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Advantages of Permanent Magnet AC Motors Over AC Induction Motors

Permanent magnet AC and AC induction motors comprise the majority of electric motors sold worldwide and total more than 70% of electric motor sales in 2020.  AC motors are used extensively throughout numerous industries to drive various devices, from fans and pumps to HVAC systems, appliances, power tools, and electric vehicles.  Demand for higher efficiencies from government agencies has driven advances in AC motor manufacturing processes and innovative motor designs that improve not only efficiencies but also motor life. This article covers the pros and cons of the two leading AC motor technologies.

Advantages of Permanent Magnet AC Motors (PMAC)

A permanent magnet AC motor uses magnets embedded in or attached to the surface of the rotor. The magnets provide a constant magnetic field that interacts with the electromagnetic field of the stator to produce motor rotation. In contrast, to create motion, the AC induction motor requires stator excitation to generate a magnetic field that then induces current within aluminum bars integrated in the rotor. The electromechanical design of the PMAC motor provides advantages associated with efficiency, power density, dynamic response, and speed range.

  • Efficiency - Permanent magnet AC motors (PMAC motors) are more efficient than induction motors due to the rotor design, which consists of permanent magnets on a laminated core to minimize rotor core losses. The stator windings of the PMAC motors tend to have lower resistance than the equivalent AC induction motors to provide additional energy savings.

  • Power Density - PMAC motors are able to deliver more power in a smaller and lighter package since the power-dense design minimizes the motor size. The nature of the induction motor rotor design leads to a larger, heavier motor for the same power output.

  • Dynamic Response - High dynamic response, a common characteristic of the permanent magnet AC motor, enables quick acceleration/deceleration and produces high output torque across the speed range (even at zero speed). The lower inertia of the permanent magnet AC motor also contributes to faster reaction times that are ideal for applications that require rapid move and settle times, such as indexing or high-speed processing.

  • Speed Range - PMAC motors operate over a wide speed range and maintain high-performance levels at both the low and high velocities prevalent in many motion control applications. Induction motors typically operate at a fixed speed based on input voltage frequency, although variable frequency drives can control an induction motor over a relatively wide speed range.

Other considerations for PMAC motors

PMAC motors have significant advantages over AC induction motors but also have many drawbacks to consider, such as cost, temperature range, demagnetization, and control complexity.

  • Cost - Permanent magnet AC motors typically have a higher initial cost than AC induction motors due to the higher magnet costs used in rotor construction.  In addition, the complexity of controls related to the permanent magnet AC motor is also a cost driver.  The lifetime savings from reduced energy consumption and maintenance, however, can offset the initial investments in PMAC motor designs.

  • Temperature Range - Magnets are affected by temperature extremes and can degrade over time if subject to constant, elevated temperatures, which can limit the use of a permanent magnet AC motor in certain thermal applications. The magnet type used in an application can directly influence a motor’s susceptibility to heat.

  • Demagnetization - While most permanent magnet AC motor systems are designed to prevent demagnetization, extreme overcurrent conditions or elevated temperatures can partially or completely demagnetize the rotor.  A demagnetized motor requires repair or replacement to restore operation.

  • Control Complexity – Permanent magnet AC motors require more complex control systems and may also require additional feedback elements.  Control algorithms used by the drivers provide precise control of the motor, especially during transient conditions or at high speeds.  Many PMAC motors utilize the flexibility of a variable frequency drive (VFD), while AC induction motors can be run off the line or with a VFD drive as well.

With additional pressures to reduce energy consumption, motor efficiency becomes an important factor in AC motor topology selection. PMAC motors provide clear advantages with efficiency, usable speed range, dynamic response time, and can reduce the physical dimensions and weight of the motors.  It's important to note that the specific advantages and disadvantages will vary due to the specific design, application, and technological advancements in motor manufacturing.

Updated PMAC vs Induction Graph

Source: Marathon Motors

Attributes PMAC Induction
Efficiency *** **
Power Density *** *
Dynamic Response *** *
Speed Range *** **
Cost ** ***
Temperature Range ** ***
Demagnetization ** ***
Control Complexity ** ***
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