Thanks Sean On 8 August 2018 at 00:27, Sean Breheny wrote: > The only kind of operation where I think the BLDC motor would work > completely fine with a typical VFD (no commutation sensor feedback) is if > the maximum load on the motor was only a small fraction of its capability= .. > In this case, the motor rotor will tend to self-align with the phase of t= he > driving current. > > Some VFDs may be able to accept position feedback input so that they can > work with BLDC motors (which are also called PMSM or Permanent Magnet > Synchronous Machines and aren't really DC motors at all). Induction motor= s > have no permanent magnetic poles on the rotor - the field from the rotor > comes from current induced in it by slip - which is the difference in the > driving electrical frequency and the rotor rotational frequency (times po= le > count divided by 2). This means that not only is there no special alignme= nt > information to know about the rotor at startup, but in fact the motor nee= ds > to be operated in a slightly asynchronous operation in order to function. > > If you were using a BLDC motor and it got out of sync, there would be 0 > torque averaged over a full rotation but there would be torque ripple and > yes, if that were slow enough (by limiting the maximum driving frequency)= , > it might be able to rock the motor free from a stall condition, but that > would still be pretty suboptimal operation. > > A third option is PM brushless motor sensorless operation. Maybe some VFD= s > can do this, too, I don't know. I designed one that can but it was > specifically designed for use with PM motors in a specific application. > There are several variations of sensorless operation. One of them starts = up > assuming no heavy load but once it is running it can maintain > synchronization with varying and heavy loads by watching the phase of the > back-EMF voltage. Another variant uses knowledge of the way the motor > inductance varies with rotor angle to obtain an initial rotor angle > estimate at startup. Obviously that needs to be matched well to the motor > it is using. > > Sean > > > On Tue, Aug 7, 2018 at 6:33 AM, Justin Richards > > wrote: > > > Outside of the OP requirements but I am curious to know if it would > perform > > favorable if the BLDC was only ever driven at relatively slow speeds by= a > > VFD turning a heavy load. > > Justin > > > > > > On Mon, 6 Aug 2018 03:22 Jason White > > wrote: > > > > > Okay, thanks Sean and Alan. I figured that probably was the case. > > > > > > On Sun, Aug 5, 2018 at 2:37 PM, Sean Breheny wrote= : > > > > > > > Most likely it would work most of the time. However, I would be > > somewhat > > > > concerned about what might happen if the load on the motor changed > > > > suddenly. In such a case the motor might stall with the VFD > continuing > > to > > > > drive an AC sequence at high frequency since it has no feedback abo= ut > > the > > > > motor movement. > > > > > > > > With an induction motor, there would still be torque in such a > > situation > > > > and if the load on the motor became slightly lighter the motor woul= d > > > begin > > > > to spin again. With a permanent magnet motor, there woudn't be much > > > torque > > > > at all when the slip is so extreme so it would require resetting th= e > > VFD > > > or > > > > manually slowing it down until there was sufficient torque again to > get > > > the > > > > motor spinning. During this time when the slip was extreme the moto= r > > > could > > > > be getting quite hot. > > > > > > > > > > > > Sean > > > > > > > > > > > > On Sun, Aug 5, 2018 at 12:17 PM, Jason White < > > > > whitewaterssoftwareinfo@gmail.com> wrote: > > > > > > > > > Hello, > > > > > > > > > > I am working on rewiring a small CNC mill. For the spindle the > > existing > > > > > unit has the following brushless DC motor with hall effect sensor= s > > (see > > > > > attached drawing). It is rated at 160V, 7A. > > > > > > > > > > Could this motor be driven by a VFD intended for conventional > 3-phase > > > AC > > > > > induction motors? As I understand it most 3-phase AC motors are > > 220Vac > > > > > which would suggest that the answer is no. > > > > > > > > > > For this application I do no care about slippage or precise speed= .. > > > > > > > > > > Thanks, > > > > > Jason White > > > > > > > > > > -- > > > > > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > > > > > View/change your membership options at > > > > > http://mailman.mit.edu/mailman/listinfo/piclist > > > > > > > > > > > > > > -- > > > > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > > > > View/change your membership options at > > > > http://mailman.mit.edu/mailman/listinfo/piclist > > > > > > > > > > > > > > > > -- > > > Jason White > > > -- > > > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > > > View/change your membership options at > > > http://mailman.mit.edu/mailman/listinfo/piclist > > > > > -- > > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > > View/change your membership options at > > http://mailman.mit.edu/mailman/listinfo/piclist > > > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .