Torques magnetic field

Direct-current motor fields are on the stator. The rotor is the armature. The magnetic field does not rotate like the field in ac machines. Current in the armature reacts with the stator field to produce torque. 

The magnetic field rotates at a synchronous speed, so it should also rotate the rotor. But this is not so in an induction motor. During start-up, the rate of cutting of llux is the maximum and so is the induced e.m.f. in the rotor circuit. It diminishes with motor speed due to the reduced relative speed between the rotor and the stator flux. At a synchronous speed, there is no linkage of flux and thus no induced e.m.f. in the rotor circuit, consequently the torque developed is zero. 

Note The speed of the motor ean be varied by vai-ying the frequeney alone but this docs not provide satisfactory performance, A variation in frequeney causes an inverse variation in the flux, for the same system voltage. The strength of magnetic field, p, develops, the torque and moves the rotor, but at lower speeds. / would be reduced, which would raise 0 , and lead the magnetic circuit to saturation. For higher s )ccds, / would be r.nised, but that would reduee which would adversely diminish the torque. Hence frequency variation alone is not recommended practice for speed control. The recommended practiee is to keep V/fas constant, to maintain the motor s vital operatin.c parameters, i.e. its torque and 0 ,. within acceptable limits. 

Let us consider the simple equivalent motor circuit diagram as shown earlier in Figure 1.15. The no-load component of the current, / , that feeds the no-load losses of the machine contains a magnetizing component, produces the required magnetic field, (p, , in the stator and the rotor circuits, and develops the rotor torque so that... 

All of these arc pha.sor quantities. 4 is the active component responsible for developing the rotor torque and / , the magnetic field. Varying /., would mean a corresponding variation in the torque developed. 

The plus sign indicates a positive sequence harmonic and the minus. sign a negative sequence harmonic. Their effect is same as for the positive and the negative seqtience components discussed in Section I2.2(v) and ctiuses pulsation in the magnetic field and hence, in the torque of a rotiitimi machine. 

Now suppose that an additional small magnetic field is applied perpendicular to Ho in the plane formed by pi and Hq, call this field (see Fig. 4-4B). Field Hi will act upon pi to increase the angle 6. If field Hy is caused to rotate around Ho at the Larmor precessional frequency of Wq, the torque produced will steadily act to change the angle 6. On the other hand, if the frequency of rotation of Hi is not the same as the precessional frequency, the torque will vary depending upon the relative phases of the two motions, and no sustained effect will be produced. 

Consider a nucleus with magnetic moment pi in a magnetic field Ho- According to classical mechanics the rate of change of the angular momentum G is the torque T. 

The external magnetic field could be due to a magnet or it could be due to the magnetomotive force induced by a current in a conductor (or another stationary coil). The relationship for torque developed when the fields of a stationary (stator) coil and a rotating (rotor) coil interact is given by... 

Because the current in each coil induces a magnetic field, the torque relationship may also be given as... 

Thus from Equation 2-215 we see that for a given dynamo geometry, the developed torque only depends on the interaction between two magnetic fields and their orientation with respect to each other. One or both of the magnetic fields may be induced by a current. If one of the fields is the field of a magnet, then it may be either in the rotor or the stator. If the rotation results from the imposition of mechanical power on the rotor, the device is called a generator. If the rotation is caused by the flow of current, the device is called a motor, i.e., converts electric power to mechanical power. 

According to electrodynamics, the force F for a non-uniform magnetic field produces on the atom a torque T given by... 

This demonstrates that the magnetic field strength of500 Gauss is enough to quench the thermal motion in xy plane. We also calculated the magnetic torque of the nanorod from the bulk magnetization of nickel to be approximately IE-15 A which produces... 

Figure 10.3 The magnetic field generated by an RF pulse applies a torque to the bulk magnetization vector and drives it towards the transverse plane.

A magnetic moment in an external field H will experience a torque L tending to turn it about the direction of magnetic field. L is given by... 

---