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Induction vs. Permanent Magnet Motor Efficiency | Auto Electrification.

Posted by Fran Hanejko - November 29, 2022

With the electrification of the automobile continuing at an accelerated pace, many wonder what type of motor is best for the modern electric drivetrain.

Could it be a three-phase induction motor or a permanent magnet motor? Both motors are currently in use in electric vehicles. Both offer high efficiency and reliable performance. But which is better? 

There’s a strong argument to the permanent magnet motor being superior to the induction motor. The inherent advantages of powder metallurgy -- potential for increased motor performance and lower overall cost -- can be an effective tool in producing these drive systems.

Let’s make a few comparisons of induction vs. permanent magnet motor efficiency to see their advantages and potential shortcomings:

  • Cost
  • Efficiency -- torque, core losses, frequency & motor speed control
  • Material opportunities
  • Uses

The fine details of electric motor design are more complex than described below, but this is a great head start for those weighing a design improvement:

Permanent Magnet Motor vs. Induction Motor Efficiency

With the electrification of the automobile continuing at an accelerated pace, many wonder what type of motor is best for the modern electric drivetrain.

Could it be a three-phase induction motor or a permanent magnet motor? Both motors are currently in use in electric vehicles. Both offer high efficiency and good performance. But which is better? 

There’s a strong argument that the permanent magnet motor is superior to the induction motor. As such, current and new designs are transitioning to the permanent magnet motor design. The inherent advantages of powder metallurgy -- potential for increased motor performance and lower overall cost -- can be an effective tool in producing these drive systems.

Let’s make a few comparisons of induction vs. permanent magnet motor efficiency to see their advantages and potential shortcomings. The fine details of electric motor design are more complex than described below, but this is a great head start for those weighing their options.

Permanent Magnet Motor Efficiency

As the name implies, an EV permanent magnet motor uses permanent magnetics on the rotor (see the graphic below). The alternating current applied to the stator results in rotation of the rotor. Because the magnets are permanently magnetized, the rotor runs synchronously to the switching AC current. The slippage necessary in induction motors is eliminated, improving your mechanical & heat efficiency.

The inherent efficiency of a permanent magnet motor is higher than an induction motor. Both motors use a three-phase design to  fully optimize performance. Induction motors, however, were initially designed to work at 60 Hz. As you increase the frequency, total losses in induction motors will be far greater than in permanent magnet motors (in particular those motors embracing SMC technology).

Regardless of how you fabricate an induction motor, a well-designed, synchronous permanent magnet motor will offer increased range, better performance, implicit with their higher efficiency.

Permanent Magnet Motor Material Use

Induction Vs- Permanent Magnet Motor Efficiency Auto Electrification - component comparison-1

(Comparison of AC induction motor design vs. permanent magnet motor)

In a permanent magnet motor, can the rotor be a solid piece or a series of thinner slices, each having magnets specific for that length? The rotor will minimize cogging torque. This motor design uses the very best of both SMC and sintered soft magnetic (SSM) technologies. 

A powder metal/SMC intensive motor concept eliminates the need for electrical steel laminations, improving the overall performance, eliminating supply chain complexities and maximizing material utilization. 

Electrification & Soft Magnetics Resource Page - Click Here


Uses for Permanent Magnets in Motors

A 50 kW (about 70 HP) permanent motor typically weighs less than 30 lbs. (Note you would still need a DC-to-AC inverter to generate enough voltage and frequency.)

Uses of permanent magnet motors in the automotive industry include the Ford Mustang Mach-E, the Ultium Platforms, all Tesla models, and the list continues to grow.

The speed of the permanent magnet motor is always greater than that of its induction counterpart due to the inherent slippage. The speed can be represented by the following equation:

  • Ns = 120 * frequency / pole count 

(Ns is synchronous speed. Pole count is the total pole count per phase, including both the north and south poles.)

Cost Vs. Performance

One major consideration in permanent magnet motors is the cost of the magnets. If you’ve used high-energy magnets (such as iron neodymium boron), you’ve felt the pain in your budget (or your boss has). The potential waste of stamping the lamination material only compounds the problem. 

Opportunities for powder metallurgy are abundant in these types of motors. The rotors of a permanent magnet motor can be made via sintered powder metal, regardless of whether you’re taking the internal or external design route. The stator can also be produced via soft magnetic composites. At the high switching frequencies expected, the losses in SMCs are lower than that of laminated 3% silicon iron, further improving the efficiency of this design. Simply put, soft magnetic composites are custom-built for high frequencies.

ac electric motor efficiency: intro to soft magnetic composite - download e-book here!

