Permanent Magnet Synchronous Motor
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:
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 can run synchronously to the switching AC current. The slippage necessary in induction motors is eliminated, improving your heat efficiency.
The inherent efficiency of a permanent magnet motor is higher than an induction motor. Both motors use a three-phase design through fully optimized performance. Induction motors, however, were designed to work primarily at 60 Hz. As you up the Hz in high-frequency induction motors, eddy current losses will be far greater than in well-made permanent magnet motors.
The design of brushless permanent magnet motors provides 2-3x more power density (torque) than induction motors, with about 50% fewer core losses. Regardless of how you bend or shape an induction motor, a well-designed, synchronous permanent magnet motor will offer increased range, better performance, and so on.
In the permanent magnet, a rotor can now be a solid piece made from press-and-sinter powder metallurgy magnetic material (for example). You can design the rotor in such a way as to have the magnets glued to the outer diameter or encased within the rotor, like below:
(Comparison of AC induction motor design vs. permanent magnet motor)
It doesn’t have to be made from electrical steel laminations! A powder metal rotor can have the slots you see in the image above designed via the net-shape nature of powder metal, eliminating any need for costly machining. By using sintered soft magnetic material, a powder metal rotor for a permanent magnet motor can achieve strength similar to competing processes.
Rotor material for induction motors, however, still consists of stamped electric steel laminations. The stamping process results in far more scrap waste than powder metallurgy, and core losses increase as you stack more sheets.
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 Chevy Volt (now discontinued), the Chevy Bolt, and a growing number of Teslas:.
The Chevy Bolt is a 200 HP design with magnets inside the rotor. It uses a 7.05-to-1, single-speed gear reducer to drive the wheels. No estimates of weight are available publicly.
Tesla Model 3
also uses a permanent magnet motor with the magnets arranged in a Halbach array. This array focuses the magnetic lines of flux to optimize MPG equivalent.
Tesla's bigger vehicles, the Model S and Model X, switched their smaller front motors to permanent magnets after seeing the Model 3's impressive range. These models use front-wheel drive while cruising and all-wheel drive when accelerating and in low traction.
Why only convert the front motors? Induction motors still produce high power due to their excellent control of magnetic fields. However, at low power, speed control of permanent magnet synchronous motors is more efficient.
The speed of the permanent magnet motor is the same as that of its induction counterpart:
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.)
Remember, the rotor won’t slip relative to the stator’s operating frequency.
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.
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.
(Related Resource: Permanent magnet motor efficiency curve vs. induction motors. This performance chart was developed at about 60 Hz line frequency. As the frequency rises, expect performance to improve further.)
The above discussion has focused on looking at permanent magnet motors using stator designs similar to those in an AC induction motor. However, there have been major developments in the design of new motor types that also use permanent magnets for improved electric motor efficiency.
Linear Labs, for example, developed a new, high-efficiency permanent magnet synchronous motor. It eliminates some of the expensive rare-earth magnets you’ve been stuck with for years.
We think the permanent magnet motor is the wave of the future. For the sake of completeness, let’s look at the induction motor design that 90% of engineers currently use.
Nikola Tesla conceived the induction motor in 1883. It’s fundamentally the same basic stator design as the permanent motor, but without the permanent magnets.
Its basic operating principle 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 your induction motor rotor design no longer needs brushes and rotor windings. Variable speed and torque control of induction motor is easier during acceleration because the voltage can be reduced at high speed.
Motors of this type are also:
Dependable
Rugged
Low-maintenance
Look at this 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 design is as common as it gets. 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.
Three-phase motor use is relatively easy in industrial applications 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.
Now, how can we control the speed of an induction motor?
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 85% to 96%. See the chart below for torque vs. slip.
(Typical torque vs. slip for AC induction motors -- courtesy All About Circuits)
Could you build a three-phase induction motor with both low voltage and high torque/speed? Technically, yes ... but no.
You'd need to control that small battery pack with a whole lot of amperage. A high-power, low-voltage electromagnetic design would not only need huge (and heavy) copper bars as windings, it'd also generate excessive heat.
Induction motors of 50-100 HP for industrial applications vary in weight from 700-1,000 lbs. Much too heavy for automotive applications, right?
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. It’s also a bit ironic that GM debuted its EV1 vehicle in the mid-90s with an induction motor that was limited by the fact it used lead acid instead of lithium ion batteries.
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.
However, the unique design of the Tesla auto motor is a bit more expensive. It’s hard to find an exact price online, but a four-wheel drive option for the Tesla adds about $4,000 to the vehicle’s total cost. You also have to consider the increased cooling requirements at these high AC frequencies.
Despite the advantages of using powder-based electric motor materials in a permanent-magnet design (SMCs are a nonfactor 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 and possibly lower weight.
The major sticking point with PM motors is the potentially high cost of the magnets. Fortunately, there are promising developments on the horizon that could eliminate this drawback.
We employ the services of a respected motor designer to assist customers with projects just like these. If you need help designing the components to fully leverage the full potential of powder metallurgy for permanent magnet AC or DC electric motor design, see our resource hub:
(Editor's note: This article was originally published in April 2020 and was recently updated.)
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