87 Frequency Converters and Motors 3.4 Operation 3.4.1 Motor Speed Control The output frequency of the FC, and thus the motor speed, is controlled by one or more signals (0-10 V; 4-20 mA, or voltage pulses) as a speed reference. If the speed reference increases, the motor speed goes up and the vertical part of the motor torque characteristics is shifted to the right (Fig. 3.4 Reference signal and motor torque relation). Fig. 3.4 Reference signal and motor torque relation If the load torque is less than the motor torque, the speed will reach the required value. As shown in Fig.
3.5 Relation current limit and over current limit, the load torque curve intersects the motor torque curve in the vertical part (at point A). If the intersection is in the horizontal part (point B), the motor speed cannot continuously exceed the corresponding value. The FC allows brief current limit overshoots without tripping (point C), but it is necessary to limit the duration of the overshoot. t Fig. 3.5 Relation current limit and over current limit