There’s an opportunity for powdered metal to provide additional efficiency to a permanent magnet motor vs. an induction motor. The 3D shape-making capabilities of powder metallurgy allow you to form the stator to totally encase all the wire in soft magnetic composite to eliminate end turn losses.  .

These are some of the many advantages that powder metal -- both sintered soft magnetic materials and SMCs -- offers.

Efficiency of the Three Phase AC Induction Motor

Historically, Nikola Tesla conceived the induction motor in 1883. It’s fundamentally the same basic stator design as the permanent motor, except for improvements in raw materials.

The basic operating principle of the induction motor is that the magnetic field generated in the stator creates an opposing current in the rotor bars. The induced rotor current then creates a magnetic field in the rotor laminations. That opposing field causes the rotor to turn -- with the stator current switching, the rotor is always lagging and causing the rotor to rotate.

The benefits of this induced magnetic field are that brushes and winding the rotor are both unnecessary. Motors of this type are:

  • Dependable
  • Rugged
  • Low-maintenance

 

Induction Vs. Permanent Magnet Motor Efficiency Auto Electrification - diagram

 

 




 

 

Above is the typical configuration of the induction motor. Note the rotor has laminations in the core and electrically conductive material (either copper or aluminum) in the rotor’s slots, the so-called rotor bars.

For most industrial applications (greater than 1 HP) and for automotive drivetrains, the three-phase induction motor is as common as it gets. In this design, the three phases are wrapped around the stator in such a way that gives smoother operation and high efficiency. Three-phase AC motors are self-starting once the voltage is applied to the stator windings. In many instances the so-called rotor bars are angled to give higher torque.

AC Induction Motor Efficiency in Practice

Three-phase usage in industrial applications is relatively easy because the incoming voltage is already three-phase. However, in automotive applications, you have to convert the battery’s DC power to three-phase AC power. This happens through a DC-to-AC inverter. 

With AC induction motors, you must consider the rotor’s speed relative to the incoming frequency of AC power. This is defined initially by the so-called synchronous speed. For an AC induction motor, the synchronous speed is calculated this way:

  • Ns = 120 * frequency / pole count 

(Remember, Ns is the synchronous speed. Pole count is the total pole count per phase, including both the north and south poles.)

For a two-pole AC induction motor operating at 60 Hz, the motor’s synchronous speed would be 3,600 RPM. However, if the rotor were rotating at 3,600 RPM in this configuration, you’d have zero torque from the motor. Ideally, there’s some slippage of the rotor relative to the frequency; typically, this is about 5%. As such, these motors are considered asynchronous motors.

Efficiency of three-phase induction motors can vary from 80% to 90%. 

Induction Vs. Permanent Magnet Motor Efficiency Auto Electrification - torque chart

(Typical torque vs. slip for AC induction motors -- courtesy All About Circuits)

Certain Tesla induction motor models claim to weigh only 70 lbs. and can generate 360 HP at 18,000 RPM. The total weight of the motor and inverter is about 350 lbs. -- still much lighter than the average internal combustion engine. 

This motor is a three-phase design with eight poles per phrase, meaning the AC frequency used to generate this power is about 1,200 Hz. At these operating frequencies, the eddy current heating of the lamination material is going to be quite high. This Tesla car motor requires considerable cooling to keep it from overheating.  

Cost of Induction Motors

A key advantage of AC induction motors for electric vehicles is cost. They’re relatively cheap to build. 

AC induction designs use steel laminations in both the stator and rotor; these can be stamped almost simultaneously from the same sheet of material. In other words, the scrap rate is much lower than your average stamping job.

Induction Vs. Permanent Magnet Motor Efficiency: The Winner Is ...

Despite the advantages of using soft magnetic materials in permanent magnet motors -- SMCs are less common in induction designs -- picking a motor type for your drivetrain is difficult. Each has advantages and disadvantages. 

Despite the AC induction motor being first developed more than 100 years ago, it’s still viable thanks to efficiency and performance improvements in the 20th and 21st century. The permanent magnet motor is a relative newcomer but promises higher performance, higher torque density, and lower weight.

The major sticking point with permanent magnet motors is the potentially high cost of the magnets. Fortunately, there are promising developments on the horizon that could eliminate this drawback.

If you need help designing the components to fully leverage the full potential of powder metallurgy for an AC or DC magnetic applications, see our resource hub:

View the Engineering Hub >(Editor's note: This article was originally published in April 2020 and was updated on November 29, 2022)

Topics: Powdered Metallurgy, Magnetics, Materials, Applications, Design, motors, automotive, electrification


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