Frequency converters are used to change the frequency and magnitude of the constant grid voltage to a variable load voltage. Frequency converters are especially used in variable frequency AC motor drives.
Figure 1 shows the behavior of an induction motor with several motor input voltages. The bold blue curve represents the electrical torque as a function of rotor speed when the motor is connected directly to a constant supply network. The blue portion of the torque curve shows the nominal load region (-1…+1 [T/TN]), which is very steep, resulting in low slip and power losses. Similar motor torque behavior with other motor input frequencies can be achieved by feeding the induction motor with a frequency converter and keeping the ratio of the magnitude and frequency of the motor voltage constant. As a result, the shape of the torque curve remains unchanged below the nominal speed (constant-flux region -1…+1 [n/nN]). In the field weakening region the motor voltage is at its maximum and kept constant, resulting in the torque curves being flattened.
Frequency converters can be classified according to their DC circuit structure to voltage-source (Fig. 4), current-source (Fig. 3) and direct converters (Fig. 2). With a voltage-source converter the variable frequency and magnitude output voltage is produced by pulse-width modulating (PWM) the fixed DC voltage, whereas with a current-source converter the output voltage is produced by modulating the fixed DC current. With a direct frequency converter the variable output voltage is formed directly by modulating the constant input voltage. At low voltage applications (<1000 data-blogger-escaped-br="" data-blogger-escaped-is="" data-blogger-escaped-mainly="" data-blogger-escaped-the="" data-blogger-escaped-topology="" data-blogger-escaped-used.="" data-blogger-escaped-v="" data-blogger-escaped-voltage-source="">
