Energy savings in Danish trade and industry by reduction of harmonics

The project has included knowledge building on the basis of literature and discussions with a Swedish and a Belgian producer of active filters. In connection with the project, tests have been made on a production line at the firm of Færch Plast, where the distortion from the harmonics, measured as THDU, was 6 to 7 %. Measurements during two days with the active filter switched on and off with an interval of 10 minutes showed a very small difference in power input in the two situations. The power was 2.7 kW higher with connected filter than without filter. As the own consumption of the filter was 5.8 kW, the consumption of the installation itself was 3.1 kW lower, equal to 1 % of the load of the directly supplied induction motors.

As a consequence of the very small differences, measured at Færch Plast, it was decided to transfer the further work to a laboratory in order to measure under so controlled circumstances as possible. Measurements have been taken at KME (Copenhagen School of Marine Engineering and Technology Management) and at TI (Danish Technological Institute). The result of measurements on a 5.5 kW induction motor with a distortion from the harmonics (THDU) of 2.65 % shows no significant change in the input power compared to the situation with almost no distortion. Neither, it appears, is there a significant difference in the motor losses in a situation, where the measurement was taken with two rather distorted mains voltages with a THDU of 6.33 % and 7.66 %, respectively.

Calculations on the basis of the equivalent diagram of the motor show that the additional motor losses in a 5.5 kW motor as a consequence of 5. harmonics of 5 % of the mains voltage (THDU 5 %) only is 2 W, while an experimentally derived formula results in a difference of approx 9 W or 0.2 % of the rated power of the motor. The results of the lab measurements correspond very well to the losses calculated.

On this basis it is concluded that harmonic currents and voltages cause energy losses in induction motors directly supplied, but that the energy losses are fairly low at the levels of distortion, a THDU of up to 6 to 8 %, which occur in Danish industry. Motors, which are supplied through a frequency converter, are not influenced by the harmonics in the supply mains.

The analysis of the motor losses shows that the losses from the load torque, caused by the 5., 11., 17. etc harmonics, are considerably lower than the (very small) current heat losses in the motor, which have been caused by the same harmonics. The analysis also shows that the current displacement in the motors reduces the currents and the losses from the higher harmonics. Considering losses consequently the lower harmonics and especially 5. and 7. harmonics are most important.