Post by Sean BrehenyThe 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 the
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 motors
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 pole
count divided by 2). This means that not only is there no special alignment
information to know about the rotor at startup, but in fact the motor needs
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 VFDs
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
Post by Justin RichardsOutside of the OP requirements but I am curious to know if it would
perform
Post by Justin Richardsfavorable if the BLDC was only ever driven at relatively slow speeds by a
VFD turning a heavy load.
Justin
Post by Jason WhiteOkay, thanks Sean and Alan. I figured that probably was the case.
Post by Sean BrehenyMost likely it would work most of the time. However, I would be
somewhat
Post by Jason WhitePost by Sean Brehenyconcerned about what might happen if the load on the motor changed
suddenly. In such a case the motor might stall with the VFD
continuing
Post by Justin Richardsto
Post by Jason WhitePost by Sean Brehenydrive an AC sequence at high frequency since it has no feedback about
the
Post by Jason WhitePost by Sean Brehenymotor movement.
With an induction motor, there would still be torque in such a
situation
Post by Jason WhitePost by Sean Brehenyand if the load on the motor became slightly lighter the motor would
begin
Post by Sean Brehenyto spin again. With a permanent magnet motor, there woudn't be much
torque
Post by Sean Brehenyat all when the slip is so extreme so it would require resetting the
VFD
Post by Jason Whiteor
Post by Sean Brehenymanually slowing it down until there was sufficient torque again to
get
Post by Justin RichardsPost by Jason Whitethe
Post by Sean Brehenymotor spinning. During this time when the slip was extreme the motor
could
Post by Sean Brehenybe getting quite hot.
Sean
On Sun, Aug 5, 2018 at 12:17 PM, Jason White <
Post by Jason WhiteHello,
I am working on rewiring a small CNC mill. For the spindle the
existing
Post by Jason WhitePost by Sean BrehenyPost by Jason Whiteunit has the following brushless DC motor with hall effect sensors
(see
Post by Jason WhitePost by Sean BrehenyPost by Jason Whiteattached drawing). It is rated at 160V, 7A.
Could this motor be driven by a VFD intended for conventional
3-phase
Post by Justin RichardsPost by Jason WhiteAC
Post by Sean BrehenyPost by Jason Whiteinduction motors? As I understand it most 3-phase AC motors are
220Vac
Post by Jason WhitePost by Sean BrehenyPost by Jason Whitewhich 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