Optimum Torque per Unit Current Control

The aim of this control strategy is to maximize electromagnetic torque for a unit stator current. By using this strategy the PM machine will produce maximum torque for a given magnitude of current. To develop the mathematical models of this strategy, consider the torque equation of the PM machine given in equation 3 and normalize it into p.u. system. The normalized torque expression is

(28)

The torque per unit stator current is defined as

(29)

The condition under which the machine produces maximum torque per unit stator current is obtained by differentiating equation 29 with respect to δ and equating it to zero, that is

(30)

Using the trigonometric identity in equation 30 gives

(31)

The solution of equation 31 gives

(32)

In equation 32, only the value of δ greater than 90° is considered so as to reduce the field in the air gap. The performance characteristics of the PM machine (parameters of the machine are given in Table 1) for this control strategy are shown in Figure 6a and the torque versus speed characteristics are shown in Figure 6b.