Adjustablefrequency Drives

Until recently, the most significant step in the improvement of adjustable drives was to replace the autotransformer and rectified variable voltage power supply with SCR drives. Although dc motors are seldom used for pump drives, except in automotive applications and other special cases, the SCR and PWM adjustable speed dc drives are very highly developed and available from a number of suppliers.

Pulse Width Modulation Frequency Inverters Adjustable-frequency drives consist of an adjustable-frequency inverter control and a constant-speed motor, as shown in Figure 9. The inverter control has generally been built in either pulse width modulation or square wave construction. An inverter control consists fundamentally of a circuit-protective device, a diode bridge, and an inverter section having an SCR and a firing-control (FC) section for the SCR, all packaged in a steel enclosure. Special power supplies, motor protective devices, and electrical protective devices complete the package.

Only low voltages are used to energize the motor bus. A diode bridge rectifies voltage and an inverter inverts the direct current into an adjustable-frequency adjustable voltage that varies linearly with frequency. This voltage is not necessarily sinusoidal; it can also consist of a number of dc pulses of positive or negative polarity, as shown between the inverter and the motor in Figure 9. The firing controls modulate the width of the pulses and the number of pulses per half cycle to vary the apparent frequency and to maintain motor voltage at a constant volts-per-cycle. The firing controls automatically introduce additional pulses or withdraw pulses as bandwidths reach their limits.

Firing circuits composed of solid-state devices trigger the SCR in accordance with logic controlled by the setting adjustment of a speed potentiometer (Figure 9) or the signal from the process. The control provides normal protection of the motor as well as short-circuit protection of the control, motor, and cables. Also, the control monitors and limits current drawn by the motor under all conditions to a preadjusted value, such as 150% of normal.

FIGURE 9 Block diagram for ac adjustable-frequency drive (General Electric)
FIGURE 10 Torque versus synchronous speed for adjustable-frequency drive (General Electric)
TABLE 5 Adjustable-Frequency Drive Data
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