‘Virtual magnet’ claims reveal why EVs need their rare-earth magnets

There have been a number of news reports recently about a startup in Bengaluru called Vimag Labs that has reportedly replaced rare-earth magnets in motors with so-called “virtual” magnets.

One such report quoted Manish Seth, the start-up’s CEO, as saying: “We remove permanent magnets, replace them with copper coils, and then through software, we generate magnetic fields inside the motor.”

To understand this claim, it helps to first see how electric motors produce magnetic fields and why they use permanent magnets in the first place.

Rotor and stator

A motor needs a magnetic field to operate. One way to create it is using a permanent magnet. Another is to pass current through copper coils that have been wound around a ferromagnet, temporarily turning it into a magnet. This is called an electromagnet — and it is this approach that the startup has used, instead of permanent magnets.

When an electrical machine converts electricity into mechanical motion, it is called a motor. On the other hand, a machine that converts mechanical movement into electricity is called a generator. In both cases, the conversion happens via a magnetic field.

In a motor, for example, mechanical energy is harvested when the electric energy is used to rotate a component called the rotor. In a generator, electricity is harvested through conductors embedded in a stationary component called the stator.

You can imagine both the stator and the rotor to be two concentric cylinders. Most of the time, the rotor is the inner cylinder. And the magnetic field is established in the gap between the stator and the rotor for the energy conversion process to happen.

An old technology

This is how the large generators in hydroelectric, thermal, and nuclear power plants work. And for a long time now, they have used electromagnets instead of permanent magnets.

In these generators, a turbine spins the rotor. In a thermal or nuclear power station, the turbine is driven by steam. In a hydroelectric power station, it is driven by falling water. The rotor rotates at a fixed speed to generate an alternating current (AC) of frequency 50 Hz. The rotor has copper coils wound around its ferromagnetic core. When these coils are supplied with a direct current (DC), they create a magnetic field, and the core acquires a magnetic north and a magnetic south pole. And as the rotor spins, this magnetic field sweeps past the stator windings, inducing an alternating voltage in them. This helps convert mechanical power into electricity. This technology has been around for 135 years.

The next question is how electricity can be fed continuously into coils that are themselves rotating. Enter: the brushless excitation system. The rotor coils need DC — but supplying it through sliding electrical contacts is difficult because the rotor is constantly spinning. The solution is to mount a rectifier, a device that converts AC into DC, on the rotor itself. To this rotating rectifier, an AC is supplied from another generator. And this other generator is coupled to the same shaft of the turbine imparting mechanical power to the main generator.

Software and magnetism

This is essentially the principle behind the start-up’s “virtual” magnet: software regulates the current flowing through the electromagnets, thus controlling the strength and direction of the magnetic field. The rectified current can be fed in a controlled manner to the copper coils in the rotor that would make the magnetic strength and direction controllable.

In other words, the software is not creating magnetism by itself. It is just controlling how much current flows through the electromagnets, and thus how strong the magnetic field becomes. There is no novelty in this design.

A permanent magnet establishes the magnetic field in the air gap of an electrical machine in a one-step process. And because the magnet already exists, no electrical energy is needed simply to create the magnetic field. On the other hand, with electromagnets, the magnetic field first has to be established, and the electric current modulated as per the requirement using software.

And every additional step here consumes some energy. There are magnetic losses in the ferromagnetic core, called core losses. The copper conductor has resistance losses. The electronic switches have switching and conduction losses. As a result, it is unlikely for a motor with an electromagnet to be more efficient than a motor with a permanent magnet.

Electromagnets in EVs

BMW and Renault have been trying to build such an electrically excited motor. The three-phase induction motor that Nikola Tesla invented in 1888 — a light yet rugged machine — was used by the first version of the Model S from Tesla in 2012. However, its efficiency was not good enough.

Every 0.1% increase in efficiency of the propulsion motor drive matters in an electric vehicle (EV). This is because any increase in the drive’s efficiency will improve the range drastically for a given battery rating. Put another way, for a given mileage, the battery size can be reduced when the efficiency of the motor drive improves. Thus, increasing the motor drive’s efficiency will bring down the weight and cost of the car perceptibly.

In any EV, the battery pack is the costliest and heaviest component, which is why the focus is on optimising the battery’s rating. In turn, permanent magnet synchronous motors dominate the EV market because their efficiency is the highest at present.

Important constraints

Electromagnets are not the only alternative to permanent magnets. Another approach is the switched reluctance motor (SRM), whose rotor contains neither permanent magnets nor copper coils. Instead, SRMs have lower rotor inertia than even induction motors. In exchange, their torque comes in spurts, which makes the motor noisy and less efficient.

Recently, Honda joined hands with a start-up from Canada called Enedym to improve the performance of an SRM to be used in an EV. Hitachi Astemo is similarly trying to use a synchronous reluctance motor in a battery-driven EV to get rid of the dependency on rare-earth magnets.

On the whole, a large amount of effort has gone towards replacing permanent magnets in the rotor with either electromagnets or just a piece of iron. However, efficiency, starting torque capability, and the maximum speed achievable are some important constraints that have kept these motors out of EVs.

At this time, we will have to wait and see how the Bengaluru start-up fares in the four-wheeler EV market.

G. Bhuvaneswari is professor, Electrical and Computer Engineering Department, Mahindra University, Hyderabad, and former faculty member, IIT-Delhi.