Re: [CAD_CAM_EDM_DRO] linear encoder protection, stepper motor 'power rating' question, motor temperature

> Contrary to the discussion on this list that higher voltage drive reduces
> the current requirement, commercial drives pump in the current they're
> jumpered for, and another sobering thought is that some of those big drives
> have 3 phase 208 voltage supply, and have big fans and heatsinks. I guess
> the electric bill would tell the story, but I have a hard time seeing 90 V @
> 13.2 A per phase (x2) = 3.19 hp. The motor is smaller than a 3/4 hp
> induction or synchronous motor...could the above numbers be 'peak hp', like
> vacuum cleaners specify?

Another way of looking at it is that a good chopper drive is really a
form of switching power supply. Just like a transformer, this takes
one voltage and current relationship and transforms it into a
different voltge current relationship with relatively constant power
(minus switching losses).

They do pump rated current through the motor, but not at the rail
voltage. Instead, the effective voltage that governs the current flow
is the rail voltage minus the back EMF produced by trying to turn on
the inductive winding. As soon as the back EMF dies down, the chopper
drops the effective applied voltage by lowering the duty cycle to keep
average current from becoming excessive. Hence the power applied by a
chopper is not all that much greater than what the motor is rated to
dissipate as heat while holding still - any power not being converted
to work is going up as heat, and too much of that would fry the motor.

What we are still missing is a good design figure for sizing the rail
supply current capability.

Chris

--
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