Thursday, August 4, 2011

Losses in Induction Motor

       The various power losses in an induction motor can be classified as,
i) Constant losses
ii) Variable losses
i) Constant losses : 
      These can be further classified as core losses and mechanical losses.
      Core losses occur in stator core and rotor core. These are also called iron losses. These losses include eddy current losses and hysteresis losses. The eddy current losses are minimised by using laminated construction while hysteresis losses are minimised by selecting high grade silicon steel as the material for stator and rotor.
      The iron losses depends on the frequency. The stator frequency is always supply frequency hence stator iron losses are dominate. As against this in rotor circuit, the frequency is very small which is slip times the supply frequency. Hence rotor iron losses are very small and hence generally neglected, in the running condition.
      The mechanical losses include frictional losses at the bearings and windings losses. The friction changes with speed but practically the drop in speed is very small hence these losses are assumed to be the part of constant losses.
ii) Variable losses : 
      This include the copper losses in stator and rotor winding due to current flowing in the winding. As current changes as load changes as load changes, these losses are said to be variable losses.
       Generally stator iron losses are combined with stator copper losses at a particular load to specify total stator losses at particular load condition.
      Rotor copper loss = 3 I2r2 R2                        ......Analysed separately
where                    I2r  = Rotor current per phase at a particular load
                             R2  = Rotor resistance per phase

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