This paper presents a novel approach to the establishment of the operational boundary
for synchronous motors in variable speed drives. This is based on the strategy of
keeping the motor total power loss constant at its rated value with rated output power.
Subject to hot spot risks, this is likely to ensure the thermal robustness of the motor at
any operating point, independent of motor current and power limits. The proposed
strategy of control adjusts the net air-gap flux to achieve the desired speed and maintain
the motor total loss within the rated value. The motor operational boundary for a typical
motor, under the proposed control strategy, is established and compared with the
corresponding boundary under conventional VIF control. Higher torques, output powers
and efficiencies are achieved. Furthermore the operational speed range can be extended
above base speed, compared with the more limited range with conventional V/F control.
Efficiency maximization at different values of part load is also investigated by
controlling the net air-gap flux. A significant reduction in power loss is obtained,
achieving worthwhile significant energy savings. The theoretical analysis is derived
based on an equivalent circuit model. Saturation, armature reaction and variation of iron
losses are taken into consideration. Experimental verification at some of the theoretical
results is obtained.