Employing electronically-commutated motors (ECMs), the input of a variable speed heat pump resembles a large switch-mode power supply (e.g., 3 kW or more). The ECM is a dc brushless motor with stator construction similar to a three-phase ac induction motor. The stator winding current is sequenced and switched creating a rotating magnetic field. This switching is performed electronically by an inverter. The rotor uses permanent magnet construction.
Conventional heat pumps have a current THD of 13% with around 9% third harmonic content; some of the newer ECM designs have current THD values of 123% with the third harmonic content of 85%.
Because of the large load, there is concern that it would only take relatively few installations of this type of heat pump to cause voltage distortion problems. In , the authors found the voltage distortion on a distribution feeder reached 10% when the penetration rate for these variable-speed drives reached 10%, i.e., when 10% of the homes installed the new ECM designs. Though a 10% THDV would be deemed unacceptable by most of us, it does give us a sense of how much of a particular nonlinear load would cause problems on a residential feeder.