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AC Drives

An AC drive is a device that is used to control the speed of an electrical motor, either an induction motor or a synchronous motor. AC drives are also known by various other names such as adjustable speed drives (ASD) oradjustable frequency drives (AFD) or variable frequency drives (VFD) or variable speed drives (VSD) orfrequency converters (FC).

The first electrical AC motor was designed in 1899. Electrical motors convert electric energy into mechanical energy by electromagnetic induction. These motors are characterized by:

  • fixed speed, determined by the frequency of the power supply
  • fixed torque

 Obviously, a fixed speed is not suitable for all processes in all circumstances; thus, the need for adjusting the speed according to need.

Industrial machinery is often driven by electrical motors that have provisions for speed adjustment. Such motors are simply larger, more powerful versions of those driving familiar appliances such as food blenders or electric drills. These motors normally operate at a fixed speed.

If speed control is required, that controller is called a (variable speed) AC drive. AC drives are used in a wide variety of industrial applications. To give an easy example, AC drives are often used with fans to provide adjustable airflow in large heating and air conditioning systems. The flow of water and chemicals in industrial processes is often controlled by adjusting the speed of pumps.

However, variable speed AC drives are commonly used in more complex and difficult environments such as water and wastewater processing, paper mills, tunnel boring, oil drilling platforms or mining.

The technology

The speed is controlled by changing the frequency of the electrical supply to the motor. The 3-phase voltage in the national electrical grid connected to a motor creates a rotating magnetic field in it. The rotor of the electrical motor will follow this rotating magnetic field. An AC drive converts the frequency of the network to anything between 0 to 300 Hz or even higher, and thus controls the speed of motor proportionally to the frequency.

1. Rectifier unit

The AC drive is supplied by the electrical network via a rectifier. The rectifier unit can be uni- or bidirectional. When unidirectional, the AC drive can accelerate and run the motor by taking energy from the network. If bidirectional, the AC drive can also take the mechanical rotation energy from the motor and process and feed it back to the electrical network.

2. DC circuit

The DC circuit will store the electrical energy from the rectifier for the inverter to use. In most cases, the energy is stored in high-power capacitors.

3. Inverter unit

The inverter unit takes the electrical energy from the DC circuit and supplies it to the motor. The inverter uses modulation techniques to create the needed 3-phase AC voltage output for the motor. The frequency can be adjusted to match the need of the process. The higher the frequency of the output voltage is, the higher the speed of the motor, and thus, the output of the process.

What is an variable speed AC drive, figure 1

Figure 1: The main components of an AC drive: rectifier, DC circuit and inverter

AC Servo Drive Basic Diagram

AC Drive Basic Diagram

 VFD types and ratings

Generic Topologies

AC drives can be classified according to the following generic topologies :
  • Voltage-source Inverter (VSI) drive topologies (see image): 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.

Topology of VSI drive
  • Current-source inverter (CSI) drive topologies (see image): 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.

Topology of CSI drive
  • Six-step inverter drive topologies (see image): 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.

Six-step drive waveforms
  • 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 (see image):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.

Topology of direct matrix converter
Some AC Drives are
Legend for this Topic
^ Inverter switching device (with std. diode rectifier)
^^ Inverter and rectifier switching device
^^^ Rotating or linear
AFE Active front end
BLDM PM trapezoid machine (Brushless DC electric motor)
CME Common mode elimination
CHB Cascaded H-bridge
CSI Current source inverter
CSR Current source rectifier
GCT Gate controlled thyristor
GTO Gate turn-off thyristor
IGBT Insulated gate bipolar transistor
LCI Load commutated inverter
LV Low voltage
MV Medium voltage
NPC Neutral point clamped
PAM Pulse-amplitude modulation
PM Permanent magnet
PMSM Permanent magnet synchronous generator
PWM Pulse-width modulation
SCR Silicon controlled rectifier
SGCT Symmetrical gate controlled thyristor
SRM Switched reluctance motor
SyRM Synchronous reluctance machine
VRM Variable reluctance machine
VSI Voltage source inverter
VVI Variable voltage inverter
WFSM Wound field synchronous machine
WRIM Wound rotor induction motor


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About engrsajidrehman

SAJID REHMAN is an Electronics Engineer and working in his field for many years. His in depth field of experience is hardware Repairing and troubleshooting, especially AC/DC Drives, Electronics Controllers, Computer hardware and other PCBs. Beside all this he is a Web Designer, Graphics Designer and Content writer. You can contact him on Google and Linkedin. Thanks

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