VFD Types And Applications?

Figure 1: Variable Frequency Drive (VFD).

Variable Frequency Drive (VFD) can be classified according to the following generic topologies.

1. Types

1.1 Voltage-source Inverter (VSI) Drive

In a VSI drive, the DC output of the diode-bridge converter stores energy in the capacitor bus to supply stiff voltage input to the inverter. The vast majority of drives are VSI type with PWM voltage output.

Figure 2: Voltage-source inverter (VSI) drive.

1.2 Current-source Inverter (CSI) Drive

In a CSI drive, the DC output of the SCR-bridge converter stores energy in series-Inductor connection to supply stiff current input to the inverter. CSI drives can be operated with either PWM or six-step waveform output.

Figure 3: Current-source inverter (CSI) drive.

1.3 Six-step Inverter Drive

Now largely obsolete, six-step drives can be either VSI or CSI type and are also referred to as variable-voltage inverter drives, pulse-amplitude modulation (PAM) drives, square-wave drives or D.C. chopper inverter drives.In a six-step drive, the DC output of the SCR-bridge converter is smoothed via capacitor bus and series-reactor connection to supply via Darlington Pair or IGBT inverter quasi-sinusoidal, six-step voltage or current input to an induction motor.

Figure 4: Six-step inverter drive.

1.4 Load Commutated Inverter (LCI) Drive

In an LCI drive (a special CSI case), the DC output of the SCR-bridge converter stores energy via DC link inductor circuit to supply stiff quasi-sinusoidal six-step current output of a second SCR-bridge's inverter and an over-excited synchronous machine.Low-cost SCR-thyristor-based LCI fed synchronous motor drives are often used in high-power low-dynamic-performance fan, pump and compressor applications rated up to 100 MW.

Figure 5: Load commutated inverter (LCI) drive.

1.5 Cycloconverter or Matrix Converter (MC)

Cycloconverters and MCs are AC-AC converters that have no intermediate DC link for energy storage. A cycloconverter operates as a three-phase current source via three anti-parallel-connected SCR-bridges in six-pulse configuration, each cycloconverter phase acting selectively to convert fixed line frequency AC voltage to an alternating voltage at a variable load frequency. MC drives are IGBT-based.

Figure 6: Cycloconverter or matrix converter (MC).

1.6 Doubly Fed Slip Recovery System

A doubly fed slip recovery system feeds rectified slip power to a smoothing reactor to supply power to the AC supply network via an inverter, the speed of the motor being controlled by adjusting the DC current.

2. Applications

2.1 Switching Frequency Foldback

One drive uses a default switching frequency setting of 4 kHz. Reducing the drive's switching frequency (the carrier-frequency) reduces the heat generated by the IGBTs.

2.2 Noise Smoothing

Some drives have a noise smoothing feature that can be turned on to introduce a random variation to the switching frequency. This distributes the acoustic noise over a range of frequencies to lower the peak noise intensity.

2.3 Dynamic Braking

Torque generated by the drive causes the induction motor to run at synchronous speed less the slip. If the load drives the motor faster than synchronous speed, the motor acts as a generator, converting mechanical power back to electrical power. This power is returned to the drive's DC link element (capacitor or reactor). A DC-link-connected electronic power switch or braking DC chopper controls dissipation of this power as heat in a set of resistors. Cooling fans may be used to prevent resistor overheating.

Dynamic braking wastes braking energy by transforming it to heat. By contrast, regenerative drives recover braking energy by injecting this energy into the AC line. The capital cost of regenerative drives is, however, relatively high.

Related Info