Studies on voltage unbalance have revealed its detrimental effects on the operation of three-phase induction motors (TPIM) due to the production of negative-sequence current and torque. Voltage unbalance has been defined in different ways over the years, and each of these definitions is limited in scope in one form or the order, e.g. the neglect or inadequate consideration of the phase angle of the supply in these definitions. The complex voltage unbalance factor captures the phase angle using the ratio of the sequence components, but there is a challenge on the interpretation of the random angle θv with respect to the supply. In this study, a novel definition of phase unbalance is proposed which considers the normal 120° displacement of the three phases of the supply. A comparative analysis of all the definitions is presented using four operational supply case studies for a TPIM, and the results may ultimately impact control strategies.