[Info-vax] OT: the Daily WTF for today is a VAX/VMS story

[Chris]

On 02/14/16 22:30, Jan-Erik Soderholm wrote:>
>
> The wood work machinery in my grandfathers shipyard had 3-phase AC
> motors. It took several minutes to rev up some of them to speed.
> You put the switch first to the "Y" position and when the speed
> of the machinery was stable switched to "D". Seems to be called
> "star-delta (YΔ) starting" in English...
>
> So I do not understand this:
>

That's because an ac motor at stall looks like a short circuit to
the line. Think of the rotor as a transformer secondary. Not so
troublesome with small motors, but big motors need current
limiting on startup to avoid overload. To do that, they
"rewire" the windings in series at startup to limit the
current to a safe level. Once the motor has spun up, back emf
from the rotor cancels out running current. proportional to
load.

There may still be many star delta systems in use, but modern
practice typically uses a variable speed drive, with an
inverter to provide programmed frequency, ramped voltage and
current, to allow the motor time to spin up to running speed...

Regards,

Chris