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 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. 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 Magnetic 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 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 increase the frequency, eddy current losses in induction motors will be far greater than in permanent magnet motors using powder metal technology.
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.
Permanent Magnet Motor Material Use
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, as shown below:
(Comparison of AC induction motor 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 power metal rotor for a permanent magnet motor can achieve strength similar to competing processes.
The induction rotor, however, still requires the stamping and lamination process. The stamping process results in far more scrap waste than powder metallurgy.
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 Chevy Volt (now discontinued), the Chevy Bolt, and Tesla Model 3.
- 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. Very little detail is available, but rumor has it the magnets are arranged in a Halback array. This array focuses the magnetic lines of flux to fully optimize performance.
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.
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.
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.
(Permanent magnet motor efficiency curve vs. induction motors. This performance chart was developed at about 60 Hz line frequency. As the frequency goes higher, expect performance to become even better. Chart courtesy Empowering Pumps & Equipment)
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 has developed a new motor blueprint combining high efficiency with robust design. 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 now look at the induction motor design that 90% of engineers are working with.
Efficiency of the Three Phase AC Induction Motor
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 brushes and winding the rotor are both unnecessary. Motors of this type are:
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 85% to 96%. See the chart below for torque vs. slip.
(Typical torque vs. slip for AC induction motors -- courtesy All About Circuits)
Induction motors of 50-100 HP for industrial applications vary in weight from 700 to almost 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.
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.
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.
Induction Vs. Permanent Magnet Motor Efficiency: The Winner Is ...
Despite the advantages of using soft magnetic materials in a permanent magnet motor -- 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 an AC or DC magnetic applications, ask our expert below: