Aaron,

They are expensive because they offer lots of features and the good ones
are usually pretty close to bulletproof.  I notice your design is
unidirectional, no feedback (either velocity or torque), no power factor
correction, no short circuit protection, no field supply, ... .  Not
that any of that's a problem if it meets your needs.  It's just that
many servo applications are produced in small quantities and the effort
(and cost) involved in designing and manufacturing a couple of (well
designed) servo amps quickly exceeds the cost of those amps.  Now, if
you need something special or are going to produce something in
quantity, that's a different story.  Hope this helps answer your question.

Best regards,

Dave

A.J. Tufgar wrote:
> Hey all,
>         I've been looking around the net for a variable speed motor
> controller for a 2HP 120Vdc motor and have found they are rather
> expensive.
>
> I decided I might try making the controller myself, but I'm thinking
> the controllers must be expensive for a reason.  Anyone know why?
>
> I've attached a rather simple design to this message and would
> appreciate any comments.
>
> I have yet to design the inrush current limiter, but will probably use
> an active mosfet design similar to the following:
> https://www.onsemi.com/pub/Collateral/AN1542-D.PDF page 2.
>
> Thanks for any input or help you may offer,
> Aaron
>
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