I dont remember seeing you or anyone else saying that.
Whilst I have never done that yet it seems a perfectly possible method.
Why shoule it not be used? Technical reply please!
Tony
because there is a danger of permanent damage to the pump due to either overheating or cavitation
To run a DC motor from an AC supply you rectify the AC and use switch mode power supply to produce a lower higher current DC voltage.
Since the desire was for greater power efficiency then I have always expected a heating pump would be using a low voltage DC permanent magnet motor because I have always expected them to be more efficient.
But of course manufacturing costs are important too.
I have not had the opportunity to examine the current range yet and see how they are achieved.
I have not had a look a the newer domestic ones either as the pumps I work on are commercial and controlled either by external VFD or pump mounted VFD, but I doubt they would be dc, you can control a standard pump anyway merely by varying the voltage, although its much better to vary the frequency also.
Light commercial pumps such as the grundfos Alpha are ac motors as they have internal frequency convertors
Matt