What makes a good electric motor for an EV?

The Basics

This latest phase of vehicle electrification is changing transportation globally. It appears that new motor technologies are invented almost daily driven by eager marketing efforts. Every EV motor used in direct propulsion, inside a transmission, or directly driving a wheel/axle, can be traced to existing motor technology. 

AC motors, DC motors, Permanent Magnet Motors, Internal Permanent Magnet Motors, Synchronous Motors, Asynchronous Motors, Induction Motors, Reluctance Motors, Axial Flux Motors, Transverse Flux Motors, and ANY combinations of these well-known technologies are all being used in EVs. There are even new combination names that have popped up in a quest for differentiation. Nothing has really changed. All these motor technologies were invented 50-100+ years ago.    

What has changed is methods for manufacturing, materials used, control methods for optimizing performance under certain application conditions, and control systems that convert power and control electric motors. These new approaches are enabling the application of existing motor technologies for use as motors and generators.

To really understand the requirements for electric motors used in electric vehicles, we first need to get back to cover some terms.

1. The Electric Motor is an electromechanical power converter that takes electrical power in the form of current and voltage and converts that to mechanical power in the form of torque and speed. 
2. An Inverter is an electric power converter that exchanges DC power to AC power, or AC power to DC power, or AC power to AC power at different frequencies. 
3. A Battery is a source of stored electrical energy in the form of current and voltage.
4. An Internal Combustion Engine, (diesel or gas), is a power converter that converts fuel and air to mechanical power in the form of torque and speed.
5. A Power Control Module is a computer that controls fuel, air, and ignition to create torque and speed for an internal combustion engine. Note: before computers, passive systems were used to control the internal combustion engine through human interfaces.
6. Fuel is a combustible liquid that stores energy. When mixed with air and ignited it produces energy in the form of heat and pressure.
7. An Electric Motor can also be used as a Generator with the proper electronic controls. These machines can be used to generate power, recharge the battery and provide electromagnetic drag to help decelerate a vehicle.
8. A Hybrid Electric Vehicle contains a system that includes all the above items and uses them when and where they are appropriate. It is limited in range to a combination of stored energy in the battery and the fuel. It is limited in peak power based on battery technology and the generator function attached to the internal combustion engine (ICE).
9. A Battery Electric Vehicle contains only the Battery as the energy source. It has one or more Electric Motors and corresponding Inverters. It is limited in range to available stored energy in the battery. It is limited in peak power based on the battery technology used.

Energy Storage and Efficiency

A vehicle’s range depends on the amount of stored energy and the rate at which the stored energy is depleted. In typical ICE fuel powered vehicle with 75 liters of fuel can produce 600 kilowatt hours of energy.  For a 2000 kg vehicle, this may result in 900 kilometers of range. This amounts to .66 Kwh/Km.

A battery electric vehicle of the same weight and size with 100 kilowatt hour of battery might have a range of 450 kilometers. This results in 0.22 Kwh/Km. The difference is in the efficiency of the electric drive train which can be in the 80% range versus ICE with transmission included are in the 20% range. 

The above numbers are generic and intended to show a basic comparison. There are always exceptions and corner cases.

Battery Usage/Storage

Battery technology is advancing, albeit slowly. The latest chemical and physical compositions are adapting to the new form required. The most common theme is to design battery packs so they can be placed low in the vehicle and lower the center of gravity. This option was not present in ICE vehicles.

Batteries are rated in kilowatt hour (Kwh). If a battery is 100 Kwh one might assume it can 100 Kw of power for 1 hour. However, batteries are not rated exactly like this. Every battery type has a discharge rate based on a set current output and time. A 100 Kwh may be rated at delivering 5 Kw for 20 hours, or it could rated to deliver 50 Kw for 2 hours. EV batteries are sometimes altered to last longer than batteries used in Hybrid vehicles. Some Hybrid batteries can be discharged and charged at a faster rate.

