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AC motor drives

An AC motor drive is a frequency converter designed to control the speed and torque of an electric motor – typically 3-phase AC induction or 3-phase BLDC motors – using specifically developed control methods such as Variable-Frequency Drive (VFD) or Field-Oriented Control (FOC).

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Variable Frequency Drive (VFD) working principles

A variable frequency drive (VFD) consists of three main elements: an AC motor(usually a 3-phase induction motor), adrive controller and a user interface

The AC motor 

The AC induction motor often has a fixed speed so that the controller is relatively simple. This supplies voltage in three phases in a sequence into the coils of a stator in the motor, creating a rotating magnetic field. This induces an electric field to drive the rotor. 

The AC drive controller 

A typical industrial AC drive controller has three half-bridges, each delivering a sine-wave voltage to the stator. For the adjustable frequency, scalar algorithms are used in the controllers to vary the voltage to determine the frequency of the phases, or volts/hertz. The use of more sophisticated algorithms is becoming increasingly popular in high-end motors. Examples include vector control or field-oriented control (FOC) that are used to control the frequency of multiple phases. 

Adding in variable speed drives increases the complexity of the control algorithms. 

These drives also need the controller to provide a soft-start capability to reduce the load on the motor and the current surge of the motor during start-up. This is essential to reduce the stress on the motor and provide the maximum lifetime for the industrial drive on the factory floor. 

This is because the motors can have inrush currents seven to ten times that of the operational current, and the required torque can be three times higher at the start compared to when in operation. A digital soft-starter controller continuously monitors the voltage during start-up, adjusting to the load of the motor to provide smooth acceleration, speed and torque control. This is used to start and stop the AC motor drive system, reducing the stress on the drive. This also puts less load on the power supply, reducing the energy consumption and leading to significant cost savings

The AC drive interface 

The interface to the AC drive can be via a number of different connections. These can vary from a 4 to 20 mA current loop, a 0 to 10 VDC voltage signal or a remote connection via Ethernet or a wireless communication link.

AC drives in industrial applications

AC drives in industrial applications

The advent of powerful semiconductor controllers has enabled a new class of industrial drives to be developed around AC electric motors, specifically using a variable-frequency drive (VFD). This has allowed many applications that previously required a DC motor with pulse width modulation for the accuracy to use more power-efficient three-phase AC induction motors instead. These can deliver significant energy savingsacross an industry, especially when combined with wireless sensing and control links as part of the Industrial Internet of Things (IIoT) delivers significant cost savings across the factory floor, while boosting productivity. 

Industrial AC drives are seen across the factory floor, from powering a fan or a hydraulic pump to a stepper motor controlling a pick and place head or a robotic arm requiring fine control. The load demands of roll-to-roll manufacturing systems, whether that is web-offset printing or thin film panels of all descriptions need careful speed control, and industrial drives are key to ensuring that the speed of all the rollers is closely aligned, requiring both high speed and high accuracy. 

Application examples 

Pick-and-place machines require both speed, accuracy and positioning. The faster a placement head can move to the exact location within a fraction of a millimeter, the quicker an entire printed circuit board or piece of equipment can be produced. 

High-torque applications such as conveyor belts need to be able to handle heavy loads and light loads within a short time, requiring fine torque control to be efficient and effective. These also need to be highly reliable, as a conveyor shutting down hits the productivity of the entire factory. 

All these applications can be addressed with the latest AC industrial drive technology. The AC induction motors are more reliable and energy efficient than the DC versions, and the latest semiconductor devices provide the levels of control that are required in industrial applications. 

Linking industrial drives to the cloud opens up a world of opportunities as part of the industrial IoT and Industry 4.0. Remote control and monitoring of the drives with predictive maintenance algorithms ensures that preventive maintenance can be carried out to avoid any stoppages before they occur. All of this delivers significant cost savings across the factory floor, while boosting productivity.

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