Re: Stepper Drivers

> From: "Matt Shaver" <mshaver@...>
>
> > From: stratton@...
> > Prototype is going to be with the big chip...
>
> What are you going to use between the step and direction signals and the
> H-bridge? Ericsson?

Probably an L297 doing step/direction to phase sequencing and chopping.
Why? Well I made it work with the little L298 bridge...

Next question: power supply sizing. Say I run a parallel-wired
stepper off a chopper drive set for 6.6 amps per phase with a phase
resistance of .29 ohms, using a 48 volt nominal supply. The motor is
consuming around I^R or 12.6 watts.

According to the user's manual for a pacific scientific drive, the
secondary of a brute force supply should be rated for 1.8 RMS amps
times the chopper setting. In other words, they want a transformer
rated for nearly 12 amps at 48 volts, or 756 VA to drive this little
12.6 watt motor.

Does this make sense? I would think that the filter caps would absorb
all the chopping-frequency alternating currents, leaving the
transformer to simply deliver the overall power needs. I can't see
where the difference between 756 VA and 13 watts is going. Up in
heat? Where? What gets hot?

Perhaps I misunderstand the physics of chopper driven motors. In
fact, the more I think about it I'm sure I do. Mechanical power is
related to torque and speed, and is related to electrical power.
Sitting still with a non-chopping driver and consuming 13 watts, the
motor may be holding against its rated 450 oz/in, but it is not
moving, hence it is not doing any work. The faster I want to make it
spin (using my chopper to keep the torque up) the more work it must
do, so the more electrical power it draws.

Perhaps I should estimate the torque/speed I need and convert
backwards into units of electrical power?

Obviously new at this,

Chris

PS -anyone know a cheap source of big transformers?

--
Christopher C. Stratton, stratton@...
Instrument Maker, Horn Player & Engineer
30 Griswold Street, Cambridge, MA 02138
http://www.mdc.net/~stratton
(617) 492-3358 home/shop