The latest battery technologies need electrical and thermal management systems. This is complicating many EVs. The battery pack is made up of many independent cells. The cells are not balanced during production and voltage and impedance vary slightly from unit to unit. During charging and during use temperature can greatly vary based on some cells being overcharged while others are under charged.

The Electric Motor Drive System

For this discussion, a drive system is a series of power converters connected together and excludes the energy source. The Electric Motor Drive System starts with an Inverter that converts energy source to electrical power in the form of current and voltage. The electrical power flows to an electric motor which converts electrical power to mechanical power in the form of rotating torque and speed. The output of the electric motor feeds the Drive Train which modifies the mechanical power and translates this power to the wheels. The wheels then convert rotational power to kinetic energy moving the vehicle.

The Electric Motor Drive System can be used with any type of energy source. The most common source for EVs is the battery. However, over time fuel cells have popped up as an energy source. Fuel and ICE and a generator could also be this source of electrical power. The Series Hybrid Vehicle utilize this strategy, where the Fuel and ICE is used as an electrical power plant and all of the propulsion is done with the Electric Drive System.  Parallel Hybrid Vehicles combine ICE power and Electric Motor Power to act on the Drive Train.

Electric Motors used in EVs

The most widely used electric motors in EVs today are considered Synchronous Permanent Magnet machines by design (a.k.a. Brushless DC). To enable wide speed ranges with the latest control systems and reduce the volume of magnet material, magnets are typically embedded into an iron structure in the rotating rotor shaft. This is commonly referred to as internal permanent or embedded permanent magnet construction. This type of electric machine works well as a motor and a generator due to its permanent magnet construction.

There are a few EVs that still use Asynchronous motors, (a.k.a. AC Induction motors). EVs from the 1990s started out using Asynchronous motors because of their natural ability to field weaken offering a very wide speed range. The largest automotive EV manufacturer still uses Asynchronous motors today. There is a debate on the advantages of Asynchronous motors versus Synchronous Permanent Magnet motors. Asynchronous motors are less efficient over their operating range and they are less efficient when used to generate power, they are also heavier and larger. On the other hand, they do not contain magnets which are currently plagued with supply chain issues and raw materials that are mined in certain politically challenged geographies. Availability may offset efficiency in the next decade.

Early EVs used DC motors with brushes, circa year 1900, when 1/3 of all vehicles were BEVs. EVs were for the elite and served as popular city vehicles. Two versions were utilized, field wound DC and permanent magnet DC. Field wound DC motors, still in use today in many Golf Carts and Fork Lifts, have that natural field weakening attribute providing a wide speed range and high starting torque.  

What is a Synchronous Permanent Magnet Motor?

The Synchronous Permanent Magnet Motor is typically comprised of an electromagnet stator and a permanent magnet rotor. Stator is a term derived from the stationary part of a motor. Rotor is the rotation part of a motor. Three electrical phases are wound around the stator creating electromagnets that can change polarity. The rotor is comprised of a specific number of magnets or magnetic poles in order to create an operational speed range required.

Each electrical phase is usually comprised of copper wire that is wound around iron stator teeth. The polarity of these stator electromagnets change based on the Inverter. The rotor spins within the stator in a traditional motor, but in some cases the rotor may spin adjacent to the stator or outside of the stator.

Torque is created when rotor magnetic fields are properly aligned with stator electromagnetic fields. In order to keep torque applied as the rotor starts to rotate the inverter changes the stator phases to always lead the rotor and cause it to rotate.  This phenomenon is called commutation. An internal or external sensor on the rotor feeds information to the Inverter allowing it commutate the phases with a goal to keep the optimum magnetic fields aligned. At perfect alignment the maximum torque can be produced and is typically a function of the stator phase current amplitude.   

Installing magnets internal to the rotor allows the minimum amount of magnet material to be used. It also creates salient poles in the rotor core and produces a reluctance torque with angle based on the interaction with the stator field. The latter allows for field weakening, a method of increasing the motor’s speed range without sacrificing low speed torque output. The internal permanent magnet topology also retains the magnets mechanically and is suitable for high-speed operation.

