In general, relationship between T (torque), K (constant), V (voltage), I (current), and F (frequency) is expressed as "T = K × (V/F) × I".
Since (V/F) is constant at up to 60 Hz, the current and torque are proportional.
After exceeding 60 Hz, V (voltage) is now constant and a constant output range is entered. To keep torque constant, it is necessary to increase the current in accordance with F (frequency).

In other words, when 60 Hz is exceeded, torque decreases in inverse proportion to the frequency. (For example, when the frequency increases from 60 Hz to 80 Hz, current must be increased for 80/60 to keep torque constant.)
Operation of vector inverter motor is almost same as the general-purpose motor.