Heating of Electrical Machines
Whenever a machine transforms energy from one form to another, there is always a certain losses in various parts. Copper losses(I²R losses) in conductors ,Eddy current and hysteresis losses in iron and friction and wind-age losses in mechanical.
The loss takes place in the machine itself, causing
- Increase in temperature
- Reduction in efficiency.
The rise in temperature depends on the balance of heat generation. and that heat is dissipation through the cooling surface.
Type of heat dissipation in machine
Thermal resistivity of sir is for more than the thermal resistivity of insulating material used in machine. If there air bubble present in insulation of machine, it may dangerous affect by heat dissipation. In conduction heat transfer is a heat is transfer by direct contact.
heat dissipation by conduction per unit area of the surface
W = (θ1 – θ2) / ρ.t watt/m²
where, θ1 = higher temp. of one the surface
θ2 = normal temp. of another surface
ρ = resistivity of material
t = length of medium
if heat is dissipated is X , then total heat dissipation is , Wt = X (θ1 – θ2) / ρ.t
Conduction is most effective in solids.
Convection is the transfer of heat by the actual movement of the warmed molecules. when any machine gets heated due to gases or liquid in contact with the heated body. the body become lighter rises giving place to other cooled body which is get heated. heat is being dissipated from the heated surface this process is called convection. shown in fig..
when gas or liquid are in contact with the heated body in natural way. heat dissipation by natural convection per unit are
W = λç θ
Where, λç = specific heat dissipation
if heat is dissipated is X , then total heat dissipation is W = λç θX
Radiation is quite different from conduction and convection. The heat transferred by photon of electromagnetic radiation. the heat is dissipate by radiation from a machine surface , depends on the temperature of surface.
The heat radiated by a black body per second per unit area is proportional to the fourth power of the absolute temperature.
where, T1 = absolute temperature of hot body
T2 = temperature of absorbing body
σ = stefan constant
e = emissivity (1 for ideal radiation)
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