Field Weakening in Brushless p-m Motors

 Field weakening is commonly used in separately-excited dc motors in order to obtain speed increase with falling torque. Thus, with armature voltage V, armature current I and armature resistance R, the armature emf E is given by the equation

 V = E + IR

 To identify the performance capability envelope, V is taken as the maximum available supply voltage and I is the maximum continuous current capacity. The equation shows that, under these conditions, the emf E must be constant. The effect of reducing the field flux F is then given by the emf equation

 E = kFW

 whence

 W = E/kF

 and it is seen that, with constant E, speed W increases in inverse proportion to the reducing flux. Furthermore, torque T is given by

 T = kFI

 So torque falls in direct proportion to the flux F. Mechanical power output P is

 P = TW = (kFI)(E/kF) = EI

 which is a constant. That is, for a conventional dc motor, a constant power capability is obtained in the field weakening mode. By analogy with dc motors, the term `field  weakening` is also applied to brushless p-m motors but the mechanism and technology are  quite different and it does not follow that constant power capability will automatically be  obtained.

Field weakening in brushless p-m motors

 A brushless p-m motor is, in essence, an inverter-fed ac synchronous motor with a p-m excited rotor.Accordingly, the variable-frequency supply to the stator is the only excitation which is available to be controlled. Field weakening is obtained by electronically advancing the phase of the stator currents to produce a demagnetizing component of stator magnetomotive force which opposes the rotor magnet flux, thus reducing the net effective flux.

 Considering only the fundamental component of voltages and currents, the phasor diagram in Fig.1 illustrates this process. Here, excitation emf E and reactances Xd and Xq are defined at base speed (n = 1) and take values nE, nXd and nXq at other speeds n. Stator resistance has been neglected, for simplicity. The p-m rotor excitation lies on the d-axis and induces the emf nE. In the field weakening mode, the current angle b is greater than 900, as shown, so that the d-axis component of stator current Id is in the opposite direction to the d-axis. The induced voltage jnXdId is in the opposite direction to the excitation emf nE, demonstrating the field weakening effect. As b is increased, the field weakening is increased and the speed n rises.

 The power capability in the field weakening mode, that is within the limits of maximum voltage V and current I imposed by the variable-frequency inverter supply system, is dependent upon the motor parameters. An ideal machine would have unity power factor (ie. j = 0 in Fig.1) and 100% efficiency over the whole speed range. Its output power would then be constant and equal to the supply volt-amperes. In contrast to the dc motor, this condition is not achieved automatically. It is necessary to design the motor quite carefully, choosing the values of E and XdI relative to V. Approximately constant power can then typically be achieved over a speed range of about 3:1.

 Rotor Types

 There are several different types of rotors which may be used. Surface p-m rotors: with modern highfield permanent magnets, which have relative permeability similar to that of air, motors with surface magnets have no saliency so Xd = Xq . Their output is produced solely by excitation torque.

Figure 1. Phasor diagram during filed weakening operation.

 Interior or buried magnet rotors: These rotors have small air gap on the q-axis so Xq is greater than Xd. This is called inverse saliency. Output torque is a combination of excitation torque and reluctance torque.

Inset surface magnet rotors: Owing to their small air gap on the q-axis these rotors also have inverse saliency. Reluctance rotors: These have no rotor excitation (ie. E = 0) so they produce only reluctance torque. b is less than 900 for reluctance motors and field weakening is achieved by increasing b, as usual.

There are many examples in the literature which demonstrate the performance of rushless p-m motors in the field weakening mode eg. [1] - [5].

 References

1]. R.F.Schiferl and T.A.Lipo: Power capability of salient-pole permanent magnet synchronous motors in variable speed drive applications, IEEE Trans. Ind. Applicat., 26, Jan./Feb. 1990, pp. 115-123.

2] S.Morimoto, Y.Takeda, T.Hirasa and K.Taniguchi: Expansion of operating limits for

permanent magnet motor by current vector control considering inverter capacity, IEEE

Trans. Ind. Applicat., 26, Sept./Oct. 1990, pp. 866-871.

3] B.J. Chalmers, R. Akmese and L. Musaba: Design and field-weakening performance of a permanent-magnet/reluctance motor with two partrotor, IEE Proc., Electric Power Applications, 145, 2, March 1998, pp. 133-139.

4] B.J.Chalmers, R.Akmese and L.Musaba: Validation of procedure for prediction of field weakening performance of brushless synchronous machines, Proc. ICEM `98, Istanbul, September 1998, pp. 320-323.

5] B.J. Chalmers and L. Musaba: Design and field-weakening performance of a synchronous reluctance motor with axially-laminated rotor, IEEE Trans. IA, 1998, 34, 5, pp. 1035-1041.

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