Alternating Current (AC) induction motors have become a more common choice among EV manufacturers. The AC induction motor has, for example, powered Tesla models up to the current Model 3. It’s the most commonly used electric motor in the world.
Induction motors don’t have magnets. Rather, they have stator windings positioned around the inside of the motor casing. The stator windings are supplied with three-phase alternating current. Because each of the three phases changes polarity sinusoidally according to the frequency of the power supply, the magnetic fields produced effectively rotate around the inside of the case.
The rotor in the middle of the stator windings consists of a ring of aluminium bars set longitudinally and all joined together at both ends by conductive end-rings. A pack of annular, laminated steel plates sandwiched together form a core into which the bars are set.
The electromagnetic fields of the energized stator windings ‘cut’ through the rotor bars inducing current and corresponding electromagnetic fields in the rotor. The rotor fields are then attracted to the rotating stator fields and begin to turn with them. The induced rotor motion becomes torque at the attached output shaft.
Current and electromagnetic fields are only induced in the rotor when the stator fields are cutting through the rotor bars i.e. when there is relative motion between them. If the rotor turns at the same speed as the rotating stator fields there is no relative motion and the rotor, no longer driven by the stator, slows down. Once it does, relative motion is re-established and the rotor becomes driven by the stator fields once again.
So, the rotor always turns more slowly than the stator fields. The speed difference between the two is known as ‘slip’.
Vector Control is a term relating to the control of AC induction motors. Basically, it means altering the frequency of the AC supply to the stator coils.
Lower frequencies mean that the fields will rotate more slowly. Higher frequencies will cause the fields to rotate at greater speed.
As explained above, the stator fields attract the rotor fields, so slowing the former also slows the latter and vice versa. This is a commonly used method for governing the speed of an induction motor.
The Permanent Magnet Synchronous Motor (PMSM) is the most widely used drive unit in current battery electric vehicles. It’s quite similar to the AC induction motor in that it operates on three-phase AC power, and as with the induction motor, the stator has rotating fields.
In one type of PMSM rotor fields are created by DC current supplied through brushes. Again with the brushes.
A better type of PMSM uses permanent rare earth magnets to create the rotor’s magnetic fields. A feature of the PMSM is that the rotor magnets become locked to the rotating stator fields and they turn at the same speed, or, synchronously. This is the type of PMSM becoming popular.
As with induction motors the rotating speed of the stator field can be regulated by vector control. PMSMs provide high torque at low speeds and can maintain the same rotational speed under varying load (within design parameters).
A Switched Reluctance Motor (SRM) is really a type of stepper motor. The rotor has no magnets or windings. It’s what’s known as a salient pole rotor, which means that the magnetically reactive sections of it extend outwards from the centre of the unit radially as discrete sectors, like spokes.
The stator has a similar arrangement but the salient poles extend inward and are energized by coils that attract the radial poles of the rotor.
As the name suggests SRM stator fields must be switched somehow. This is done by means of electronic switching circuits but this is challenging for a range of reasons related to the mechanical features of such motors. The advantage SRMs offer is that there’s no need for expensive rare earth magnets.
Synchronous Reluctance Motors (SynRM) run synchronously like PMSMs.
They are small and light and offer a number of advantages. As this type of motor is developed more it will probably be used more widely in EVs. They have unique rotor designs that improve saliency and increase power factor. Power factor is an indicator of how much input power is used to do intended work, compared with power lost in doing so.
Power density is another fairly common term but it refers to how quickly a motor can convert energy into work. A variation on SynRM is the PM (Permanent Magnet) Assisted Synchronous Reluctance Motor. The addition of permanent magnets can increase the efficiency of the motor without the cost of increased back-emf.
The Axial Flux Ironless Permanent Magnet Motors are quite different from radial flux motors. The motors we’ve outlined so far have been of the radial flux type.
Axial flux motors are lighter for a given power rating and generate more torque because the reaction between the coils and magnets occurs further out from the centre of rotation.
Other advantages include better utilisation of the copper in the motor. Conventional radial flux windings fail to make use of the overhanging ends of the copper windings. Wasted copper can be as much as 50 percent in some motors.
One weakness in the axial flux design is cooling but Magnax, manufacturer of the motors shown, says its design places the winding in contact with the aluminium side covers, which is much more effective.
It’s a very interesting time in electric drivetrain engineering. There are many things to learn and it’s better to start earlier than later.