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AC Induction Motor (ACIM) Overview

The AC Induction Motor (ACIM), sometimes called a squirrel cage motor, is one of the most popular motors used in consumer and industrial applications. Induction machines are by far the largest group of all industrial electrical machines, converting approximately 70-80% of all electrical energy into mechanical form. The have a very robust rotor construction, which makes them suitable for high-speed applications. With proper design, they have good overloading and field weakening characteristics.

  • The ACIM is comprised of a simple cage-like rotor and a stator containing three windings
  • The changing field produced by the AC line current in the stator induces a current in the rotor which interacts with the field and causes the rotor to rotate
  • The rotor does not have any moving contacts, which eliminates sparking
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Recommended Products for AC Induction Motor Control
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Microchip’s award winning 8-bit PIC® MCUs are an excellent solution for VF AC Induction motor control.

  • For 8-bit PIC MCU’s for AC Induction Motor Control Start with the: PIC16F917

For advanced closed loop AC Induction motor control Microchip’s dsPIC® family of digital signal controllers (DSCs) offer DSP performance and advanced motor control peripherals.

  • For dsPIC DSC’s for AC Induction Motor Control Start with the: dsPIC33EP64MC206

Microchip offers both MIPS and ARM Cortex MCU's for high performance, 32-bit motor control. These devices feature high performance peripherals tailored for high speed, closed loop motor control.

Key Characteristics of the AC Induction Motor
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  • Low cost to manufacture and maintain
  • Simple, low-cost design for fixed-speed applications
  • Lower efficiency than other motor types
  • Speed proportionate to line frequency (50 or 60 Hz)
  • Complex control for variable speed and torque
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How it Works
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The AC Induction motor is comprised of a simple cage-like rotor and a stator containing three windings. The changing field produced by the AC line current in the stator induces a current in the rotor which interacts with the field and causes the rotor to rotate. No brushes are necessary in this design. The base speed of the AC motor is determined by the number of poles built into the stator windings and the frequency of the AC input voltage. Variable speed control of an AC motor can be accomplished by increasing or decreasing the input frequency.

A load on the motor causes the motor to "slip" in proportion to the load. The slip occurs when the rotor turns at a slower speed than the rotating field produced by the stator. This slip is responsible for energizing the rotor. The ACIM is available in single-phase and 3-phase versions. A 3-phase ACIM is usually the best choice for variable speed applications. For variable speed and torque, things get more complicated.

ACIM Control & Characteristics
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Input:

  • Speed, frequency, torque, position, direction

Control:

  • Smooth control at low speeds
  • Efficient control at high speeds
  • Complex control for variable speed and torque
  • Must know rotor position (velocity) for slip and vector control
  • Rotor position sensor is eliminated for sensorless vector control strategies
  • Sensorless control does not work at low motor speeds

Feedback:

  • Quadrature encoder, phase current

Driver:

  • H-bridge for single phase
  • 3-phase inverter for 3-phase motors
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Design Flowchart
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Applications
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  • Air conditioner and refrigerator compressors
  • Home appliances
  • Pumps
  • Blowers
  • Automation/industrial applications
  • Power tools