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[DPRG] Big Robot Base Advice Please.

Subject: [DPRG] Big Robot Base Advice Please.
From: Randy M. Dumse rmd at newmicros.com
Date: Tue May 23 16:52:31 CDT 2006

>  The big question I have is whether 2x a parallax HB 25 motor
controller
> driving a 350W scooter motor will get the job done,
> ...
> I want to use Parallax’s new HB-25(amp) motor controllers.
> (http://www.parallax.com/detail.asp?product_id=29144) It can
control a motor
> up to ½ Hp.  They are relatively inexpensive and I like
parallax’s products,
> but I am having a hard time finding motor/gearbox combos that
are close to ½
> Hp.  Any suggestions here would be fantastic.

Jonathan, I'd be careful about these bridges actually delivering
25A. Pololu made one with the VNH3SP30 and most only get 7 to 9A
out of it. We know the chip on the board because we have also
tried it. The most we've been able to get continuous with a fan
is about 15A. It was rated by ST at 30A but I don't know how
they made that claim. Also it was rated up to 36V operating with
10KHz. Parallax might have a later version of the chip, a
VNH3ASP30, which we haven't gotten yet. It has a lower high
voltage, limit 16V, and a higher PWM rate 20KHz. So. maybe, its
problems are fixed. But I would be very skeptical. (We're
checking on availability of the newer part again. Nope, no one
has them yet we can find.)

Now, if it is indeed 16V and 25A as they claim, you have 400W
which is your half horse power. But I doubt you can get but
about 2/3 of that continuously. You'll have trouble (as you are
finding out) matching a 1/2 horse motor to that 16V upper limit.

You know, a human can barely reach 1/2 horse power output for a
brief moment. I think generally moving around (briskly), we only
use something like 1/7 horse power. I think you are over
designing for power anyway.

Randy
www.newmicro.com



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