In the early days of variable speed drives, Nema rated motor contactors were used. The more economical IEC rated contactor had not be introduced. As variable speed drive technology improved and sequencing the opening of the contactor to coordinate with the termination of current flow to the motor became reliable, the need for a contactor to break an arc took second place to the use of a contactor for isolation purposes. This meant that the more robust and more expensive Nema contactor was not required.