Difference between revisions of "AC Drives"

[[Category:Actuators, Drives]]{{Knoppen}}

[[File:Ac drive.gif|right|thumb|AC Drives]]

The word "drive" is used loosely in the industry. It seems that people involved primarily in the world of gear boxes and pulleys refer to any collection of mechanical and electro-mechanical components, which when connected together will move a load, as a "drive". When speaking to these people, an '''AC drive''' may be considered by them as the variable frequency inverter and motor combination.  

People in the electrical field and electrical suppliers usually refer to a variable frequency inverter unit alone, or an SCR power module alone (when discussing DC drives) as the "drive" and the motor as the "motor".

==Control Performance==

AC drives are used to bring about process and quality improvements in industrial and commercial applications' acceleration, flow, monitoring, pressure, speed, temperature, tension and torque.

Fixed-speed operated loads subject the motor to a high starting torque and to current surges that are up to eight times the full-load current. AC drives instead gradually ramp the motor up to operating speed to lessen mechanical and electrical stress, reducing maintenance and repair costs, and extending the life of the motor and the driven equipment.

Performance factors tending to favor use of DC, over AC, drives include such requirements as continuous operation at low speed, four-quadrant operation with regeneration, frequent acceleration and deceleration routines, and need for motor to be protected for hazardous area.

==Generic Topologies==

AC drives can be classified according to the following generic topologies:

===Voltage-source inverter (VSI) drive topologies===

===Current-source inverter (CSI) drive topologies===  

In a CSI drive, the DC output of the SCR-bridge converter stores energy in series-reactor

connection to supply stiff current input to the inverter. CSI drives can be operated with either PWM or six-step waveform output.

===Six-step[e] inverter drive topologies===

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.

===Load commutated inverter (LCI) drive topologies===

In a 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.

===Cycloconverter or matrix converter (MC) topologies===

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.

===Doubly fed slip recovery system topologies===

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.

==Sources==

[http://en.wikipedia.org/wiki/Variable-frequency_drive Wikipedia Variable-frequency_drive]