Stator copper losses (I2R) –
These are the losses caused by current flowing through the windings of the stator. As seen by the formula, these losses vary with the square of the current through a conductor, and inversely with the conductor material resistance and cross-sectional area.
These losses appear in the stator and rotor. The copper losses in the stator can be exacerbated by higher frequency where the AC currents crowd in outer edges of the conductors (skin effect).
A very common misconception is that a motor with lower voltage and higher current will have higher I2R losses than an identical motor with a higher voltage. Typically the voltage of a motor is changed by changing the number of turns in the windings. The total current flowing through the slots is the same, and the losses and efficiency will be about the same.
Eddy current losses –
These are I2R losses of currents induced in the stator and rotor from AC flux. These losses are why the magnetic steel of the stator, and generally rotor are laminated, to reduce the circulating path for these losses and greatly reduce the losses. Generally these losses are in the magnetic steel laminations, but they can be in any conductive material, for example the finger plates, end bells and frame. These losses vary with the flux density and frequency of the AC flux.
-These losses appear in the stator principally because of the fundamental AC flux, and in the surface of the rotor where although principally DC flux is present, but some AC flux is induced from slot passing.
Hysteresis losses –
These losses result from a the “resistance” (not electrical resistance) the magnetic material has to being magnetized. The magnetic domains in the material must align to be magnetized and pass flux. These rotating these domains to align takes some energy. Where AC magnetic fields are present, each zero to (+) magnetizing to zero to (-) magnetizing cycle takes some energy to align the domains and this is the hysteresis loss. Different magnetic materials with different crystal structures have very different hysteresis loss characteristics. Hysteresis losses vary proportionally to flux density and frequency.
– These losses appear in the stator in a synchronous machine.
Windage losses –
These are losses from moving air around inside the machine. The rotor “whips” air around and the air resistance causes losses. The rotor fans move air as well. Sometimes the rotor fan losses are included in windage and sometimes they are calculated separately; however, the physics of both are the same. Windage losses vary with the airspeed relative to the motor surfaces squared. In low speed machines these are often trivial, but in large high speed machines (like flywheels and some 400Hz generators) windage losses are a dominant loss. To reduce windage losses and improve cooling, large power generators are sometimes sealed and cooled with hydrogen rather than air.
These losses are induced into the air itself.
Bearing losses –
These are the losses from friction in the motor bearings. Typically these losses vary proportionally with the speed. In hydrodynamic bearings, bearing losses vary with the speed squared.
– These losses are in the bearing material. In hydrodynamic bearings they are in the lubricating fluid.
There are also a number of other losses that are more subtle in synchronous machines they are –
Cross-slot losses –
The magnetic flux prefers to flow through the stator iron material. However, as the stator teeth have higher flux at higher loading, they begin to saturate and the flux will cross through the slots and cut the copper. This will cause additional eddy current losses in the stator copper.
Dielectric losses –
Like hysteresis in magnetic materials, dielectric materials (the insulators) resist the presence of the electric field across them. As the electric field in applied (each cycle of AC voltage) the charge is distributed across the insulators (acting as capacitors) and no capacitor is ideal. They leak charge and they take some small amount of energy to become charged. This amount of energy varies significantly from material to material, and the feature is referred to as the “dissipation factor”. This loss mechanism varies proportionally to frequency and the square of voltage. In high voltage machines (utility generators and transformers) it should not be ignored, and in high-frequency machines, it may not be trivial and sometimes overlooked.
Stray losses –
This sounds like a technical term, but it is really a collection of losses that are from unknown sources that are characterized empirically.