There are many methods of adding magnets and shaping the rotor by adding more magnets in certain places to optimize torque, or field weakening, etc. In figure 1 below, the housing and shaft have been removed. The core electromagnetic part of any motor is its rotor and stator. The stator, a universal term, contains the coils and electromagnets. The rotor, in this case spinning internal to the stator, contains the permanent magnets.

Figure 1 below shows a cross section of a Synchronous Permanent Magnet motor. The configuration of this motor is considered a radial airgap motor or radial flux motor. It is by far the most widely used.

Figure 1 Axial View Cross Section PMSM with Internal Magnets

As mentioned in the introduction there are many motor types of these motors used today in EVs. Axial Flux motors are getting lots of press lately. In some cases the torque output for axial flux motors can be higher than radial flux motors. Axial motors are also Synchronous Permanent Magnet Motors. The flux path is axial compared to radial shown in Figure 1.  The axial flux motors can also have internal permanent magnets. The advantage of axial flux motors is the ability to stack multiple torque producing active motors on one shaft. The disadvantage is thermal conduction of the heat to the outside world.

Manufacturing Techniques and Improvements

For many years motor manufacturing has been a labor intensive process. Some automated winding machines existed for certain permanent magnet synchronous motors, but many motors still include a labor intensive process for installing the phase winding.

A recent development has been hairpin stator winding. This method using fewer thicker wires inserted into the motor stator slots has become very automated. Almost all automotive motors are either using this or moving to it as a low cost high performance technique.  Below is an automotive alternator stator with hairpin winding compared to a similar stator with hand stuffed winding.

In order to take advantage of hairpin winding techniques, the stator design has changed to have a higher number of teeth, each with a small number of wires in it. This make for a more efficient motor construction and allows for automated winding techniques.

Early synchronous permanent magnet motors used asynchronous motor stators (like the stator on the right) with a permanent magnet rotor. For a short time, machine winding forced the number of stator teeth to reduce to allow for needle winding. This has quickly changed to an increase in stator teeth to reduce the number of coil turns on each tooth and allow hairpin winding technology.

Hairpin windings are inserted in between each tooth and welded at the rear of the motor. Thick rectangular magnet wire can be utilized and shaped to optimize motor fill without having to stuff wire into the slots.

Rotors have become much simpler in construction also. Most synchronous permanent magnet motors have surface mounted permanent magnets on the outside of the rotor (or inside of the rotor if the motor is an outside rotating construction). Today, the minimum amount of magnet material is used and inserted deep into the rotor core, forming magnetic poles in the steel at the surface.

Figure 3 Surface Mount PM Rotor vs Internal PM Rotor

Field Oriented Control/Sinusoidal Control

By far the most advances in electric motor operation have come on the Inverter/Motor Controller side. More reliable MOSFET and IGBT transistors (as well as other new technologies) have become available with smaller sizes and lower losses. Processors and DSPs have allowed for lots of high speed calculations and better faster control execution.

Field Oriented Control (FOC) is classical Asynchronous motor theory that decouples torque and flux by transforming the stationary phase currents to a rotating frame. When applied to Synchronous PM motors with internal permanent magnets, it can allow them to operate above base speed. The process involves control of direct and quadrature currents in the motor through the use of a rotor sensor and field weakening the rotor fields in order to increase speed.   

In an AC Induction or Asynchronous motor, the rotor field is induced by the stator field and is shifted by the conductors in the rotor. The position of the rotor shaft is not always aligned with the magnetic field in the airgap. Indirect FOC uses a software estimator algorithm to predict where the actual rotor magnetic field is and uses that to optimize the phase angle between the rotor and stator. Asynchronous motors naturally field weaken as the speed increases producing a constant power output and very high speeds. The FOC control for Synchronous PM internal magnet motors attempts to mimic what is natural for Asynchronous motors.