Estimation of motor efficiency at full load:-Method-1

Full load efficiency of the motor is estimated in this method.
Chronological order of measurements is as follows.

1. If the motor has been in operation prior to this test, stop the motor, decouple the load from the motor and keep the motor idle till the it cools down to ambient temperature. Usually, it may take about 2 hours.

2. Measure winding resistance R _ph-a at cold conditions. Record the ambient temperature T_a

3. Apply voltage across the motor at no load and start the motor.

4. Measure line voltage (U), line current (I_nl), frequency (f), energy (E_nl) and time duration (t).

     From measured energy (E_nl), estimate power consumption (Pi_nl) by dividing E_nl by time duration.

    Direct power input measurement (Pi_nl) can also be done using power meter instead of energy and time measurements.

5. Calculate phase current (I_ph-nl) from line current (I_nl) as given below.

    For Delta connected windings, phase current, I_ph-nl = I_nl/√3

    For Star connected windings, phase current, I_ph-nl = line current

6. Calculate stator copper loss at no load and subtract this from no load power to get constant losses.

     No load stator Copper loss, P_cu-st-nl = 3 x I_{ph-nl ²} x R_ph-nl

     Constant loss, P_k = Pi_nl - P_cu-st

7. Estimate friction & windage losses, P_fw. Generally it is sufficient to assume the friction and windage losses       as follows.

     For Drip proof motors, friction & windage losses 0.8 to 1.0% of motor rated output

     For TEFC motors, friction & windage losses 1 to 1.5% of motor rated output

8. Estimate core losses

     Core losses, P_fe’ = P_k -P_fw

     Correct core losses to the rated voltage, U_r, by multiplying with the factor (U/U_r)²

                                   P_fe = P_fe’ x (U/U_r)²

9. Calculate stator winding resistance at full load. i.e. at temperature as defined in the class of insulation as given in Table 4-1.

                                   R_T = R_ph-a * [(235 + T_R) / (235 + T_a)]

10. Estimate Stator copper losses at full load, assuming nameplate full load current and corrected stator resistance at full load.

                                   P_cu-st-FL = 3 x I_ph-FL² x R_T

11. Obtain stray losses, as a % of input power from fig.5.4 corresponding calculate stray loss, P_s by iterative procedure.

12. Calculate full load slip (s_FL) from the rated speed (N_FL) and synchronous speed (N_s) at the rated frequency.

                                     s_FL = (N_s - N_FL) / N_s

13. Calculate rotor input power from rotor output at full load.

               Power input to rotor, Pi _rot = Rotor output / ( 1- s_FL)

                                                         = P_mech / (1- s_FL )

      Rotor output at full load is the nameplate output kW rating of the motor.

14. Calculate rotor copper losses from full load slip and rotor input

                Rotor copper loss, P_cu-rot = s_FL x Pi _rot

15. Total losses at full load is sum of all the above losses

               Total losses, P_T = P_fw + P_fe + P_{cu-st -FL} + P_s +P_cu-rot

16. Efficiency at full load is obtained from rated output and estimated total losses as

             
                Efficiency at full load, η_FL = [ P_mech / (P_mech + P_T)] *100 %