

#### jaiii

• Sr. Member
• Posts: 114
##### Re: Direct wire
« Reply #2 on: 05/11/2013 11:50:12 »
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

#### evan_au

• Neilep Level Member
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##### Re: Direct wire
« Reply #3 on: 05/11/2013 20:29:28 »
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

#### The Naked Scientists Forum

##### Re: Direct wire
« Reply #3 on: 05/11/2013 20:29:28 »