Stray Load Loss and Mechanical Loss in an Electrical Machine

Stray Load Loss: As we know that, when an electrical machine is loaded, load current flowing in the armature winding of the machine produces an mmf which distorts the space distribution of mmf produced or set up by the field winding in the air gap. This distortion of space distribution of mmf or flux density wave in the air gap of machine leads to increased loss in core as well as in armature conductors (how in armature conductor? You will get the answer after reading the complete post) from it�s no load value. This increment in the core loss caused by the distortion of air gap flux plus the increment in ohmic loss i.e. I²R loss due to non uniform distribution of conductor current is called Stray Load Loss.

Read Armature Reaction in an Electrical Machine for better understanding of distortion of flux density wave.

In other words, we can say that Stray Load loss consists of two components: a) one originating in the iron part and other in the armature conductor. Again, in iron part of machine, the Stray Load loss consists of Eddy Current Loss in the Stator Cover, End Frames etc. caused by the armature leakage flux under loaded condition and b) increased loss in the teeth due to distortion of the flux density wave.

In conductors, the Stray Load loss is due to the circulating currents set up by the alternating leakage flux produced by the load current in the conductors. Theses circulating or Eddy Currents make the conductor current distribution non uniform and as a result the effective resistance of the conductor increases. Because of this, extra ohmic or I²R loss is produced which is called Stray Load loss. It shall be noted that, in DC Machine, Stray Load loss also occur in the coil undergoing commutation. The Stray Load loss is proportional to the square of the Load Current. But it shall be kept in mind that this is only an approximation as Stray Load loss cannot be calculated accurately.

By convention, in DC Machine Stray Load loss is normally taken as 1% of the rated output for rating of DC Machine above 150 kW. For Synchronous and Induction Machine, Stray Load loss is taken around 0.5% of their rated output.

Must read,Losses in Electrical Machine - Core Loss and Eddy Current Loss

Mechanical Loss in Electrical Machine: Mechanical Loss in an Electrical machine basically consists of Bearing friction, Brush friction and Windage loss. The windage i.e. wind friction loss is the power required to rotate the air through the machine and ventilating ducts. Windage loss is approximately proportional to the square of the speed of rotation of the machine.

Brush friction loss occurs in machine fitted with brushes like in DC Machine. As in Squirrel cage Induction Motor, as no brushes are there, hence brush friction loss shall not be considered for Squirrel cage Induction Motor. Brush friction loss depends upon the brush pressure, coefficient of friction between the brush and rotor body and speed of the machine.

Bearing friction loss is the power loss occurring in the bearing provided in the rotor of machine in the form of heat. This loss depends upon the type of bearing used and the lubrication provided. Better the lubrication of bearing, lesser will be the bearing loss in electrical machine.

It shall be noted that, whenever we talk of Mechanical loss, it shall be understood that we are considering windage, bearing and brush loss (if applicable). Mechanical loss is calculated by just measuring the power required for running the machine at rated speed without providing any excitation and load. Mechanical Loss is basically frictional in nature.

At this point, No Load Rotational Loss shall also be discussed. No Load Rotational Loss is basically sum of mechanical loss and open circuit core loss. This loss can be determined by running the electrical machine as an unloaded motor at rated speed or frequency and with armature voltage equal to normal generated emf. The total power input to the unloaded motor minus a small no load armature ohmic loss (mind that no load armature ohmic loss is applicable only for induction machine) gives the magnitude of No Load Rotation Loss. For quick and better understanding of losses in an Electrical Machine, please carefully observe the figure below and understand. In the figure below, losses are categorized into two parts, one which is considered constant, hence called Constant Loss and other which are dependent on load current and hence they are Variable Loss.

Notice that, as Stray Load loss depends upon the load current therefore it is categorized under variable loss while Mechanical Loss (independent of load current) comes under constant loss. No load Iron Loss is constant and therefore it is under Constant Loss.