Performance Evaluation - Design of Induction Motors

Performance Evaluation:

Based on the design data of the stator and rotor of an induction motor, performance of the machine has to be evaluated. The parameters for performance evaluation are iron losses, no load current, no load power factor, leakage reactance etc. Based on the values of these parameters design values of stator and rotor can be justified.

Iron losses: Iron losses are occurring in all the iron parts due to the varying magnetic field of the machine. Iron loss has two components, hysteresis and eddy current losses occurring in the iron parts depend upon the frequency of the applied voltage. The frequency of the induced voltage in rotor is equal to the slip frequency which is very low and hence the iron losses occurring in the rotor is negligibly small. Hence the iron losses occurring in the induction motor is mainly due to the losses in the stator alone. Iron losses occurring in the stator can be computed as given below.

(a)  Losses in stator teeth:

The following steps explain the calculation of iron loss in the stator teeth

(i)                Calculate the area of cross section of stator tooth based on the width of the tooth at 1/3^rd height and iron length of the core as A'_ts= b'_ts x l_i m²

(ii)             Calculate the volume all the teeth in stator V_ts = A'_ts x h_ts x S_s m³

(iii)    Compute the weight of all the teeth based on volume and density of    the material as

W_ts = V_ts x density. ( density of the material can be found in DDH) (7.8 x 10^-3 kg/m³)

(iv)           Corresponding to the operating flux density in the stator teeth of the machine iron loss per kg of the material can be found by referring to the graph on pp179 of DDH.

(v)             Total iron losses in teeth= Iron loss /kg x weight of all teeth W_ts ie result of (iii) x (iv)

(c) Losses in stator core

Similar to the above calculation of iron loss in teeth, iron loss in stator core can be estimated.

(i)      Calculate the area of cross section of the core  as A_cs = d_cs x  l_i  m²

(ii)             Calculate the mean diameter of the stator core below the slots as D_mcs= D + 2 h_ts + dcs m

(iii)           Compute the volume of stator core as V_cs = A_cs x π D_mcs  m³

(iv)           Calculate the weight of the stator core as W_cs = V_cs x density

(v)             Corresponding to the operating flux density in the stator core of the machine iron loss per kg of the material can be found by referring to the graph on pp 179 of DDH.

(vi)           Total iron losses in core = Iron loss /kg x weight of core W_cs ie result of (iv) x (v)

Total iron losses in induction motor = Iron loss in stator core + iron losses in stator teeth.

In addition friction and windage loss can be taken into account by assuming it as 1- 2 % of the out put of the motor.

Hence total no load losses = Total iron losses + Friction and windage loss.