About the torque-free charging operation

Question

I and one of my friends are working on understanding the below context (paper link):

The study provides an extensive overview of on-board integrated chargers for electric vehicles that are based on multiphase (more than three phases) machines and power electronics. A common attribute of all discussed topologies is that they do not require a charger as a separate device since its role is transferred to the already existing drivetrain elements, predominantly a multiphase machine and an inverter. The study demonstrates how additional degrees of freedom that exist in multiphase systems can be conveniently utilised to achieve torque-free charging operation. Therefore, although three-phase (or multiphase) currents flow through machines’ stator windings, they do not generate any torque; thus the machines do not have to be mechanically locked. Cost and weight saving is achieved in this way, while the available space is increased. For each topology operating principles are explained, and its control elaborated in detail for both charging and vehicle-to-grid mode. Finally, the validity of theoretical considerations and control algorithms of some of the existing charging solutions is experimentally verified and experimental performance of all discussed topologies is compared.

But we don't understand the meaning of the below sentences:

The study demonstrates how additional degrees of freedom that exist in multiphase systems can be conveniently utilised to achieve torque-free charging operation.

Especially we don't get the meaning of the torque-free charging operation.

So, if possible i have asked here to have some answer To better understand the above text.

Thanks.

Answer 1

The paper is about using the EV motor windings as a transformer or inductors as part of the charger when the vehicle is charging instead of using a separate component.

A common problem of doing so is that the current flowing through the motor windings will cause it to produce unwanted rotation or vibration(torque) while the car is being charged. The study describes the various approaches used to avoid that situation.

The only cars I've heard of that use the motor as part of the charging circuit are some Renault/Nissan EVs that are sold in Europe.

Answer 2

The author is talking about charging an electric vehicle by re-using its drive train electronics and the motor windings as parts of the charging circuit.

This means that during charging, currents flow through the motor's windings, which could cause the motor to create mechanical torque and potentially cause the car to move.

It therefore has to be locked in place to prevent it from spinning. The paper describes ways to ensure that the currents through the motor flow such that it doesn't create mechanical torque (and therefore won't spin even when not locked).

Answer 3

Additional phases increase the resolution and frequency of nulling the DC current with AC pulses thereby preventing torque motion of the EV motor.