In a general sense, motor losses can be classified as either mechanical or electrical.

Mechanical Losses:

Mechanical losses come primarily from bearing friction and any windage relative to the rotating rotor

Pure friction losses are a linear function of speed, and windage, the displacement of air by any rotating component in the motor, accounts for a large proportion of mechanical damage. Minimizing the effective frontal area can bring huge benefits in reducing windage losses. Motors with smooth rotors, such as permanent magnet synchronous motors and induction AC motors preferred in electric vehicles, will have less windage losses than equivalent sized motors with windings in the rotor (such as all DC and wound rotor AC motors).

Electrical Losses

Electrical losses can also be divided into two major categories, traditionally referred to as “copper” and “iron”, with the stator winding conductors of the motor being made of copper and the magnetic structure/frame being made of steel. Copper losses include any power consumed to produce the magnetic field. This includes the stator and rotor in an AC induction motor, any additional armature current required to achieve field weakening in a permanent magnet AC motor, more obvious resistive losses, and less obvious AC losses (from skin effect and proximity effect).

Resistive losses, also known as I2R losses, tend to dominate the motor losses in electric vehicles, which often run at high currents and low speeds. In this case, the product of speed and torque, i.e. the total motor power, is very low, and I2R does not care about the speed (voltage) component, so the efficiency of the electric vehicle’s motor will be very poor when starting a load from a complete stop.

Purely resistive losses occur at frequencies from DC to light, while the skin effect and proximity effect can be thought of as resistive losses that increase with frequency. The skin effect is the tendency of current to become increasingly confined to the periphery of a conductor as frequency increases, caused by tiny current loops induced in the conductor (eddy currents) by the alternating current flowing through it. Such eddy current loops are proportional to the magnitude of the source current and also proportional to the rate of change of the magnetic field (i.e. the frequency of the source current). These eddy currents impede the flow of current in the center of the conductor and add current at the periphery, which is why the current becomes increasingly confined at the periphery.

The usual solution to the skin effect is to split a large wire into many small wires that are insulated from each other but parallel, but this causes more losses from the proximity effect, which is basically the same as the skin effect, except that it is eddy currents caused by AC currents from other nearby conductors. Basically, the more layers of windings there are, the higher the proximity effect losses.

Eddy currents are generated because any time-varying magnetic field will induce currents in any nearby conductors (including the source conductor). A time-varying magnetic field will induce a voltage in nearby conductors (including itself), and this voltage will cause current to flow in a loop around the source conductor. For a given separation distance, loop area, and rate of change of magnetic flux, the induced voltage is fixed, so the current generated will be inversely proportional to the loop resistance and directly proportional to the loop area and the source current frequency. Therefore, eddy currents in better conductors such as silver and copper will be higher than in poorer conductors such as electrical steel or ferrite (which are almost insulators). Electrical steel is an iron-silicon alloy specifically designed to maximize the conductor resistivity without excessively compromising its magnetic properties, such as hysteresis losses and saturation flux density.

The absolute resistivity of electrical steel is quite low, and the resistivity of ferrite is very high, but it also has a much lower saturation limit (typically 0.35 T compared to 1.3-1.5 T), making it less feasible for use in the armature of a motor. Fortunately, it is possible to reduce the loop area by simply breaking up a monolithic structure into a stack of laminations that are insulated from each other (usually with a thin lacquer or oxide coating). The thinner the laminations used, the lower the eddy current losses, and as the laminations get thinner, their insulating coating becomes a larger and larger proportion of the total thickness, so there is a practical limit to how thick the laminations can be used.

The last of the iron losses is hysteresis, which is basically the resistance to changes in magnetization direction or flux density. The armature in all motors is excited by an AC current, whether provided by an external inverter or brushes and commutator, and its magnetic circuit repeatedly experiences large fluctuations in flux density between opposite polarities. Magnetic materials that tolerate such operation need to be “soft”, that is, easy to magnetize (low coercivity) while not retaining magnetic moment (low remanence). Conversely, materials that are difficult to magnetize (and demagnetize) are classified as “hard” materials, and they tend to make good permanent magnets. Hysteresis losses are basically a measure of how soft the magnetic material is, and it depends on the flux density.

Finally, there are various “stray” losses, the most notable of which is leakage flux, which is basically any flux that is not connecting the rotor and stator together. It is not doing any useful work. This unconnected flux also subtracts from the effective AC voltage exciting the armature, which translates into inductance. The final loss mechanism considered here is common-mode capacitive coupled currents, which tend to have very little actual power loss, but can corrode bearings and damage insulation on phase windings, and can also cause the vehicle to fail EMI/RFI radiated tests.