AC motor drives are defined as amplifiers or frequency inverters that interface between a controller and an AC motor. They convert step and direction input from the controller to currents and voltages compatible with the motor. These units are sometimes called variable frequency drives, referring to a majority of AC motor drives which adjust input frequency. In industry, a 'drive controller' is a motor drive which incorporates functions of a controller and drive to determine the speed, torque, horsepower, and direction of an AC motor.

AC motors tend to require less maintenance than DC motors, making them preferable for hard-to-service locations. They are better suited for high speed operation since no brushes are involved and commutation is not a problem. They are also generally smaller, lighter, and more commonly available than DC motors.

View Teardown of ABB ACS150 1 HP AC Motor Drive

Types of AC Drives

AC motor drives are classified based on the type of AC motor being used. The most common types include induction, synchronous, sensorless vector, and servo drives.

Induction motors derive their name from the fact that current is induced into the rotor windings without any physical connection with the stator windings (which are directly connected to an AC power supply); adaptable to many different environments and capable of providing considerable power as well as variable speed control. Typically there is "slip," or loss of exact speed tracking with induction motors.

Synchronous motors operate at constant speed up to full load. The rotor speed is equal to the speed of the rotating magnetic field of the stator; there is no slip. Reluctance and permanent magnet are the two major types of synchronous motors. A synchronous motor is often used where the exact speed of a motor must be maintained.

Sensorless vector drives employ independent control of both the voltage and frequency supplied to the motor for good speed control, and low-speed torque output approaching that of DC motors. Sensorless indicates that no feedback sensor such as an encoder or resolver is used.

Servo motors are typically permanent magnet synchronous motors that can often have low torque-to-inertia ratios for high acceleration ratings. They frequently employ brushless commutation with feedback provided by Hall Effect sensors, and sinusoidal winding excitation.

AC motor drives can also be classified based on types of control functions (e.g. integral motion controllers, variable speed drives, motor speed controllers, etc.).

Design Specifications

There are a wide range of design specifications to consider when searching for AC motor drives. The most important of these include:

• Continuous current - the current applied to the motor during continuous operation.
• Supply voltage - the voltage supplied to the drive.
• Output voltage - the range of output voltages of the drive.
• Input frequency - the AC input frequency accepted by the device.
• Power - the rated power output of the drive motor system.
• Operating temperature - the operating temperature of the power supply.

Other important characteristics include the mounting configuration and accompanying features.

Mounting Configuration

Drive mounting configuration is important for compatibility with the motor system. Drives can be mounted in various ways based on the design, including onto a PCB (printed circuit board), PC board, panel, DIN rail, or rack. Other drives may be stand-alone devices or designed to be incorporated into specific products by original equipment manufacturers (OEMs).

Features

AC motor drives may also include various features which may be important for certain applications. These include:

• Regeneration - Method of braking in which the motor is disconnected from the power supply and power generated from the rotating motor is sent back to the supply.
• Programmability and configuration - Device can be programmed with routine configurations and commands for greater functionality and process control.
• Auto restart - Drive is designed to automatically restart operation after a stall.

References

Joliet Technologies - Adjustable Speed Drives

SDT Drive Technology - Glossary of Motor Drive Related Terms

Drive Systems, Inc. - AC Drives

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ABB Drives and Motors | Eaton Corporation | Panasonic Electric Works